Q&A / Tech

Mailbag: Tuning for More Off-Idle Power

Q: I own a 1966 Pontiac Lemans with a 1970 400-cubic-inch engine, a two-speed automatic, and an open rear axle. I recently rebuilt the engine to stock specs, adding a Crane cam (290-degree advertised duration, .454-inch lift), an Edelbrock Performer intake, and a 600 cfm Holley.

When I punch the gas pedal from a standing start, the car doesn’t have the power I think it should — the thing should be cooking the tires. It seems like the engine makes power when I downshift at cruising speeds (40-50 miles-per-hour). Is it possible the two-speed transmission and the rear axle are holding the car back? Could the distributor’s vacuum advance have an effect if it’s not working properly?

A: There are any number of possibilities why you don’t feel any power off-idle. The cam in your 400 has a power range of 2,000-2,500 rpm. If you installed the cam “straight up” without degreeing it in, it may be in a retarded position. Advancing the cam 4 degrees would drop the power range by about 500 rpm.

Another possibility is that your 600 cfm carburetor may need a good tuneup. You didn’t tell us what your rear axle gear ratio is or what stall speed converter you are using, but these will greatly affect low-end response. Distributor advance also has an effect: with your cam, you should have around 12 degrees initial (static) advance with the vacuum hose disconnected.

This is another in a series of weekly Q&A Mailbag sessions with Summit Racing‘s tech department, in which there are hundreds more. Click here to see them all.

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