Best exhaust diameter for KO3

[GTI_Kyle]

Already got a 3" decat downpipe mated to a stock system & silencer delete. Whats the best catback diameter to maintain torque etc, heard 3" is unnecessary & 2.75" is better for the KO3?

Advice is appreciated 

[pudding]

I've seen 2.5" systems on 500hp VR6 turbos, so nope, 3" isn't necessary in terms of flow, but it'll certainly keep restrictions to a minimum.

Personally speaking, if you've already got a 3" DP and deleted a silencer, I wouldn't bother on a K03.   Save your cash!

[GTI_Kyle]

Won't they be too much back pressure with stock catback though?

I'm already stg2, but will probably go stg2+ eventually, is it worth putting money into?

[AJP]

It's the downpipe that causes the restriction. There's really not much to be had from a cat back apart from sound.

[pudding]

Yup.  The first cat is right next to the turbine wheel and is what causes the most restriction.  The second cat is fine.

[Dan_FR]

The second cat is not fine, both of them need to go for performance, especially if mapping for the cat removal, running a leaner mixture etc.

A single 2.5" pipe on a VR6 turbo running 500hp would be a massive restriction, and the car would be capable of a lot more with a bigger pipe. Twin 2.5" pipes are a different story.
Just because a car with a huge turbo can force 500hp's worth of air through a small pipe, does not mean it is not restrictive on a smaller framed turbo that is balls out on its limit where every bit of efficiency is critical when chasing those extra few numbers.

A 3" downpipe and de-cat is ample, as would be a 2.75" exhaust. 3" is not necessary on a K03. A better flowing catback would help a little, but not a lot. The rear silencer design is not great - having this removed would be enough.

All depends on budget really, how much you want to throw at the car and how loud you want it to be

[GTI_Kyle]

is it worth putting money into in your opinion Dan_FR? If its not a necessity for stg2+, id much rather fit a better flowing backbox instead & put the money into a fuel pump.

Don't want it too loud & boomy, its loud enough now with a straight through centre section & the stock backbox on.

[pudding]

Depends what you want.  If you want a civilised road car that's quiet then 2.5" is fine in the majority of cases. 

The VR6 turbo example was my own car, which I built and mapped myself.  It made 400hp and 350lbft from 11psi of boost (GT3582R turbo).   3" DP down to a large generic cat and then into a 2.5" system with 2 silencers.   It was extremely quiet and at no point during mapping and dyno testing did the exhaust system cause a big restriction.  If it were, EGTs would have been excessive meaning the timing would need to be retarded, but it pulled hard all the way to 7500rpm with decent amounts of advance.

Would I have found more power and torque with a 3" system, yes, about 30hp at the same boost pressure.  Did I want the drone and noise from a 3"? Nope.  Would I feel an extra 30hp when I had 400 to begin with? Nope.

Compare the compressor maps between a GT3582R and a K04 turbo and see the huge difference in lb/min flow rate.  To get that much power from such low boost through an exhaust system (with a cat!) which everyone told me was too small made me proud 

[Dan_FR]

Depends what you want.  If you want a civilised road car that's quiet then 2.5" is fine in the majority of cases. 

The VR6 turbo example was my own car, which I built and mapped myself.  It made 400hp and 350lbft from 11psi of boost (GT3582R turbo).   3" DP down to a large generic cat and then into a 2.5" system with 2 silencers.   It was extremely quiet and at no point during mapping and dyno testing did the exhaust system cause a big restriction.  If it were, EGTs would have been excessive meaning the timing would need to be retarded, but it pulled hard all the way to 7500rpm with decent amounts of advance.

Would I have found more power and torque with a 3" system, yes, about 30hp at the same boost pressure.  Did I want the drone and noise from a 3"? Nope.  Would I feel an extra 30hp when I had 400 to begin with? Nope.

