2.5" or 3" Bore
 

At what power will a 2.5" become restrictive? Is a 3" more suited for 360bhp+

We seem to have all the same questions lol

Anything up to 350 and 2.5 is better IMHO :)

Thats because I think we have similar cars and wanting more POWER :lol1::lol1:

Why is that?

I'm running 402 bhp with a 2.5

Have been tempted to switch to a 3" system to see how much gain is made

Mine has been OK 2.5" up to 350bhp and really if you are passing 400bhp i would suggest 3" but until stick with 2.5"

Andy F run with 2.5" 450bhp.


jura

I have got a 3" milltek and am running 340bhp.

i noticed a big difference changing to a 3inch milltek.....

I had 400 bhp on a APS 3" catted system.

It's more about spool I think so post ya graphs

Hmmh... any enlightenment ?
 

Yup I'd like to know too the exact differences...

I guess the 3" system will make more mid & top end power - but how much more 10 -15bhp ?

And whats the pay off ? Some say worse spool - is this true ? Some say less throttle response ? Some say once the exhaust gases are out and under the car (significantly cooler) it makes v little diff at all ?

Yes we know of a few cars run by 'tuning experts' that have been at 450 bhp with 2.5" systems - with standard top mounts too... So are these cars daily drivers or just for the track ?

A

Just thought.... 'I think' the high power 2.5" systems are on 2.0 litre lumps.

I guess a 2.5 litre would want more airflow ?

A

So if I'm running a 3" and only producing 287bhp am I losing out on power because of the larger bore exhaust then? :(

Would a 2.5" give me more power or not too mess around and stick with the 3"?


Thanks

Im running 420 bhp with my trusty 2.5" afterburner.
Never had any problems with output so far.

3" pointless on your car, yes you are missing out on a punchier torquier car :)

Just to throw something in to the mix :D I run a 3" downpipe that meets a 2.5" inlet to the centre section :rolleyes: That then becomes a 3" system again after 6 inches or so. What's that doing for my power and what's that going to do when I'm aiming for 450bhp? :wonder:

I'm planning on getting the 2.5" section cut out and replaced with a 3" section but will that harm my spool up?

I'm sticking with my 2.5 system and just doing a cat back. That will do me fine. Thanks for the response.

Paul I used to run a 3" system but I changed to a 3" dp which couples at a 3" reducer 2.5" centre pipe and BB. Ran much torquier and spool was better. Also when I put a 3" NurSpec on my SPecD it felt down on torque, much less pokey.

Also depends on your turbo size 2.5" (63mm) is perfect IMHO for standard turbos TD05 63mm outlet.

If you're going for a bigger turbo with bigger outlet it makes sense to make the corresponding exhaust turbo back bigger also :) IMHO.

Hi Stu,

It sounds like keeping the 2.5 "restriction" might work in my favour because my turbo is an MD321S and I'm still on a 2.0 so it's likely to be laggy so we are looking to promote spool wherever we can :lol1: I must say though that my car on my current engine & exhaust etc goes very well for the spec :D

Hoping to free up the injectors soon for you mate ;) I think we are only weeks away from the motor being ready to drop in :eek:

Cool, I actually happened upon some PH2 440 injectors which I am gonna get Rich to flow to 860, but I am still happy to honour buying yours:) I hope to fit the airbox before the Motor museum run next sunday so I will see if Rich is going and give him the induction :)

If you don't want them that's cool and I'll sell them on, I'm not that worried to be honest as long as I get rid and recoup some cash :)

What is the motor museum run? Have I missed something? Glad you got the airbox mate :thumb:

Makes no sence at all that 2,5" should help spool. Every turbo car I've had has spooled up better after 3", even low pressure Saabs with tiny TE05-12B turbos. It's the 3" downpipe that makes the biggest difference, you can go down to 2,5" catback if running below ca 350 hp (not a definitive limit, you can go higher with 2,5" but might be restricting power). N/A engines are a bit different though.

Only reason to stick with 2,5" is to reduce noise.

I thought 3" was better for spool too

I run a 2.5 inch with centre resonator and prodrive back box, decat down pipe.
So 500bhp is easily attainable.
Andy Forrest uses a 2.5 aswell on his newage race car.

<------------:D

I would be interested in graphed figures too.

I had a Japspeed 3" system and swapped over to a 3" Miltek and lost 0.5 bar, this was soon sorted on the map tweak, but surely 3" would be best suited to almost any Subaru so long as the headers and up pipe are correctly set up?!?!

Rob

https://www.scoobynet.com/scoobynet-...power-run.html

That is my car

:thumb: https://www.scoobynet.com/middlesex-...eum-hayes.html

Ok thanks for that :)

Looks like I maybe selling a 3" HKS system soon then :( lol

There are lots of 400 + BHP cars running 2.5" systems. At what point a 2.5" system becomes restrictive I do not know. However what I do know is that turbo cars (as opposed to naturally aspirated) want the least restriction in the exhaust as possible. After the downpipe, where the gas has cooled substantially, the exhaust diameter is less critical. I always use a 3" open neck downpipe as the cost difference is small, if atall and the car is future proofed.
It is a falacy that a smaller diameter exhaust on a turbo car adds torque but the diameter of the headers and uppipe are critical to spool, response and torque.