In short I don't believe a CAT back system will make any difference at all. - But that's just my opinion. I should add that I've never bought one so I could easily be shot down in flames on this, but I will tell you what I base my opinion on.
I have an B5 S4 which AMD chipped. Three weeks ago I called Miltek and spoke to their Managing director and told him I didn't believe their CAT back exhaust would give the web site claimed 8-10% increase in power.
He agreed, and went on to say that I might see 5-8bhp increase as opposed to 24-32bhp.
In short, I challenged them to the following:
I said I would buy one of their CAT back systems if, and only if, after a before and after rolling road session, I saw a 5bhp-8bhp increase in power. Only this would justify the £450 to replace the substantial stock exhaust which is thermal aluminium sprayed.
I basically didn't want to shaft my specialist tuner, by asking him to order an exhaust then turn around and tell him, "No it makes no difference so you can keep it." and he would be stuck with the exhaust.
Millteks MD told me he was not Authorised to send an exhaust on the basis that it may be returned as used which no one would buy! (The real boss was on holiday) I told him the RRP may be £450 but the unit cost was probably considerably less. And as member of both Audi-net and RS246 and a subscriber to Audi driver the advertising potential was substantial.
I continued to explain the questions/dilema we all faced on considering a system such as theirs that all the AMD and Forge advertising illustrate performance gains for a remapped car in combination with a Milltek. There is no way to differentiate the power increase from the exhaust alone.
After several conversations Miltek called my tuner and never returned my call. I called my tuner. He told me Milltek had declined to take the challenge!
I went ahead and had my car rolling road checked as part of the service anyway. I got 302bhp and 387ftlbs of torque on a 23Deg C. day. The original AMD rolling road test post chip was 323bhp and 374ftlbs in November.
Different rolling roads and ambient temps aside I'm making the figures every other chipped and Millteck S4 is making. My tuner said he would be surprised if a Milltek could improve on that.
Quite frankly the only thing I can see that will make a huge difference is the high flow sport CATs. Since there are 4 on the bi-turbo these are the real restriction, but not just on the bi-turbo but on any CAT exhaust system.
However, a very interesting point I picked up from a fellow posterlast weekend. He bought the full Milltek system for his S3 and that didn't even make double figures - which is really disappointing. However, I believe the full system will make a noticeably difference by extending the rev range as there should be reduced back pressure.
I'm sure Audi world have an article where various exhausts etc are tested by means of a rolling road and claims of +20bhp. Interesting, but that would give my S4 a theoretical 350bhp. Which I'm confident is never going to happen.
I have no axe to grind, If my stock one falls off tomorrow I'd buy a Milltek simply because it's a well made unit in stainless steel and is cheaper than the stock unit.
I just hate being taken for ride with false claims and certainly when my hard earned cash is involved. - Nothing to do with being Scottish. If I thought I could get more power for £450 I'd have one in a heart beat!
You decided!
