how much hp does a catback exhaust system add?

Posted by c5chris, Jan 24, 2003
Newbie Forum - Beginner DSM modification questions and discussions. Common, repetitive, and general discussions will get dumped here from other tech forums. Unless you're a Probationary member you shouldn't post threads here.

  1. c5chris

    c5chris Proven Member

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    im not a believer in exhaust systems other than it's just all sound and no added hp - well maybe 2-5 hp tops. i rather add headers and do something before the cats than add something after it. but thats for my other car. for the dsm say, i add a greddy catback or an rsr catback, how much power will i add? i want to hear from people who added them and dynoed to prove it.
     
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  2. cucumber

    cucumber Proven Member

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    , Knoxville_(School)_Murfreesboro_(Home)Tennessee
    On a turbo car, the exhaust system and how free flowing that exhaust is is very crucial to the performance of turbo-charged vehicles.

    I've hard of people adding upwards of 20 horsepower with 3 inch downpipe, test pipe, and 3 inch cat-back, i.e. 3 inch turboback exhaust.

    Some will say they've seen more, so we'll let them add their .02 cents too.
     
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  3. c5chris

    c5chris Proven Member

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    Thanks for the insight cucumber. im off to get my exhaust now.
     
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  4. cucumber

    cucumber Proven Member

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    , Knoxville_(School)_Murfreesboro_(Home)Tennessee
    Have fun Chris, I know my goal with my car was to go fast with class i.e. a Corvette, I hope you can do the same with your GSX.
     
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  5. DSMNightmare

    DSMNightmare Proven Member

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    Exhaust is more of a multiplier than a hp adder. When I added my downpipe and test pipe I did gain almost an entire second off my 1/4mile. While it might add 10-20 hp to a T-25, the difference would be much greater if you were running stock exhaust with a 16G. Like cucumber says go with new exhaust. A test pipe will help but you will be a lot louder.
     
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  6. Prodigy

    Prodigy Proven Member

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    yeah alot louder...my car sounds like and 18 wheeler... i would say 20hp maybe fly wheel...no way 20 rwhp....
     
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  7. ProStreetOnline

    ProStreetOnline Proven Member

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    We had a customer come in with a 91 GSX. We added a catback, testpipe, boost controller, and an ACT clutch. He ran 13.7@99 on pump gas.

    Not bad.

    Exhausts do help a lot on turbo cars.
     
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  8. XxXEcLiPsE93

    XxXEcLiPsE93 Banned Member

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    what size was the catback and testpipe? 2.5" or 3"?

    also with the boost controller what was the boost set at?

    is that all the mods he had on his car or were there more , and if so, what were they?
     
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  9. Slappster

    Slappster Proven Member

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    Well, ya. Since no dsm's are rwd then ya thats impossible.
     
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  10. ProStreetOnline

    ProStreetOnline Proven Member

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    2.5" Testpipe, 3" A'PEXi N1 catback, boost was around 19psi.

    No more power mods, just ACT Clutch.
     
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  11. Laser-man

    Laser-man Proven Member

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    i was told that removing the exaust on a stock car gains 40 h/p but thats just what i was told, dont quote me on it.
     
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  12. MNGSX

    MNGSX Proven Member

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    Either NA or TC'd the gains of the exhuast depend on the other parts of the car.

    Breathe in Breathe out. If it's having trouble breathing out because the exhaust is stock but the turbo was swapped and it was cammed or something. You will gain alot. If something is choking the intake side or you don't have enough fuel for the amount of air you can flow now you wont get much.

    The only major point of restriction on a turbo car should be between the exhuast ports and the turbo. From the turbo gradually expand from the turbine exducer diameter to the diameter of exhaust that matches the power level you are looking to produce.

    NA cars typically run best with equal length headers that have primary and collector tube sizes matched for the power band the intake and cams are setup for.
     
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  13. Prodigy

    Prodigy Proven Member

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  14. UNFORGIVEN GSX

    UNFORGIVEN GSX Proven Member

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    A typical cat-back adds about 5 hp and they cost an insane 500 bucks or more depending on what you get. NOW, like somebody said before they are a horsepower multiplier in that you need a free flowing exhuast to extract the horsepower from other modifications. I suggest getting not only a cat-back, but also getting a dp. A full 3in system multiplies HP quite a bit. Getting rid of the cat or getting a high flow cat is a huge advantage over the stock system. In fact, if you have an old cat it may be clogged. In one of my turbo books, I have read where a clogged cat can take away 10% hp. However, if you get a full 3in DP with no cat like I did you will run into the problem of boost creep. In order to get rid of that problem you either need an external wastegate or port the hell out of your O2 housing and turbine housing and get a 34mm WG valve. Even that doesn't stop boost creep in the winter. But, now that it is summer, I get rock steady boost. On top of adding hp, free exhuasts relieve back pressure (dur) and allow almost all of the gases to escape the cylinder during the exhuast stroke. This lowers cylinder temps and lowers the chance of detonation. Thus, it gives you more leeway when tuning. Exhaust systems also add style and sound to your car. I have a full Apexi N1 system and absolutely love it. It sounds top notch!:thumb: Free flowing exhausts are key to extract HP from turbo cars.


