Street Stock Engines - The Vacuum Trials

Godbold says that once you learn how much overlap you can get by with, you can begin playing with your durations and LSA to maximize power, while maintaining your minimum vacuum level. He also recommends making your lobe profile as aggressive as possible. Your duration may be limited by the amount of overlap you can get by with, but by raising and lowering the valves as quickly as possible inside those parameters, you can get more air into and out of the combustion chambers.

"You may get more wear and have more valvetrain noise, but a very aggressive cam is definitely the way to get more power while still maintaining vacuum," Godbold says. "As long as you aren't slinging parts everywhere, that's OK. The only way that it can be too aggressive is if you start losing control of the valves at, say, 6,000 rpm, when the engine is capable of running up to 7,000 rpm with a milder cam.

"Sometimes racers will end up running some of our most aggressive solid lifter designs even though they have a hydraulic lifter rule at their track and have to run hydraulic lifters. You don't care whether it was originally designed for a hydraulic or a solid lifter application, you just want the cam that will get you the quickest possible lift off of the seat so that you can get as much flow as possible with a limited duration."

If you are considering going the route of ultimate aggressiveness, you may also want to consider running a nitrided cam. The nitriding process puts a very hard, very slick surface on the camshaft that is much more resistant to wear than any standard hardening process alone. Most vacuum-rule classes also require flat-tappet lifters, which limit how aggressive your lobes can be. With a very aggressive camshaft and flat-tappet lifters, you run the risk of the edge of the lifter digging into the surface of the lobe. Aggressive lobes also require stiffer valvesprings, and when you combine that with flat tappets, the chances of flattening a lobe are also increased. Nitriding helps reduce the chances of both and can help you not only get through the tricky break-in period but also help your cam live longer overall.

To get a better idea of how much cam lobe design affects vacuum, we decided to try our own tests on the dyno. Our test mule was a Chevy 350 built at KT Engine Development for the Strictly Stock class. It's a typical motor for vacuum-rule classes raced at tracks all across America. It features stock heads cut down so that the chambers measure 64 cc's and flat-top pistons to make the compression a mild 10.2:1. The intake is a stock cast-iron dual-plane unit and the carburetor on top of that is a 350 cfm two-barrel Holley.

Comp Cams helped us pick out two cams for the test. The first cam was chosen for the typical rule set assuming there's no vacuum rule. Total lobe lift was held to 0.283-inch, and combined with 1.6:1 ratio rockers, the valve lift came out to 0.453-inch. Duration at 0.050 is 238 for the intake and 242 for the exhaust, and with a 110-degree LSA the overlap measured in at 20 degrees. The lifters are stock Chevy diameter hydraulic lifters provided by Comp Cams.

On the dyno this cam produced 352.1 lb-ft of peak torque at 3,800 rpm and 305.7 hp at 5,200 rpm. Nice numbers for the very restrictive rules, but it also was only capable of managing 12 inches of vacuum at 700 rpm.

We gave the engineers at Comp a goal of 17 inches of vacuum at 700 rpm, and the second cam tested was ground to meet that target. Like before, this cam was ground with 0.283 lobe lift and a 110-degree LSA. But this time, the duration at 0.050 was much shorter at 212 for both the intake and exhaust. And while there technically is still some overlap, if measuring when the valves are 0.050-inch off the seat, the lobes are separated by 8 degrees.