interesting exhaust note: removing rear muffler does not increase noise at all >>
08-06-2002, 10:04 AM
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interesting exhaust note: removing rear muffler does not increase noise at all >>
..untill you floor it, but overall the noise level under normal driving condition was not affected. We did this test last night to a '93 S4 and a 2000 S4 by simply hacking them off :P.
If you have a turbo car, I would highly recommend ditching the muffler in favor of a straight pipe.
If you have a turbo car, I would highly recommend ditching the muffler in favor of a straight pipe.
08-07-2002, 08:25 PM
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Disagree in part . . . install high-flow cats, then mufflers . . .
The down pipe usually does not matter that much, because most Audi's flow exhaust smoothly until the cats then flow slowly through the restrictive mufflers.
The A4 12v has a factory "water-inch" restriction of 24; whereas a passenger car engine of around 200 hp needs only about a 14 to operate properly. Thus, Audi cats and mufflers create far too much backpressure. (HP and backpressure have a inverse relationship--a 4000 hp dragster needs no backpressure; a 850 hp racecar needs about a 5, etc.)
High-flow Cats and mufflers alone will significantly reduce the backpressure to a more a realistic number even without going to bigger pipes.
Get bigger down pipes only when you have added so much horse power that it becomes necessary to accomodate the exhaust flow; otherwise, you actually increase the backpressure by slowing the flow of exhaust gases. Remember, when moving a gas rather than a liquid, the gas always fills the entire pipe no matter how much you have, whereas the liquid obeys the laws of gravity and will only fill the portion of the pipe required by the volume of liquid. The bigger the pipe, the more gas it requires to fill it; thus, the slower the gas moves, because it requires more to fill-up a larger pipe volume before it can begin to move down the pipe.
The A4 12v has a factory "water-inch" restriction of 24; whereas a passenger car engine of around 200 hp needs only about a 14 to operate properly. Thus, Audi cats and mufflers create far too much backpressure. (HP and backpressure have a inverse relationship--a 4000 hp dragster needs no backpressure; a 850 hp racecar needs about a 5, etc.)
High-flow Cats and mufflers alone will significantly reduce the backpressure to a more a realistic number even without going to bigger pipes.
Get bigger down pipes only when you have added so much horse power that it becomes necessary to accomodate the exhaust flow; otherwise, you actually increase the backpressure by slowing the flow of exhaust gases. Remember, when moving a gas rather than a liquid, the gas always fills the entire pipe no matter how much you have, whereas the liquid obeys the laws of gravity and will only fill the portion of the pipe required by the volume of liquid. The bigger the pipe, the more gas it requires to fill it; thus, the slower the gas moves, because it requires more to fill-up a larger pipe volume before it can begin to move down the pipe.
08-08-2002, 11:36 AM
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Disagree to your disagree :)
I guess it depends on what your configuration is.
What you say mostly applies to NA cars. Turbo cars are completely different, there is a turbo in the way, and it does effect the exhaust gas.
The exhaust gas comes out of the turbine extremely hot, and very turbulent. You want this to enter into as big of a pipe as possible, this allows the exhaust room to expand, cool, and straighten out. This is why I HATE the cat being right on the end of the turbo on the 1.8T It's a terrible design, at least from a performance point of view.
I noticed a HUGE difference on my XR4Ti when going from a 2.25" to 3" downpipe, but it has a T3 turbo, and it's also a 2.3 liter motor.
On a K03 1.8T, a 3" downpipe is probably overkill, but a 2.5" downpipe, and moving the cat under the car (or removing it), would make a big difference.
My T3/T4 2.0 has a 3" downpipe, no cat, and I wouldn't have it any other way
What you say mostly applies to NA cars. Turbo cars are completely different, there is a turbo in the way, and it does effect the exhaust gas.
The exhaust gas comes out of the turbine extremely hot, and very turbulent. You want this to enter into as big of a pipe as possible, this allows the exhaust room to expand, cool, and straighten out. This is why I HATE the cat being right on the end of the turbo on the 1.8T It's a terrible design, at least from a performance point of view.
I noticed a HUGE difference on my XR4Ti when going from a 2.25" to 3" downpipe, but it has a T3 turbo, and it's also a 2.3 liter motor.
On a K03 1.8T, a 3" downpipe is probably overkill, but a 2.5" downpipe, and moving the cat under the car (or removing it), would make a big difference.
My T3/T4 2.0 has a 3" downpipe, no cat, and I wouldn't have it any other way
08-08-2002, 08:30 PM
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boohoo
exhast flows slower in a bigger pipe -
yes but isn't the volume of exhaust what matters? bigger HP makes more exhaust and hence you need to flow more exhaust volume - and bigger pipe is less restrictive to higher volume of flow.
have to the cat moved to underneath the car -
you mean like where the resonator is? i don't see how the cat is gonna work - the exhaust gas won't heat up the cat to burn all the unburned fuel. its like not having a cat at all. cat is too far away from hot exhaust.
well thats what i figure. pleas correct me if i am wrong.
yes but isn't the volume of exhaust what matters? bigger HP makes more exhaust and hence you need to flow more exhaust volume - and bigger pipe is less restrictive to higher volume of flow.
have to the cat moved to underneath the car -
you mean like where the resonator is? i don't see how the cat is gonna work - the exhaust gas won't heat up the cat to burn all the unburned fuel. its like not having a cat at all. cat is too far away from hot exhaust.
well thats what i figure. pleas correct me if i am wrong.
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08-09-2002, 09:09 AM
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Most cars have the cats under the car....
the reason manufacturers are moving it closer to exhaust manifold is for a decrease in light-up time for the cat (the cat has to be above a certain temp in order for it to work) - This is for decreased emissions.
It will still work with no problems under the car.
Exhaust volume is not what matters the most. Exhaust temp, pressure, velocity, and turbulence are also factors. And those factors are going to vary depending on load and RPM. As always, it's a balancing act.
It will still work with no problems under the car.
Exhaust volume is not what matters the most. Exhaust temp, pressure, velocity, and turbulence are also factors. And those factors are going to vary depending on load and RPM. As always, it's a balancing act.
08-09-2002, 09:19 AM
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We mostly agree, b/c if you make the power you need the bigger downpipe . . .
This goes for any set-up, as the engine is sucking in and combusting a certain volume of air and fuel. The greater the volume of air+fuel, the greater the hp, the greater the volume of exhaust.
The question becomes how cool will the exhaust become in the larger downpipe?, because if it becomes too cool, it will become heavy and slow the exhaust flow causing the exhaust to back up all the way to the turbo (just as the cats do on the 1.8T's sitting directly after turbo).
Some people believe that bigger pipes provide better performance on all applications; however, like anything else in life, the truth is always more complex.
The question becomes how cool will the exhaust become in the larger downpipe?, because if it becomes too cool, it will become heavy and slow the exhaust flow causing the exhaust to back up all the way to the turbo (just as the cats do on the 1.8T's sitting directly after turbo).
Some people believe that bigger pipes provide better performance on all applications; however, like anything else in life, the truth is always more complex.