the muffler and exhaust mod thread
 

Anyone who reads this site regularly knows that periodically someone posts a question about what effect swapping their stock muffler for an aftermarket one will have on their fuel economy. Others ask about exhaust pipes or headers.

Some ask whether removing the muffler entirely will help fuel economy or power. Sometimes people post items for sale online that make unlikely-sounding FE claims. Some ask broadly what effects a mod might have.

The consensus answer tends to be that aftermarket muffler claims for fuel economy are usually bogus, exaggerated, or unproven. Changing the muffler size or the pipe size will have little benefit for fuel economy unless coupled with a general exhaust tune (including calculated pipe sizes) to maximize benefits. There are exceptions. One exception, noted in comment #4 below, is the effect of exhaust mods on a turbo diesel engine (corroborated here and here). Another exception is that shields and wraps of the exhaust header are effective in reducing engine bay temperatures and some have argued improved high-rpm power and/or fuel economy may result if the velocity of exhaust gasses increases as a result of preserved heat (though the FE claim is unproven, see here and here).

A well-known chart on pipe sizes often is suggested as an aid to judging the proper pipe diameter. A caution about this chart is that it always refers to the MAX horsepower a car might produce. MAX hp is reached at very high RPMs (such as 6000rpms). Unless you are modding for racing, you are not likely to be at the max hp for more than perhaps 1% of the car's life. The calculator will not help much in planning for FE gains or everyday driving.

I hope to use this thread to consolidate the best information into one place, through links such as those above and any comments you offer below. Please stay on topic: mufflers, exhaust pipes, exhaust headers, pipe diameter, and how to use them to benefit fuel economy or emissions or (preferably) both. I'll periodically edit this post to include links to other threads or products suggested below.

My own muffler is on its last legs, rusting out and patched around the tailpipe. In researching the claims through this site, I decided to post this thread.

Thanks for reading this long post.

[EDIT: I have installed a Thrush Turbo muffler because it is lighter and flatter than the stock Bosal and will help me with a rear diffuser or belly-pan install. It will not benefit FE or power. I weighed both: 9lbs for the Thrush and pipes and 18lbs for the OEM Bosal. I think it is quieter than the Bosal, but it required welding so it was not cheaper. The Bosal would have bolted on.]

And anyone who has read these threads knows I will come on and remind that an extra .000001 mpg (IF a muffler delete gives that) is not worth ruining the quality of life of everyone in the neighborhood for.

That said, especially if you have instrumentation, go ahead and do a proper a-b-a muffler-straight pipe-muffler or good muffler-fart can-good muffler test.

I had one Tempo with a very nice mandrel bent stock system and one where the muffler rusted off. I put a straight "test pipe" on there for a while and noticed no discernable difference in fe.

One of the basic myths is "it will flow better with XXXXXX (insert larger pipe 70% of the time).
Since we are ecomodding, we are trying to use the engine in the lower rpms. There is NEVER a time when a larger than factory size will be beneficial.
Thanks for providing the exhaust pipe size chart!
On my particular car, (02 infiniti Q45) there was a very large muffler in the rear. commonly refered to as 'the suitcase muffler'. It wieght about 40lbs. There were already 4 cats, 1 resonator and 2 mufflers beofre this last one. I removed it PRIMARIALY for some weight savings.
I could see where someone (with an older car or a top of the line car) could reduce some weight and still have a relatively quite exhaust. as long as they keep the piping at factory size.
here are some pics:
I have removed tj=he rear dual muffler and installed minis
http://i678.photobucket.com/albums/v...nterior029.jpg

I di remove the resonator...there was a crack developing and installed the x pipe.
http://i678.photobucket.com/albums/v...nterior027.jpg

that lead into the factory mufflers:
http://i678.photobucket.com/albums/v...nterior028.jpg

The rear is covered now with a rear belly pan

About the only application where adding larger pipe and muffler or removing the muffler is going to help will be a turbo diesel.
Turbos like as little back pressure as possible and diesel engines love having their cylinders stuffed with air mass even at light load.

Excellent point. Exhaust effects on turbodiesels can be quite different than on naturally aspirated gasoline engines.

thanks for the clarification on the diesel!

Elimination of back pressure has benefits in gas turbocharged engines as well, but top-end power is where this is primarily seen. In regards to fuel economy, I don't know of any proven effects either way.

I might, eventually, modify my exhaust, and if I do, I'll be sure to track any changes in FE.

Some great reading here on back pressure:
http://www.google.co.uk/url?sa=t&rct...ZXjwugSHi3jtxQ

Also a research paper on marine diesel engines which tests and proves that less exhaust back pressure is a good thing on turbo diesels.

http://www.google.co.uk/url?sa=t&rct...6ftTzqrFO90QRw

I want to add something to Post #8. What oil pan 4 was saying in regards to diesel turbos "loving their cylinders stuffed with air" would apply to at least some gas turbos. I was doing some more research on my car's tuning, and its AFR range is from 9-11:1 from factory. By eliminating back pressure in the exhaust, the AFR does improve. I'm not sure how much fuel efficiency can by gained by increasing from, say, a 10:1 AFR to a 12:1 AFR, but that seems to be a pretty common shift.

I added this point, and all of these: