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Old 03-19-2012, 06:35 AM   #53 (permalink)
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Intake runners and exhaust port/primary (ie. the parts of the exhaust system that are still just being fed by one cylinder) is where bigger is absolutely not better.

The diameter/CSA (cross-sectional area) of the intake and exhaust ports and runners absolutely change where peak torque happens, how well fuel is mixed, where reversion (exhaust going back into intake runners) occurs in the RPM range, etc. However, once you've merged the individual cylinder pulses into one big tube (the collector, as opposed to the primary exhaust tubes) diameter has a much smaller effect.

The difference in where and how much advantage your exhaust collector is giving you between, say, a 3 inch collector that effectively ends (large disruption or size change like the H-pipe, mufflers, cats) 30 inches after the primaries and a 2 inch collector of the same length is negligible and difficult to calculate accurately. Add to it the fact that a 3 inch collector of a given length may behave similarly to a 2 inch collector of another length and IMO it stops mattering much when you're talking part-throttle low RPM in streetable applications with minimal from-scratch fabrication.

A set of headers/exhaust manifolds with an excessively large primary diameter (say 2 inch instead of 1 5/8 on a near-factory small block Ford or Chevy) would be harmful to economy unless you did some very creative packaging. But a similarly excessive 3.5" collector/exhaust pipe diameter would likely see no engine efficiency loss compared to the same exhaust routing in a more reasonable/traditional mild performance size of 2.5".

The exception to all this is if you're paying big money (think $1000+ just for the headers) to an exhaust guy who can set up all your exhaust for a certain RPM. That's probably not realistic for the RPM ranges most people want though. A 45 inch primary length and 30-40 inch collector is a decent rough length (computer simulations and experience would get you much more exact) for good tuning at 5000RPM. Now keep in mind that longer lengths generally= lower rpm tuning advantages, and you can see the packaging issues with an economy-based setup like this. To have peak tuning efficiency at, say, 2300RPM, you'd likely need a system that was muffler, cat, and sharp kink free all the way past the rear axle for the collector to end where you wanted. Not practical.

Anyways, that's all getting a bit too technical and honestly get near the end of my knowledge outside of the specific area of small block fords, but I stand by the original point that it's a misnomer to say that restricting the exhaust piping or making it smaller is a potential FE boon. Pumping losses and general volumetric efficiency gains will almost universally beat out any waveform tuning type disadvantage in a system that isn't designed from teh ground up to be tuned in such a way. And no, your old pickup truck or VW diesel or what have you does not have it's cats or the start of the exhaust manifold collector placed with these tuning advantages in mind. It's all packaging and NVH in a compromise with restriction/pumping losses when you're talking non-performance OEM setups.
2001 Prius - 170,000 KM - just got it (no consistent FE numbers yet)
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