Aftermarket muffers and performance/efficiency

[mechtech2]

Back pressure is NEVER wanted or good.
How this myth perpetuates is beyond me.
A straight through muffler should produce more power, but it would be at full throttle and high RPM that the most benefits would occur, with some at mid power and RPM.

 

[Brandon1048]

Originally Posted By: SteveSRT8
Actually the majority of owners in my community have nothing but problems with anything other than stock cats. They constantly throw codes, collapse internally, etc.

I can't really say that I know the answer to your question. On my tuner that is not an option. I can adjust the mixture and spark timing but only within narrow ranges.

I've seen cats overheat from a misfire, so it would seem richer mixture=hotter cat, but yet many cars have a "cat protect" mode involving a super rich mix that actually cools them under load.


I've had aftermarket cats on two vehicles, one was a magnaflow and the other an off brand, with no problems. But I did not use them for a normal service life of a converter and my vehicles were in sound mechanical condition.

A lean mixture is more associated with overheating parts; think white burnt exhaust valves and an excessively enriched mix causes cooling of parts and the black soot found on fouled spark plugs.

Originally Posted By: mechtech2
Back pressure is NEVER wanted or good.
How this myth perpetuates is beyond me.
A straight through muffler should produce more power, but it would be at full throttle and high RPM that the most benefits would occur, with some at mid power and RPM.


This myth perpetuates because if you increase the exhaust flow on an engine with a fuel system that isn't adjusted for the increased exhaust scavenging it will run lean. Too lean and you will experience a mild version of the bogging that happens when fuel pressure is too low or un-metered air gets in.

Most engines with modern EFI can account for a wide range of intake and exhaust flow and air density. I had a toyota pickup with a 22re engine that lost all low end torque with an open exhaust. sometimes a bigger, better flowing exhaust will adjust the torque curve to a higher rpm. Aftermarket exhausts often sacrifice two or three times as much power in the midrange than the peak power it will gain.

Whether a free flowing intake or exhaust will help fuel efficiency has many many variables. The more air you flow the more fuel you burn, and the more power you make. I think that a nonrestrictive exhaust or intake will burn more fuel every time there is no load on the engine with the stock exhaust (idling/cruising) With the engine under load you may not have to open the throttle as much and it will might use less fuel or the same amount of fuel. I think it is too much of a case-by-case basis for a definitive answer.

 

[Brandon1048]

Originally Posted By: SteveSRT8
This is very vehicle specific. But most factory systems are very well engineered and make excellent power without much noise.

Beware as many cars these days react poorly to lowered back pressure and or velocity changes. You may require tuning to develop any additional power at all, and the gains are always very small.

As an example, on my car a cat back with drastically better flow numbers results in a very slight gain in HP at the upper rev range and a huge drop in torque at the bottom! Generally the car is quite a bit slower than a bone stocker!


A full exhaust system or especially a cat back on almost all cars is a waste of money and just causes excessive noise. I find loud exhausts annoying on anything: buzzy economy cars, screaming sportbikes, harleys that go potato-potato-potato, or v8 trucks and muscle cars. They are for kids who don't know any better or people who want attention. When I was 16 I had a flowmaster on s-10, and at 17 I had a 110 decibel roar emitting from the rear of my ninja 250, eventually I realized that the noise is rude to everyone else on the road and no gains could be quantified. At 24 years old, I bought an old chevy truck with a 350 v8 which had dual cherrybombs on it. After I installed a single 2.5" exhaust with TWO TURBO MUFFLERS ONE AFTER THE OTHER, yes that correct there two of the worst flowing but quietest mufflers butted up to each other, The truck accelerated exactly the same as before; it got to 55mph from a stop in the same time and distance as with a horrific droning straight through cat back.

All that being said, I am torn between keeping the stock exhaust or installing a freer flowing one alongside an intake. On cars with forced induction those mods are actually worth it. The Mazdaspeed miata actually gains about 50 horsepower from an intake and turbo-back exhaust.

 

[dwcopple]

Originally Posted By: Brandon1048
actually gains about 50 horsepower from an intake and turbo-back exhaust.
WOW! Mazda should be ashamed of themselves.

 

[mechtech2]

Brandon -
Wrong #1 and wrong #2 in your posts.