Compare the compressor maps between a GT3582R and a K04 turbo and see the huge difference in lb/min flow rate.  To get that much power from such low boost through an exhaust system (with a cat!) which everyone told me was too small made me proud 

But a GT3582R is capable of a lot more than 400hp, so you would be nowhere near the edge of the compressor map and you're not forcing the power at that PSI on that size turbo. Any more boost/power with that level of exhaust restriction and the EGTs

Remove the restrictions in breathing, and so long as you have the fuel, flow through the head etc. I would bet you would have found a lot more than 30hp. Even 30hp is a fair chunk to find, especially if that is a gain across the whole RPM range and you would definitely notice it in that case. 30hp for the last 1000rpm - possibly not

It's a different ballgame entirely on a K03 or even a K04 and running at the edge of the compressor map, you really cannot compare. It's like when people say the airbox on a mk4 Golf is good for over 300hp, because big turbo TDIs, charged R32s etc. can make that figure, but when you have a baby K03s which is balls out at 230hp with the standard airbox, remove that restriction and you suddenly find an extra 10-15hp across most of the rev range.

Anyway, we digress

is it worth putting money into in your opinion Dan_FR? If its not a necessity for stg2+, id much rather fit a better flowing backbox instead & put the money into a fuel pump.

Don't want it too loud & boomy, its loud enough now with a straight through centre section & the stock backbox on.

Standard catback is fine. I only replaced mine as my backbox fell apart internally and rattled like crazy so I had a turboback made. Fuel pump and some R-Tech magic will make a much bigger difference, as the K03 is all about the midrange rather than top end/peak hp.

[pudding]

But a GT3582R is capable of a lot more than 400hp, so you would be nowhere near the edge of the compressor map and you're not forcing the power at that PSI on that size turbo. Any more boost/power with that level of exhaust restriction and the EGTs

Remove the restrictions in breathing, and so long as you have the fuel, flow through the head etc. I would bet you would have found a lot more than 30hp. Even 30hp is a fair chunk to find, especially if that is a gain across the whole RPM range and you would definitely notice it in that case. 30hp for the last 1000rpm - possibly not

It's a different ballgame entirely on a K03 or even a K04 and running at the edge of the compressor map, you really cannot compare. It's like when people say the airbox on a mk4 Golf is good for over 300hp, because big turbo TDIs, charged R32s etc. can make that figure, but when you have a baby K03s which is balls out at 230hp with the standard airbox, remove that restriction and you suddenly find an extra 10-15hp across most of the rev range.

Anyway, we digress

I know it is - about 550hp with 0.82 hotside on the VR6 - but I didn't want more than 400hp.  Mainly to keep the gearbox in one piece and also because it was a daily so needed low noise levels and good road manners.  I ran it at over a bar for a while and no problems.  I forgot to add, I used the stock Corrado VR6 airbox, with paper filter, too.

I know what you're saying and that's fine if you're pushing for the maximum of everything, but in my experience, exhaust and induction is massively over-hyped, hence why there is such a huge market for that stuff.  It's mostly sales guff.

[GTI_Kyle]

Thanks Dan_FR  would a different backbox increase power in any way?

[AJP]

Thanks Dan_FR  would a different backbox increase power in any way?

No. Spend your money elsewhere if you're after power.

[GTI_Kyle]

Thanks Dan_FR  would a different backbox increase power in any way?

No. Spend your money elsewhere if you're after power.

Wont the stock catback/backbox restrict power at stg2+?

[AJP]

No. Maybe, and that's a huge maybe, there's one or two bhp to free up.

99% of exhaust restriction on these cars is at the cat(s) and downpipe.

Generally, the reason people go for a full exhaust from the likes of Milltek or Powervalve, rather than just the downpipe and cats, is to achieve a bit of a sweeter exhaust note along with the extra power from the downpipe.

A HPFP upgrade is the best next step for power after doing the downpipe.

[AJP]

If you're at 2+ you should have uprated the downpipe, fuel pump, intercooler, and intake.

Next step then is bigger turbo.