Exhaust Piping Diameter

[JamJay]

Hi all.

I have a 3" DP & Sports Cat and the time is nearing to put the rest of the system on from the Cat-Back. I will be going custom as I hate the gigantic size of the popular 6"x4" oval that's so common, sorry, so i'll be choosing something a little more understated like the OE tailpipe.

Currently the piping on the car goes from 3" at the turbo to the end of the DP, 3" at the start of the Sports Cat which reduces to 2.5" at the Cat-Back connecting flange and then the OE Cat-Back system is 2.5" all the way through.

From what I understand, the DP & Cats are the restriction on the OE system so with those removed, I should be good to go however, will there be any benefits/drawbacks to the pipe diameter I choose for the new Cat-Back?

I have a choice of 2.5" (matching the rear Sports Cat flange), 2.75" or 3". If I go for a larger diameter than the 2.5" Sports Cat Flange, is this causing a bottle neck or is the change in size between the pipes so minimal that it makes no difference? Will be holding the car back at all by adding a 2.5" system instead of a larger diameter?

 

[ADAM_84]

To be honest I'd see what milltek and other exhausts manufacturers use on theirs as it tried and tested with excellent results both sound and performance. Alot of people asks if exhausts are droany at motorway speed which the milltek isn't and I assume it comes down to what size pipe they use so a bigger pipe could make it very noisey at 70mph, just something to consider from my opinion could be totally wrong lol

 

[JamJay]

As far as I know Milltek is 2.75" as is BlueFlame, Scorpion is 2.5" as is Piper. My main concern is the rear flange on the Sports Cat as it's 2.5" and I am not sure if it's a restriction or not, going from 3" to 2.5". If it is, I might have to look at the possibility of cutting it off and rewelding a 2.75" or 3" flange but i'd like to avoid this.

 

[ADAM_84]

Yeah it's a tricky one, as you said though milltek and blueflame are smaller then what you have so surely it just means their restriction is just after the turbo where as yours is at the back box, so there both restrictive in a way but just at different places, providing the flow is smooth going into the smaller size pipe then I don't see it being any different from where it comes off the turbo then funnels down to 2.5" or 2.75"

 

[ADAM_84]

Sorry just re-read your post and the restriction being at the cat not the back box, would reccomend getting a de-cat fabricated as that is what I had done ;-) then just fit the cat with smaller flange at mot time but have it made with the bigger flange on it obviously.

 

[wayne lcr]

3 inch all the way you no you want to jay

 

[HallycupraR]

3" all the way mate u know what mine sounds like

 

[JamJay]

The thing is, no off the shelf Cat-Back is 3". Hally, our PipeWerx sports cats have a 2.5" flange at the end where it connect to a cat-back. You could go 3" all the way through but you'd still have the 2.5" flange on the sports cat which then opens back up to 3" into the cat-back.

I'm not going to go for a decat because I want my car to be legal

 

[HallycupraR]

Very true well go blueflame or piper unless u go custom mmm choices mate

 

[motty225]

im going for 3" pipewerx downpipe with decat and a 3inch non res custom system

 

[Bobby.Gee]

im going for 3" pipewerx downpipe with decat and a 3inch non res custom system

thats what im eventually going to have.

 

[JamJay]

So general opinion is that even though one flange is 2.5", opening it back up to 3" isn't going to be an issue?

 

[DJ-LIAMC]

So general opinion is that even though one flange is 2.5", opening it back up to 3" isn't going to be an issue?

I would also like to know this

 

[lukecupra2001]

im going for 3" pipewerx downpipe with decat and a 3inch non res custom system

christ that will be loud!
i knew a lad a long time ago that had an ibiza cupra with a 3" pipe all the way through from the turbo back with no silencers
sounded good on full throttle but normal driving was like a tractor