100 cell cat and twin vs single centre pipe

[Jamo]

Guys has anybody confirmed an S2 will pass an MoT emissions with a 100 cell cat?

Also what is the think is 1 3" pipe better than 2 2.5 inch centre pipes?

Looks like i'm going to have to remove the sh*t that was put on the Avant to get it through an MOT.

Can't be doing with this problem every year.

[FITZY]

Just had my cats removed tuesday, and I'm having it emission tested early next week just to see how far out it is now (homefried 94 aby) Will let you know how it gets on. hopefully this may give us some indication as to what 'flavour' cat will be required, because I've got 11 and a half months to get something sorted for myself. What was really shocking for me was that I knew at least one cat was on the way out, but at MOT time last month emissions were fine. When my cats were removed tuesday, the left hand cat's internals were jammed in my milltek centre pipe completely blocking it on that side, and half of the right hand cat was floating around in the right hand side Will let you know results when I get them, and I'll ask what they recommend cell wise when they test it.

HTH

[Jamo]

Get ready for a shock...........

I did when I got mine MOT last year

[faaaaast]

I'm also wanting to know whether 1x3" is any different than 2x2.5". I reckon from what knowledge I already have on the subject that the 2x2.5" pipes might in fact be less restrictive.

There is a benefit to having one large pipe, in that there is less overall surface area interacting with the exhaust gasses and therefore less 'drag' on them.

However, the overall cross-section of 2x2.5" pipes surely makes up for, or even exceeds this effect:

Since the area of a circle A = (pi)r^2:

2x2.5" pipes = 2 x (3.14 x 1.25^2)

= 9.8 sq inches

1 x 3" pipe = 1 x (3.14 x 1.5^2)

= 7.0 sq inches

You'd have a hard time convincing me that a 3" system is more efficient than a dual 2.5" system.

It's a very complex subject though, one that I'm still studying. Although for motobikes, this article is interesting:

http://www.harley-performance.com/harley-exhaust.html

James

[Jamo]

Well according to fluid dynamics even though 2 x 2.5 has a greater cross sectional area the transition from one to two pipes causes fluctuation in gas flow and the y-point can become a bottle neck

A single 3" keeps gas flow at a constant high speed and there is no bottle neck.

Problem is this is subjective as why would Audi on the RS2 up the pipe to 2.75 and then have a dual pipes to the centre silencer?

I've asked this question before at exhausts places and I get conflicting answers.

[faaaaast]

I don't know for certain either, but I guess it may have something to do with the detrimental effects of resonance/thermal waves carrying the exhaust gasses back to the combustion chamber.

Larger diameter pipes will have lower resonant frequencies; smaller diameter pipes higher. I guess Audi might well have literally been tuning the exhaust system. Theoretically, if it were tuned accurately for the specific engine signature frequencies, it would provide optimal gas flow, or at least that's my understanding.

It's partly for these reasons I'm considering keeping the large diameter downpipe, but taking the middle/rear sections back to twin pipes of greater combined diameter. I also think the engine sounds better this way...

James

[Fat Audi 80]

Just had my cats removed tuesday, and I'm having it emission tested early next week just to see how far out it is now (homefried 94 aby) Will let you know how it gets on. hopefully this may give us some indication as to what 'flavour' cat will be required, because I've got 11 and a half months to get something sorted for myself. What was really shocking for me was that I knew at least one cat was on the way out, but at MOT time last month emissions were fine. When my cats were removed tuesday, the left hand cat's internals were jammed in my milltek centre pipe completely blocking it on that side, and half of the right hand cat was floating around in the right hand side Will let you know results when I get them, and I'll ask what they recommend cell wise when they test it.

HTH

I had Exactly the same thing on my car. It doesn't half rob you of some power doesn't it!

I run no CATs all year and put them on for the MOT. 2 hours work each way for me...

Cheers,

Steve.

[Fat Audi 80]

I suspect the RS2 twin pipes may be a "packaging" issue...

[Jamo]

I had Exactly the same thing on my car. It doesn't half rob you of some power doesn't it!

I run no CATs all year and put them on for the MOT. 2 hours work each way for me...

Cheers,

Steve.

That's fine if you have a standard system, when you have one that is modified and has no provisions to install a cat it's different and a bit frustrating

[s2driveruk]

The guy that started longlife told me that a 100cell cat would be fine for a late 3b (4 days into the simple cat test) newer cars subjected to the more stringent emissions test might struggle. A 100cell cat is almost the same as having no cat from a flow point of view he said....

[Fat Audi 80]

That's fine if you have a standard system, when you have one that is modified and has no provisions to install a cat it's different and a bit frustrating

True, I wasn't suggesting that you couldn't be arsed to change it every year or anything like that!

Oh, the joys of standard cars...

Cheers,

Steve.

[s2driveruk]

That's fine if you have a standard system, when you have one that is modified and has no provisions to install a cat it's different and a bit frustrating

Well to be honest you could just flange up either side of a standard length cat replacement pipe and do the same as Steve - regardless of customer or oem setup?

[dan_b]

hee hee, you said "flange"

[s2driveruk]

[faaaaast]

Uh huh huh huh... flange...