[SSDiv6]

Originally Posted by Greg Reimer 7376

Back in the scary days of the early '70's, I worked at a Chevy dealer beginning in 1975 up till about mid 1980. The cars they made between those years were not exactly exciting as to drivability and gas mileage. Beginning in 1971, the typical 4 barrel 350 that you got in most cars, came with 9.0 to 1 compression by using a piston with a small dish and a large CC combustion chamber in the heads. The '71 engine had the same camshaft as the 67-70 engines, .390" intake lift, .410" exhaust lift. Those cars still ran pretty well. It seemed that NOX emissions were a real concern, and excessive heat or combustion produced it, so the quick fix was to drop the compression down to 8.5 or so to 1, pull ignition timing out of it, and they ground a different cam with more lift,.398I,.430E, as per the NHRA engine specs in the guide. The problem was that the exhaust valve opened early and closed late. Also, the exhaust system in the car was quite restrictive. The sleeve on the right side exhaust manifold where the exhaust do-nut went had a noticeable restricted opening. This was an attempt to cause back pressure in order to trap a portion of the exhaust in the combustion chamber as a precursor to the EGR systems that came out in '73. Also, the presence of an inert gas in the mix combined with the retarded timing (4-6 degrees initial ) was designed to get the NOX emissions low enough to pass some federal standard. This resulted in a car that just didn't have it compared to its predecessors. This also was the start of a rash of 350 Chevy motors that used to get flat camshafts in about 20-30,000 miles. The factory replacement cam was the earlier cam, so when the line mechanics at the local dealer got one of these cars,'72-80 for a camshaft and lifter replacement, the earlier cam went in, and the customer got his car back with a noticeable improvement in drivability. We always set the timing to the factory specs and marked it so that the state smog gestapos wouldn't come visit the dealership and fine us for "tampering". In 1970-72, we had no idea the horrors awaiting the auto industry regarding upcoming emission control constraints! Now, if you have a Stock Eliminator car, all attention is being paid to making all the cylinder pressure and thermal efficiency you can for maximum power at the wheels when the car needs it most. The whole science of camshafts and their effect on engine performance is a huge field, and sometimes understanding what it takes to make an engine work better is to review why some engines didn't work right in the first place.

You are correct. There are many variables from one engine manufacturer and year to the other. One limiting factor, especially in Stock Eliminator is the lifter diameter. The lifter diameter on GM engines limits the lobe and ramp when compared to Ford, Chrysler and AMC that uses a larger diameter lifter, hence, allowing more aggressive lobes.