| Cam Theory |
I have heard it said before and it is worth repeating for this article that if a Briggs motor has a heart it is the camshaft. More questions are raised over this one reciprocating piece than any other part of the motor.
There are many areas to consider in examining cams. They are lift. duration, timing, profile, center lines and the every popular Briggs easy spin. We will examine all of these aspects of camshaft function and design in this article and I will leave you with my specific thoughts on the best cams for Briggs stock classes as the rules currently permit. We will first try and discuss cam dynamics in a very general form and then move on to the unique aspects of the Briggs camshaft under WKA stock class rules.
A general description of valve train function in any race engine is relatively straight forward. The valves must be lifted as high as possible to give maximum air flow both in and out of the cylinder. The rate of valve lift(relative speed of opening) must be as fast as possible to give the maximum amount of air flow for the maximum amount of time.
The opening of the intake valve must start upward at a slow rate in order to take up valve clearance as well as reducing shock to the valve train. Once this has been accomplished, the lob can push as hard as possible to raise the valve as fast as is possible, but as the lifter nears the nose of the cam the rate of valve lift must slow down so that the valve spring is not overcome and allow the components to loose contact. On the backside of the intake, the lob can let the valve drop as rapidly as possible. Since the intake valve is being cooled by the rush of cold methanol, the lobe doesn't need a long closing time frame. Because of the relative cool temperature of the intake valve it can slam home against the seat without fear of warpage unlike we have with the exhaust.
On the exhaust side we have a different situation. The lobe must again start slowly taking up valve slack. Once this is done, we still want the valve opened as rapidly as possible in the shortest amount of valve timing. It is the backside of the exhaust lobe that is different. Because of the red hot temperatures experienced by the exhaust valve the cam must let the valve down on the exhaust seat in a rather soft manner. This will prevent warping or possible fracturing of the valve and seat. Due to this problem the closing of the exhaust must be fairly long in order to gradually slow down the valve closure and set the valve down gently on it's seat.
VALVE TIMING
More than any other thing, overall performance from a camshaft is determined by the timing events of the cam. These are the points when the intake and exhaust valves open and close. This timing is described by measuring the relative point in crankshaft rotation. The points that these vents occur as well as the overall duration(relative time the valve is open) is a standard method for comparing camshafts.
Many factors play in determining the best camshaft for a given motor. These include the efficiency of the intake or exhaust port, the carburetor, the exhaust header and on and on. More than anything else the valve timing is affected by engine speed. At slow cycling speeds the relative elapsed time between the valve events is long. However as the cycle rate is increased the timing for each event gets smaller and smaller. As engine speed increases, the time between the events will decrease to such an amount that efficiency is impacted. The only way to counteract this is to increase the duration between the valve events. It is very common in high speed racing engines to extend the valve open duration.
What is the effect of extending the duration? First, the longer duration raises the effective engine speed (rpm) range. If there is a larger duration between valve events there is more relative time between events at higher engine speeds. This allows the rpm to go higher before we reach restrictive event timing. This will only give more horse power if the intake or exhaust system can give increased flow or fuel mixture. As we increase the duration, low speed power will be DECREASED. This is caused by both the increase of the duration and the phasing of the intake and exhaust cycles.
There is a small period of valve timing where the exhaust valve is closing and the intake valve starts to open. Both valves are actually open at the same time. This event is called OVERLAP.
Interesting things happen during overlap. Since the intake valve is being opened while hot exhaust gases are rushing out of the exhaust track, an effect known as intake draw-though can occur. This effect can actually 'pull' the intake mixture into the cylinder. This effect actually increases with rpm. The faster velocity in the exhaust during high rpm will create a lower pressure in the exhaust track(trust my physics prof). If the intake is opened as the last of the exhaust gases are being pulled from the cylinder the intake charge is sucked into the chamber. This effect can be enhanced by opening the intake valve earlier in the cycle. This is great until the pressure in the exhaust gets lower and draw though actual begin to pull some of the intake mixture out of the exhaust pipe! This is an important consideration in a restricted motor.
Exhaust opening happens at the changeover from the power cycle to the exhaust phase. Generally it is to your advantage to open the exhaust valve sooner. If the valve is opened during the period of still high combustion pressure, the pressure will zip past the valve creating an initial surge of flow. This event is sometimes called exhaust blow down. Open of the exhaust is a compromise . Open it too soon and combustion pressure(power) will be lost out the exhaust port. In general very little effect will be seen from changes in the opening in a race engine.
Exhaust closing will have a much larger effect on engine performance. If the valve is closed later the engine will generally want to rev up better than a motor whose exhaust closing is earlier ( 22 degrees ATDC verse 15 degrees ATDC). Later exhaust closings will also help to cool the motor in comparison to early closings. The only disadvantage is that we are increasing overlap by closing the valve later. The will again hurt a restricted motor and low end torque.
Intake closing is possibly the most important timing event. Close the valve early(82- 78 degrees BTDC) and torque will be helped in the lower rpm ranges. For higher rpm motors a later closing(75-65 degrees BTDC) can be used, as the engine will be operating in a higher rpm range and the overall duration will need to be increased. This logic also applies to the easy spin opening if you belive what Briggs says about the easy spin basically being the closing point for the intake valve. Later openings such as 51-55 degrees will help a higher rpm motor. |
| Cam Application |
Cam Lobe Center lines are really pretty easy to figure and I'll give you a couple of ways to go about it. Lobe Center lines give you a relative perspective of how advanced or retarded a cam is in relation to top dead center(TDC). In a Briggs motor under the WKA/IKF cam profiles it is possible to have an intake center line from 98 to 103 degrees. Exhaust center lines run from 114 to 109. An intake center line of 98 is considered to be the most advanced and generally gives the most torque. The 103 center line will give power in the upper rpm range.
Exhaust centerline of 114 is the most advanced while the 109 is the most retarded. Again an advanced lobe will give power in the lower rpm range while the retarded lobe will have it's power range extended in the rpm range. For practical terms, most Briggs ground cams are in the range of 100 to 103 on intake and 110 to 112 on the exhaust.
