If you are required to race...
If you are required to race flat-tappet lifters, always try to use the largest lifters available. This requires honing the lifter bores, but it is often worth it. As you can see from this diagram, the smaller lifter (light gray) is limited to the rate of lift in the lobe before the edge starts digging into the camshaft. The larger lifter can be used with a much more aggressive lobe. Courtesy of Comp Cams
Currently, one of the greatest limiting factors when it comes to aggressive camshaft profiles is the requirement many tracks and sanctioning bodies have mandating flat-tappet lifters. A flat tappet limits how quickly you can raise the lifter because the lobe angle can only be raised a specific amount before the edge of the lifter begins digging into the side of the lobe. Increasing the diameter of the lifter allows the lifter to slide over the face of the lobe again, so if your rules allow, try running a larger lifter combination. Godbold provided us with a few interesting numbers concerning maximum velocities for lifters. For example, with a stock Chevy 0.842 diameter lifter, the maximum velocity is 0.00700 inch per degree. If you use a Ford 0.875 lifter, that increases the maximum lift to 0.00735 inch per degree of rotation. That may not sound like much to you and me, but it's enough to make a cam designer drool. Of course, those numbers pale in comparison to a roller lifter, which doesn't have the same limits and is definitely the way to go in a racing application if the rules allow it. The maximum velocity for a performance roller lifter is typically around 0.009 per degree. Now we are talking about a really big difference.
One thing that confuses many new racers is that cam companies typically list both an advertised duration and a duration measured when the lifter is 0.050 inch off of the seat. The problem is that different manufacturers use different points to determine duration. There are different reasons for this, but few of them concern anyone other than the cam designers and their respective marketing departments. That's why lift at 0.050 has become an industry standard.
The good news is that when...
The good news is that when it comes to high-performance cams, companies such as Comp Cams have hundreds of lobe grinds on file that are mixed and matched to fit your engine package. Sometimes they aren't even in the catalog. Don't be afraid to get in touch with someone in the tech department. Give that person the engine combination you plan to run and the rules for your class, and use that person's experience to help you determine what works best.
Usually, a valve doesn't begin flowing a significant amount of air until it has been raised several thousandths off of the seat. Also, differences in lash make it difficult to determine the exact moment a valve leaves the seat. Finally, duration at 0.050 inch of lobe lift is easier to measure and makes life easier for anyone setting the cam timing with a degree wheel. " . . . it's easier to measure the 0.050 duration than the advertised duration because the tappet velocity is much higher after it has had some time to accelerate," Godbold explains. "When using a cam degree wheel and a dial indicator, there is far less uncertainty about where the degree wheel is oriented when the dial indicator reads exactly 0.050 inch of lift than with lifts in the 0.004 to 0.020 range."
By using special timing sets, you can change the angle of the cam relative to the crankshaft. Spinning the cam forward so that the valve opening events happen sooner is called advancing the cam. Retarding the cam is just the opposite. Most camshaft manufacturers grind in around 4 degrees of advance into their cams so that it is automatic when you install your cam with the zero marks on the timing set. This is very common with street cams but varies with different race cams. Make sure you know what you have.
"Typically, engines respond better with a few degrees advance," Godbold explains. "This is likely due to the importance of the intake closing point on performance. Earlier intake closing leads to increased cylinder pressure and better responsiveness." As a general rule of thumb, advancing the cam will help low-end torque, but if your engine is dying by the flag stand, retarding the cam a few degrees should help extend high-rpm power a bit.
The information we've provided is a lot to digest, but don't worry. Most cam manufacturers have helpful tech departments to work you through the rough spots. To make things just a little easier, here's a cheat sheet for cam changes and the typical result. Remember, all engine packages are different, and your results may vary. These are only general guidelines.
|Cam Change: ||Typical effect |
|More Lobe Separation: ||Wider powerband, more peak power, |
|Less Lobe Separation: ||Increased mid-range torque, faster |
acceleration, narrower powerband
|More Duration: ||Powerband moved higher in rpm range |
|Less Duration: ||More low-end torque |
|More Overlap: ||Improved signal to carburetor, lower fuel |
efficiency, potential for reversion
|Less Overlap: ||Improved low-rpm responsiveness, better |
fuel efficiency, engine may run hotter