mufflers and exhausts - what they do...

Nightmage82

Nightmage82

1,494
the exhaust fell off one of the cars i had the pleasure to drive once :P and it was quite a large section. what baffled me was why the car suddenly lost a great deal of power... i understand back pressure could have something to do with it but does anyone know exactly what an exhaust does? (besides the obvious). is a longer/shorter one better? is there an optimum length for a particular engine type? or are they just to keep noise and emissions down and what i experienced was just a coincidence? :D

paul
 
It's happene to me before. But I noticed it in the opposite way. It was gone for so long then I got it fixed and the car seemed so much quicker... a 1976 Ford LTD mind you.
 
Just happened last week to my 323. My dad hit a bump and broke the converter off at the front flange. Loud as hell, and slow too.

I think it has everything to do with back pressure. You would think that it would make it faster with less resistance, but I guess it only works on some cars.

It also could do with the fact that the car is so loud that you can't really feel the power anymore. In the 323, since it has no tach, it was impossible to hear what the engine was doing. It was a constent loud noise, and it seemed really slow.
 
Originally posted by Nightmage82
the exhaust fell off one of the cars i had the pleasure to drive once :P and it was quite a large section. what baffled me was why the car suddenly lost a great deal of power...
Click to expand...

If the exhaust fell off in an uneven manner, causing a kink or bend in the piping, then the now smaller exhaust port would kind of explain why there was a loss of power. If that wasn't the case, and the muffler fell off cleanly...well, it could be the MCU realizing something's wrong, and is drastically reducing fuel injection.


i understand back pressure could have something to do with it but does anyone know exactly what an exhaust does? (besides the obvious). is a longer/shorter one better? is there an optimum length for a particular engine type? or are they just to keep noise and emissions down and what i experienced was just a coincidence? :D
Click to expand...

Primarily, they're for noise and emissions. The other thing is to keep exhaust fumes from running into the passenger compartment. If not for those factors, then you could probably just run open-port heads (as most 70's race cars did).

Given that a street car has strict noise and emissions standards to meet, sizing one (length and diameter) depends on how you want your engine to respond. Generally, though, a larger exhaust creates less backpressure, which allows the engine to flow more freely, which allows for more power. There does have to be some backpressure, otherwise unburnt fuel starts to flow out, greatly reducing efficiency. As for the specific math involved, you'd have to ask someone else. ;)
 
Originally posted by Hooligan
If the exhaust fell off in an uneven manner, causing a kink or bend in the piping, then the now smaller exhaust port would kind of explain why there was a loss of power. If that wasn't the case, and the muffler fell off cleanly...well, it could be the MCU realizing something's wrong, and is drastically reducing fuel injection.
Click to expand...

200th post! yeahy. heh. anyway thanks for that hooligan yeah you could be right, i wasn't the one who fixed it so i didnt see the actual break, but it was very much louder than before. sounded great even though it was a 2 litre engine. 1989 vauxhall (opel) carlton, i dont think it had an MCU.

as far as i know the pipe broke off very close to the engine block so umm back pressure was probably the reason then.
 
Backpressure is actually what helps get rid of spent fuel and such after ignition has occured in a cylinder. After the piston moves up, backpressure helps force it back down and push all the fumes out. Exhaust goes hand in hand with intake though. The more air youre taking in, the more you need to move out.
 
