I realize this topic has been beaten to death, but humor me. Some people state that the big HP gains come from cats, some state that headers give the gains. To me, this should be a undebatable subject. It is either one or the other. To further complicate, how come on the X51 package for the carrera S, they replace the headers and not the cats to get their power increase (albeit by changing intake, etc as well).

Point is: which one of these parts actually makes the bigger power difference and which one is more of a luxury rather than a necessity when it comes to increases in power.

Thanks in advance for your guidance.

 

swapping out the cats with higher flow units will give the larger gains. But for the most part might as well do the entire thing to maximize the relatively small gains to begin with.

X51 is OEM thus they have to stress about stricter smog issues vs. the aftermarket. They could be mandated to use specific sizes vs. the 100 or 200 type the high flow units use also - I dont know specifically but thats my guess.

 

It's all about restriction... You can change the headers and get a minor gain to superior design. You can change cats for slightly lower restriction for minor gain. Or you can change both for a better gain than either one alone. That's why so many change the intake for more air, and headers, cats, and mufflers to allow that extra air to burn and exit the engine faster, thus increasing performance.

I was after performance, weight, and sound (to know where the engine is by sound instead of instruments on the track) so I knew I would change either all 3 headers, cats, and mufflers, or none.

 

Mass the reason why they didn't change the cats in the X51 (which would help these cars immensely) is that the car won't pass the manafactuers smog certification. There are 2 cats for each bank of cylinders (total 4) with the H6, one is a preheater which brings the cats to temp quickly for startup emmissions. With aftermarket us end-users won't have to worry about this, to pass the sniffer as long as the car is at temperature and tested on a chassis dyno. To be honest if I wouldn't get a CEL or had software to delete the rear 02 code I wouldn't run any cats or just 100cpi cats. With the M3 this would kick off the SES light but wouldn't affect the cars performance in anyway. With the Porsche there is no SES light (emissions related malfunction) just the 1 CEL and you need this to report more serious issues.

To answer your question if you were going to one or the other..I say the cats..that really is the weakest link in the exhaust. Going to a 100-200cpi metallic cat will also wake the cars sound up quite a bit. I am ending up with X51 headers, AWE cats and mufflers. The mufflers may get switched out when the Borla hits. I like it very aggressive sounding...The AWE's seem a little to quiet for my taste, but I am witholding judgement until I put the cats on.

The AWE and Fabspeed use a very good cat the HJS 200cpi metallic cat, although I believe that Europipe really has the best setup though using a 100cpi cat with no CEL's but the stuff is expensive. You can't use just their 100cpi cat pipes as the diamater of the tubing is some 5mm larger. Price vs. performance I chose the AWE catpipe since I had AWE mufflers already. Had they have been Fabspeed mufflers I probably would have gone with the Fabspeed cat pipe as they were the same price.

I did my whole exhaust for under $3k which was my goal...I expect 15-20rwp with good gas and this is mainly from the cats. The headers probably help in midrange and low end torque but don't do much for topend I believe. The X51 headers aren't equal length headers but they do have the largest primary tubes though I believe at 48mm.

Dave

 

Interesting Stuff!

While on the topic of Headers/Cats, which one would make the exhaust louder? I figured that cats were limiting the sound .. but people have said that headers make a sound difference.

 

I think its the catalytics that limit the sound, whilst the Headers might increase the sound marginaly.

 

Headers dont do as much cause OE headers are pretty well designed, the cats are going to make the most difference in sound and a little on performance

 

I have seen a solid 18rwhp with a full exhaust on a independent dyno. While I had my M3 dyno'd there was a 05 997C2S there with a full fabspeed exhaust..his best run there was 301rwhp after he did the exhaust he did 319rwhp on 91 octane. He drove the car around for a week for full adaption, and weather was very similar to his baseline dyno so I feel it was pretty accurate. I feel with 93 octane he would have been a mid 325+rwhp. This shop wasn't a BMW or PCar shop it did Subi's with a all wheel drive DTS dyno, to bad it isn't in business anymore.

Dave

 

Cats do, but headers do give the car a more "tuned" sound with the cats. Also the headers will help in throttle response and midrange power a tad.

Dave