On a C6 good for 20-30 rwhp.
http://forums.corvetteforum.com/showthread.php?t=931343

 

While I agree one should insist on documented evidence that any bolt on really does provide the performance claimed, Chevy did leave something on the table with respect to headers. Long tube headers are proven performance enhancers, but Chevy couldn't use them because of the quick light cats they needed to pass emissions requirements.

The problem is that the quick light cats need to be close to the engine to get enough heat to light quickly. Long tube headers allow exhaust gases (at cold start, idle, and part throttle where the feds do emission tests) to cool off too much by the time they hit the cats to quickly light them off, or keep them lit. So Chevy had to use shorty manifolds, and that won't let the engine produce maximum torque.

Now, OTOH, I don't know of any long tube headers for the C6 that have received a CARB number, so if you live in a state that does smog testing, you'd probably be up a creek if you put headers on a street driven car (they'll generally be ok if the car is only for off-road, ie racing, use).

You certainly will be up the creek if you change out the cats (against federal law to change or alter them on a street driven vehicle, unless they fail, and then they may only be replaced with exact replacement units). There's a hefty federal fine if you get caught with non-complying cats on a street driven vehicle.

I don't think that any of the turbo or supercharger companies have a CARB number for their products on a C6 either. So those are technically illegal now too. But adding an oxygenate (NOS) isn't illegal as far as I know (EPA is *requiring* oxygenates in some non-attainment areas).

Now on the third grasping member, Chevy did do an excellent job with the air box and the catback exhaust systems. They did leave very little on the table in those areas as several people are already discovering. So changing out the airbox or catback may make more noise, but it is unlikely to make any significant power gains.

Note that any gain or loss of less than 5% is well inside the normal tolerance range of chassis dynos. In other words, the gain or loss may not be real if it is that small. So it is hard to document such small changes.

 

The figures look great. 20/22 a good gain especially in mid range. But the test was with headers AND hi flo cats. To get that gain you need both. Headers alone will not do it. I have a line on the hi-flos but what do the headers cost Parts & install?

 

Wow. Thanks shopdog. Now it makes sense to me. What I heard you say is that, Long-Tube headers will gain much, but may not pass emissions. OK no emission test in So VA. You also said that the high flow cats were not necessary because the OEMs are just fine. Did I get that right?

 

I assume you mean that headers alone with stock cats won't do it right?
Of course headers alone with NO cats at all should yield even more gains than they do with high flow cats.