Replacement Exhaust/muffler 97 Metro

[Ryland]

I have not had a chance to compare a metro xfi muffler to a normal metro muffler, but I have been able to compare a crx hf muffler to a normal crx muffler and the higher mpg crc hf has smaller exhaust pipes all around, and that would appear that the people who designed it read the same types of books as I have on the shelf that say that just like air intake, you don't want to large around if you plan to run low rpm's, it's not restriction, it's a matter of maintaining air velocity as air velocity in a proper sized tube like a exhaust pipe is kind of like having the air equivalent of a fly wheel that is helping to pull that exhaust gas out of the engine.
the only way I've seen a cherry bomb muffler help with gas mileage is tha they are so loud people drive less and hey drive slower so they don't get a ticket.

[an0nymous]

Just thinking here but what about SuperTrapp: Performance Exhaust? Apparently they are very tunable, like all the way down to stock. Using a scangauge you could probably nail down to a high degree of accuracy the best amount of backpressure.

[Formula413]

As has already been said, a low restriction exhaust will really only provide benefit at high RPMs, which is basically irrelevant to FE. A simple explanation would be that under light loads the engine does not have to push very much exhaust gas out the exhaust, so whatever restriction exists in the exhaust is likely not hurting efficiency, while under heavy throttle at high RPMs there is much more exhaust gas being produced, and a low restriction exhaust means the engine will not have to work as hard to push all this exhaust gas out. There are other factors that come into play but that is the simplest explanation. This is why I have left my Escort's exhaust unchanged, but have added a very low restriction muffler to my Firebird. Doing this produced no noticeable change in FE but did improve power. Plus it sounds cool.

I have heard it suggested that running an exhaust with too large a diameter for a given application will actually allow atmospheric pressure to work against the engine. I have no idea whether this is true or not.

[john_bud]

Originally Posted by Formula413

As has already been said, a low restriction exhaust will really only provide benefit at high RPMs, which is basically irrelevant to FE. A simple explanation would be that under light loads the engine does not have to push very much exhaust gas out the exhaust, so whatever restriction exists in the exhaust is likely not hurting efficiency, while under heavy throttle at high RPMs there is much more exhaust gas being produced, and a low restriction exhaust means the engine will not have to work as hard to push all this exhaust gas out. There are other factors that come into play but that is the simplest explanation. This is why I have left my Escort's exhaust unchanged, but have added a very low restriction muffler to my Firebird. Doing this produced no noticeable change in FE but did improve power. Plus it sounds cool.

I have heard it suggested that running an exhaust with too large a diameter for a given application will actually allow atmospheric pressure to work against the engine. I have no idea whether this is true or not.

Very nice explaination.

A different way to look at it is that mufflers are rated (big rig ones are anyway) for flow rate at a specific pressure drop across the muffler. More flow and less drop = less energy wasted pushing the piston UP the cylinder on the exhaust stroke. If you have 5 psi in the exhaust pipe for instance and a 4" diameter piston you have 12.5" of area and 5 psi so basically 60 pounds of force holding the piston back. That means you need to burn fuel equal to 60 pounds force extra (times the number of cylinders) to overcome the exhaust back pressure.

But as Formula413 correctly pointed out, running for high mpg's means you are only using the minimum fuel and generating a minimum amount of exhaust gas. So the pressure is very close to zero and the force is very close to zero. So the gains from the exhaust are also close to zero.

High HP and High rpm engines generate more exhaust, so the pressure is higher and the gains can be significant.

[Katmandu]

Originally Posted by Formula413

As has already been said, a low restriction exhaust will really only provide benefit at high RPMs, which is basically irrelevant to FE. A simple explanation would be that under light loads the engine does not have to push very much exhaust gas out the exhaust, so whatever restriction exists in the exhaust is likely not hurting efficiency, while under heavy throttle at high RPMs there is much more exhaust gas being produced, and a low restriction exhaust means the engine will not have to work as hard to push all this exhaust gas out.

That is EXACTLY right on the money.

[Frank Lee]

Well then what is so damned cool about taking the muffler off?

[Katmandu]

Originally Posted by Frank Lee

Well then what is so damned cool about taking the muffler off?

Since you don't know. You must not be cool.

[Frank Lee]

Thnx for the help

[fonque]

this is weird for me to read a thread like this. I have been into getting more performance out of my cars since I started driving. Getting the best MPG forces me to throw out most of what I have learned.

On my NX2000, it has been proven that with a custom intake plenum, the right cam profile, you can make power naturally aspirated with up to a 3.5 inch exhaust. But with the logic in this thread I would have to go back to the stock 1.9 inch pipes.

[Frank Lee]

I'd venture to guess that those of us who are willing to putter around at 2000rpm or less would benefit from smaller than stock pipes.