To figure the intake lobe center line do the following: While profiling the intake lobe rotate the motor until you are at TDC and read the dial indicator over the intake valve(always use a dead valve or one with 0 clearance). It should be somewhere between .055 and .065 lift. Write this number down. Now rotate the motor until you reach the same lift before the valve closes. Note the degrees on the degree wheel. Now add 180 to the degrees noted and divide the sum by 2. This will give the intake center line.
Example: Rotating the motor you find that at TDC the dial indicator reads .058. After continuing to rotate the motor and stopping at .058 on the down side of the lobe you find the degree wheel reading 22 degrees ABDC. 180 + 22 = 202/2 = 101. This is the center line.
To figure the exhaust center line do the following: Right after the valve passes through the .050 lift point on the closing side and before reaching .020 lift you will pass TDC. At TDC read the dial indicator and mark this number down. It will probably be somewhere around .026 and .034 lift. Now continue to rotate the motor until the valve closes and begins to open again. Stop the motor when you reach the lift noted above. Read the degree wheel. Take this reading and add it to 180 and divide by 2. This will be the exhaust center line.
Example: While the exhaust valve is closing the lift noted at TDC is .034. Continuing to rotate the motor and when the valve starts it's opening cycle stop the motor at .034 lift. You read 44 BBDC. Adding 180 +44 = 224/2 = 112. 112 is the exhaust center line.
If you want a really simple and slightly less accurate way of finding your center line do the following. After profiling the cam to compare it against the WKA/IKF specs look at the .220 lift numbers. On the intake simply take the .200 number going up such as 34 and subtract it from 180 and add the .200 number going down such as 56. Divide the total by 2 and you will get a quick look at your center line. You can do the same on the exhaust but in reverse. Always subtract the smaller number and add the larger.
Cam duration is simply the opening point of the valve + 180 + the closing point. A cam opening at 22 degrees BTDC and closing at 75 degrees BTDC, would give a duration of 22+180+75 or 277 degrees. Most cam builders will tell you to calculate the intake closing at the point where the easy spin starts. The easy spin was developed by Briggs to help make cranking the motor easier. WKA says the easy spin must start between 45 and 60 degrees ABDC. Rules also say the lift at the point of easy spin must be between .013 and .019. The easy spin must be a minimum of 30 degrees in duration. Most Cams today have around 33 - 39 degrees.
Specific Recommendations:
For rookie motors and any restricted class keep the overlap to a minimum. Opening less than 20 degrees BTDC and closing around 80 degrees BTDC on the intake and exhaust openings around 58 degrees with a closing around 15 degrees ATDC will give good torque and minimum overlap. The intake center line should be 99 -100 with the exhaust around 112. For purple motors keep the rpm to around 4500 and for gold plate don't go above 5400.
For seniors or stock heavy, use a very similar cam to the rookie profile with the exception of possibly opening the intake earlier and closing the exhaust later. Keep the rpm in the 56-5700 rpm range.
Stock medium and stock light can take advantage of a higher rpm cam with center lines around 101 on the intake with 110 on the exhaust. Intake opening can be as much as 30 degrees but current practice is to limit this to around 25. intake closing should be around 75 degrees BTDC. Exhaust opening should be around 60 degrees with a closing around 20. This will allow the motor to rev better and produce power in a higher rpm range. Rpm range should not go above 6000 as you simply don't have enough air through the stock carburetor.
Super Stock motors can use intake openings above 30 degrees, but you will be very surprised how well the stock light cam profiled above will run.
Large tracks of 1/4 mile or so like cam intake center lines around 99 with a exhaust line of around 110. This cam will have a broad power band. Smaller tracks can use rather peaky power bands with the key to success being gearing. Peaky cams are generally ones that have both retarded intake and exhaust center lines.
Please give Mike Bordeaux at DYNO cams a call and ask for his free catalog. This will give you a great chance to look at many different profiles and combinations. It will become a good reference for profiling any cam. DYNO cams can be reached at 910-655-9035. p.s. Don't be afraid to ask them to grind a cam for you with any combination of intake and exhaust profile from any of their cams. tough to beat the 95-3 for a good all aroudn cam for almost any application. I like it in the plate classes as well as hte light classes. The 99-3 is a killer heavy cam as is the 96-3 |
| Cam and Ignition Timing |
In this article, we will attempt to explain how to setup and degree in a camshaft to insure it meets the makers specifications as well as the tech guidelines. You will first need some basic tools to do this.
The major tool needed is a degree wheel. These are available from most all kart supply houses as well as local shops. They run the gamut from a 7" diameter plastic model to a much larger and more expensive 11".
If you are just doing your own motors the plastic wheel will do just fine. The advantage of the larger wheels is durability(most are aluminum) and precision(easier to read the exact degree you are on). All degree wheels are marked for both TDC(top dead center) and BDC(bottom dead center) with 180 degrees marked in between each making up the full 360 degrees in a complete circle or one rotation of the crankshaft.
The easiest method for bolting on the wheel is to use the bolt and washers you use on the end of the PTO shaft for your clutch and bolt the degree wheel up to the PTO side. You will also need a 1 " dial indicator and a bracket that will position the indicator over the valves. These can be purchased from most kart shops or supply houses.
The bracket is around $18 and the dial indicator can run from $20 to $70. You will also need some new valves that have not been clearanced or some older valves with their seats ground off to allow the valve to sit on the lifter with 0 clearance.
You will need to make or purchase a pointer which will attach to the block and be used to reference the exact position on the degree wheel. I have seen everything used from a piece of coat hanger bent 45 degree with a loop on one end that is bolted to one of the head boat holes to very fancy stainless steel pointer attached to the magneto bolt holes.
The deal here is to have a stable pointer that will not move while you are degreeing in the cam.
FINDING TDC - The first step in setting up to degree a cam is find TDC of the crankshaft rotation and make your degree wheel pointer reference TDC on the wheel at this point in rotation. Where is TDC?
TDC occurs on the compression stroke of any 4 cycle motor. As you rotate your motor forward you will notice it goes through cycles of the intake and exhaust valves opening and closing. TDC is when the piston reaches it's most upward movement during the cycle after the intake valve closes.
