1998 GTR

I understand that the cats are a major point of exhaust restriction. Can someone answer a few questions for me? And maybe give me some advice? But first here are some details on current mods; 383 forged stroker (built by Katech), 11:1 compression, stock Z06 heads, 224/228 comp cam (113 separation, and .585 lift I believe – don’t have the cam card in front of me), Halitech AF and air bridge, TPIS long tube headers, stock cats and cat backs, stock throttle body, stock intake and injectors, recent A&A dyno tune (about 412rwhp/450rwt). California car.

Basically, I’m thinking the next logical step in milking out a few more ponies is to open up the exhaust to some degree. I don’t mind the noise much – Actually kind of like a much deeper rumbling and throaty sound. This is a weekend go-out-and-play car, so it’s not like I have to listen to it every day or on long trips.

(1) If I were to install an old set of cats and gut them out, would it still pass smog (California) as far as exhaust gasses are concerned? (Assuming engine is properly tuned and runs clean) What does that do in regard to O2 sensors and the PCM? Would any PCM re-programming be required? Would I gain much HP with this strategy?

(2) If the above would not pass smog, would it be practical to have a bolt-in section of the exhaust that can be run without cats normally, and then just bolt the cats in for smog testing? Oh . . . Ummmm . . . Yeah . . . Let’s say this is a track car I’d like to run on the street occasionally, and would of course bolt the cats back in anytime I’m running on the street. And again, would a tuning change need to be made when the cats were bolted back in? I do have the LS1 Edit software. And though I’m no tuner, I could have A&A run two different tunes and I could then easily switch between the “no-cat” tune and the “with-cats” tune.

(3) Next step along this same line of thought would be to go straight exhaust all the way back. At a recent cruise there was a guy running 3” all the way back, no cats, and Borla straight pipes. He also had a 338/342 cam, which I know will put any smog machine into cardiac arrest. But is sure did sound nice! But it was a bit too loud at WOT – Maybe some sort of muffler might tame it down just enough to be bearable, and to not get the LEO’s on your butt for noise. So first of all, does anyone know if the TPIS long tubes have a 3” collector on them? I’ve not measured them yet? Are there prefabricated bolt-on straight pipe systems out there? Or would I need to have them made at a muffler shop? Does a good cross pipe help performance, or does it just reduce the popping at deceleration?

(4) What sort of performance difference would I see between a 3” system vs. stock? What is the stock diameter? Is it 2-1/2”?

(5) At what point does it make sense to go with a larger throttle body, larger injectors, and/or a performance intake manifold? And what HP gains would I see form each?

(6) Any other suggestions to gain any more HP? I think my compression ratio is too high for SC or turbo. I know some AFR heads would help, or porting & polishing these heads – How much added HP would you guess I’d add with the AFR’s or porting/polishing? Other than that, am I about at the limit without moving up to more cubes?

THANKS!

Jackal

Ok here is what I would do. (BTW that is a ton of questions..)

1. Get some AFR 225 heads and go with the same CR.

2. Get some 1-7/8 Headers (kooks or LGs) w/the cats and 3" all the way back.

No cats dont gain you much if anything, and it will not pass smog. Long tube headers wont pass visual, (unless you know someone), but they will pass the sniffer with the high flow cats. Headers+high flow cats+borlas or GHLs = VERY LOUD.

The Throttle body and manifolds in my opinion are non-essensials, but if you have the cash by all means go for it. The injectors on the other hand can be very benificial. What injectors do you have now? I would say if they are stockers, you should at least have 32lb Lucas injectors and then get it tuned.

Jackal

To give you an idea, I have these specs:

Halltech TRAP
LS6 intake
224/224 581 114 cam
West coast racing heads (Stage 2 LS1 castings)
Dynatech headers w/ high flow cats
Balanced and blueprinted Block/rotating assy, stock cubes.
32 lb lucas injectors

I just had mine tuned, and he actually pulled 3 degs. of timing and kept my A/F ratio at 12.86 across the board and I got
411 RWHP
380 RWT

I also had a bad lifter at the time (found after the first dyno run)that probably lost me some top end.

With what I suggested above, you should be hitting closer to 480 RWHP and 500 RWT.