What really happens is people put a larger pipe on the exhaust, and the volume and mass of the internal gas increases. This larger pipe causes INCREASED back pressure at low flow/RPM, when the driver thinks it is less! Any benefits are at higher RPM and throttle settings.

More fuel can be needed because of more flow, but not leaness.
More flow [to it's limits] is compensated by the FI system, and more flow through a carburetor automatically gives the proper fuel [if it was set right originally].

 

[SteveSRT8]

Lucy, you've got some 'splainin' to do!

That first one is a whopper. Larger pipe diameter equals MORE back pressure? That just is not true.

And for the sake of limiting ridiculous examples let's limit this to N/A motors, as turbos,etc. are just to different.

 

[dwcopple]

 

[morris]

years ago i had a 1963 chrysler 300J, with a 500ci maxi wedge system about 550 hp. took it to a muffler shop. he made the pipes 2 1/2 id. two pipes. then he said to reduce noise, reduce the last 2-3 inches down to 2 inches. it worked very good, not much back pressure. i was very happy

 

[mechtech2]

Originally Posted By: SteveSRT8
Lucy, you've got some 'splainin' to do!

That first one is a whopper. Larger pipe diameter equals MORE back pressure? That just is not true.

And for the sake of limiting ridiculous examples let's limit this to N/A motors, as turbos,etc. are just to different.


No, you are wrong. And I am not ridiculous, I am scientific.

The larger pipe has more internal volume. This means more exhaust gas volume and mass. This impedes flow at low RPM throttle.
Or, how could more mass NOT affect the flow? Well, it HAS to!
things change at high RPM and throttle, of course.

Now, who is ridiculous?

 

[SteveSRT8]

You misunderstood me. I was not calling you ridiculous, I was trying to get turbos left out as they are not comparable to N/A motors and their reaction to exhaust mods.

So all the physics guys are stupid now. ANY fluid has lower pressure at higher volume if all else stays the same.

You are in essence insisting that a 3 inch exhaust will have more back pressure than a 2 inch? If so, I can't possibly run with you, as things are very different in your world!

And we haven't even gotten into velocity yet!

 

[jimvegas]

Originally Posted By: SteveSRT8

And we haven't even gotten into velocity yet!


That's the key.

Backpressure and velocity get confused all the time. Too large of a pipe slows velocity as the gas expands and cools in the larger pipe, slowing it's exit. This effects scavaging and torque at lower rpms.

Sure, with a larger pipe you get more top end power, all depends on what you are looking for.

For your average newer vehicle, the exhaust is tuned for the usual rpm range it will be driven at.

 

[mechtech2]

Yes - pushing a heavier column of a gas takes more power - there is more resistance with more mass of gas to move. Or does more mass take less to move it?
Same thing for a cold exhaust - it is not as efficient due to a denser and heavier mass of gas to move.

For the third time, I am referring to low RPMs. Ram tuning and scavenging are not in effect, of course.

 

[Klutch9]

A free-flowing exhaust, in my opinion, will work better without tuning on a turbocharged car than on an N/A car. The turbo already provides backpressure to the engine itself, and although the turbo also benefits from a small amount of backpressure, a large diameter, free-flowing exhaust can help build boost quicker with less resistance.

 

[Brandon1048]

Originally Posted By: Klutch9
A free-flowing exhaust, in my opinion, will work better without tuning on a turbocharged car than on an N/A car. The turbo already provides backpressure to the engine itself, and although the turbo also benefits from a small amount of backpressure, a large diameter, free-flowing exhaust can help build boost quicker with less resistance.


Many people with small displacement turbo cars run a small diameter exhaust wrapped in the heat shield/header wrap to keep the exhaust density down and the exhaust velocity up. All I'm saying here is that larger diameter exhaust pipes are not universally better.

 

[OtisBlkR1]

Originally Posted By: SteveSRT8
This is very vehicle specific. But most factory systems are very well engineered and make excellent power without much noise.

Beware as many cars these days react poorly to lowered back pressure and or velocity changes. You may require tuning to develop any additional power at all, and the gains are always very small.

As an example, on my car a cat back with drastically better flow numbers results in a very slight gain in HP at the upper rev range and a huge drop in torque at the bottom! Generally the car is quite a bit slower than a bone stocker!


couldnt say it much better myself..