The most accurate way to find TDC is to use a positive stop. This involves stopping the piston slightly before TDC. There are many ways you can do this but the simplest is to take a 5/16 bolt and large washer, mounting it on one of the head bolt holes near the middle of the cylinder.
Now take dime or any thick washer and place it on the piston so that it will hit the washer we just bolted onto the block as the piston is moving up. This should stop the piston and crankshaft movement prior to it reaching TDC. Take notice of the point at which this stop occurs on the degree wheel. Lets say it's 12 degree BTDC.
Now rotate the crankshaft in the reverse direction until we again hit the washer and stop the crankshaft movement. Take note of where this occurs, say 14 degrees after TDC. TDC is exactly between the two points we have noted. In this case we are very close and only need to move either the point or degree wheel over 1 degree to split the difference between the two reference points(12 and 14). Keep doing this cycle until you have both markings the same number of degrees before and after TDC.
At that stage your point will be referencing exact TDC.
DEGREEING THE CAM - Once you have found TDC, we can proceed on to degree the cam. Both WKA and IKF have the same cam timing reference points for a legal Briggs camshaft. Each point is at .050 height intervals as the camshaft raises and lowers the valve. These cam specifications give you some play at each .050 point. Following are the specs for both the intake and exhaust valves.
LIFT | INTAKE | LIFT | EXHAUST | | | degrees | | degrees | 050 | 7 - 0 BTDC | 050 | 38-33 BBDC | 100 | 10-17 ATDC | 100 | 21-16 BBDC | 150 | 29-36 ATDC | 150 | 2 BBDC - 3 ABDC | 200 | 55-64 ATDC | 200 | 21-31 ABDC | Max Lift | .233 | Max Lift | .233 | 200 | 43-33 BBDC | 200 | 76-65 BTDC | 150 | 13- 6 BBDC | 150 | 48-40 BTDC | 100 | 6-13 ABDC | 100 | 28-21 BTDC | 050 | 23-31 ABDC | 050 | 10- 4 BTDC |
To be legal the camshaft must fall in between the degree numbers on the chart, so let's begin. With the intake valve in the closed position and the dial indicator positioned over the valve and set at 0, rotate the motor in the forward direction until the valve just begins to move or open. Now slowly rotate the motor until the degree wheel points at the makers recommended open point(generally somewhere from 35 to 16 degrees BTDC). Once you reach this point stop and look at your dial indicator and make note on the exact reading. This height will be the valve clearance you will need for the valve to open at this precise point (This should match your cam maker's specs). I like to take note of the degree wheel position at several indicator points, so I can change the valve opening, if I am going to later experiment with this. I will generally note the degree wheel point from .003 to .006. Most cams today have recommended clearances in the this range. After finding the valve opening point continue to slowly rotate the motor until the dial indicator is at .050. Read the timing off of the degree wheel at this point. It should fall between 7 - 0 degrees BTDC. Now continue rotating the engine until you reach the next point on the chart, 100 in this case. Take the reading here and write it down also. Continue to rotate the motor and mark down the degree wheel points at each of the following reference heights(150, 200). After the .200 point slowly rotate the motor looking for the maximum lift of the cam. Most cams will be from .232 - .233. The legal limit is .233. Once you have noted the max lift, continue rotating the crank marking down each .050 point on the downward side of the intake valve. After you record the .050 point on the closing cycle slow down and begin to look for the easy spin beginning. Easy spin? What the $^##@@ is easy spin. Remembering that what we are dealing here with is a tiller motor, Briggs developed a way to make cranking the motor a little easier for the general public. They did this by stopping the closing of the intake valve for a period of time just before closing the valve. As the close cycle of the intake valve is on the compression stroke of a 4 cycle, what this does is bleed off a little bit of compression making it easier to pull the starter. If the valve closed with no easy spin compression would have a point sooner within this cycle to build within the cylinder. Back to degreeing the cam. As you approach 45 degrees ABDC slow down the rotation and look for the point where the dial indicator stops it's downward decent. This is a bit tricky and you may need to back the motor up and try again to find the exact point. If you have to back up, go back to at least .150 on the dial indicator and start again. This will take out any slack in the crankshaft movement. The is also true if you miss one of the .050 points you previously recorded. To be WKA legal the easy spin must start between 45 and 60 degrees ABDC and last for a minimum of 30 degrees. Lasting for 30 degrees means the valve can only move .001 during this event, up or down. Most easy spins on today's ground cams start around .015 - .016 lift remaining. The legal variance here is from .013 to .019 . After you find and mark down the easy spin starting point, rotate the motor until the valve drops or raises 1 degree from the starting easy spin point. Now take note of the degree wheel position. this will be the easy spin stop point. The difference between the start and stop must be the 30 degrees. Most cams have more than 30 degrees giving you some room for wear. This is the spot on the cam that seems to wear out the most so be sure to check this carefully on any used cam. Quoting directly from the WKA manual " If ez-spin starts at .015, it may drop to .014 and move around between .014 and .015 but not go above .015." Also " if ez-spin starts at .015 and rises to .016, it may move around between .015 and .016, but at no time fall below .015" At no time can the ez--spin or the .001 travel go above .019 or below .013". IKF specs are different so check their tech manual in this area. After we are done with the easy spin, let's continue on looking for the point where the intake valve will close. This will be the point on the degree wheel where we encounter the same height we marked for our intake valve opening. If your cam let's say had a recommended clearance of .005 you will want to look for that height as the valve is closing and mark down this point. Generally this is somewhere between 85 and 65 degrees ABDC. If you marked other opening heights earlier, mark the same closing points now for future reference. After finishing the intake valve, move your dial indicator over to the exhaust valve and zero it on top of a closed valve. Now you repeat the above process marking each .050 lift point as the valve raises and lowers. There is no easy spin on the exhaust valve. Well there you have it. Hopefully you will be able to degree in a cam at this point. It may take you a couple of times to really get the feel for the process but stay with it. If you have a cam that will not come into spec call the maker and ask for his advise. You will find small variances from the manufacturers specs due to the differences in blocks. SETTING TIMING Setting the timing of a Briggs is fairly straight forward. What we are going to do here is set the static point within the compression cycle at which we want to fire the spark plug. This is always BTDC as it takes a little bit of time(movement of the