I got a chance to buy a very nice set of very lightly used Kooks stepped headers (they don't make them anymore!) for a very reasonable price. Here's what I have in a 2006 GTO. LS2 long block is stock.Engine-wise it is:
1. professional dyno tune
2. FAST 102 LSXR (ported), not really a mod but also has a new LS3 TB
3. SLP 25% underdrive pulley
4. JBA shorty headers and JBA high flow cats
5. Magnaflow catback
6. OTRCAI, DuSpeed
7. Relocated fast acting IAT sensor
8. NGK TR6 spark plugs (helps with the tune apparently)

Did 379 RWHP on a Dynojet on a 95+ degree day. Tune is not on the ragged edge.

I'll be adding Kooks 1-7/8"-1-3/4" stepped headers and Kooks high flow cats. Not sure how well or if the Kooks 3" cat pips will fit to my 2.5" Magnaflow system. Tuner thinks no or only minor adjustment of the tune will be needed. Will have him check it out once the Kooks are installed. I'll dyno it again later this year when he has a dyno day just to see if it improves. I've always been more than happy with my JBA shorties and HF cats, but for the price I paid for the Kooks I just couldn't pass it up. Also gives me more room to grow.

 

Almost as much as going from stock mani to long tubes. Stock headers ran close to shorties iirc.

 

Eh, in thinking about it, I'm not sure how much you'd get in terms of actual HP/TQ, but the gain should be noticeable.

 

Roughly 10rwhp give or take a few just a guess...

 

I'd take 10. Expecting improvement through the whole RPM range. Kooks stepped are supposed to be some of the best.

I put off putting on a FAST for a long time, just couldn't see the benefit given the cost (they are stupid expensive for what they are). I had seriously ported a stock LS2 intake along with a ported LS2 throttle body and figured that was nearly as good as a FAST would be so why bother. But last year I got an unexpected bonus at work and didn't have any other plans or needs so I got the FAST. I was impressed with how it improved the output, especially noticeable in the 2500-3500 range and well up through the entire RPM range. The tune done following that cleaned it up and made it even better. Anyway, my point being, even moderate upgrades from one thing to another can make a pleasant improvement.

I just hope it will bolt up to my Magnaflow 2.5" catback. Guy selling it to me said he had it bolted to a stock cat back without any problems.

 

Reading notes on Summit Racing, they say the Kooks mids are for connecting Kooks headers to an OEM 2.5" catback so I should be good.

What header gaskets are the best? When I installed the JBAs folks told me the OEMs even used would work the best and that's what I did and they have worked great. I think I have a new set of OEMs around here somewhere.
d.

 

you GTO owners are all the same, tryin' to get 400 whp with bolt-ons without doing longtubes first.

PFFFFT.

 

(i say get that EWP and the vac pump while you're at it. go for an ATI balancer, too.)

 

I already have an underdrive crank pulley. I'm not considering the electric water pump at this time.

I never had a specific goal to get to 400 RWHP (but it would be nice). I've always known the shorties would be limiting me in that regard but they fit so well, kept costs down and honestly did work well. The JBA high flow cats are where the real benefit comes from.

Even with LTs I doubt I'll make it to 400 but we will see eventually if they help and if so by how much.

 

It sounds like you have made your mind up so I hope you are going with the long tubes. The shorty headers have been proven back to back for years that they make very minimal gains on stock or performance engines. Long tubes are they only ways to go. You spent the money on getting the air in but it can come out properly with shorties.

 

Oh, the LTs are paid for and will be on the way soon.

 

The thing about shorties is they are not worse than cast manifolds, and can be the only solution where manifolds may not fit right.

 

Probably won't gain much HP, but may gain abuncha torque.

In general, header length is tuned; shorter ones resonate at a higher frequency, i.e. higher RPM. Not always of course, just, in general. The length is a factor in the RPM range in which they're most effective.

But also of course, details matter. Length alone isn't the only factor.

 

i think in general, a well designed long tubes will get you a lot in the lower and midrange vs a shorty style. depends on the application, sometimes with a well designed shorty you won't see much difference in peak hp, but overlaying the two graphs you will see a nice gain under the whole curve with a good set of longtubes.

 

so much for the clue, or maybe you know and are like RAWR DIS IS LS1TECH NOT LS1GTO

 

I think the longtubes would gain most at the top of the rpm range due to the 3 inch collectors vs the 2.5 inch collectors of shorties.

 

If the current shorty headers are 1 3/4 inch primary the peak hp gains might be around 5hp to 10 hp however there will be nice overall gains in the hp & torque curves above ~4,000 rpm with a quality. 1 3/4 to 1 7/8 long tubes.

If the shorties are 1 5/8 inch primary the gains will likely be more on the peak numbers.nice overall gains in the hp & torque curves above ~4,000 rpm with a quality. 1 3/4 to 1 7/8 long tubes.

 

The current JBA shorties are 1-5/8" primaries with a 2.5" collector. The new long tubes are a 1-3/4" stepped to 1-7/8" primaries with a 3 inch collector.

 

Sounds like 10 to 15 hp gain peak and a nice gain under the curve for torque & hp over 4,000 rpm. Maybe more with additional mod's etc.

 

You are already on right track, but since no one responded to your question, a vote of confidence here. Yes, OEMs. Forget anything else.