January 2011 Chevy High Performance Q&A - Performance Q&A

Tom Truty Dec 9, 2010 0 Comment(s)
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Again, if you're looking for a street camshaft, we'd run one of the Comp Cams Thumpr series to give the car the right sound for the period. Comp has three levels to choose from starting with the Thumpr, then the Mutha Thumpr, and the largest is the Big Mutha Thumpr. If you're looking for the true Stocker sound for your Mouse, the Mutha Thumpr (PN 08-601-8) will deliver the goods in both sound and performance. It specs out at 235/249 degrees duration at 0.050-inch tappet lift, 0.522/0.509-inch max lift, and is ground on a tight 107 separation angle. Use the complete camshaft valvetrain package offered by Comp under PN K12-600-4; it comes complete with camshaft, lifters, springs, retainers, keepers, and a double roller timing chain set. This will ensure that you have the proper spring set for the camshaft design. You'll need to check your piston-to-valve clearance when assembling the engine. A good benchmark is to maintain at least 0.080-inch on the intake and 0.100-inch clearance on the exhaust so you won't have any issues if you miss a gear or two. Please don't ask what we run our Stocker intake valves down to. (0.038 inch!) Sources: compcams.com, nhra.com

CFM vs. HP
Q: I currently have a '67 Camaro RS convertible with a nice 327 with a mild Lunati cam, headers, and a Tremec five-speed with 3.31gears and am in the process of building a torquey small-block 406. I do not see it going over 6,000 rpm on any regular basis. I have noticed articles that mention the cfm to support horsepower and am leaning toward a set of AFR 195 Eliminators and mild a hydraulic roller with max lift somewhere in the 0.550-inch range and duration at 0.050-inch tappet lift of 230 degrees. Is there a published formula, like compression or engine cubic inches?
Jeff Foster
Via email

A: Well, we could easily get into trouble with this one. First, there isn't a direct correlation to cylinder head flow on a flow bench and the horsepower it will support. We use the flow bench as a yardstick to compare cylinder head flow figures against each other. Airflow on a flow bench is constant flow. The flow of air and fuel into a cylinder of an engine is interrupted flow, with many factors affecting its ability to fill the cylinder to its fullest. If an engine achieves 100 percent volumetric efficiency, it is completely filling the cylinders with exactly the amount of air and fuel that the engine specifically displaces. It's our goal to overfill the cylinders through tuning and sizing the runners, manifold, and headers and camshaft tuning to achieve in excess of 100 percent. Any time you can achieve over 100 percent volumetric efficiency (VE), the engine is larger than the engine truly displaces.

Back to your question. We think you want to know if your AFR 195 Eliminator heads are going to support the 406-cid engine you're building? Yes, these heads will give you a very well-performing 406 with great torque and horsepower potential. Is there a simple formula, like a compression ratio or cubic inches? No. To come up with the theoretical 100 percent VE number for your 406, multiply the cubic inches by the max engine speed (rpm), then divide by the constant 3,456. This gives you a cfm number for your 406 at 6,000 rpm of 705 cfm. This is, again, at 100 percent VE. Most street performance engines may achieve right at 100 percent VE at or near torque peak. As the engine speed increases, the VE falls off due to the time it takes to fill the cylinders, and that the intake runners and exhaust headers are tuned to peak around torque peak. This gives you the broadest powerband for a nice driving package. If this were a race-only engine, you would tune all components around a very narrow powerband where you wished to achieve peak horsepower.

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