S3 decat or sports cat?

Scoot Yo

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What's more beneficial, legality and mot's aside?
 

JJ.Foulds

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well obviously something with no resistance will be better, so decat.
 

Scoot Yo

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That's what I thought, but on that basis why do so many people choose sport cats? Surely changing an exhaust once every 12 months isn't the only reason?
 

smurfworth

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Having to drop the propshaft everytime might have something to do with it
 

JJ.Foulds

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well id choose a sports cat, because i wouldnt want to have to change it over as id have to pay someone.

But apparently the standard cat back isnt too restrictive and a few just get half the system.
 

XFi

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That's what I thought, but on that basis why do so many people choose sport cats? Surely changing an exhaust once every 12 months isn't the only reason?

Surely you aren't so surprised? I've been there done the decat thing on a low car where no jack can get under the car and then putting a cat back on for MOT and then it's a pain as it didn't always seal correctly. A sports cat gives the performance bonus without the hassle. As much as I love decats I don't have the time to be changing it out every year for a day!
 

Scoot Yo

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I'm not really surprised but just curious. People fit hid kits, wide wheels, stretched tyres and a whole host of other MOT failures. I appreciate the sports cat is more 'fit and forget' but with so many people being so hung up on dyno figures I would just expect the decat system to be more commonly used.

I know that an exhausts back pressure is important and was also wondering if the decat offers and 'real world' gains, which turns out from people's opinion here it does.
 

S3Alex

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You really don't need a decat at anything less than a big turbo build.

Once you're into all of that there's a point where it's required but other than that I wouldn't bother.
 

Tom5190

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I managed to get through uni without a 'proper' MOT on the car I had at the time so de-cat was no issue but the S3 I sent through a legit one. I couldn't be chewed changing over the cat each year like and I've got a unit with all the tools etc. CAT free MOT's aren't too hard to come by mind.

I'd like to see the difference between a well designed sports cat and a de-cat pipe, a straight through de-cat isn't always best for exhaust gases any way, depending how well the system is designed in the first place the positioning of a box might be quite critical to the pulse.
 

Scoot Yo

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Well I've no intention of ever going past stage 2+. Long term plans are exhaust, fuel pump, intercooler induction kit.

Stage 1 with induction system at the moment. Obviously the decat system is also cheaper which is a benefit in itself as it means what I save on the sports cat would go towards the fuel pump.
 

Lewis583

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On N/A cars a decat isn't always beneficial purely because N/A requires some exhaust gas resistance to aid the scavenging which helps clear all exhaust gases out of the cylinders.

However it is the opposite on Turbocharged cars. Downstream of the turbo you want as little restriction as possible (decat/big bore etc), so the bigger the better. Though you do get to a point of diminishing returns in that you will find that, for E.G a 3inch system causes little to no resistance so going bigger would be of no benefit.

So though decat is better for outright performance the better option (IMO) would be something like a 100/200cell sports cat which, as said earlier, gives performance benefits with the assurance that no faffing around changing exhausts is need come MOT time. Like Smurf said propshaft drop is needed and it's not the nicest of jobs (even with a ramp and all the tools)
 

S3Alex

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Put another way, mine turned out the same figures to just about everyone else's give or take a few percent,which is what you'd expect.

The hassle of a decat system comes around MOT time and of course can be dealt with but as I said the gains really aren't there to be seen when compared with a sports cat on a good well designed exhaust.
 

smurfworth

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Being childish here, but surely you'd get pops and bangs from a straight through decat?

I like it when the standard exhaust pops, just wish there was more of it.
 

S3Alex

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Being childish here, but surely you'd get pops and bangs from a straight through decat?

I like it when the standard exhaust pops, just wish there was more of it.

You wont unfortunately.

Fuel management on the engine is so tight that the old thing of fuelling during overrun which would make some spectacular flames and bangs,is long gone.
Even mine doesn't do much of this.....spits the odd flame but not much else,and thats on a generously fuelled car plus decat.
 

s3_trev

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Il be going for a sports cat when I finally get around to ordering my full BCS powervalve system! Decats dont seem worth the hassle TBH, as Alex has pretty much said.
 

smurfworth

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You wont unfortunately.

Fuel management on the engine is so tight that the old thing of fuelling during overrun which would make some spectacular flames and bangs,is long gone.
Even mine doesn't do much of this.....spits the odd flame but not much else,and thats on a generously fuelled car plus decat.

So how come the standard exhaust pops as the revs drop?

Surely these pops are gonna be louder/more frequent with a straight through or decat?
 

S3Alex

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So how come the standard exhaust pops as the revs drop?

Surely these pops are gonna be louder/more frequent with a straight through or decat?

OK...what I'm saying is you will get the occasional pop or bang,and it may even be a little louder,but you aren't really going to get more of them.

Mine is probably one of the more heavily fuelled and modified 8Ps on here,and it just doesn't do much of that.
 

45bvtc

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ALL cars are now built to a strict set of parameters that are listed in that vehicles Type Approval document: where that vehicle is manufactured with 3-cats (1-primary and 2-secondary) as in the RS3, then those 3-cats (if the MOT is done correctly) will be 'counted' during the MOT. So if you remove the 2-seconadry cats (as I have) and present your car to an Audi dealer for MOT then, irrespective of the lack of NOx being emitted, the car will FAIL its MOT.

So sports cats yes, by-pass pipes no.

Interestingly, Milltek affix plates onto their by-pass pipes that clearly state "Not for road use" and where these pipes have been fitted to RS3s with the plates attached then the Audi tech has refused to drive the car on the road. My plates fell off prior to the pipes being fitted.

Notforroaduse_zps950528c3.jpg


All said, you may well have a friendly MOT garage that will overlook the absence of cats, but he'll not be undertaking MOTs for much longer if found. My cats will be re-fitted for the MOT.

Take care; legislation is now getting draconian.

 
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