Camshafts - Application

Bob Evans

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 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 in the lower RPM ranges. The retarded 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.
The best method to determine you centerline with today’s cams is to go off maximum lift. This method will work on either lobe. With your degree wheel installed and TDC found, rotate the engine until the valve you wish to check is at max. Lift. Now zero a dial indicator on the valve. Rotate the engine backwards until you have gone .050 on the indicator. Record the reading on the wheel(such as 47). Now rotate the engine forward passing TDC until you again show a .050 drop on the indicator. Read your wheel degrees again(24). Take the higher number and add it to 180(180 + 47 = 227). Subtract the lower number(24) from the previous result(227-24 = 203) and divide by 2. Your centerline in this example is 101.5.
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 .200 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. Remember, we’re dealing with a tiller motor.
WKA says the easy spin must start between 45 and 60 degrees ABDC. Rules also say the valve lift at the point of easy spin must be between .013 and .019 taken with no clearance in the valve train. The easy spin must also last a minimum of 30 degrees in duration. Most Cams today have around 33 - 39 degrees to allow for some wear in this area and still be legal. Most cam manufacturers today also flame harden their cams to help with durability. This easy spin area is the one place most cam become illegal through wear. In the chapter covering timing, we will show how to measure the easy spin when profiling a cam for legality.

Slapper Cams
I’m sure many of you have heard of ‘Slapper’ cams currently being made by most of the cam manufacturers for stock class motors. Cams of this type would include the Dyno 95-3, 99-3, ,00 series , Comp Cams 9330, and the Precision Cams 143-150. What these cams allow to happen is to slap the intake valve so hard and accelerate the valve to a speed during the opening cycle that it actually overcomes the intake valve spring’s ability to keep the valve tappet on the surface of the cam lobe. The valve is literally slung up off the lobe surface and obtains a higher overall lift than would be allowed within the stock cam profile rules. More lift equals more air/fuel mixture equals more HP in the upper RPM range.
Most of these Slapper cams will obtain a lift well in excess of the .233 maximum lift specs. The valve also will keep this higher lift longer than normal, as it will come down onto the back side of the intake lobe some where around the easy spin area. Overall effect is much higher lift and longer duration at this lift. The key to obtaining this effect is valve spring pressure. An intake valve spring pressure of around 14 lbs.(seated), will get you into the needed area for a stock class motor. Less pressure would be needed on restricted motor to give this effect at a lower RPM range.. This spring pressure can be obtained through back facing of the valve spring area(legal in WKA) and using older exhaust springs on the intake side. The exhaust valve side should not be floated in this manner. Just an exhaust spring with enough pressure to keep the valve along the cam profile. Remember, too much spring pressure eats HP. This is definitely something to remember with restricted motors.
One of the best ways to figure all of this out is to mount a degree wheel on a motor with the head removed and watch all of the events unfold as you rotate the motor forward through it’s 4 cycles(intake, compression, power and exhaust).
Specific Recommendations:
For rookie motors and any restricted class keep the overlap to a minimum. Openings less than 20 degrees BTDC and closing around 78-80 degrees BTDC on the intake and exhaust openings around 50 - 58 degrees with a closing around 15-16 degrees ATDC, will give good torque and minimum overlap. The intake center line should be 99 -100 with the exhaust around 112. For purple plate motors keep the rpm to around 4800-53000 and for gold plate 6100-6500 for ovals. 5600-6000 for blue plates. A savvy old timer once told me to keep dropping teeth on the back end until your lap times slow down. This is very true in the restricted classes.
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 65-6900 rpm range. I love the 99-3, 96-3 and 04-3 for heavy.
Stock medium and stock light can take advantage of a higher rpm cam with center lines around 101 on the intake with 110-111 on the exhaust. Intake opening can be as much as 30 degrees but current practice is to limit this to around 22 degrees or less. Many of the more popular fast ramp cams are opening the intake valve around 16 degrees. Intake closing should be around 72-75 degrees BTDC. Exhaust opening should be from 50 - 55 degrees with a closing around 16 - 20. This will allow the motor to rev better and produce power in a higher rpm range. Rpm range should not go above 6700 on ovals typically but some of the newer cams (dyno 7X) are designed to run above that level but maintenance (rod/piston) becomes an issue. *** Today many stock engines are being turned above 6600 on ovals with great success. The new 7X from Dyno is designed to run around 68-7000 with spring pressures around 20(fish scale). The 99-3 will easily turn in the 6800 range with a bit less pressure. These cams do very well, but keep your maintenance in mind. If you turn your engines above 6800 then you’re going to have to check/replace things more often than not. Most are replacing the stock rods after 4-5 weekends and obviously doing a quick re-build in the process. New billet rods will run all year without issue
Most cams made today are of the highest quality. I have to admit I’m rather prejudiced toward the excellent cams done by Dyno and Precision Cams. One of the best attributes of both, is that they will grind a cam in any combination you like, using any of his many intake and exhaust profiles creating a cam of you choice. A classic example of this, was a cam run by quite a few people many years prior to it being made a production unit, the 98-3. This cam was originally done from the 95-5 intake profile combined with the 95-4 exhaust profile. Today my favorite cam for longer tracks is the 99-3 or 7X intake profile with the same 99-3 or 96-3 exhaust. Give them a call and ask for their catalog as it has complete profiles for each cam detailing both intake and exhaust profiles. The 97-3 exhaust profile is also a winner when coupled with one of the slapper intakes. With either the 97-3 or 95-4 exhaust profile you will not have any problems breaking the exhaust lifter boss. Some of the new 00 series cams from Dyno are using a much later exhaust close and are similar to these combinations. These cams are designed to run in the higher RPM ranges. The new PC 146 is a combination cam made from the 145 intake and the 147 exhaust profiles. Precision and Dyno currently give coil bind, seat pressure and installed height recommendations with there cam cards. Very nice.
Some of the most popular Dyno and Precision grinds today are:
Restricted motors: 95-3, 95-6, 96-3 for purple and the 95-3 or 96-3 in blue / gold plates. With the new move in 99 by WKA to allow billet lifters, look at the 96-2 intake with the 95-3 exhaust for something different. I also like the 00-3 and 96-3 cam in the larger plates(blue/gold). The PC 145 and 2k6 work well for the larger plates. The Dyno 95-3 is by far the easiest one to set up for and its the most popular
Stock classes: 95-3 (most popular Dyno grind), 96-3, 99-3(heavy) and 98-3(road course). The Precision 145, 150 and 2K6 are also very good stock class grinds. For larger tracks the 00-X series of Dyno cams are very good. The new 7X is a high RPM(6900) cam from Dyno and has fast become the most popular cam dyno sells. The 04-3 is a great high RPM cam with better bottom end for the heavier classes compared to the 7X.