Originally posted by Nightmage82
the exhaust fell off one of the cars i had the pleasure to drive once :P and it was quite a large section. what baffled me was why the car suddenly lost a great deal of power... i understand back pressure could have something to do with it but does anyone know exactly what an exhaust does? (besides the obvious). is a longer/shorter one better? is there an optimum length for a particular engine type? or are they just to keep noise and emissions down and what i experienced was just a coincidence? :D

paul
Click to expand...
You have it pretty much nailed. Without back pressure, a cars/engine HP drops
 
:rolleyes: Guys, If you want to know ANYTHING about Exhausts/Backpressure , then I'm ypur man!!
First; A broken off pipe/muffler WON'T cause UNBURNT air-fuel mixture to get blown out UNLESS your car ISN'T tuned properly!!
If that's happening ...then it's running too RICH or your IGNITION sucks, and can't burn it!
Second; Backpressure is the exhaust trying to go BACK INTO your system. If you have too much, your engine works harder than it should to expell the gasses. when you DECREASE the backpressure, it's easier to get tha gasses out...therefore your engine can pump them out quicker. This helps it rev easier AND ingest MORE AIR!
Since more BURNT gasses are getting out of the combustion chamber, the mixture isn't as contaminated...So you get MORE POWER!! due to a better air/fuel mixture.
That's why a decent flowing exhaust/muffler gives you extra HP!
There's a term "VOLUMETRIC EFFICIANCY", basically it's the percentage of how an engine burns the air/fuel mix compared to how well it SHOULD.
In a "perfect car it would be 100%, but in reality a really well tuned street car is lucky to achieve 65-75%!
 
As far as the broken pipe goes, did it break BEFORE the O2 sensor or AFTER it? Now that's a different deal. that would make the cumputer go NUTS! Or if it broke off before or after the catalitic converter . Normally ,that would tend to INCREASE HP, NOT DECREASE it!
Also, pipe diameter WILL alter the powerband & tone/loudness of your engine.
A SMALL one has good torque, but sacrifices top end power.They also tend to "RAP" &"crackle" when you step on it. BIG pipes are louder(depending on the muffler) and can KILL your low end punch...but INCREASE top end pull (max HP).Most important of all in an exhaust is FLOW & VELOCITY!!! If you can maximize them for your particular car you'll be impressed at the results. I'd be glad to answer any questions if anyone here needs some help. I'v specialized in exhaust systems & exhaust technology since the mid eighties, so I know what I'm saying... and if I can't answer a question...I'll find it for you guys. I hope I helped out with this.
 
Originally posted by Turbo2
As far as the broken pipe goes, did it break BEFORE the O2 sensor or AFTER it? Now that's a different deal. that would make the cumputer go NUTS! Or if it broke off before or after the catalitic converter . Normally ,that would tend to INCREASE HP, NOT DECREASE it!
Click to expand...

hey thanks for your comments turbo 2, it certainly sounds like you know what your saying. but i've had two different cars that it happened to, neither of which had O2 sensors, or catalytic convertors. one of them didn't even have fuel injection, just carburettors :D they were certainly not high tech cars. the exhaust was MUCH louder, very raspy and fairly crackly, but there was a MARKED decrease in power. it wasn't just me pressing the accelerator less because it was too loud... it felt like the car was running on 10hp or something, barely enough to accelerate at all..

everything you said has made sense to me but im telling you what i experienced.

are you sure it isn't as shobthabob said "Backpressure is actually what helps get rid of spent fuel and such after ignition has occured in a cylinder" ?
 
The decrease in backpressure SHOULD have "uncorked" your car, but I can give you more precise info if you give me Specific details about it. let me know what year/make/model/engine it is and I'll try to find the cause for you.
It sounds odd, but I love a good challenge , besides, I might learn something new...you never know.
 
my friend eliminated the catalics on one honda 1800cc
for gain hp on his bike (10/15 according to him)

btw turbo can this be truh or not ?

cause another friend have 1800cc and according to the cattalog they have a muflers can give 20 hp on that bike.

i know that is a lot hp gained, but that is what he said.
 
It's definately POSSIBLE ,depending on the tuning of the bike. But that increase in HP is most likely on the top end, plus depending on the type of bike , the stock mufflers can be VERY RESTRICTIVE!! So it might be the truth. 20 extra HP on a bike is a real bump in power, that's like 100HP for a car, because the power to weight ratio is so much greater for a bike!
 

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