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Straight Pipe Or HF Cat 97 GSX

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rofljerry

Proven Member
85
2
Aug 18, 2014
MUSKEGON, Michigan
Hey guys, I have a a HF cat on my 3in exhaust system, i needed one for california since I was out there, but now I'm back home in Michigan. Should I straight pipe it or not? I dont know everything about the air or exhaust science behind cars but my buddy was saying that if I take of the HF Cat I would lose back pressure for my turbo and that I would actually decrease performance rather than gain. But I always figured straight pipe would help me gain HP but I'm not sure what to do. Any help please.
 
it is true sorta. back pressure is a good thing to have. there is a power loss with less back pressure. these cars were tuned for the stock exhaust size. to compensate, you can put a stock size pipe in for back pressure. its sorta like a fuel line, the flow is determined by the smallest fitting
 
In a turbo system back pressure is NEVER good. At all. In any area of the torque curve/rpm band. It's always bad.
If you can get away with it make your exhaust as free as possible.

This has been something that refuses to die. The idea of backpressure being good. This isn't a 2 stroke.
 
In a turbo system back pressure is NEVER good. At all. In any area of the torque curve/rpm band. It's always bad.
If you can get away with it make your exhaust as free as possible.

This has been something that refuses to die. The idea of backpressure being good. This isn't a 2 stroke.

Well my gsx is turbo'd, so yes straightpipe it?
 
Well my gsx is turbo'd, so yes straightpipe it?

Yes. If you don't DD it and want the best then have a short pipe run to exit through the bumper. Problem with that is typically you're gonna have to relocate the alternator so bye bye AC.
You can take it back a few feet and have it angle to fire exhaust gas down(so you don't burn anything) and that's pretty good. Shorter, wider exhaust is ALWAYS better with a turbo.
 
Don't forget to port your o2 housing or you will most likely get spiking or surge. I haven't gotten around to it yet, so in the meantime I just adjusted my MBC to about 14psi (on stock turbo) to minimize it

I've heard about this on smaller turbos a lot. Boost creep. Yea you need to port out the hotside a little to allow the wastegate flapper to open completely up. The port work on the o2 housing definitely helps too from what i've heard. I ran a 5558 with internal gate. Didn't touch the turbine housing just ported the hell out of the o2 housing and never saw a spike. I could turn it down to 12 to 14psi during break in with zero problems.
 
it is true sorta. back pressure is a good thing to have. there is a power loss with less back pressure. these cars were tuned for the stock exhaust size. to compensate, you can put a stock size pipe in for back pressure. its sorta like a fuel line, the flow is determined by the smallest fitting

You kinda have the idea, half way there. NA cars benefit from scavenging, a lot of people mistake that for backpressure. In a turbo car you never want backpressure, the less backpressure the better so the exhaust side can breath better/ spool.

OP I would get a 3" test pipe fabbed up with flanges, any exhaust shop should do it pretty cheaply. If you don't like it (get sick of the smell, see no hp difference) put flanges on the HF cat and put it back on.
 
it is true sorta. back pressure is a good thing to have. there is a power loss with less back pressure. these cars were tuned for the stock exhaust size. to compensate, you can put a stock size pipe in for back pressure. its sorta like a fuel line, the flow is determined by the smallest fitting


No sir.... You want to flow bc turbos make their own backpressure. In a n/a motor yes, but this is a turbod car. You can run a full straight or like mine and straight out the side oft front bumper.
 
Just to add, on a turbo'd car, the turbo acts like a cat on a N/A motor creating the backpressure.
 
I'm amazed nobody has mentioned the slight lag that will develop from the loss of backpressure.. Is backpressure bad for a high performance engine? Yes. For a stock-mild street build? I don't believe so. I've only had 3 dsms but I am armed with a welder and a lot of boredom so I have spent DAYS playing with different exhausts and found that a 2.5-3 inch exhaust with no cats and a high flow muffler will affect your spool time, not drastically, but if you have a 16g or smaller it will be noticeable as they are not laggy turbos by nature.. Personally I would keep the cat unless you plan on doing serious upgrades.
 
I'm amazed nobody has mentioned the slight lag that will develop from the loss of backpressure.. Is backpressure bad for a high performance engine? Yes. For a stock-mild street build? I don't believe so. I've only had 3 dsms but I am armed with a welder and a lot of boredom so I have spent DAYS playing with different exhausts and found that a 2.5-3 inch exhaust with no cats and a high flow muffler will affect your spool time, not drastically, but if you have a 16g or smaller it will be noticeable as they are not laggy turbos by nature.. Personally I would keep the cat unless you plan on doing serious upgrades.
Incorrect. Part of the way a turbo makes boost is by the pressure differential across the turbine. In all situations the less pressure after the turbine, the quicker the turbo will spool, and the more efficient it will be. The less pressure after the turbine, the less drive pressure is needed to reach the same shaft speed.
 
Back pressure isn't good for a naturally aspirated car either. As mentioned above scavenging is the principal they use.

When an exhaust pulse travels down a pipe, a low pressure area travels behind it. If the header pipes, and exhaust are sized, and placed correctly, this low pressure wave helps draw the exhaust gas out of the cylinder when the exhaust valve opens. This works great if everything is timed correctly for the rpm range you want to make peak torque at.
 
It makes sense to me but I've still experienced a noticable difference in spool times, especially so when i switched from a stock 1g manifold to the FP manifold, not trying to dispute but I would like to hear why, The more I opened my exhaust the more my spool time suffered. I have researched it and initially was concerned; all of my searching basically ended with "FP manifold+larger exhaust causes you to lose low end and gain top end due to the larger runners and free flowing exhaust" (Paraphrasing) Which would be fine if I was running a much larger turbo but for a street car I would imagine it's the mid-end you would want to maintain.
 
It makes sense to me but I've still experienced a noticable difference in spool times, especially so when i switched from a stock 1g manifold to the FP manifold, not trying to dispute but I would like to hear why, The more I opened my exhaust the more my spool time suffered. I have researched it and initially was concerned; all of my searching basically ended with "FP manifold+larger exhaust causes you to lose low end and gain top end due to the larger runners and free flowing exhaust" (Paraphrasing) Which would be fine if I was running a much larger turbo but for a street car I would imagine it's the mid-end you would want to maintain.
A manifold with larger runners can slow spool.
 
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