piston) before the ignited air and fuel actually reach their maximum point of expansion(power). While you still have the degree wheel on the motor and referenced to TDC, mount your flywheel and magneto. I generally mount the mag with .015 clearance. the best way is not to use feeler gauges but fine some hard paper that is the thickness you want for the clearance. Place this between the flywheel and the bottom legs of the magneto. Tighten down on the mag bolts and you have your clearance. One of the things we need to know when setting the timing is where the magnet is on the Briggs flywheel. We will use the right most edge of the magnet as reference point in this process. The magnet is housed within the aluminum part of the flywheel. On an older rusted flywheel I like to sand the magnet area to highlight it. As the magnet it is not aluminum you can also blue the magnet to make it stand out from the lighter aluminum. You can use tool makers layout bluing or get some instant gun blue from your local Kmart or gunshop. To set the timing we are going to use the rightmost edge of the magneto leg and line it up with the right most edge of the magnet. If you look closely at the magneto's leg you will see a slight indention on the bottom. This is the point I like to use for lining up the magnet. It is a little more exact that way. After you line up the magnet and magneto take a look read your degree wheel. This will be the static timing point. This is generally set from 24 - 30 degrees BTDC on a methanol Briggs. For restricted motors, I like to set it between 24-26 and on a stocker set it 28-30. You can vary the timing by changing the offset keys used in mounting the flywheel or by just changing the position of the flywheel when you bolt it up on the motor. Some builder will use the offset key while others will simply mount the flywheel without a key. Either method will work fine if you torque the flywheel to at least 70 Ft. Lbs. I use the offset keys and have never had a flywheel slip timing. The offset keys are available from any kart shop or mail order house. They come in .010 increments which does not mean they change the timing by 10 degrees. The increments are the amount of offset on the key. You always want to mount the key with the lower side toward the valve side of the motor, or with the flywheel turned slightly clockwise in relation to the crankshaft. Keys vary from maker to maker and different flywheels will have their keyways cut different from one another, so don't put a .30 offset key in a motor and expect it time the same as the last one you did!!! The most common keys you will use will be from .020 offset to .050 for restricted and stock motors. This method seems to be the standard among current engine builders. Some use other reference points but as long as you always use the same method on your motors you will have a reference point to experiment from. You can also measure and set your timing without having the degree wheel set up. This method is called the 'in the hole' method of measuring timing. Mount the flywheel as you would normally with the offset key you want to try. Now set your dial indicator up over the piston. What we are going to do is measure the travel to TDC when the magnet and magneto are lined up. Once you have them lined up as described above, zero your dial indicator on the piston. Now rotate the crankshaft forward until you reach the upper most height of piston travel.(TDC). Read you indicator and match it's reading to the closest one on the chart below. This chart will give you the corresponding degrees for a given height.
.1157- 22 .1262- 23 .1372- 24 .1486- 25 .1603- 26 .1725 -27 .1851- 28 .1981- 29 .2115- 30 .2252- 31 .2394- 32 .2539- 33 .2687- 34 .2839- 35 I generated this chart from an old BASIC program on my PC and will put it on the web page for everyone to use a little later. The calculations use a standard Briggs stroke of 2.437 and a rod length of 3.873(center to center) Good luck and happy tuning-- Jimmy Glenn |
| Carburetor |
The second most important piece on a Briggs stock class motor (cam is #1) is a good carburetor. I will attempt to go through the basics of building any stock carb and then get into a few items that are a little on the secret stuff side.
After pulling the carburetor and tank off a new motor remove the three screws that hold the carb onto the tank. It's my option that you should reuse these screws as they have lock washers on them and work quite well. Many builders will trash these and use allen bolts for replacements. All well and good If you include lock washers. If not keep the stock screws as these will not back out under vibration!
If present next remove the swirl from the throat. The secret here is to grasp the swirl with a good pair of pliers and twist the swirl slightly clockwise as you pull the swirl out. The only time you may want to keep the swirl is in the purple plate restricted class. Due to the small amount of air flow with this plate I believe the swirl will help with the mix of air and methanol.
With a 1/2" wrench you can remove the high speed needle and nut as a unit. Next take a good flat screwdriver and unscrew the brass jet. Us a good screwdriver as brass is very easy to damage. Keep the jet as we'll modify it later for use in a methanol motor. Now remove the four screws retaining the diaphragm cover and carefully pull the plate, diaphragm, cap and spring from the carb. Take the cover and surface the side that butts up to the diaphragm. The easiest method is to lay a piece of 400 wet/dry sand paper on a flat surface such as glass and using a light oil, rotate the plate in a figure 8 pattern until the mating surface is flat. This will insure good fuel pressure and no leaks! You should change the diaphragm about every 6 races. Remember it's your fuel pump.
Back the idle screw out until it no longer is engaged. Using a long screwdriver reach down the throat of the carb and remove the screw holding the butterfly. Be careful not to mare the screw as it is directly in the air flow line. After removing the screw turn the carb throat down and shake out the butterfly. Final remove the throttle shaft. Don't loose the felt/foam washer.
Using the long screwdriver knock the plug out of the back of the carburetor.
Now we are ready to really work on the carb. First measure the backside thickness of the throttle shaft. WKA rules permit it to be a minimum of .086. Most stock shaft are around .095 so you can CAREFULLY take a small jewelers file and bring the backside down to around .088. Again this piece is directly in the air flow so every little bit helps. 96 rules now state that the front edge can be no smaller than .040 so not much here.
Next lets move to the area left from removal of the jet and high speed screw. You will see two holes. For methanol racing these should be enlarged as follows.
Use a #71 drill bit to enlarge the smaller hole. This bit is .026 and will keep the hole under the WKA minimum of .028. For the larger of the two holes use a # 53 bit. This will keep the main metering hole under the .062 required.
NOTE**** Do not use an electric drill for this modification.
Get a jewelers hand bit holder for this operation and do it by hand. It's simply to fine an operation for an electric drill. Generally after you do this step if you look down the throttle bore you will see small rises where the drill bit came through the bore. These are illegal but we will get these out with the next step.
