Now for the intake side of things. Here, a carb and manifold decision had to be made. Whatever was chosen had to work well at both ends of the rpm range. Also, in terms of a match between heads and intake manifold, the manifold had to accommodate four different sizes of ports. The intake chosen because of very promising previous dyno tests was Dart's 180cc two-plane design. To accommodate the port size variance, the 180cc heads had a small chamfer applied to the intake ports of the heads, as the manifold was slightly larger. From here, on the manifold runners were opened up over about the first inch into a size just shy of the port size in each successively larger head.
Feeding the fuel to the system was a race-prepped 950 Holley. With this induction system, the engine had access to sufficient airflow for a good top end, while still catering to whatever low speed the smaller port heads might deliver. Undoubtedly, had we used a single-plane race intake such as a Victor Jr. or the like, the bigger port heads could well have shown a greater top end advantage over the smaller ones. Countering this, a race-style single plane could have compromised the smaller port heads' ability to deliver a stronger low speed output. In all, the induction system used proved to be a globally effective compromise.
Quoting port sizes by volume seems to have come about for two reasons. Way back when the only choice of heads was those produced by the factory, ports for street applications were about it. With less than ideal castings to start with, I can say from experience it took a lot of grinding to maximize the castings' capability. To give the customer an idea of how much work had been done, the head porters would quote port volumes as a measure of what had to be taken out to achieve the end product. Bigger equated to more work and thus more money.
When the castings were as limited to the extent those early ones, the more that came out, the faster the car went, and the sooner the customer would be back for more ported heads to replace those that cracked. And believe me, if you wanted to go fast, cracking had to be an accepted outcome.
So demonstrating the amount of work done on castings was one reason for quoting port volume. The other was to get some idea of the size of the cross-sectional area of the port. Since the port area changes substantially as the port progresses from the manifold face to the valve, quoting size in square inches is not practical
The other option, since the traditional small-block Chevy head has a five-inch long port, is to quote port size in terms of its volume. The bigger the volume, the bigger the mean cross-sectional area of the port is.
So why is port cross-sectional area important? If the area is bigger, the flow surely goes up, and that's what we want, is it not? Sure, the engine wants as much airflow as possible, but much of the flow through depends on port velocity and the generation of pressure pulses. This means an overly large port can hurt power even though it may, on the flow-bench at least, flow better.
As can be seen from the flow tests (Fig. 1), the bigger port does flow more up at the higher valve lift numbers. Part of this is due to a bigger intake valve, but about 70 percent of the additional flow in the 0.500 range up is because of the bigger port, not the bigger valve. So the big port/big valve combo flows the biggest numbers. The question is ,how does this work out on the dyno?
Now's time to look at a load of curves on our graphs. So you can better see what's going on here, the torque and hp graphs have been separated. The effect any particular head has on low speed output can be more clearly seen by considering the curves shown on the low end of the torque graph. To see what happens at the top end look at the high speed results on the hp graph.