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Mufflers and exhaust systems

Does a performance muffler make a
Difference in gas mileage? I looking to replace mine …

Not much, if at all. There are rare exceptions, but if a company could get more mpgs with a simple change to their exhaust system, they would aready have done it. And don’t count on much change to performance, either. Mostly just louder.

30 years ago it could make a difference in certain vehicles…but with today’s computer controlled vehicles…not-so-much.

Some do sound nicer though.

A muffler only, likely no change. Borla and other companies make exhaust systems, pipes and mufflers and claim to increase performance. Not sure if those claims are real or wishful thinking. Borla systems do sound nice on a V8.

We have a muffler shop in our area that specializes in “tuned” exhaust. They make the vehicle a little louder and the engine has a beautiful exhaust note. I don’t like loud but I love the sound of an engine with a throaty growl. I agree with the assumption that modern vehicles do not usually have fuel saving benefits when a performance muffler is installed.

Performance exhaust systems do a lot to enhance public revenue in areas where sound ordinances are enforced. Fuel economy claims are dubious, especially when they are accompanied by claims of increased horsepower.

Performance exhaust systems only affect gas mileage in that they can allow the engine to exhale a bit more freely, allowing it to take in a bit more air and fuel…reducing mileage when you romp it. Depending on the engine, the difference can be minimal (for most cars) to significant (supercars). The reason it’s more significant in high powered cars is simply that they’re using forced induction and their fuel injection systems are capable of putting as much fuel into the induction stream as the engine can handle. The injectors in my tC, for example, are limited, and the induction system is naturally aspirated (not forced), so no matter how much better I allow the engine to breath it’s only capable of minimal gain…at a minimal reduction in mileage.

Performance exhaust systems, performance anythings, add power (from a bit to a lot) at the price of mileage.

And keep in mind when looking at the horsepower gain claims, that 1) that’s best case scenario and 2) they are not additive.

In other words, if you add a K&N filter (10hp gain!) and an intake (5hp gain!) and a header (15hp gain!) and exhaust/muffler (10hp gain!) you will not end up with 40 extra horses.

With motorcycles the gains can be non-existant. Lots of companies sell ‘high performance’ exhaust systems for many of the ‘crotch rockets’. Cycle World’s test of some of them showed decreased HP and torque. But they were LOUD…

Did Cycle World rejet the carburetors after they installed the high performance exhausts? If you install a custom exhaust without rejetting, I wouldn’t expect any improvement. How do you solve the rejetting problem on fuel injected engines?

I don’t remember whether or not they re-jetted the carbs (they are pretty competant on this kind on thing, I’d think they did what was recommended). As for fuel injection, some ship with modified ‘maps’ I think. But it would be better to work that out on a dyno with a pc, etc.

On fuel injected engines, the same action as “rejetting” is actually accomplished by replacing the injectors with larger capacity injectors, capable of spraying more fuel per timeframe. Injectors cannot be “rejetted”. This also requires “remapping” of the computer such that with the new injectors the air/fuel ratio remains correct. The result is that the volume of air/fuel mix entering the cylinder is greater at WOT, but the mix is still readily combustable. To put in larger injectors and keep the same mapping would cause constant rich operation and possible flooding.

For bolt on parts, generally existing engine components and computers can handle things without having to be replaced or remapped. We’re talking about a fairly negligible difference - at best the power gains are the equivalent of driving down from the mountains to sea level. That crap you see in the tuner mags that implies sticking a muffler and a big intake pipe on there will make your Civic run with Porsches is just that - Crap.

Such increases as you get from these very mild tweaks are well within the specs for stock components. Once you move much beyond boltons, you’re probably either reflashing or putting a different ECU in there anyway.

The best I have seen is 1-3 mpg increase. I will bet the increase was do to the old muffler being some what plugged up. A friend who has a muffler shop did say that after cat duals would sometimes give 1-2 mpg increase. This was on trucks. As a rule I would not expect any. Now if you want to spend a lot of time with a computer and remap the ECU to fit your driving you may get some were.

Well, Shadow, I agree sort of. Many would consider turbochargers, sold as bolt-on kits, as bolt on parts. A good and proper kit includes new injectors and software, but there are a lot of kits out there that are deficient. There’s a lot of junk on the market.

I guess I’d feel more comfortable saying that stock system can handle any parts that don’t force feed the engine with air. That would include the CAI stuff, “high flow” mufflers, and the like. Real gains from these things are minimal, but they do admittedly give the kids (like me, for example) a sense of satisfaction.

In short, the program in the ECU will open the pulsewidth a given amount for a given engine speed, throttle setting, and airflow, and then trim it to where th oxygen sensor is happy. Improve the airflow beyond the outside range to which the ECU will open the injectors for a given engine speed and throttle setting, or (worse yet) push air in beyond the capacity of the injectors to provide fuel, and it won’t run right. It may even run lean…and suffer the consequences of doing so.

So I would warn anyone playing with forced air systems (turbochargers or superchargers) to either know what you’re doing and spring for a good, complete kit, or have a spped shop do the job for you.

In essence I agree with you. But am basically adding a warning to those looking for bolt-on horsepower. Be careful. Understand what you’re doing.

– nevermind.

Oh, yeah, I don’t consider boost of any kind to be bolt on.

I agree. But man, there sure is a lot of half-arsed junk out there.