[johnunit]

There is a very real restriction in most factory exhaust systems, even as low as 1500RPM.


A general guideline among hotrodders is that 1PSI is about where exhaust backpressure causes measurable efficiency/power loss at a given RPM. In performance circles, logically, this means that about 1psi is the max you want to see at redline/shift point. People here rightly point out that restriction at high RPMs is not an issue in most economy builds. However, I've personally seen 3-5 PSI on STOCK engines being free-revved (in neutral) at below 2500RPM. What does this mean? These engines WERE seeing a significant restriction at cruising RPM. One I remember vividly was a Nissan 350Z, which has a decent-looking exhaust system.

An even more in-person example is the CLEAR seat-of-the-pants difference to everyone who had known the car in my 1985 Crown Victoria going from 2 inch dual exhaust, iron exhaust manifolds, and stock mufflers to 1 5/8 inch primary headers and 2 1/2 inch with relatively free flowing mufflers. The gains were evident as low as 2000RPM in terms of acceleration. I also saw fuel economy improvements, especially on the highway, but I don't keep close enough track on that car to give you numbers. That's not even that restrictive a setup I had to begin with but the gains were still clear and WERE present at lower RPM's.



As for talking about exhaust tuning pulses and the like? Generally you need longtube (think 35 inches minimum) headers, and the ability to to choose primary tube diameter and preferrably collector diameter and length. Without that, the odds of exhaust resonant pulses lining up to give you a significant boost at cruising RPMs is very low.


I'd bet large amounts of money that virtually every car on the road, driven the same way before and after, would see fuel economy improvements from going to a larger diameter exhaust system than stock with less-restrictive mufflers. The only caveats to this would be:
1. Cars that can't self-adjust to compensate for more flow and where the owner can't/won't tune it to compensate themselves. Mostly an issue on early EFI setups.

2. If you went with such an absurdly larger exhaust system that the extra weight or aerodynamic drag negated the positives.


For further proof, go to almost any forum for a specific vehicle, be it the aformentioned 350Z, Crown Victoria, TDI VW's, anything, and ask whether people see fuel economy improvements with less restrictive exhausts.

In all honesty, I think removing restrictions to the exhaust and intake tract are probably where most of the people on this forum are missing out on gains. Not a shot at anyone in particular, but if you've spent 100 hours on coroplast aero experiments and you still have a stock 1 1/2 inch tailpipe on, you're really going about this game the hard way.