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I'm considering replacing the exhaust on one of my '94 Aeros. I've been looking at the JT exhaust system, and comparing that to having a local shop do it. The two would end up costing about the same, but the JT setup arrives without a catalytic converter, and I'd need one (I don't feel like removing and installing a pipe once a year), and the locally built piece would be 2.5 inches vs. the JT's 3 inch size.

So, two questions :

1. Any recommendations on a high-flow cat?
2. Is a 3 inch setup really that much better in terms of performance than a 2.5 inch setup from the turbo on back?

For #2, assume that I'm keeping the stock TD04.
 

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1) I've run a 3" cat bought for small $ from Summit Racing for 10yrs now, makes big power and passes annual emissions tests, just check for correct length. Think I got one that can flow a small block V8, no back-pressure.

2) On same car & d/p I've run fine with cat-back at 2.5", this includes lots of German autobahn long WOT runs with an accurate exhaust gas temp gauge.

I then changed everything cat back to 3" pipe, replacing the OEM middle resonator with a Magnaflow and still retaining the same rear box, and Abbott Racing 2.5" s/steel one. Result was a lot more growl and back-crackling, and about 10hp plus significantly lower EGT on long hard WOT runs.

So I'd say it is nice at the 300hp-ish level, but not at all essential unless you expect to spend plenty of time above 5000 rpm in 5th gear, which can be hard to do and retain your liberty.
 

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Here's a hint: after the turbo you want as little back pressure as possible. This does not apply to naturally aspirated engines. The factory headers look to be decent size, honesty would not change those
 
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