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Discussion Starter · #1 ·
I'm looking into some new exhaust components and was wondering at what point improved flow actually reduces tourque. I never really understood why backpressure is so important. Also, does anyone know the diameter of the stock 2.0 downpipe?
 

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Re: Need a lesson in why too much exhaust reduces tourque? (Volkschnell)

Torque is mainly effected by the velocity of the intake charge. The higher the velocity of air per a given volume the more torque. It's the small straw vs big straw analogy. A small straw in order to flow the same amount of air as a big straw must move the air through much higher velocity. By decreasing back pressure with a larger exhaust you decrease the velcity of the exhaust gases. This in turn decreases the intake charge velocity because the slower exhaust gases don't create as much of a siphon effect helping to pull the new intake charge into the combustion chamber.
So basically an exhaust is tuned to the amount of air flowing through the engine. The real reason we don't run 4" exhausts is because a 2.0L will never flow enough air to keep that size of exhaust in it's efficiency band.
Also it should be mentioned that many aftermarket exhausts don't lose as much torque as say bolting up a larger stock exhaust because things like mandrel bends and different muffler designs allow for higher gas velocities. Camshafts also play a role in the size of exhaust and backpressure due to the effect of valve overlap and scavenging but that's a whole other can of worms.
Oh and I'm not sure about the downpipe sizes but I believe mine were both just over 1" each since it's a dual downpipe. Techtonics makes a race downpipe for the 2.0 but only expect 2-3hp with a decent cam.
 

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Discussion Starter · #3 ·
Re: Need a lesson in why too much exhaust reduces tourque? (Fast2.0L)

Thanks for the reply Fast 2.0L ! That was fast!
I'm installing a TT 268 cam (W. GIAC chip) and wondered if I should go with the race pipe and high flow cat. I guess I'll let my wallet be my guide. My cat-back exhaust is 2.25" so based on your insights I don't think I'd loose much (if any) tourque by upping the diameter of the downpipe and cat. Question is, "are they worth the money?"
 

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Re: Need a lesson in why too much exhaust reduces tourque? (Volkschnell)

I am running a 270 Shrick and noticed a huge drop in low-end torque when I put on my high-flow cat, 2.5" exhaust and muffler. I put the stock muffler back on and it seems to be the perfect setup for me now. Got back some of the low-end, but retained most of the high-end flow.
Passatrcr
 

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Re: Need a lesson in why too much exhaust reduces tourque? (passatrcr)

Fast2.0L hit the nail on the head. Just a quick clarification though - exhaust piping that's too large will make you lose low-end torque, but of course, you gain torque in the higher rpm's (and thus gain peak hp).
 

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Re: Need a lesson in why too much exhaust reduces tourque? (94Teg)

So guys I have a Bosal/Brospeed cat back that came with my car, is this a bad exhaust? Not many people have it I realize! Am I losing torque with it?
 

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Re: Need a lesson in why too much exhaust reduces tourque? (Fast2.0L)

Would larger intake/exhaust valves also reduce low end torque? in an ABA? Is an ABA worth modding? Whats the best thing I can do to my exhaust system then if not by a hi flo cat back system?
 

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Re: Need a lesson in why too much exhaust reduces tourque? (jcadman)

OK.. so Torque might get reduced becoz of big exhaust. So how come it gives more Horse?
I always thought that Torque created in a car by converting HP of the engine... please someone clear things up for me! Thanks
 

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Re: Need a lesson in why too much exhaust reduces tourque? (aak97)

Horepower is a function of torque and rpm. A larger exhaust will reduce low end by shifting the torque curve up the power band. By shifting torque up the power band you've effectively changed one of the variables in the HP equation. That is the higher rpms with the same amount of torque = more hp. It is true that maximum peak hp is obtained with as little backpressure as possible but too big an exhaust will just loose more low end torque for not substantial gains peak hp gains.
I should mention too that I left thermodynamics out of all this but they do come into play with combustion temperatures, entropy flow characteristics, and exhaust gas energy.


[Modified by Fast2.0L, 11:16 PM 12-6-2001]
 

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Re: Need a lesson in why too much exhaust reduces tourque? (Fast2.0L)

quote:[HR][/HR] entropy flow characteristics, and exhaust gas energy.

[Modified by Fast2.0L, 11:16 PM 12-6-2001][HR][/HR]​
You sounded good until you popped up with "entropy flow characteristics!"
Now, I figure you're just trying to convince the unsuspecting that you know more than you do because you use alot of big words.
Entropy! Yes.
Entropy flow! No!
 

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Discussion Starter · #12 ·
Re: Need a lesson in why too much exhaust reduces tourque? (J. Daniel)

I don't know about "Entropy Flow" either but When my Kharmic (pronounced "car"-****) flow is increased by the square root of Pi times the diameter of my exhaust piping (you know, by helping an old lady across the street, or taking my dog for a walk) I can pull 11.698's in the quarter. Really!
 

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Discussion Starter · #13 ·
Re: Need a lesson in why too much exhaust reduces tourque? (Volkschnell)

Oh yeah,
Water pumper, if I remember correctly you've got a 268 and chip as well. Did you dyno your car? I'm getting the above setup installed today (!) but with 2.25" exhaust. How's the tourque curve?
 

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Re: Need a lesson in why too much exhaust reduces tourque? (waterpumper)

okay so what's the quick and dirty on replacing the mkIV's 2.0L exhaust? i am not going to replace the cat, just from there back. want to do it for a little talk when i am rompin on it (but not obnoxious), basically stock at idle if possible (maybe a fuzz louder) , and look good. hp gains would be great, but i am not willing to sacrifice too much low end torque to do this. if it's not going to run at at least a little better than stock, it's not worth the cash for me. i was looking into a Thermal Research Design or a Remus exhaust per one of the issues of european car or automobile mag, i can't remember which. anyone got any suggestions? thanks!
 

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Re: Need a lesson in why too much exhaust reduces tourque? (Rickdrum01)

had 2. 25" exhaust ans 268 cam and garett. car ran 15.7's and 15.8's at the track on street tire with a 2.3 60'. I founf high-flow cat made a difference, but when on the dyno we noticed the stock header made the touque fall rastically after 5500 rpm, so your best shift point would be at 6250 instead of letting it rev out to 7k like the chip allowed. If ou get a header you will regain your lost tourque and you can let that car rev out to 7k and get better 1/4" mile times.
 

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Re: Need a lesson in why too much exhaust reduces tourque? (passatrcr)

quote:[HR][/HR]I am running a 270 Shrick and noticed a huge drop in low-end torque when I put on my high-flow cat, 2.5" exhaust and muffler. I put the stock muffler back on and it seems to be the perfect setup for me now. Got back some of the low-end, but retained most of the high-end flow.
Passatrcr[HR][/HR]​
I had the exact same experience with a Magnaflow. I put the stock back on and everything was better. http://****************.com/smile/emthup.gif
 
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