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Old 01-31-2008, 07:51 PM   #1
Meat Supply
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Default Fixed exhaust- is it torquier?

so i got my exhaust fixed from the cat back and being loud, getting pulled over 3 times in 2 weeks. so now she's super quiet again and it also seems like it has a bit more torque. would this be from added back pressure-or just that it goes without being noisier than hell?
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Old 01-31-2008, 08:28 PM   #2
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i was running my last project car on an open header (straight 6) and as soon as i put a exhaust on the thing really woke up.. theres science behind it. having an properly tuned exhaust aids in cylinder filling, by effectively removing exhaust gases before IVO and during overlap.
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Old 01-31-2008, 09:06 PM   #3
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It works because the manifold isn't tuned. Normally in a tuned system you have the header consisting of the 4 primary pipes from each exhaust port, of a particular diameter and length, and coming together into a collector area, also of a particular diameter and length.

In theory, everything that controls the actual flow behavior and tuning of the exhaust is done via the header/collector only. Everything after this is essentially meaningless, if done right.

Now on most stock setups, there really is not much for tuning. Most primary piping is not equal in length, nor is there any real collector area nor any termination of the collector piping. The whole exhaust turns into part of the "system," and it can influence the overall behavior.

http://www.superchevy.com/technical/...exh/index.html
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Old 01-31-2008, 09:10 PM   #4
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ah! thanks!
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Old 01-31-2008, 09:10 PM   #5
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Quote:
Originally Posted by Back Road Runner View Post
It works because the manifold isn't tuned. Normally in a tuned system you have the header consisting of the 4 primary pipes from each exhaust port, of a particular diameter and length, and coming together into a collector area, also of a particular diameter and length.

In theory, everything that controls the actual flow behavior and tuning of the exhaust is done via the header/collector only. Everything after this is essentially meaningless, if done right.

Now on most stock setups, there really is not much for tuning. Most primary piping is not equal in length, nor is there any real collector area nor any termination of the collector piping. The whole exhaust turns into part of the "system," and it can influence the overall behavior.

http://www.superchevy.com/technical/...exh/index.html
^
sounds better than what i said. although i do dissagree that everything after a collector is meaningless, especially on a car with more than one cylinder bank. H and X pipes have been dyno proven to increase power.
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Old 01-31-2008, 09:33 PM   #6
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Quote:
Originally Posted by max427 View Post
^
sounds better than what i said. although i do dissagree that everything after a collector is meaningless, especially on a car with more than one cylinder bank. H and X pipes have been dyno proven to increase power.
Too add...
And tubing diameter is important on n/a cars to keep the exhaust gas velocity as high as possible without being restrictive... also, changing tubing diameter can hurt, sometimes a lot. changes in velocity, from slow to fast or fast to slow will hurt performance and the more changes in velocity (changes in tubing diameter) the more it will hurt performance. I dont know any formula to determine optimum diameter but i im sure there is one. the 2.5L subaru n/a motors are best around 2.25" ID under 200HP. The guys @ TWE said if aggressive cams are used or other mods that will make power over 200hp, a larger diameter tubing will be beneficial. seems like a rough rule of thumb but probably a good one since they are just about the best when it comes to subaru exhaust.
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Old 01-31-2008, 09:39 PM   #7
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oh yeah, my new pipes are a bit smaller than the "stock" ones that literally fell off. maybe they're better this way.
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Old 01-31-2008, 09:41 PM   #8
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The back pressure will help keep the air fuel charge in the cylinder on the very begining of the intake stroke, instead of it flying out the exhaust valve.....during valve over lap.
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Old 02-01-2008, 12:54 AM   #9
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But this is what you want. You need that flow from the intake side to clear out the spent charge from the combustion chamber. This is the whole idea behind scavenging.

Backpressure is always bad. You're running a big air pump. You don't want restrictions. Restriction = inefficiency.
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Old 02-01-2008, 12:58 AM   #10
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i experienced the same thing.i definetly noticedmore torque gains
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Old 02-01-2008, 01:13 AM   #11
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Quote:
Originally Posted by Back Road Runner View Post
But this is what you want. You need that flow from the intake side to clear out the spent charge from the combustion chamber. This is the whole idea behind scavenging.

Backpressure is always bad. You're running a big air pump. You don't want restrictions. Restriction = inefficiency.
Yes the whole air pump thing....You know it's more complex than that. Engines that dont make Peak VE around 7K rpm normally show a torque increase from an engineered (Atleast to a certain extent) exhaust manifold and post exh. manifold exhaust system. You want the intake charge to 'push' some of the very last exh. gas out of the cylinder, but you dont want the air fuel charge to leave out the exhaust manifold.
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Old 02-01-2008, 05:46 AM   #12
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Wait...are you talking about exhaust scavenging? Cause...it's the exhaust gases leaving the exhaust that creates the slight vacuum to pull in fresh air from the intake side accelerating the air/fuel mixture...
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Old 02-01-2008, 11:02 PM   #13
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Quote:
Originally Posted by Back Road Runner View Post
But this is what you want. You need that flow from the intake side to clear out the spent charge from the combustion chamber. This is the whole idea behind scavenging.

Backpressure is always bad. You're running a big air pump. You don't want restrictions. Restriction = inefficiency.
NO... backpressure is not bad... the correct amount of BP is a very good thing. You want some pressure to increase the velocity of the exhaust gas. Having a large very UN-restrictive exhaust will not keep your exhaust gas (and pulse waves) moving quickly which creates less of a low pressure area behind the pulse wave. this low pressure area is what helps "pull" the spend exhaust gasses out and the new a/f mixture into the cylinder. Having some of the new a/f mixture pass through the cylinder is a good thing because it ensures that no, or much less spent exhaust is left in the cylinder. It might waste a tiny bit of gas, but that is very neglegeable and if you are tying to make more HP, then fuel efficiency is something you have to sacrifice to some extent. Back to the pulse wave idea... tuned length headers (not just EL, but also tuned) will be a specific length from exhaust port to collector to create a reflected pressure wave to bounce and be back at the exhaust port ready to scavenge the next time the valve opens at the RPM the header was tuned for. Just wanted to share that so no one buys a 3.5" turbo exhaust for the n/a and then wonders why they dont make any power until just about redline.
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Old 02-02-2008, 12:53 PM   #14
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have a 97 L 2.2 SOHC, Borla 4-2-1 header, new oem converter, 2.25" mid-pipe with no resonator and 3ARacing muffler with silencer in it, noticeable torque gain with the silencer in, extremely doggy with it out, not sure why
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