    Rob
     
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  15. EvolvingGST

    EvolvingGST Proven Member

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    :thumb:

    Well the great thing abt N/A mods is that sometimes the parts stack in HP, increasing 1 part is not enough, but increasing many parts make them all more HP since they are increasing free flowing airflow. so you will need downpipes, free flowing cats and such... Its never just an exhuast. its an "exhuast system" System being key word, you change 1 link little is done you change the whole then everything is done.

    P.S. well they also have other functions too, like take intakes for example, they help spool ur turbo up faster but they also dont add that much HP... And i believe exhuasts are supposed to release backpressure?
     
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  16. l0gan

    l0gan Proven Member

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    Think cops would care if you took of the cat? I mean I know its illegal in most if not all states... but would they care / know?
     
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  17. HyperGSX

    HyperGSX Proven Member

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    What do you mean if the cops won't care? If they find out you don't have a cat on the car they probably will give you a hefty fine and possibly impound your car/suspend your license depending on your area. But then again it all depends on the cop....
     
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  18. aovsi

    aovsi Proven Member

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    Does anyone know some solid figures about how much performance you can extract out of a 2.5" mandrel-bent turbo-back system with cat? In terms of track times and HP?

    Actually I plan on getting 3" press-bent turbo-back w/3" cat. Reason for 3" press-bent, so I can use the 3" cat with 3" inlet and outlet, opposed to only 2.5" inch inlet/outlet on cat. And it stays _approximately_ equal to 2.5" mandrel in terms of flow.
     
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  19. 91TSi FWD

    91TSi FWD Proven Member

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    No other power mods huh? So he was running 19psi on the stock injectors, fuel pump, and wiring? Thats smart:rolleyes:
     
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  20. ProStreetOnline

    ProStreetOnline Proven Member

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    Yep... his car happened to be one of those "super rich" from the factory cars, so he was fine.
     
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  21. 2GeNTSi

    2GeNTSi Proven Member

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    Are you talking about with the stock catback? Can you try to explain boost creep in lamen terms, I'm not too knowledgable about all the exhaust flow and backpressure crap. I only know of the physical parts of the exhaust system.
     
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  22. Turbo Demon

    Turbo Demon Proven Member

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    Boost creep is rising boost due to not enough exhaust bypassing the housing. For example, you're set for 13 psi, and when you floor it, you get a quick rise to 13 or 14 psi. But then as rpm's climb, the boost creeps higher and higher: 14, 15, 16, 17, etc. Enlargening the wastegate hole is one cure for boost creep.

    Spiking is when your wastegate hole is large enough, but your actuator can is taking too long to fill with compressed air, so you *temporarily* overshoot your boost level. Example: you're set for 13 psi, and when you floor it it quickly rises to 16 psi, but then gradually drops downward towards 13 psi again.

    A common problem with modded engines is a combination of spiking AND boost creep. That's where you're set for 13 psi, and when you floor if it overshoots to 16 psi, *begins* to drop back down, but as the rpm's rise it goes up again!

    Normal boost rise when set to 13 psi = shoots up to 14, then 13, 13, 13...
    Spiking = shoots up to 16, then 15, 14, 13, 13, 13...
    Boost creep = shoots up to 14, then 13, 13, 14, 15, 16, 17...
    Spiking and boost creep = shoots up to 16, then 15, 14, 13, 14, 15, 16,17...

    Set a manual boost control to 11 psi and floor it slowly in high gear. It should go to 11 psi and hold it pretty well. If it starts to slowly climb over 11 psi as the rpm's get high, that's "creeping". The boost creeps higher at high rpm's because there's too much exhaust flow for the wastegate hole to bypass.

    Now go to 11 psi again, but tromp on it quickly! It will shoot to 13, or 14, or 15 psi, then drop down to 11 and stay there. That's "spiking".

    Spiking is a quick flash of high boost that goes a little beyond your set boost point. If you are set for 11 psi, and you tromp on it quickly, this might happen: boost "spikes" to 14, quickly drops to 11, then slowly "creeps" back to 14 or more as the rpm's climb. Now you are seeing spiking followed by creeping! :dsm:
     
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  23. '89beretta

    '89beretta Proven Member

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    , Eau-Town_WI
    Jumping in late. But to answer AOVSI's question:

    With air filter and MBC @ 16psi I ran 14.6 @ 93.9 mph. Added 2.5" downpipe, hi-flow cat, and 2.5 cat back (mandrel bent) and ran 14.0 @ 100.6. Both runs were in similar cool Fall conditions. Haven't been able to repeat that run, usually trapping around 97-98 now (summer).

    Don't know how much wheel horspower increase that is, but you can definitely feel it. Downpipe is a must.
     
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  24. leet

    leet Proven Member

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    Yeah, about two years too late.
     
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  25. exile

    exile Proven Member

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    Haha, it's still a really informative post. It just saved me the trip to vfaq, thanks guys =)
     
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