It's a spinoff rule that came from trying to make racers use stock components. Engines in street cars produce a lot of vacuum in the intake manifold because they are designed to spend the majority of the time at rpm levels barely above idle. Puttering around town. Looking for the best parking place at the grocery store. Stuck in traffic on your way to work. That sort of thing. The result is a cam with short intake and exhaust durations and very little valve overlap.

Vacuum may be good for powering the brake booster in your everyday car, but besides providing good signal to the carburetor, we have little use for it in racing. Instead, camshafts are optimized for high-rpm operation. That means more duration for both the intake and exhaust lobes, because at high rpms it can be difficult to get air and fuel into the combustion chambers and exhaust gasses back out in time. The longer lobe durations are combined with more overlap to use the effect of scavenging to actually pull the air/fuel charge into the combustion chambers. Together, those qualities in a camshaft harm the amount of vacuum any engine will pull in the intake manifold plenum.

That's not a problem unless your racetrack has implemented a vacuum rule in your class. Vacuum rules are most common in Street Stock-level race classes where the idea is to force the racer to use as many stock or stock-replacement parts are possible. The idea is that the requirement for a certain amount of vacuum-measured at the intake manifold plenum with the engine at idle speed-will require smaller cams which will cut power.

So the obvious question becomes how do you meet the vacuum rule at your track while maximizing horsepower at the same time? Well, it turns out with a little camshaft savvy you can use the vacuum rule at your racetrack to gain an advantage on your competition. You can also influence vacuum with other components, such as carburetor size, different types of intake manifolds, and even engine compression, but since all those things are usually controlled by the rule book as well, the cam almost always is the best area to work on if you are hoping to get the best of both worlds.

"Making vacuum is all about overlap," explains cam designer Billy Godbold of Comp Cams. "But there is no way I, or anybody else, can tell you exactly how much you need because how much vacuum your engine will run depends on a lot of factors like the compression ratio, how well the rings seal, port size, the header size, and all kinds of other little things. But we can get you close, and if you're trying to run the absolute minimum amount of vacuum your rules allow, you may have to be willing to get it close with a test cam and then dial it in with a second."

Overlap is a tool to help improve engine performance at high-rpm levels, but it can also hurt how well an engine runs at idle speed when the engine is moving relatively slowly. Basically, during overlap both the intake and exhaust valves are open, so the piston moving down in the cylinder produces no vacuum in the manifold.

So the trick is to figure out how much valve overlap you can have and still meet your minimum vacuum number to pass inspection. Overlap can be changed by adjusting the camshaft in one or two ways. The first is to change the duration that the lobes hold the valves open. Shortening the duration-and the amount of time the valves are open-generally reduces overlap. What you have to avoid is extremely long intake duration where the intake valve is held open past bottom dead center. When that happens, the piston starts moving back up the cylinder and pushing air back into the manifold. The second option is to change the lobe separation angle (LSA). The LSA is the distance between the centerlines of the intake and exhaust lobes for a single cylinder. It is measured in degrees of crankshaft rotation. Most performance cams use an LSA in the 110-degree range. And as you might expect, a greater LSA reduces overlap (because it spreads the intake and exhaust lobes apart) while a smaller LSA increases it.