loss of torque from straight cat?

[FrostBite]

i recently put on a thermal 3 inch catback exhaust and i hollowed out my cat. ever since then it seems like i lost alot of low end torque but i gained alot of high end. also my boost used to shoot up to 11 really fast but now it goes up slowly to around 10 and then at around 4 or 5k rpm it slowly goes up to 15. is there anything i can do to get my torque and faster spool up back?

 

[CodeMonkey]

I've heard a few people say gutting a cat will have that sort of effect.

Just get a straight/test pipe. They're only about $60.

 

[psychlow]

Depending on how you gutted the cat, you may have altered the way air flows through the exhaust and created turbulence that the exhaust flow now has to overcome before exiting.

Larger exhaust on a turbo car should lead to better performance througout the entire RPM band, including faster spool due to reduced exhaust pressure. I would try either getting a high-flow cat, or as the above poster suggested, get a test pipe.

 

[FrostBite]

do you think i could put a pipe through my cat to get rid of the turbulence

 

[Twinturbo2800]

do the high-flow cats have this same effect ???

 

[psychlow]

do you think i could put a pipe through my cat to get rid of the turbulence

Yep. I'm thinking that'll fix your problem if it is indeed exhaust-related.

do the high-flow cats have this same effect ???

If it's from a reputable company, I severely doubt it.

 

[91Bomb]

Larger exhaust on a turbo car should lead to better performance througout the entire RPM band, including faster spool due to reduced exhaust pressure. I would try either getting a high-flow cat, or as the above poster suggested, get a test pipe.

I disgree. Larger exhauast = less backpressure which means more turbo lag. Backpressure is desireable at low rpm's to help with faster turbo spool time. Peope who autocross will generally use a smaller exhaust for this reason (smaller than 3"). The loss at low rpm's with 3" pipes sounds normal. I noticed a significant low end loss when I put on a 2.5" dp + test pipe leaving the rest stock. Keep in mind that 2.5" pipes are good up to 300 HP and if your HP is going to remain lower than that then 3" is going to be a waste of useable low end power.

 

[suicidal2af]

I disgree. Larger exhauast = less backpressure which means more turbo lag. Backpressure is desireable at low rpm's to help with faster turbo spool time. Peope who autocross will generally use a smaller exhaust for this reason (smaller than 3"). The loss at low rpm's with 3" pipes sounds normal. I noticed a significant low end loss when I put on a 2.5" dp + test pipe leaving the rest stock. Keep in mind that 2.5" pipes are good up to 300 HP and if your HP is going to remain lower than that then 3" is going to be a waste of useable low end power.

Uh, what? I think you're a little confused.

The only reason for backpressure is to promote scavenging(well, they're 2 different things, but you can't have one without the other). The basic idea is that higher velocity pulses are going to "suck" the exhaust of another cylinder out as its valve opens, resulting in less exhaust gas remaining in the cylinder and burning again. For higher velocity, you either need hotter air, smaller diameter piping, or more pressure.. Backpressure and scavenging go hand in hand.

Now, here's the thing: your turbine is creating backpressure. Rather than the exhaust gas freely flowing out of the exhaust, it's now colliding against the turbine and building more pressure, and increasing velocity. Backpressure/scavenging achieved.

Now what about backpressure against your turbo? It's going to slow your spool. Always. Try this...go stand behind a buddy's car and have him put it in neutral. Start pushing it. Now, imagine that you moving the car is the exhaust gas moving the turbine. Now drive over to a hill, and have your buddy park facing uphill. Get behind the car, have him put it in neutral, and try to push it up the hill. Gravity just became your example of backpressure against the turbo. I'm sure it'll take you a *little* longer to push it up the hill, no? The turbo is going to take longer to spool, everytime.

As far as autocrossing goes, that is a VERY general(and incorrect) statement. How you design your car is going to depend on what class you run. If you're running in any class that disallows modification of boost levels, you're going to run a monster sized exhaust with as little restriction as possible. Why? Boost creep just became your friend. If you're running in SM, you don't want anything to hinder your turbo's performance.

The only time people run smaller exhausts are when it's an NA motor. For a turbo, bigger is better.

 

[noz]

ok from experience.
with open downpipe I notice slower spool, less torque and lowend, but pulls "A lot" harder after the first gear.

now with downpipe connected to catback you do get a faster spool but not any of that high end power.

