Removing Mufflers - Hp

[RyanSingler]

Ok so today some how me & my two friends go on the subject of mufflers,they are 2 of those guys that think that they know every thing about cars even though that pro couldent even spell car.anyways one of them was talkin about his car & i was like if i had that i'd cut the mufflers off it or put straight pipes on it, & him & my other friend were like if you do that you will lose hp & backpressure & i was like no you wont you idiots you will gain abit & then we got in this big argument & i was like if it takes away Hp then how come dragsters & drag cars have no mufflers, so anyways im right rite.

[2.2 Straight six]

not neccessarily. some engines benefit from backpressure. highly tuned engines need as little flow resistance as possible. basically non-tuned engines sometimes benefit from the backpressure. tuned engines do benefit. dont take this as a solid rule though.

removing the mufflers on non-tuned engines wont really make much difference anyway.

[TheSilentChamber]

depends on the muffler, but upgradint to a bigger/better flowing exhaust will usually hurt your low end but give you more high end. Its really a traid off.

[curtis73]

Its a balance between mass flow of exhaust and velocity of exhaust. The best flow occurs at peak velocity, but if you are providing more exhaust gasses than the exhaust can handle, it will be a restriction. Removing restrictions from your exhaust will therefore only add power if they were causing your exhaust to reach that peak velocity too soon.

Conversely, if you have too much flow capability, that peak velocity is never achieved and therefore the benefits of exhaust scavenging are not fully attained.

A proper exhaust makes best average power, not the exhaust with the greatest flow.

[Akira13126]

It really just depends on the size of the engine and the ablity of the engine to flow. Something like a 4-cyl, like what I have benefited a little bit from removing the mufflers and placing a cherry bomb on it. As long as you don't go over kill on the pipe size and don't remove the cat's it should be fine. The only gain on a stock car is the sound.

[UncleBob]

it sounds faster, so it must be so!

There is a lot of science to exhaust tuning, just like any variable in engine building. Its not just a question of back pressure, but also pulse resonants, and don't forget, any changes to the system can and will cause changes to the fuel management requirements. Although many modern cars are self-learning in that area, that doesn't mean it would be taking advantage of such a modification, let alone make the situation worse.

It really depends on the vehicle. There are many vehicles that have such highly tuned systems, that ANY change to the exhaust will cause at best, no change in power. Just as likely to cause a loss in power.

The only exception is anything with a turbo. They don't have any tuning aspect post-turbo, and the less back pressure the better. If you could run it with literally no exhaust immediately after the turbo, it would be optimal...assuming you took care of the fuel management at the same time of course.

[Reed]

someone give him a basic explaination of exhaust scavenging and pulse tuning. I would like to learn a little more also.

[curtis73]

I won't go too deeply into it here since I'm on the way to the airport But I can maybe sum it up. Look in the FAQ sticky at the top of the forum for two threads added on 1-3-06 about velocity, flow, and "backpressure."

The basic idea is that the faster the exhuast gasses are moving, the more inertia they have. The more inertia they have, the more intake charge they can suck in during the brief period that both valves are open. That is called scavenging and its basically a way of getting more air and fuel in the cylinder. In this manner, the cylinder starts filling with fresh intake charge before the piston starts to drop. In the same way, intake runner size affects cylinder filling. The smaller the runner, the faster the intake charge is moving, and therefore will continue to fill the cylinder even though the piston has stopped its downward motion.

The downside is that the smaller the runners and exhuast are, the more restrictive they become at higher RPMs. The generally accepted maximum speed is about 300 mph. Above that speed, friction and restriction are exponentially greater.

So basically, you want to choose an exhaust that flows enough so that the flow inside it reaches 300 mph at about the same time as the RPM range you intend to use. So, while a 5000-rpm big block might benefit from a dual 2.5" exhaust, a 1.8L 4-cylinder might be fine to 6000 rpm with a single 1.75"

The idea of "backpressure" isn't a very accurate description of what is happening. Backpressure would always be bad, causing the opposite of what scavenging is trying to do. If you had backpressure when the valves were open, it would push exhaust back into the intake.

[beyondloadedSE]

I remember MM&FF did a comparison a few years back between about 10 different mufflers and of course no mufflers. Just about every since muffler gained more hp compared to no mufflers at all.

[curtis73]

Yup. Mufflers absorb sound waves. When you absorb the vibration energy out of the exhaust, the resulting flow can be greater than before. The secret is designing a muffler that can cancel sound without restricting too much flow.

[Rufe]

Hey Curtis,

Interesting post. Can you direct us to a book or website that discusses the sound waves?

Thanks,

[curtis73]

Honestly, I'd just be directing you to Music Theory textbooks. I have a music degree, and one of my areas of intense study was overtones and harmonic series. Its just basic physics applied to exhaust tubing viewed as a musical instrument. The same exact harmonic pulses that exist in an exhaust or intake are present in musical instruments, whether its a human voice or a Euphonium. In an instrument you develop certain frequencies based on diameters, lengths, weights, and materials. The same holds true for exhaust and intake tuning on cars.

[Rufe]

I once walked into a room with thousands of tiny holes in the walls. It effectively absorbed all of the sound. I swear I could feel a pressure differential in my ears.

What few mufflers I have seen torn apart seem to have these holes as well.
Do you think a muffler is more effective the closer to the engine it is?
Or, does the location of the muffler initiate an additional pulse in the system, that can be tuned ?

My guess is the mufflers are at the back of the car for lack of space reasons.

One more question, increasing the exhaust flow speed at the end of the system should lower the pressure, so a gentle taper in the system would act in a scavenging manner, right?

[curtis73]

Yes. Its actually sorta common. Not a taper exactly, but a reduction. My Impala SS for example has 2" tubing back to the resonators, then its 1.75". But its not for the sake of increasing velocity, its for the sake of maintaining velocity as the exhaust gasses cool and reduce their volume. Assuming that the exhaust gasses didn't cool, a taper might increase velocity, but at the expense of an exhaust that is too big near the valves where scavenging is at its peak. If you start with the "right" size for scavenging and taper it down it would become a restriction. If you end the taper at the "right" size then you have an exhaust that's too big at the valve. But tapering to maintain velocity as the exhaust cools would be neat. I'd like to see tests on that.

Mufflers can amplify certain frequencies while they cancel others, so placing them closer to the engine can mean that the frequencies you've left active (or amplified) will continue to grow with the long tailpipe. For instance, one of flowmaster's and dynomax's claims is that deep rumble. If you put the muffler right near the engine and then leave those lower frequencies free reign in 10 feet of tubing, the drone might get pretty intense.

Also, as you pointed out, the hotter gasses near the engine have a greater volume, so the noise cancelling would have to take more of a back seat to flow when its closer to the engine.

Depending on the muffler design and which wavelengths you wish to cancel, typically the rearmost installation of the mufflers will cancel the most sound while permitting the least restriction.