The cats are the most restrictive part of the system. On blown engines, there is much more sensitivity to exhaust restriction, so I don't doubt the gains at all. Just look at the hideous right angle of those factory cats! Good call!

 

No the least of which is that that was a much older XKR, mine is a 2004, different engine, transmission, intake, blower etc. They made a lot of changes in the 2004 model year. But I really haven't done that much to the car with regard to the engine. Granted this is a year later. In any case when I put on the new intake and exhaust I'll run it again for a new base line. I know the differences are significant because I was sent a series of powder coated parts from a late 4.0 engine and nothing fit.

 

Half true.

In turbocharged - yes, turbocharger is a part of exhaust, free flowing exhaust is the best way to improve turbocharger spool.

In supercharged - no, the exhaust is as important as in N/A engines. You do need a backpressure to get good torque and hp curves, especially i low and midrange engine speed.

 

@Enderle,
The differences you mention aren’t that big on the engine, and my experience actually was also with the same engine as you have where I would guess at max about 10 to 15 rwhp. For another 4.2 Car like yours (from 2005), it was about similar, even though he had also a larger exhaust on top of the sport cats. Am also not convinced that the stock cats are so bad, 1st of all the area is bigger (/diameter) than the sport cats, meaning you just can’t judge the flow purely based on the amount of cells alone. The lower bend is almost as a funnel, as the pipe is over the full lower length of the cats.

A 30rwhp is something noticeable, so that I certainly believe, but the question is why so many others only report max half of that gain, and you more. So chances are that there maybe was something else at play why your 1st run was lower.

Anyway, please keep the info coming, the more people share dyno results, there more we all learn. Even though if not all results will be as/or above expected.

 

Thank you sir...much appreciated.

 

Let's keep things civil, please.

 

Just did the 200 cell Nameless cats on my 03 XKR at 127,000. Coming up on smog and barely passed last time, so I knew one of my cats was shot and I wouldn't pass so I pre empted.

I previously did the over axle delete and removed the mid boxes as well which I think enhanced the benefit from the NP cats.

I agree with Enderle's 30 HP numbers: there's an appreciable increase in response. Not huge, but definitely noticeable.

On a final note, the car snarls and crackles and pops like the F-Type now.
Money well spent..