You can now turn you attention to the bore. WKA specs say the maximum bore size is .695. To get the bore to as close to this as possible many builders us an adjustable reamer. Personally I purchased an .690 reamer from a great tool company, MSC Industrial Supply, located in Plainview Ny. They can be reached at 800-645-7270 and will be glad to send you their 3" thick catalog! If you love tools this catalog will keep you busy for several evenings. Anyway back to the bore. Take the reamer and starting from the front of the carburetor ream the bore out by reaming all the way through in one pass. I bolt the reamer in a vice and rotate the carburetor. Use a light oil and keep blowing our the scraps of aluminum being shaved by the reamer. Pull the reamer out the back after the single pass.
Now you are left with a fairly rough bore. Not good for air flow, so let's now hone the bore smooth. The best tool I have seen is a Flex-hone. These are the absolute best finish hones on the market. They have small balls of abrasive on the ends of flexible rods enabling the hone to conform to any bore size. These are also available from MSC as well as a company in Ca. named Cylinder Head Abrasives(800-456-5474). The size you are looking for is 18mm. (.709) After obtaining the hone you must cover certain parts of the carburetor. Only the actual bore can be honed or scratched! You can cover the front of the bore as well as the back portion under the venturi by carefully placing clear sealing tape over these areas. Just be sure to clean the carb first with a good degreaser so the tape will stick. Now using a slow speed drill such as a portable, hone the bore in a back and forth manner for about 25 cycles. Remove the hone and wash the carb in a good cleaner.
You can now plug all of the holes in the venturi end of the carb. I use plain old blue silicone gasket sealer. Be sure the sealer does not protrude into the bore.
Now you can assemble the carb in reverse order. The final step is to adjust the throttle shaft angle to give the maximum air flow. This is where you really need a flow bench. If you have a Kart shop near by many times they will allow you to use their bench for free or a very reasonable price. By bending the throttle stop you can adjust the angle of the butterfly until you get the best air flow. If you can't gain access to a flow bench try and get the butterfly straight in the bore. This will get you very close. If you are setting up for any of the restricted classes you will need to offset the butterfly and not let it be straight in line with the bore. Maybe a 1/4" offset. If you have a flow bench and are flowing for restricted classes use a gold plate to flow with as the other plate restrict air flow to the point that it is very hard to see any change in throttle angle on the bench. In a future article, I will tell you how to build your own flow bench for under $150 using high quality gauges!
When bolting the carb back on the tank be sure and use two tank gaskets. This will insure a good seal and keep the long pickup tube from possibly touching the bottom of the tank and ruin fuel flow.
As a final step obtain several new jets from your local Kart shop and drill them with the following drill bits
1.25mm bit = .050 size hole
# 55 bit = .052 size hole
# 54 bit = .054 size hole
# 53 bit = .059 size hole
As a good starting place, the .052 or .054 jets will do for 99% of all applications. During the summer as the air heats up and becomes worse (thinner) you can move to the .050 jet, as long as you can keep your motor temperature at a reasonable range (<420 degrees). I really like to keep mine around 380-390. The .054 will work well during the winter when the air is denser. For a purple plate motor, use the .059, as you need to pull whatever fuel you can into the motor given the limited air flow.
nbsp;If you are really into looking for a good carburetor, try and find an older number 2 or 4 casting number. This number is located on the top side of the carb opposite from the diaphragm. The number will be sideways. The vast majority of carbs are number 5. While there are many good #5 carbs out their there are lots of really average #5s to boot. Frankly some carbs just plan flow better than others. Briggs has several molds for these, and some molds are better than others! I know of one Kart shop that will go through 100 new carbs just to get 10-20 good ones! Most #2s, and 4s I've found are better than average carbs. The number 2s usually are found on older tiller motors. The number 4s were made in small supply about 5 years ago. GOOD LUCK.
While I'm thinking of it be sure and always use a air filter! Yes there is a very very slight air flow restriction even with a clean K&N filter, but unless you get all of your internal parts for free, you will be well advised to run the filter. Always clean and re-oil your filters after a weekends race. I actually use 2 or 3 during one days race, changing as they get dirty.
As a final note be sure and protect the carb you have just built by using a tank brace. I have used the one that bolts to the bottom tank bolt and uses a large hose clamp to secure the top portion around the tank. I have never broken a carb and I run on some pretty rough dirt tracks. Good insurance |
| Piston, Rod, and Ring Prep |
After getting the right camshaft for your application and getting the best carburetor you can possibly obtain, the third most vital piece in setting up a Briggs stock class motor is making it loose. Loose is fast but you must also have good ring seal so as to not lose compression. Fun huh.
THE FOLLOWING RECOMMENDATIONS ARE FOR THE OLD STYLE PISTON AND NOT THE NEWER RAPTER III**********(See out new book for the new pistons recommendations)
As Bob mentioned in an earlier article get the block line bored by a good Kart shop. This is very important in obtaining a round cylinder. After this operation hone the block for piston clearance. On cool bore motors you can go to .007/.008 and on the I/Cs keep it to .006/.007. This will give you enough clearance for heat expansion. You can buy very expensive bore gauges to measure this with OR simply use an old engine builder trick by using long feeler gauges. These gauges are available from MSC supply mentioned in an earlier article and are very quick and accurate.
Now you are ready to setup the piston rings. The larger single contributor to piston drag is the oil ring. Now that WKA has dropped the check for minimum inside diameter it is no longer best to use cut rings. The best method for reducing drag is to heat shrink the ring. You can do this by placing the ring into an old cylinder making sure it is square in the bore. I use the piston with the second ring installed to push the oil ring down just far enough to square it up. Now take a propane torch and heat the ring to a dull read. You can start in one place and as the ring heats up slowly move around the ring, obtaining the dull red color as you go. Using the Block to do this is very slow, as it takes quite a while to heat the block.
The best method if you are going to do many of these, is to obtain a replacement sleeve for the Briggs from a supply house. You will find that this will heat up much quicker and yet retain the roundness you need. It is very important to use a round fixture for this no matter whether you use the block or a sleeve. I always use a standard size Briggs oil ring in all my motors no matter the bore size (up to .030). Always use a new oil ring for this operation. If you compare a new ring against an older used ring you will note the difference in the size of the actual surface contacting the bore. The new ring will have very small edges and thus contribute to less drag after we shrink it.
