Does Exhaust Noise Play a Role in Fuel Economy?

[Formula413]

Originally Posted by Frank Lee

My question is does exhaust noise attract hot babes looking for action? Many seem to think so.

I am doing some long term testing of this theory in my Firebird.

When this has been discussed before I think the consensus was that freer flowing mufflers will only produce performance gains but not FE gains, since driving for FE generally involves using light throttle, and the increased airflow available from a performance muffler is not providing any benefit with the small amount of exhaust gas being generated. The best thing you can do with the exhaust for FE is optimizing the diameter of the pipe for the power being produced IMO. Too large a pipe will hurt velocity.

[Frank Lee]

Originally Posted by Formula413

I am doing some long term testing of this theory in my Firebird.

bwaa ha ha hyuk hyuk hyuk

[RH77]

I had the same questions with respect to exhaust, flow, and FE.

The best rule of thumb to use: "Stock is Best". It was finely tuned by experts -- any change in that system will effect the bottom line. Quiet is good.

Here are some great links on the Science and myths of Exhaust, by Team-Integra.net:

Exhaust Basics
Advanced Exhaust Tech I
Advanced Exhaust Tech II

RH77

[RobertSmalls]

I've heard conflicting things over the years. If no ECU modifications are made, will a car get better FE with reduced exhaust backpressure? Is backpressure ever desirable?

[Formula413]

Originally Posted by RobertSmalls

I've heard conflicting things over the years. If no ECU modifications are made, will a car get better FE with reduced exhaust backpressure? Is backpressure ever desirable?

My understanding is that no, it is not. I read an article once about someone who put a butterfly valve into an exhaust pipe and set out to determine what was an optimal amount of back pressure for different engine loads, with the goal of using the butterfly to create the right amount of backpressure. Turns out that no extra backpressure was helpful at any engine load.

Another theory I heard was that an exhaust pipe that is too large in diameter for the application will actually allow atmospheric pressure to work against the engine. I have no idea if this is true or not.

[Peter7307]

RH77,
thanks for the links to the articles.
They are a real find.

Cheers , Pete.

[Christ]

Originally Posted by Formula413

My understanding is that no, it is not. I read an article once about someone who put a butterfly valve into an exhaust pipe and set out to determine what was an optimal amount of back pressure for different engine loads, with the goal of using the butterfly to create the right amount of backpressure. Turns out that no extra backpressure was helpful at any engine load.

Another theory I heard was that an exhaust pipe that is too large in diameter for the application will actually allow atmospheric pressure to work against the engine. I have no idea if this is true or not.

Yes, it is. The exhaust must combat atmospheric pressure to exit the pipe. When it comes out of the engine, it's obviously under more than atmospheric pressure. As it goes through the exhaust, it slows, cools, and loses pressure.

While back pressure is absolutely not necessary, under any circumstances, exhaust diameter and length are necessarily tuned for optimum specifications.

I can't remember the formulae, or where I wrote them down, but there are some equations that will let you determine the optimum size/length of pipe based on VE (or a number representing exhaust flow as volume).

You basically want to fill the pipe, but not create more pressure than what is coming from the engine's exhaust pulse. You want the exhaust gasses to stay hot as possible, for as long as possible, so they're forced to exit the pipe as quickly as possible, which creates a scavenging pulse at the valve, helping to suck in more air, or reduce pumping losses.

I once tried the backpressure thing, but for a different reason: I was attempting to make an auto-tuning exhaust.

I originally theorized, that based on engine vacuum (which approximates load) a vacuum solenoid could open/close a section of the exhaust pipe just after the manifold collector to effectively "tune" the end of the manifold. No muffler or pipe was used after the collector, and the whole system failed to work as I determined it should, so I never bothered to try to dyno tune it.

I might still work on it some after I get rid of a few projects.