Anyone experiment with high flow mufflers?
 

I really dont want to add one to my metro because I dont want it to sound like a ricer car but if it helps with mileage Im all for it. I just wont get and oversized tip.

If you're adding a muffler since yours is rusted out, I would go with the oval shaped muffler instead of the cylinder shaped import car type. The oval one will sound much deeper, and still be high flow enough for a metro.

IMHO, big mufflers don't help FE. They reduce pumping losses somewhat at high RPM and WOT. But if you want good mileage, you never stomp on the gas like that.

I've often thought that a smaller muffler would be better for FE on my truck, because the OE muffler is such a big aero obstacle.

If you're trying to increase FE by increasing exhaust or intake capabilities -- that means you're not driving for FE. You should NEVER outflow your stock exhaust or your intake (of course, I'm sure there's a couple very rare exceptions) ;)

Exception:

Turbocharged cars can see mileage gains from enlarging exhaust pipe diameter and using high flow components.

:)

go for a light weight muffler that is quiet. like the one off a late 90's honda civic. its big, but thin and light. and covics are quiet cars. plus a muffler designed for a 1.6 engine has got to be basically no backpressure on a small metro engine.

If you could decrease exhaust backpressure while increasing exhaust velocity at low rpms where your cruise rpm is then that should help to increase FE.

To do this though you'd probably have to go with a slightly smaller than stock manderal bent exhaust system with a very free flowing muffler. It would increase low end torque at the expense of high rpm horse power.

That is true, but from my experience, the parts store universal mufflers are always about half the weight of the OEM Honda muffler. Just for that reason, I would probably go with a cheaper/new universal "Oval" muffler instead.

I fitted my Volvo 240 wagon(non-turbo) with an OE cat-back exhaust from a 240 turbo. Basically, its only a ~.5" difference in pipe diameter...but the turbo system uses only 1 muffler while the n/a system uses 2. The exhaust note isn't obnoxious and it still retains the OE look...so no hastles from local law enforcement.

Ther is an efficiancy point when it comes to backpressure. The only way to know what works it to experiment which is unrealistic or go with something someone else has proven. On my Camaro I put dual 2.5 exhaust on it and gained both power and gas milage. At the time I was only looking for power and the MPG increase was the benifit.

If you are running a V8 truck I know any exhaust mods will increase FE and power. I am not sure if small economy cars will benifit.