You can also shrink the second ring on the Briggs in the same fixture with the same method. The top ring gets a little tricky. Most builders will use an over size ring for the bore size and simply cut the end gap down to .005 or so. The problem here is that we have a lot of drag caused by the oversize rings wall tension. You can shrink an oversize ring down to fit but you must be careful not to go too far. You will need a larger fixture to do this in as a standard sleeve will be too small to accept even a .010 over ring. I simply have taken the sleeve and had it cut by a machine shop into several pieces and then honed the pieced to the exact size needed for each bore size marking the piece as to .010, .020 etc. You will want to leave the ring just a little tight so you can cut the end gap to the exact size you want (.005 for a stock class) (.010 for restricted).
When cutting the end gap on the top ring don't use a file. It is a rare person that can actually file one size and keep it square with the other. I actually use my Dremel tool with a sanding disc installed. I simply squeeze the ring together on the rotating disc. This will cut both sides at the same time and keep them concentric.
Another area to work on is the rod and crank clearance. Typically a new crankshaft from Briggs comes in at .999 or .998 in diameter. A standard rod out of the box will usually be at 1.000 to 1.001. So you can see that you can have from .001 to .003 clearance if you do not check the components. Using a combination of a quality dial caliper and micrometer check the clearance. I work to have .003 on a new motor. If you have above .006 you are going to start braking parts(rod!!). If the crank is down to .995 use it in the fun motor on your neighbors Kart!!! It seems expensive but really a replacement crank is cheaper than a whole new motor!. Also use a hardened crank. This is just a durability issue no speed here. If you can find some of the older Briggs heat treated cranks from their industrial motors of days gone buy get them. You can recognize them by the dark edges around the crank journal. These are hard as a rock and just never seem to wear!. I have nothing positive to say about the new cranks from Briggs that are cyrogenically treated. They seem to wear just as quickly as a standard crank.. I also like to use the old style crank in all of my motors. They are lighter and cheaper.
As to how to obtain the proper clearance, I use the flexi-hone mentioned in my carb article. These can be obtained in 1" as well as 1/2" for use on the large and small ends of the rod.. Take the rod and torque the bolts down to exactly 100 inch lbs. before honing the journal end. This will insure roundness after you are finished. Simply use plain motor oil mixed with kerosene for honing oil. Hone the journal end for the .003 clearance we talked about and then hone the piston pin send of the rod. This end should be honed just enough to allow the pin easy travel back and forth. Do the same operation to the piston. Hone the pin holes just enough with the 1/2" hone to obtain free movement of the pin. This will give you a free floating pin.
As to maintenance of a Briggs there are as many opinions as engine builders. I take my personal motors and go through them after three races. A properly prepared motor will begin to fall off after that. What is happening is the top rings end gap will be wearing to the point of losing compression and these are torque motors no matter what. It doesn't hurt to check the other components in the motor at the same time, such as valves and your spring retainers. Use good oil (I like cool power) and change it often. It's cheap insurance. Don't worry about all of these fancy fast? oils as there is very little to be obtained here. The only really fast oils have illegal additives and are probably dangerous to your health any way.
One last area concerns the flywheel and type of cranker. It has always amused me that people will spend lots of time and money looking for the lightest possible flywheel from older motors and still put on the old style cranker. Just look at the rotating weight difference between the two styles of crankers!!! I have always used the new style with no problems. I also don't have to carry a cranker in my hip pocket all day.
Well there you go . My ideas on setting up the rings, rod and piston. Keep in mind to use good parts(new) when building or rebuilding a Briggs and you will have little problems on race day. That will give you time to work on the most important part of Kart racing - handling! |
| Porting |
Porting of any motor takes careful thought and generally not a heavy hand. This is very true with the Briggs motor in the stock classes. BIGGER is not better, flat out. What you are always looking for with porting is improved AIR FLOW. Big ports do not mean better air flow.
General rule for porting the Briggs motor is to leave the size of the intake port alone and work on the upper left hand corner. I have developed a easy method for getting started on any intake port. Take an old carburetor that you don't care about and bolt it up to the port with the back plug knocked out. If you have done carb work out of my previous articles you will have in your possession a .690 reamer. Take the reamer and insert it through the carb bore until you hit the left hand side wall of the intake port. Now rotate the reamer using a wrench or vice grip until you have cut a good bit of metal out on this side. What you are doing is making a straight shot through the bore of the carb into the port.
Remove the reamer and carburetor and rough finish this side with a 5/8" deburring bit. What you want to end up with, is the left side opened to the existing side wall. You'll want to move the cut slightly upward toward the valve seat. BE CAREFUL not to hit the seat as the tech inspector will throw you and your motor out if you do.
After roughing out the basic cut, we want to move to a sanding disc. You can use the small 1/2" disc that come for the Dremel tool or I really prefer the larger sanding arbors available from MSC. These are great for smoothing out the cut and slightly radiusing the upper portion of the intake port. You just want to radius this edge for a slight improvement. Be careful not to damage the leading edge of the port.
Don't mess with the hump on the right hand side of the intake. This is a support area for the carb bolt. If you remove this, you WILL kill air flow. You will also do damage if you hog out the lower right hand side of the intake. It's ok to lightly remove casting flaws, but nothing heavy here. Again don't enlargen the overall size of the intake runner. As it comes stock, it is less than the .880 maximum size allowed by WKA but still much large than the bore of a blueprinted carb (.695). Keep in mind that as air and fuel move through the carb bore and hit the wider opening at the front of the carburetor and enter the even larger port, air speed will DECREASE! If you don't believe this look at a stream sometime where a small area of flow runs into a larger still area. You will see how the flow of water slows when it hits the open area. Air flow is the same.
