Damocell

iTrader: (1)

I am in the process of upping the power on MY99 uk car looking for 320/320

Mods will be

VF29/35
STI 8 TMIC
Walbro fuel pump
Full samco induction and turbo hoses
3 port boost solenoid
Panel filter
ported headers

My question is, is it better to go for a full 3 inch decat or a 3 inch down pipe to a 2.5 inch centre and rear silencer?

Will the turbo need the extra back pressure? will extra back pressure equal more torque?

I realise 3 inch are generally used for 350+ bhp but are there any down sides to fitting them on a 320bhp build?

If it helps I am looking for mid range shove over top end poke.

Damo

bighead

iTrader: (1)

you want a VF34 then (midrange )

Fat Boy

2.5" = less lag, quiet torquey mid range, but runs out of puff eventually
3" = opposite - bit more lag, better top end.

I stuck with the 2.5 and VF28 on my P1 as I wanted a punchy road car first and wasn't chasing big figures so running around 300/ 300 ish.

I think that you'd struggle to get 320/320 on 2.5" but others will know better.


For bigger power, it's pretty much always a trade off - less back pressure is good but it will (generally) get laggier as the power rises. Torque interrelated as usual.

scoobyman95

you don't want a vf28 or 35 you want the 34 that is far better and take high boost good for 450bhp i have one on my wagon and i get 420/400 but spent over 20k also I would go for the 3" breaths far better and better top end cheers good luck

andy97

iTrader: (1)

I believe the VF34 is rated at 350bhp max.

Jay m A

iTrader: (2)

As far as I'm concerned 0 backpressure after the turbo is ideal, you're never going to achieve that but typically a 3" will flow more than a 2.5" although it obviously depends on the design.

Regarding torque with 2.5" vs 3" thats an old wives tale IMO. How restrictive do you think the blades of a turbo stuck right in the exhaust stream are relative to a 2.5" system at 320 BHP? Much more restrictive I'd say.

Getting more torque (on the same turbo) basically means getting to your target boost ASAP, which has more to do with all the componets pre turbo, mapping, heat management etc as long as the exhaust system can flow the HP.

All IMO

scoobyman95

vf 34 will do over 400 i have the proof of it and it was built and maped by scooby clinic???

p1mark

Agree with everything Jay says. I cant see that (unlike a NA engine) there is any scavenging advantage to having back pressure on a turbo'd car.

Common theory is that the 2.5 will not be restricting things until over 350 BHP, i.e at 300 bhp there would be no difference, but at 400 there quite possibly will.

Scoobyman, i am not sure even you know what turbo you have on your car, as it was a TD05/18G yesterday? (ref post #2)

https://www.scoobynet.com/private-sa...ml#post6551481

Whichever it is, i dont think either will get you anywhere near 450 BHP.

cookstar

iTrader: (6)

rumbled

Damocell

iTrader: (1)

Damocell

iTrader: (1)

I realise what you are saying, so is it better to have negative back pressure?

The only reason I ask is that I'm booked in for the work and have a 2.5 inch exhaust at the mo. But I have bought a 3 inch in search of power and spool.

But I will sacrifice a little top end power for spool and mid range torque.

Question to everyone!

If you were looking for 320/320 and had the option of a 3 inch or a 2.5 inch without paying any extra what would you do?

Damo

Neilo

if its cheap enough, get the 3" itll help make the power easier, the map will sort out any lag issues.

Damocell

iTrader: (1)

I have both at the moment, I'll loose a little money which ever I keep as I will have to sell the other, so the money side isn't an issue.

What power are people running with a 2.5 inch or a 3 nich exhaust with what mods?

Damo

New_scooby_04

iTrader: (4)

320bhp shouldn't be a problem at all with apropriate other mods on a 2.5 exhaust.

I have 319bhp on my MY99 (Recorded on a day with 30 degree ambient temps)

Ns04

p1mark

As Jay says, anything downstream of the turbo is going to have very little effect on Spool.

If you are sure you will not be wanting anymore than 320 ish in the future then save a few pennies and get a 2.5".

If you are a serial modder and always want more ( me and plenty of others on here are like this - admittance is the first stage of being cured) then put a 3" DP and system on to stop the nagging doubt in the back of your mind in the future - "IS MY 2.5" DP A RESTRICTION" "

Neilo

agreed, you can do 320 on a 2.5 system fairly easily, they done become a restriction (unless you have a very well designed one) until you get to around 350-370, then the 3" will come in to its own.

I currently have a 3.5" APS system on my car, and im actually downsizing the a 3" knowing that its still capable of flowing enough for the potential 600+ that the turbo can provide.

Damocell

iTrader: (1)

I am a serial modder, so I think I will go for the 3 inch, the only reason doubt is in my mind, is that my tuner (well respected) advised a 2.5 inch exhaust the only reason I am thinking 3 inch is that I have the exhaust and will probably want more power in the future!

I realise I should really take the advice of the tuner but I think that the 3 inch will allow for more mods in the future.

Damo

Damocell

iTrader: (1)

Is the APS for a newage?

Damo

Neilo

yes it is