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 C3 Corvette.
Q. I have a Chevy Corvette 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.
In the early ’90s I bought a reman Buick v6 from a reputable machine shop for a bargain price; as the shop was closing due to the owner’s health. I dis-assembled the engine, checked all the specs etc. … all was good. I installed a moderate performance cam,performed mild port work on the heads, installed a 450 cfm 4bbl on a stock 4.1 L (252 ci) manifold, and a set of headers. Solid budget performance build, right? One would expect a 40-60 HP gain.
It was a DOG! Worse than stock! After almost a year of head scratching I finally discovered that many of the stock timing chains of that era were retarded some 15 degrees! That explains why I had to advance the timing to the moon just to get it to run. I used the timing chain provided when I re-assembled the engine not knowing this at the time. I replaced it with a performance double roller chain, and this time checked it with a degree wheel. Cured!
Sound familiar? The Vette owner may want to check (or double check) the cam phase…
Matt in Bonney Lake, Wa.
Build looked good except the 202 valves and the 750 holly 194’s and a 650 would have done better. They were too much 202’s and a 750 is more like 12 :1 comp and a torker intake and poor idling for street use I call these miss matched parts for greater than 7000 RPM; Everything else was built around getting a 6800 rpm max range you want to build around what RPMs you would like to achieve on the street including ignition again that you did not mention. A good High Voltage coil, Cap, rotor, and correct set up using a curve manual advance kit set up on your stock distributor. Then very important you would need at least 2400 to 2800 stall converter on that 700R4 unless you mean it is a 4 speed manual trans.