Cat By pass ?

brf2005 · 09-11-2007, 14:38

Hi

I was just wondering whether a standard cat bypass on a cat back exhaust system is street legal as it seem to me to be a grey area. Also most 'street'suppliers have a tendancy of selling you back boxes etc. that don't conform as legal on the road??

P.S Cheers for your comments an advice

ScoobyChris · 09-11-2007, 14:43

If you drive a diesel, you don't need a cat as the MOT only measures particulates and not emissions.

Chris

Fezboy · 09-11-2007, 16:29

Yes you better off getting a custom exhaust made with just 1 stright pipe(Or as straight as it can be)

I brought a Miltek with race cat for my Ibiza PD and ended up chopping it off and fitting a £70 piece of pipe

vRS Dovit · 09-11-2007, 23:04

So, no cats are needed at all? cool never knew that! guess what i'm going to be looking into lol lol lol

skudmissile · 09-11-2007, 23:11

Quote:

Originally Posted by vRS Dovit

So, no cats are needed at all? cool never knew that! guess what i'm going to be looking into lol lol lol

Be very careful if your mapped and have the standard turbo.

Pm me for details

devonutopia · 09-11-2007, 23:17

I run quite a bit of power. My exhaust is made up simply of a custom big bore downpipe and a 2.25" bore stainless pipe right out the back. Cost me very little compared to "branded" exhausts, and is ultra-efficient.

vRS Dovit · 09-11-2007, 23:19

PM sent lol

vRS Dovit · 09-11-2007, 23:20

so enlarging the bore will help NOT to do any damage?

Decron · 10-11-2007, 00:49

No, thats not how it works.

If you decrease the back pressure (As the exhaust creates it, widening it removing the cat will reduce the resistance) then you are giving the turbo a little bit less to work against meaning it can spin up quicker and easier. Decrease it too much and the turbo goes outside it's tolerances and fails early (There are several ways in which this can happen and none of them are good).

Jason runs a larger turbo and a custom map meaning his system is setup to make the most of that exaust. Just bolting a 2 1/2" pipe onto the manifold is not going to have the effect you think you will get on a car with standard turbo etc. You will need to work within the tolerances of the turbo and get it mapped accordingly.

Removing the cat will give you a reasonable amount of pressure relief and will aid the turbo to spin up quicker and there could even be modest power gains if mapped accordingly, the great bit about this is it can be done with little expense. It does however still carry a small risk of the above mentioned issues.

09-11-2007, 14:38	#1
brf2005 Briskodian Join Date: Nov 2005 Location: North Wales Posts: 41 Thanks: 3 Thanked 0 Times in 0 Posts	Cat By pass ? Hi I was just wondering whether a standard cat bypass on a cat back exhaust system is street legal as it seem to me to be a grey area. Also most 'street'suppliers have a tendancy of selling you back boxes etc. that don't conform as legal on the road?? P.S Cheers for your comments an advice

09-11-2007, 14:43	#2
ScoobyChris Briskodian Join Date: Mar 2003 Location: Andover Posts: 18,093 Thanks: 15 Thanked 322 Times in 280 Posts	Re: Cat By pass ? If you drive a diesel, you don't need a cat as the MOT only measures particulates and not emissions. Chris __________________ Holding up the traffic in a little Italian..........

09-11-2007, 16:29	#3
Fezboy Briskodian Join Date: Aug 2006 Location: Watford Posts: 2,228 Thanks: 18 Thanked 106 Times in 100 Posts	Re: Cat By pass ? Yes you better off getting a custom exhaust made with just 1 stright pipe(Or as straight as it can be) I brought a Miltek with race cat for my Ibiza PD and ended up chopping it off and fitting a £70 piece of pipe __________________ Octavia VRS A standard looking car with a twist of Impossible Performance Support Briskoda join Freedom ======>

09-11-2007, 23:04	#4
vRS Dovit Smoggy Hooligan Join Date: Oct 2007 Location: Kilmarnock Posts: 648 Thanks: 61 Thanked 28 Times in 28 Posts	Re: Cat By pass ? So, no cats are needed at all? cool never knew that! guess what i'm going to be looking into lol lol lol __________________ I NEED MORE......... A big kick in the to the Enviorment...... "......and they all lived SMOGGILY ever after......"

09-11-2007, 23:17	#6
devonutopia Mongreller Join Date: Apr 2003 Location: Ooo-ar, Oi be from Devonshire Posts: 29,107 Thanks: 60 Thanked 280 Times in 226 Posts	Re: Cat By pass ? I run quite a bit of power. My exhaust is made up simply of a custom big bore downpipe and a 2.25" bore stainless pipe right out the back. Cost me very little compared to "branded" exhausts, and is ultra-efficient. __________________ Jason - Mongrel owner! WOOOOF! Grrrrr Custom Fabia Bonnet Lifter Kits - £45! Click Me. See the mongrel's 260bhp Dyno Video!

09-11-2007, 23:19	#7
vRS Dovit Smoggy Hooligan Join Date: Oct 2007 Location: Kilmarnock Posts: 648 Thanks: 61 Thanked 28 Times in 28 Posts	Re: Cat By pass ? PM sent lol __________________ I NEED MORE......... A big kick in the to the Enviorment...... "......and they all lived SMOGGILY ever after......"

09-11-2007, 23:20	#8
vRS Dovit Smoggy Hooligan Join Date: Oct 2007 Location: Kilmarnock Posts: 648 Thanks: 61 Thanked 28 Times in 28 Posts	Re: Cat By pass ? so enlarging the bore will help NOT to do any damage? __________________ I NEED MORE......... A big kick in the to the Enviorment...... "......and they all lived SMOGGILY ever after......"

10-11-2007, 00:49	#9
Decron Want a go? Join Date: Oct 2005 Location: Cwmbran - South Wales Posts: 3,112 Thanks: 47 Thanked 165 Times in 136 Posts	Re: Cat By pass ? No, thats not how it works. If you decrease the back pressure (As the exhaust creates it, widening it removing the cat will reduce the resistance) then you are giving the turbo a little bit less to work against meaning it can spin up quicker and easier. Decrease it too much and the turbo goes outside it's tolerances and fails early (There are several ways in which this can happen and none of them are good). Jason runs a larger turbo and a custom map meaning his system is setup to make the most of that exaust. Just bolting a 2 1/2" pipe onto the manifold is not going to have the effect you think you will get on a car with standard turbo etc. You will need to work within the tolerances of the turbo and get it mapped accordingly. Removing the cat will give you a reasonable amount of pressure relief and will aid the turbo to spin up quicker and there could even be modest power gains if mapped accordingly, the great bit about this is it can be done with little expense. It does however still carry a small risk of the above mentioned issues.

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