To finish up the port, you can make you a simple tool for using scotch bright to polish the inside of the port. I very much like the finish scotch bright puts on the port. It's not super smooth and this is exactly what you want. A smooth surface will not help your air/fuel mixture. It will not help air flow as there is little surface tension anyway. What you want is a slightly rough surface to keep the air/and fuel mixed. Many builder for larger engines will actually put bumps in the intake runner to help keep the fuel and air stay mixed in low pressure areas. This is not legal in WKA stock classes. Fuel has a tendency to collect in these lower pressure area as large droplets. Large drops of fuel do not burn as well as a small atomized mixture. For fun take a spray bottle of water and spray a slick glass surface. You will see the water collect into larger droplets.
The tool for using a small square piece of scotch bright is easily made from a 4" 3/8 bolt. Cut off the bolt head and then cut a 1" slot in one end big enough to put the scotch bright into. Put this into a drill and have at it. You will end up with a dull looking port but who cares if it flows well! Scotch bright is easy to find in any grocery today.
On the exhaust side, we will not do much. The port is already too big. All I do today is take out the bump in the upper right hand corner. That's it.
A good header will mean more to you in improving exhaust flow than any porting you can do. Some general thoughts on headers:
small diameter headers will help torque. (.930 <)
large diameter headers should be used only in limited classes.(.990 >)
multi-stage headers do help over a standard single size of pipe.
Use a .910 or smaller for the purple plate class, with .930 working well in the red and gold plate restricted classes.
Don't be afraid to try several different headers with a single carb/cam setup.
Changing cams will most often cause you to re look at the header you are using.
Look to Robinson's Torque tubes and Dover Power for some very good headers in all classes. I have used these for years with excellent results. They are high quality pipes. They also have pipes made specifically for muffled classes. |
| Maintenance |
Once you have a race ready motor, maintaining peak performance is an absolute must. Why spend all of that time or money on a brand new motor just to let it wear out over the course of a few weekends racing. Ever wonder why the Kart shops always run well. Well, other that always getting the best of everything, they generally are rebuilding their motors every week!!!!!
Well here we go. These are just my guidelines to keeping a motor fresh and performing at it's peak horsepower and reliability. If your motor builder suggests other follow his guidelines.
Generally we suggest rebuilding motors after 3-4 weekends of racing. This would cover racing at one track one day. We have found that after about 3 weekends the motor is beginning to go downhill on horsepower. Generally the top ring is worn and has opened up it's clearance to above .015. Using good oil(coolpower) and changing the oil after every 20-25 laps or so will reduce the wear.
On our rebuilds we do the following:
Check the block for fatigue around the cam support area.
Check the cylinder for clearance and roundness. Bore or hone to next size if needed.
Lightly hone the cylinder to enable the new ring to seat.
Check the crank journal for roundness and wear. Replace if worn to .996. If you do not do this, you will break a rod, as too much clearance is the root cause for this happening.
Check the rod for wear and excessive crank clearance( > .004) Replace if needed.
Check the piston for wear around the skirt and top edge. Also check for cracks around the piston pin area. Replace if worn.
Replace top ring with a new one gapped at .003-004. Check the second and oil rings for wear based on tech rules.
Check the valve lifters for wear and fatigue. Replace if questionable.
Check the valve stem for wear and replace if needed.
Replace the intake valve spring if it is over 8 races old on stockers. On restricted motors don't worry about this.
Check the valve clearance for proper valve timing(re-profile) and lap the valves for good seal. Even if you do nothing else this will tend to keep your performance up!!!!!
When you check the cam profile, be sure and double check the easy-spin. This is the one area that wears out of spec the fastest.
Clean the head by soaking it carb cleaner and rubbing with a shop cloth. If you really want to clean one up, sand blast it.
Clean up the intake port with a cloth soaked in carb cleaner.
Replace carburetor diaphragm and clean the carb up.
Clean the rest of the motor and reassemble.
Well, there you have it. Just keep after your motor and change that oil for long life. P.S. Don't complain the next time your motor builder charges you $180-220 for a freshen-up. A lot of labor and parts go into this job. Also clean your motor before you take it to him. He will love you for it. |
| Raptor III Piston |
NEW RAPTOR III Piston:
The same style piston and ring set has been used since the introduction of Briggs and Stratton 5hp racing. All of this has changed this year(99) with the introduction of the new RAPTOR III piston. This piston was designed for Briggs by the A.E. Goetze division of Federal Mogul. Major changes in the design and makeup of the piston have introduced a whole new ball park for the engines builders this year. In this portion of the book, we will go over the specifications of the new piston compared to the older style and give specific installation recommendations for it's use in racing.
The new piston is made of a new alloy called Eutectic. The term Eutectic refers to the composition of the piston as it relates to the percentage of Silicon in the aluminum alloy. Silicon addition to aluminum is best related to adding sugar to iced tea. You can continue to add and mix sugar until the mixture reaches it's saturation point. At some point adding additional sugar will result in it not mixing with the tea and simply falling to the bottom of the glass. Silicon in aluminum is very similar.
Silicon can be added to the aluminum up to the point of saturation. Any additional silicon added will simply precipitate out in the form of hard particles. The point of saturation in aluminum is known as the Eutectic and occurs around the 12% level. Aluminum alloys with less than the 12% are known as Hypoeutectic and alloys with silicon contents above the 12% level are called Hypereutectic. The new piston is Eutectic in makeup, so it contains right around 12% silicon.
Eutectic pistons show improved strength and are economical to produce. In addition to improved strength, the silicon reduces the expansion of the alloy when compared to straight aluminum and aids to it's ability to reduce friction and scuffing. This reduced expansion will come to light a bit later in this chapter when we discuss cylinder clearance.
As you can see in the accompanying pictures of the two pistons, the new styles design is vastly different from the old style. The barrel shape leads itself to several improvements.
Improved stability in the bore
Aids in creating an oil film against the cylinder
Reduced friction(less surface area against the bore)
Allows for closer piston/cylinder wall clearances.
In addition to the physical makeup of the piston, the wrist pin hole has been offset to reduce torsional stress throughout the length of piston travel. The design also incorporates a shorter (less weight) wrist pin to help control flex and yield improved strength. The recommended installation of the piston has the arrow on top pointing to the Flywheel side of the block.
The ring package on the new piston also is new and unique to the piston. While the rings appear to be of similar design when compared to the old style, they are much smaller and lighter. They also have much less initial sidewall pressure eliminating the need to shrink the rings to reduce friction. The top ring has a bevel face to improve break-in, while the second and oil rings are very similar in design to the older style. Other than setting the end gap and breaking the sharp edges of the rings very lightly they are good to go from the factory.
