I would like to know if there will be any benefit to port matching my headers to the exhaust ports of my ’73 Challenger’s 360? There is a big stepup. Here’s the engine specs: fresh mild 360 with Erson Hi Flo cam (2700‑5800) and heavier valve springs, smog 360 heads with small ports (I have plugged the ‘pulse port’ under the exhaust ports), nothing else done to heads. It’s about a 8.5:1 CR. Edelbrock Performer, port matched to heads. Thermoquad with 1/4˝ open spacer which I need with this manifold, TTI headers 1‑5/8˝ into 3˝ collector. Twin 2½˝ system with X‑pipe and flow‑through low restriction mufflers. TF904 with TCI shift improver valve body, ‘more‑stall’ converter (about 2800 stall) 8¾˝ with 3.23 gears.
Most people are saying‑ “just leave it, it will work fine.” Sure it will work, but I have a die grinder so it won’t cost me anything, and if doing this port match will improve flow and efficiency, why not do it? Some people even say that the step will help things…? Have you seen before/after dynos of the benefits of this?
About 15‑20 or so years ago, some “tuners” were espousing the benefits of exactly the mismatch you describe, calling it “anti-reversion.” From what little I know about pressure, pluses, yada, yada, I always thought a sudden enlargement was actually worse for flow than a sudden restriction, causing all manner of turbulence and other nastiness, all of which is sure to cost you HP.
This disco‑era theory (deliberate mismatch) seems to have fallen by the wayside, and for good reason: It was bogus. Get out that die grinder and go to town. (While you’ve got the mask and goggles on, those heads could use lots of other help, too, just stay away from the short‑side radius unless you really know what you are doing doing—for basically, the same reasons as above.)