Exhaust advice - custom system

[beetos]

My car is soon to go to the exhaust shop for a new system. The boomy dual 2.5"system is getting a little hard on the hearing and I need to put a cat in.

I have a 3" downpipe which will come out for some welding round the turbo hot housing flange as it blows a bit.

The previous owner built the 2 x 2.5" system that splits right after the downpipe, with 3 small magnaflow can on each leg. The car dyno'd 503hp at the rears with it. I was wondering if anyone feels that a single 3" will be pushing it to support this power. If anything I think adding a 3" cat will be the major restriction so staying 3" for a lighter simpler system seems reasonable.

An alternative is to stay with the dual 2.5" theme to allow 2 x 2.5" cats to be used and put is slightly longer resonators to kill the drone. I'd love to hear from those with experience in custom set ups.

Cheers

[genphreak]

I think 3" is fine, but your cat would probably be generating too much heat and as you say would be the main restriction. I would stick with the dual 2.5" and use a stainless resonator AND 2 hot dogs after it, as well as the rear muffler. If you just go for a resonator in the middle, not round the bend to where the pipes make it under the trailing arm, you will still get the boom, though at different load/speed than you do now. The waves are long enough to be annoying in my my 2" system (without the hot-dogs), but I am sure that using a 2.5" system the waves will be even longer and more powerful.

Interestingly, the supersprint distributor (in SA)recently sold a 540i cat back stainless setup on eBay for AU$1200, perhaps this is not a bad idea (sure its not bad ass, but as close as you can get to an engineered factory setup that is less restrictive). The guy that bought it then sold it on for $750 odd I think), perhaps worth a think if you want less noise... but it may lose power on the dyno.

Then again.. perhaps you could ask the question of supersprint in the US and get their considered opinion? You might be surprised what they acheive through careful design. (One would hope- I've seen a lot of seemingly good (and expensive) exhaust setups that are in reality very plain in the performance and noise dept).

[Booster]

I do know that the abcence of a cat will GREATLY increase the Decibles of your exhaust. I just did a cat delete on one of my cars and added a larger muffler to compensate......NO GO>
It was as if I didn't add anything back there. If your feeling that a cat will be a restriction......try stepping up to a larger I.D. cat such as a 4" with entry and exit step ups. The cat alone will cancel a huge amount of drone and make your car capable of passing the SMOG tests if your exposed to this.
Perhaps the upsized cat will flow great and keep you compliant too !
Best of luck.........vinny:)