The new pistons are also approximately .002 larger than the old style for a given size piston. For example the standard bore new pistons measure 2.5610 at it's largest point(1.378 inches from the top of the piston) compared to an average of 2.5589 for the old style. This .002 size difference runs through the entire new line of RAPTOR III pistons. Speaking of the line of pistons, we now have several new sizes to play with. The old pistons came in standard, .010, .020 and .030 over. With the introduction of the new parts, we now have seven sizes of replacement pistons. This should give us more life out of our motors as we move from one bore size to the next. The new pistons and their Briggs part numbers are listed in the following table.
Size Part # Ring Set Size
Std. 555478 555485 Std.
.010 555479 (Use .015 ring set)
.015 555480 555486 .015
.020 555481 (use .025 ring set)
.025 555482 555487 .025
.030 555483 (use .035 ring set)
.035 555484 555488 .035
As you can see Briggs is not making a ring set for each new piston size. Given the fact that engine builders are going to fit each piston and ring set to a given motor, simply using the new larger size ring set will work in most cases. The one missing piece today is a .040 over top ring for the .035 piston. The Briggs .035 ring set top ring has too much end gap for most builders (.011). If you purchase the pistons as parts be aware that for the standard, .015, .025 and .035 sizes the ring sets that come with them in the box, will have a top ring with too much end gap. This forces you to use the next size up top ring. Top rings are available from Briggs as a unique part # these days. Also Burris and Power Products have aftermarket top rings for the new piston. These will be legal in 2000 for WKA.
End gap recommendations should go along the same guidelines as the older style piston with the top ring being installed with something around .0015 - .002 end gap(be sure you have a straight bore). The second ring should be around .002 - .004 and the oil ring around .004 -.010 although it can be tighter. As to cylinder clearance, that's a whole other story folks. Big changes here. Due to the design and makeup(Eutectic) of the piston it will not expand under heat as much as the older piston, so we can run much tighter bore clearances than we are used to doing. The offset wrist pin also comes into play here as too much clearance will allow the piston to rock in the bore ,creating several problems(excessive ring wear and scuffing).
Briggs recommends the following procedure for installation of the new style piston. First, measure down from the top of the piston 1.378 inches to the center of the trust faces(90 degrees from the piston pin). Due to the sides of the piston being barrel shaped, this will be the point of the largest diameter. From this point measure the diameter of the piston with a quality 3" micrometer.
Using the size found above, finish the bore to a size from .0005(yes that's 1/2 thousand of an inch) to .0025 clearance. They recommend using these clearances on both cool bore and I/C blocks. Now folks, .0005 scares me to death, given the many warped cylinders I've seen over the years. The Briggs blocks have a natural tendency to settle(warp) a bit as they are heat cycled during their early life. They also seem to settle down after this initial heating and cooling. So what does our experience to date show. Somewhere around .002 to .0025 starting cylinder clearance seems to work very well in both style blocks. My personal experience, as well as, the experience of other builders I know, has seen everything from .007 to .001 tried with varying degrees of success. The larger clearances seem to work, but ring wear is very rapid and forces you to rebuild the motor more often than necessary. I did have a good cool bore that was run with the new piston at .005. After three races the cylinder had an actual imprint of the piston shirt at BDC!! I'm currently using .002 with good results. After a bit of experimentation, it seems the new rings like a very fine cylinder finish. Something around 320-400 to 600 grit stones should be used for the final finish. Either honing stones or the Flex-Hones work well in achieving this plateau finish.
As I noted earlier, the piston is recommended to be installed with the arrow(on crown) pointed toward the Flywheel side. We have run them in the opposite direction with no failures. Which way is better for a given style of motor(restricted or stock)? I'll leave that up to your testing.
One of the reasons for the new piston was the relative high failure rate of the old piston pulling out the wrist pin at higher RPM levels. As the new Slapper cams (Dyno 95-3,99-3,96-3,PC 150) have come into popularity, we are turning the motors to RPM levels we would not have dreamed of 5 years ago. With these higher RPMs came more piston failures. The increased strength of the new piston should address this issue and in fact we have used the new piston in Super Stock motors turning them over 7000+ with no failures to date(kiss of death). They are not a cure all for sloppy clearances but they are an improvement.
The one thing to be very CAREFUL with on the new piston is the wrist pin retaining clip installation. The new piston has very little free space between both clips and the wrist pin when installed on the rod, making a bit more difficult to properly install the clips into their slots. Make darn sure you have both clips installed correctly. I like to examine both very closely and rotate them in their slots to insure they are seated properly. Seems the grooves are a bit shallower than the old styles to further complicate the issue. The latest pistons are coming with thinner clips which should help.
One last bonus for the new piston is, that as a total package, is lighter than the old style. Below is a chart of weights and measurements taken from two standard size piston of both designs. I would like to thank Jim Stone of Stone Racing Engines in Greenville SC, for his time and effort to gather these figures.
PISTON CHART
NEW OLD
Weight(Grams)
Piston 136.9 131.7
Wrist pin 1.735 1.945
Rings 27.2 30.5
Wrist pin 14.8 29.6
Circlips .8 .8
Total 179.7 192.6
Dimensions(inches)
Piston height 1.674 1.878
Diameter(max.) 2.560 2.558
Pin bore .4895 .4895
Top to pin bore .941 .941
Compression Ring
Height .106 .124
Width .0585 .0781
Second Ring
Height .106 .124
Width .0585 .0781
Oil Ring
Height .085 .116
Width .1000 .1857
Following is the current WKA tech inspection specifications for the new piston and ring set.
Piston minimum length 1.673
Top of piston to top of wrist pin minimum .937
Ring lands for top two rings .0603 to .0612
Ring land - oil ring .1020 to .1032
Top two rings minimum width .090
Top two rings thickness .058 + or - .005
Oil ring minimum width .070
Oil ring thickness .100 + or - .005
Wrist pin length 1.732 + or - .005
Outside Diameter maximum .490
Inside diameter maximum .281 |
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