 

[noz]

I disgree. Larger exhauast = less backpressure which means more turbo lag. Backpressure is desireable at low rpm's to help with faster turbo spool time. Peope who autocross will generally use a smaller exhaust for this reason (smaller than 3"). The loss at low rpm's with 3" pipes sounds normal. I noticed a significant low end loss when I put on a 2.5" dp + test pipe leaving the rest stock. Keep in mind that 2.5" pipes are good up to 300 HP and if your HP is going to remain lower than that then 3" is going to be a waste of useable low end power.

agreed

 

[4geze63]

ok from experience. with open downpipe I notice slower spool, less torque and lowend, but pulls "A lot" harder after the first gear.

its usually faster spool with open dp, first gears going to pull hard regaurdless since your a GST like me you dont want first to have alot of power anyways or we just end up spinning through 1st...2nd....and 3rd

 

[noz]

its usually faster spool with open dp, first gears going to pull hard regaurdless since your a GST like me you dont want first to have alot of power anyways or we just end up spinning through 1st...2nd....and 3rd

hmm...yeah with open downpipe it only spins first gear.
but try this though, on the highway, 5th gear and just smash on the gas pedal.
with open downpipe it usuallytakes a little bit longer to reach 15psi or whatever u r running. with downpipe connected its quite faster but then you dont' feel as much power as with the open downpipe and slower spool.....

 

[TurboGeoff]

Yes, it is a proven fact that you loose off-boost torque with bigger exhaust, however, it increases top end.

 

[91Bomb]

Uh, what? I think you're a little confused.

The only reason for backpressure is to promote scavenging(well, they're 2 different things, but you can't have one without the other). The basic idea is that higher velocity pulses are going to "suck" the exhaust of another cylinder out as its valve opens, resulting in less exhaust gas remaining in the cylinder and burning again. For higher velocity, you either need hotter air, smaller diameter piping, or more pressure.. Backpressure and scavenging go hand in hand.

Now, here's the thing: your turbine is creating backpressure. Rather than the exhaust gas freely flowing out of the exhaust, it's now colliding against the turbine and building more pressure, and increasing velocity. Backpressure/scavenging achieved.

Now what about backpressure against your turbo? It's going to slow your spool. Always. Try this...go stand behind a buddy's car and have him put it in neutral. Start pushing it. Now, imagine that you moving the car is the exhaust gas moving the turbine. Now drive over to a hill, and have your buddy park facing uphill. Get behind the car, have him put it in neutral, and try to push it up the hill. Gravity just became your example of backpressure against the turbo. I'm sure it'll take you a *little* longer to push it up the hill, no? The turbo is going to take longer to spool, everytime.

As far as autocrossing goes, that is a VERY general(and incorrect) statement. How you design your car is going to depend on what class you run. If you're running in any class that disallows modification of boost levels, you're going to run a monster sized exhaust with as little restriction as possible. Why? Boost creep just became your friend. If you're running in SM, you don't want anything to hinder your turbo's performance.


The only time people run smaller exhausts are when it's an NA motor. For a turbo, bigger is better.

People with Na motors certainly benifit from larger/free flowing exhausts. If you take the '05 6 cyl mustang single exhaust for example and were to put on headers with a dual exhaust would you benifit?

And if bigger is always better for turbo cars why do they sell 2-1/2" exhaust for turbo dsm's?
Why doesn't everyone just buy a 3" exhaust?

I can tell you from experience thet when I went from the 2" dp to a 2-1/2" it took longer for the turbo to begin to spool -- more turbo lag. Once spool began though it ultimately did spool up faster and there was a gain in power throughout the upper mid and higher rpm range but lag was increased which hurt the low end and isn't a good thing especially with an auto trans in around town driving.

If we were strictly talking about 1/4 mi racing then I might agreee that bigger is always better.
IMO for everyday driving you're better off matching your exhaust with the turbo and amount of boost you plan on running. If I'm only running 12 or 13 lbs on a 13g a 2-3/8" exhaust would be my choice for the reasons that turbo lag is less than with a bigger exhaust which gives me boost sooner, the backpressure/scavenging effect gives better fuel mileage when on boost (and less lag), and when running those lower boost levels w/ small turbo (12-13lbs w/13g) the bigger pipes might benifit but it would only be on the very high end and not enough for me to sacrifice low and mid end response. Of course the increase in power over 2" stock would be the main reason I'd go bigger but only to the point where I achieve this goal while retaining the benifits of a 'smaller' exhaust. I'm talking about a 4-spd auto trans here and if I had a manual trans then I might be thinking a bit differently.

If I had a t25 (bigger than a 13g) running 15lbs then I'd opt for the 2-1/2" pipes.

Why would I use a 3" exhaust if I was running 275 HP or less when all the upgrade paths say that 2-1/2" is good up to 300 HP? I don't want to make the low mid/mid rpm range completely useless, do I?

 

[91Bomb]

i recently put on a thermal 3 inch catback exhaust and i hollowed out my cat. ever since then it seems like i lost alot of low end torque but i gained alot of high end. also my boost used to shoot up to 11 really fast but now it goes up slowly to around 10 and then at around 4 or 5k rpm it slowly goes up to 15. is there anything i can do to get my torque and faster spool up back?

You're dp isn't stock is it? If it is get a bigger one.
Maybe try a test pipe and see if there's any difference. If not, you could try a high-flow cat -- it will add some backpressure which should help with that slow spool and should help a bit with that boost creep you're getting.