Mr_Diesel
08-06-2006, 11:45 PM
Okay, peeps... I just straight piped my GTI... I was on vortex looking for some exhaust sound clips of straight piped VR6's and I came across a post where this guy did the same thing. All the uneducated schmucks decided to chime in and said he was "ghey" and stupid for doing this cause he lost all this torque blah blah blah. Finally I got pissed off and hit the books. I probably spent 4 hours studying back pressure, manifold and header design, nascar and formula 1 exhaust systems, mufflers, cats, pipe diamter, forced induction, NA performance, oldschool auto technology, etc, etc, etc... The list goes on and on. You should get the idea of what I found out by reading the post I created in response to all this. Enjoy.
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*Back from the dead*
Quick story...
Back when I got my 94 jetta it was completely stock. For the fun of it, I decided to get a base line dyno test. 89hp and 111 lbs of torque. TO THE WHEELS in Salt Lake City (elevation 5,000 ft). Later on while going to the market, my muffler finally rotted away and fell off the car. I was a teenager and VERY poor at the time. I decided to straight pipe until I could afford a cat back system or something. I used 1 3/4 inch pipe (stock size) from the cat until just after the rear axle. Then I used 3 inch pipe from the axle to the bumper (about 2 or 3 feet) just to deepen the sound a little bit. At idle the 2.0 sounded like a V8, and sounded like a 14,000 rpm crotch rocket at redline WOT. It was very loud, don't get me wrong, but it still sounded good.
With all this BS about loosing torque and crap, I decided to get another dyno test at the same place on the same dyno. Results. 83hp and 122 lbs of torque. I actually LOST hp and gained torque, still trying to figure that one out... ANYWAY...
Regarding exhaust BACK PRESSURE:
For those of you who just say "your engine needs back pressure" just because they heard it from some other schmuck, I will describe back pressure as best as I can, so next time somebody asks you, you can provided an educated response instead of passing on baloney.
Textbook Definition: During the exhaust stroke, an easy way for an engine to loose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted.
The reason why your engines produce less torque is that the reduction in back pressure is causing more air to be drawn into the combustion chamber than before.
Way back in the day, earlier cars with a carburetor often could not adjust because of the way that back pressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced back pressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if back pressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
Any modern car (including VW's) with electronic management compensate for this increase in airflow which is caused by a decrease in back pressure, and adjust the fuel mixture accordingly. This back pressure MYTH was true for old school carbureted vehicles, but it is completely irrelevant with modern electronic engines. You WILL gain power and you won't loose any bottom end by straight piping your car.
Another thing that makes me laugh :D Is people will sit around and tell you all day long that removing your muffler is bad because it reduces back pressure, but in the same breath will recommend you install a header. (which is SPECIFICALLY DESIGNED TO DRASTICALLY REDUCE BACK PRESSURE AND INCREASE FLOW).
Argument in a nutshell: Back pressure restricts natural airflow in your exhaust stream which consequently consumes power from the moving piston, which reduces power produced at the crank..... YOU WANT ZERO BACK PRESSURE IN ANY MODERN CAR.
So... why don't you guys lay off his back??? I guarantee he gained power from it. The reason I found this thread was because I just did this exact same thing to my 95 GTI. I was looking for comparing exhaust clips. His car sounds awesome, and my car sounds INCREDIBLE. Also, now my tires spin half way through 2nd gear instead of chirping. I am AMAZED at how long this argument has gone on in the modern automotive world. One guy with 2500 posts will post garbage about why back pressure is good, and suddenly you have 30 guys installing crappy mufflers on their cars to increase back pressure because they heard it from somebody on the internet... they act on that without doing any reasearch... Come on peeps... :banghead:
------------------------------
*Back from the dead*
Quick story...
Back when I got my 94 jetta it was completely stock. For the fun of it, I decided to get a base line dyno test. 89hp and 111 lbs of torque. TO THE WHEELS in Salt Lake City (elevation 5,000 ft). Later on while going to the market, my muffler finally rotted away and fell off the car. I was a teenager and VERY poor at the time. I decided to straight pipe until I could afford a cat back system or something. I used 1 3/4 inch pipe (stock size) from the cat until just after the rear axle. Then I used 3 inch pipe from the axle to the bumper (about 2 or 3 feet) just to deepen the sound a little bit. At idle the 2.0 sounded like a V8, and sounded like a 14,000 rpm crotch rocket at redline WOT. It was very loud, don't get me wrong, but it still sounded good.
With all this BS about loosing torque and crap, I decided to get another dyno test at the same place on the same dyno. Results. 83hp and 122 lbs of torque. I actually LOST hp and gained torque, still trying to figure that one out... ANYWAY...
Regarding exhaust BACK PRESSURE:
For those of you who just say "your engine needs back pressure" just because they heard it from some other schmuck, I will describe back pressure as best as I can, so next time somebody asks you, you can provided an educated response instead of passing on baloney.
Textbook Definition: During the exhaust stroke, an easy way for an engine to loose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted.
The reason why your engines produce less torque is that the reduction in back pressure is causing more air to be drawn into the combustion chamber than before.
Way back in the day, earlier cars with a carburetor often could not adjust because of the way that back pressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced back pressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if back pressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
Any modern car (including VW's) with electronic management compensate for this increase in airflow which is caused by a decrease in back pressure, and adjust the fuel mixture accordingly. This back pressure MYTH was true for old school carbureted vehicles, but it is completely irrelevant with modern electronic engines. You WILL gain power and you won't loose any bottom end by straight piping your car.
Another thing that makes me laugh :D Is people will sit around and tell you all day long that removing your muffler is bad because it reduces back pressure, but in the same breath will recommend you install a header. (which is SPECIFICALLY DESIGNED TO DRASTICALLY REDUCE BACK PRESSURE AND INCREASE FLOW).
Argument in a nutshell: Back pressure restricts natural airflow in your exhaust stream which consequently consumes power from the moving piston, which reduces power produced at the crank..... YOU WANT ZERO BACK PRESSURE IN ANY MODERN CAR.
So... why don't you guys lay off his back??? I guarantee he gained power from it. The reason I found this thread was because I just did this exact same thing to my 95 GTI. I was looking for comparing exhaust clips. His car sounds awesome, and my car sounds INCREDIBLE. Also, now my tires spin half way through 2nd gear instead of chirping. I am AMAZED at how long this argument has gone on in the modern automotive world. One guy with 2500 posts will post garbage about why back pressure is good, and suddenly you have 30 guys installing crappy mufflers on their cars to increase back pressure because they heard it from somebody on the internet... they act on that without doing any reasearch... Come on peeps... :banghead: