Finally, other than a jet...
Finally, other than a jet change in the carburetor and a timing sweep (both cams worked best at 38 degrees advanced), the engine was exactly the same for both runs.
This time around, the Chevrolet pulled 17.2 inches of vacuum at 700 rpm. It just barely met the vacuum minimum, which was exactly what we wanted. You never want to give up any more than you have to. But the power curve the new cam produced is really what was interesting. Predictably, the shorter durations and reduced overlap moved the peak power numbers lower in the rpm range, but the smaller cam actually helped torque substantially. This time around peak torque showed up 200 rpm earlier and pulled 377.0 lb-ft. That's an improvement by nearly 15 points! And truthfully, peak torque showed up as soon as we began the dyno pull at 3,600. We could have pulled the rpms down lower, but there is little point in a race motor.
Peak horsepower also moved down the rpm ladder, this time by 600 rpm. Unfortunately, the shorter durations also hurt how well the engine could pull in the air/fuel charge as the rpms climbed, and peak power dropped 21.1 points to 284.6. Although nobody was clicking their heels over giving up horsepower, it's still respectable for a Strictly Stock motor and expected considering how much duration we had to give up to get the vacuum number necessary.
Still, the lessons learned about using aggressive lobes to overcome short durations will be very valuable in any vacuum-restricted class. And with more experimentation, we probably could find a cam to help make back some of that horsepower we lost. But first, we'll allow KT Engines to hand this Chevy over to the customer and see how it fares on the racetrack.
Lash Your Hydraulics
Hydraulic lifters require no lash in normal operation, but Comp Cams' Billy Godbold passed along this power tip that might be helpful depending on how and when your vacuum-restricted car must pass through tech. In vacuum applications, Comp often recommends staying away from the high bleed-down lifters. Instead, use a set that doesn't bleed down as quickly-such as Comp's 8400-16 hydraulic flat tappet lifters-because they increase stability. And if you have trouble meeting your track's vacuum requirements, you can set them with a couple of thousandths of an inch of lash. It won't damage the cam or lifters in the short term, but the extra lash will make the cam act as though it has less duration and will increase vacuum. Then, if you have the chance, take the lash back out before the race for more power.
All reputable cam manufacturers send a cam card with all their camshafts that include most of the pertinent information you will need. But many don't specifically tell you how much overlap is ground into the cam. You can figure it out yourself with a little math, but an easier way is to use the "Overlap Calculator" Comp Cams provided us while working on this story. It is a Microsoft Excel worksheet, so it should work with most Windows-based computers, and it can tell you much more than just your overlap. Simply plug in the information in the yellow boxes (duration numbers at 0.050, lobe separation angle, and even advance if you like) and the program spits out not just the overlap, but also the exhaust and intake centerlines, and even how much tappet lift you should see at 0.050. All of this information is very helpful when degreeing in your cam. You can get a copy of your own by going to www.circletrack.com and typing "vacuum" in the search box.