Replacement Exhaust/muffler 97 Metro

[garys_1k]

No, there's no FE BENEFIT from smaller pipes or more backpressure, it's just that putting in low restriction stuff yields no additional benefit over stock.

Having said that, it's possible for an engine to be "over-scavanged" if all sorts of things come together just right, but again for FE driving that's not going to happen. When it does, raw air and fuel are swept right out of the exhaust valve during the valve lift overlap period, so some additional backpressure could help limit that. But again, not an issue for what we're doing.

[Frank Lee]

woont ya get better low end torque?

[garys_1k]

Originally Posted by Frank Lee

woont ya get better low end torque?

No, adding backpressure won't add crankshaft torque. But we're talking small gas flows here, compared to the max. design point (full load, at full speed), so small increases or decreases won't count for much.

[Frank Lee]

no, not backpressure, improved low rpm scavenging from the small pipe

[garys_1k]

Originally Posted by Frank Lee

no, not backpressure, improved low rpm scavenging from the small pipe

Ah! Yes, if the higher speed increased the gas' momentum enough, then it could reduce avereage pressure at the valve ports. Good catch, and it would make an interesting dyno test.

[Ryland]

I'm surprised this is still a topic that is being talked about, it has been studied, books have been written about it, exhaust pipe size should be chosen based on engine size and RPM, the smaller the engine or slower the speed the smaller the pipe should be, it's not about restriction, it's about fluid dynamics, science and math.

[garys_1k]

I think it's being talked about because the question is if it's worth mod'ing the stock system for better low end torque w. smaller pipes. I have seen nothing about how to calculate the "best" pipe size, but I suspect it gets into acoustic and other issues. Complicated stuff.

[whatthe]

I would try to build a custom header if you really want to help things out. It does start getting into period of valve opening, wave tuning, etc..

As these guys have said, the backpressure from the factory muffler is not likely to be a concern at cruising speeds and I would expect little gain to be had for any muffler at fuel economy rpm/throttle levels.

If it's turbocharged that's another story.

[garys_1k]

I'd think a model built with GT Power could help calculate both the gas dynamics (momentum) and acoustic effects. It would be interesting to see how one could tune the exhaust to get best torque at a given cruising speed.

That is an issue, of course. All tuning gives one sweet spot speed, and unless you're at that speed you may actually make things worse than an untuned stock system.