I just took delivery of some Heddman Hedders from Summit that are designed for C-4 Vettes and found that the exhaust ports are round.
The exhaust ports on the heads are rectangular. Will this cause a large perfromance drop? This car is being built as a track only car to run around a track just for fun. I am adding true dual 3" exhaust, but if the port mismatch causes a huge depreciation in power I may have to upgrade to Hooker$.
Unless the header ports are smaller than the head ports, the shape mismatch provides an advantage unavailable by more closely matched ports. Any reversion, reverse pulses, in the exhaust system will be inhibited from entering the head as it travels back up toward the head, when it encounters the "brick wall" of the mismatch. The same is true of "D" shaped exhaust ports. A close match of shape between the head and the header is not only unnecessary it isn't even the best scenario. If the header ports are smaller than the head ports, THEN, you have a problem.
The Hookers have 1 3/4" primary tubes. The Hedmans have 1 5/8" tubes. The smaller tubes will start to "work" at a lower rpm, and for the vast majority of SBCs won't sacrifice power on the upper end.
A small mismatch can be "adjusted" with a grinder, if the header is smaller than the head port. Once again, if the header is bigger than the head port, the mismatch is an advantage.
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