This is Mark Gjavenis's thumpin' 540ci big-block Chevy in his CC-featured turquoise '65 Ch
Jim Homick, via CarCraft.com: I am building a 454 Chevy with a D-1SC ProCharger. It will be used primarily for the street, but I will be bringing it to the strip. Which style of cylinder head should I be using? I already have a set of oval-port heads upgraded to bigger valves that are otherwise stock. I'm thinking I should use a rectangular-port head to really move some air. This will be a budget-oriented project.
Jeff Smith: Generally, the words budget, big-block, and supercharger are most often mutually exclusive, but we're intrigued with the idea. A set of rectangle-port heads will certainly present less flow restriction on the inlet side. This will result in better cylinder filling, which is the whole idea behind adding a supercharger. But as usual, there's more to the story. If you were to run a dyno comparison of the supercharged, rectangle-port heads against a set of oval ports, the first thing you might notice would be a significant reduction in manifold pressure-or boost. This might lead one to think that the change to the larger ports is a mistake, but the opposite is actually closer to the truth. Boost as shown on a pressure gauge is the indication of manifold pressure. It could also be used as a measure of flow restriction. As an example, ProCharger rates the D-1SC supercharger as capable of moving 1,400 cfm of air, but this is with no restrictions to flow. Since the intake ports represent a restriction, the blower is capable of producing more airflow at a given rpm than the inlet side of the engine is capable of accepting, so the air (mixed with fuel, of course) begins to stack up in the inlet tract, creating pressure. This is what is indicated on the boost gauge. Because most (but not all!) rectangle-port heads can flow more air than oval-port heads, the boost will read lower, and in theory at least, the engine should make more power.
The key to a blow-through supercharged engine is a properly designed carburetor. In our ex
So now that we have the theory out of the way, let's get into the details. We'll assume your oval-port iron heads are the more attractive 119cc, open-chamber, oval-port heads with casting numbers ending in 049, 781, 241, or 359. These heads have small 2.06/1.72-inch valves but offer excellent flow potential with just a little bit of help. It sounds like your heads are already fitted with larger valves, but we have some additional ideas here. The hot ticket is to have a quality machine shop open up the seats to the larger rectangle-port valve sizes of 2.19/1.88 inches. If you are bold, you can even step up to a 2.250-inch intake size. Once the valve seats have been enlarged, there will be a sharp edge directly below the new seat that must be carefully blended into the throat to take full advantage of those larger valve sizes. One of the problems with production big-block heads is a sharp angle of approach on the short side radius of the intake valve seat. With the larger valve diameter, this radius increases. With careful blending, this really improves the low-lift flow potential of the heads. While some may think this extra work isn't necessary when using a supercharger, the truth is that anything that improves airflow through the engine will enhance the power that much more when adding a blower. This is especially important for the exhaust side of the cylinder head. Consider adding a 30-degree back cut on the exhaust valve to improve low-lift flow and help scavenge the exhaust gas after the combustion is even. This increases the amount of fresh air that will enter the cylinder on the following intake stroke. This means adding a good size primary tube set of headers. For a 454, the minimum primary pipe diameter is 1 3/4 inches, but 1 7/8 inches would be even better. Also consider adding 3-inch exhaust pipes to the mufflers and including a cross- or H-pipe to the system. Exhaust noise can be near offensive with a full 3-inch system, so consider adding 2 1/2-inch tailpipes that will tend to dampen the noise a bit.