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1987 Pearl White Turbo R154
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Discussion Starter #1
Hello, I'm looking into going full 3 inch turbo back. currently running a 3 inch cat back. I have a 3 inch downpipe that I am waiting to put on, but it isn't as long as the stock one as it leaves out the cat or room for a test pipe. I'm thinking I wanna leave the main cat for legal reasons. I talked to an exhaust shop and they said they could cut up the main cat on the stock downpipe and make it work with my setup. Only thing is the stock cat is 2.5 inches. I'm just wondering whether or not I should use the stock cat or buy a high flow 3 inch. Since it will be slightly restricted at the cat is it worth it to spend another couple hundred bucks for a 3 inch cat? Or will the .5 inch restriction not make a huge difference? Please let me know what you guys think. Car is stock no boost controller or anything like that.
 

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1987 Pearl White Turbo R154
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9 Posts
Discussion Starter #3
As far as performance and spool-up goes, ANY cat will kill spool-up. A turbo exhaust should have as little backpressure as possible. But for legal reasons, a 3" high flow cat is the way to go.

Al
hmmm ok maybe I do just say fuck it and run no cats. I feel like I've heard running no cats will shorten the life of the engine... Is there any truth to that or is the quicker spool worth it?
 

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1987 Pearl White Turbo R154
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Discussion Starter #4
also If i decide to run no cats is there any potential for boost creep to be an issue?
 

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Setting an engine up to run well won't shorten the life of it. Boost creep is a possibility, not sure what turbo/wastegate setup you have. A wastegate that is too small or marginal in size is susceptible to boost creep with a free-flowing exhaust. Cold weather can also play a part in boost creep - a car that's fine in the heat suddenly has boost creep on a 40 deg night.

Al
 

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1987 Pearl White Turbo R154
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Discussion Starter #6
Setting an engine up to run well won't shorten the life of it. Boost creep is a possibility, not sure what turbo/wastegate setup you have. A wastegate that is too small or marginal in size is susceptible to boost creep with a free-flowing exhaust. Cold weather can also play a part in boost creep - a car that's fine in the heat suddenly has boost creep on a 40 deg night.

Al
ok. Still on stock wastegate and boost, if I do end up running into boost creep issues, I could always shim the wastegate or buy the drift motion one right?
 

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ok. Still on stock wastegate and boost, if I do end up running into boost creep issues, I could always shim the wastegate or buy the drift motion one right?
Boost creep happens when the wastegate can't flow enough exhaust to keep boost levels down. The remedy is a larger wastegate, not exactly an option with the stock turbo. Also not likely to be a concern with the stock turbo as it tends to choke down up in the higher RPM ranges.

With a better flowing exhaust your boost level will typically rise due to the decrease in flow restriction behind the turbine outlet, allowing it to spin faster and thus produce more boost.
 

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I have a stand alone and full tune, if I delete my cat also would I need to worry about the boost mapping?


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