Muffler Question - Fuel Economy, Hypermiling, EcoModding News and Forum

rsx2002 · 08-01-2008, 02:57 PM

A friend on mine is considering buying a 96 Metro. He will have to buy a muffler for it as soon as he buys it(assuming he does). I told him a muffler with smaller diameter inlet and outlet pipes would benefit his FE. His dad disagrees with me. So I need to know who is right. So I did some reading to find out the answer. This is my case for why it will benefit and if nothing else not hurt FE (remember he has to get a muffler of some sort anyway). 1.) Larger diameter pipes are to increase HP and high end torque. They accomplish this in large part by allowing air to flow more freely and efficiently. The more air flowing through the more gas used. To maintain proper air/fuel ratios. So wouldnt smaller pipes have the opposite effect? 2.) More restriction increases low-end torque. Which would allow you to shift into higher gears at lower RPMs. Obviously higher gears get better MPGs. 3.) The third reason I think it may help is because of back pressure. Back pressure would not allow all of the exhaust air out of the engine and therefore not allow as much new fresh air (along with new fresh fuel) into the engine. This is where i suspect you would lose some HP but also burn les fuel. 4.) My last point is more of a question but it seems to me that the more efficient cars have smaller exhaust. I know that part of that is due to smaller engines. However I am refering to differences in the same model. Is that correct? Well please let me know how you guys weigh in on this. We are most likely going to check out that metro sometime today. Thanks!

Frank Lee · 08-01-2008, 04:08 PM

IMHO, the whole system from port to outlet would have to be re-sized to get any potential benefit, and not just the muffler. I'd go stock.

Ryland · 08-01-2008, 04:11 PM

you friend is only correct if they don't believe in science and math, if a free flowing exhaust was really all that great then why don't gas engines run right without any exhaust, a properly sized pipe will HELP to draw out the exhaust gasses from the engine, allowing it to work less and use less gas, a larger pipe requires more flow for this to work properly, so to larger of an exhaust pipe will create that back presure you spoke of and make the engine work harder.

aerohead · 08-01-2008, 04:47 PM

Go with OEM specs.

stigler4profit · 08-01-2008, 05:31 PM

If you size an exhaust correctly, meaning not too big.. you can potentially gain MPG. Because the engine, being a pump, will not have to pump as hard to expel gases as it would with a smaller diameter pipe. I personally saw gains of 1-2mpg with going from factory 2" to a 2.5" dumped exhaust with a turbo (free-er flowing) muffler than with the factory exhaust on my (now gone) Explorer Sport. In addition I ran straight exhaust on my Mustang GT, and saw 1-2mpg better by doing so.. without going larger in diameter. On my wifes little MX-3 it gets the same mileage with the 2.5" catback into a quiet universal muffler as it did with the factory sized pipe installed, yet it does accelerate much better.

Bicycle Bob · 08-01-2008, 05:50 PM

Another correspondent put a 2" mandrel-bent system on his Metro, and gained a noticeable percentage in mileage. Pumping against back pressure requires energy. Only rarely does it overcome a worse problem in tuning. Beware of false associations.

johnmyster · 08-01-2008, 05:50 PM

Not to rain on a parade, but...

The mileage gains attributed to dumped exhaust, loud mufflers, and straight pipes, could very well be due to another factor. The exhaust got louder, so you began to drive without your foot on the throttle as much. Mileage gain magic.

stigler4profit · 08-01-2008, 05:52 PM

No magic, simple math. A pump pumps better with less to pump against. Less backpressure the easier it is to pump.

johnmyster · 08-01-2008, 05:58 PM

Yes, but I don't spend much time driving a WOT at maximum RPMs. Does a typical ECOMODDER style driver spend much time under high load conditions anyway? Most pressure drop scenarios in fluid dynamics relate backpressure to a function of fluid velocity, somewhere between the square and cube depending on compressability. So my WOT 2000 rpm backpressure is less than my WOT 6000 rpm pressure by a factor of 9 to 27.

I just don't think I really spend much time driving under conditions where backpressure is significant, and it's hard to say that the "louder pipes, less throttle" relationship could not have had any effect at all.

You said it yourself, your MX3 accelerates better but doesn't get better mileage. A high flow setup may have decreased backpressure under high acceleration loads, but under normal MPG style driving, perhaps the stock system did just as well. Perhaps there wasn't any backpressure under those scenarios to begin with.

stigler4profit · 08-01-2008, 06:04 PM

Perhaps, but I did expand the size from a factory size of 1.something on the Mazda MX-3 to 2.5, now that I think of it probably 2" ( I have too many cars) cat back, into a quite muffler (universal) and it averages as good or better mpg than stock, and it runs better as well. And my wife drive it like a grandmaw. I view the engine as a pump and if I can aid it in it's internal parts moving with more ease.. so be it, every little bit helps.