You’ve got questions, we’ve got answers. We work with the Summit Racing tech department to help you tackle your auto-related conundrums. In this week’s Mailbag, we’re helping a reader sort out trouble with the 350 small block Chevy in his 1978 Corvette.
Q. I have a 1978 Chevy Corvette Pace Car with an L-82 350 and a four-speed transmission. The engine was recently rebuilt, and has ported 57cc chamber heads with 2.02″ intake valves (heads milled 0.030″), a COMP Cams High Energy cam, a 750 CFM Edelbrock carburetor on an Edelbrock Performer intake manifold, and Hedman headers going into a 3″ exhaust with Flowmaster mufflers.
Ever since I installed the engine, the car is less responsive than stock, won’t pass a smog check, and is running very rich. I live 5,000′ above sea level, and have tried jetting the carburetor for high altitudes. There has been no improvement. I plan to install a half-inch phenolic carburetor spacer and an MSD Ignition to help boost power (I hope).
A: Everything you have done so far has increased airflow into your engine. It sounds like you need to boost low-end torque and throttle response. Try a different cam, something with at least 112 degree lobe separation. This will help increase cylinder pressure, which translates into more torque.
You may want to try a smaller carburetor too, perhaps a 650 CFM Holley with vacuum secondaries. I don’t know how much porting you did on the the heads, but it could be that you made the ports too large. That reduces air/fuel charge velocity, especially at your altitude.
You didn’t mention the distributor, so I assume it is the factory HEI with no modifications. You can add an MSD ignition to increase voltage at the spark plugs, but you need to modify the factory mechanical advance curve to slow down and limit total timing to better match the engine modifications you have made.