2.5" vs 3" exhaust systems
 

Im looking to change the exhaust on my sti v4 engined uk wagon, currently got a 3" bellmouth tapering down to a 2.5 to join a straight through exhaust with a scoobysport backbox. sounds great but id like something a bit deeper. im also wondering if there are any advantages of a 3" over a 2.5 for performance. im running pretty standard power for on the sti v4, vf28 and 75 ecu with boost controlled with a HKS evc and set to 0.8 bar low and 1.2 high boost settings. I am planning to go frontmount soon and vf34 or 35 then a remap.

cheers

Rich

No difference at this level, just noise...!

3" won't really benefit until you reach 400bhp + , stick with 2.5"

2.5" is good up to 480bhp.

A 3" will have a deeper and louder note, but IMO that sounds ****e! If you stick with a decent 2.5" straight through you'll get a more track freindly raspyer noise.

im at 430 with 2.5"...

sounds better.... looks better...

Far too many people dont read up and fit a 3" without asking why....

Agreed!

When fitted to a standard car along with an induction kit it lets you advertise it on ebay as "my mate says it must be at least 320bhp".

Im at 440/440 with a very conservative map on a 2.5"! sounds great and doesnt exceed any track limits.

I guess Andy Forrest is wrong then.

most tuners say around 350hp is when to go from 2.5" to 3"
or at least andyf does..(quote taken from afp)
"A fully decated 2 ½” bore system is adequate up to 350 bhp. After this a 3” system is recommended. Sports cat systems are available but ultimately still limit power compared to a fully decated setup"

Thats similar to what i was told. So i bought a 3 inch cobra system and it does have a very deep rumble.

How is the fit of the cobra systems?

I'm looking fur a trackday friendly exhaust and they make one.

Cheers
Lee

Tubby is the Cobra a loud system?

I was looking at their track day system, but am not sure if it will be loud enough, but dont want anything too loud either.

Thanks

If you want loud Iv got a cobra 3 inch with 2.5 flanges for sale with a Magnex down pipe decat. Made 407 on V power only using an RCM 400, so I don't think the exhaust was holding it back.

Yup its a bit too loud tbh.
At bedford on sat it was 101db at 4500 rpm at static test.
But it is a full 3inch turbo back non resonated version.

Thanks, think I'll stick with the track day system.

Just ordered the cobra track day resonated system. . Picking it up tomorrow so will let you know how it sounds. I'll post a video too for those that might be interested. Heard good things about cobra so should be good.

mines a bit too loud, im going to buy a resonated centre section in the summer

Will be good to hear how you find it as I'm looking to buy the same one soon. Can you also let us know how good the fit is?

Cheers

Will someone ask AndyF to enlighten us what figures he got out of his 2.5" System.....??

i think you'll be impressed.

And accept what AndyF said but theres no mistaking my power figures on my 2.5"

Well it turned up tonight. What a nice FED X man!

I'll update you all when its fitted.

Attachment 57958

Cheers mate, thats very helpfull of you, are you going for a sports cat too?

I have a remap booked with AndyF for the start of july, triple checked his website, and it does say that a 2.5 is ok for 400bhp. Dont know if thats catted or not though.

I don't think he means a 2.5" won't make good power but it just won't be quite as efficient as a 3" when getting to 400bhp and above.

There's a lot more to the dynamics of power than assuming the size of the exhaust and its increase means more power.

There's an informative thread on NASIOC about sizing exhausts to turbos etc...

I choose 2.5 to keep the noise down (got a baby) and due to the fact it wouldn't effect spool nor reduce top end power.

Rhetorical question: if bigger means better, why do we not all have 3.5 or even 4" systems on as standard.

Same question with FMIC.... bigger does not mean better.

90% of user here buy products off the shelf, fit and remap.... its a matter of convenience and finance.... then accept the results of what happens during the map.

The rest trial and error, R+D with products to find the right balance and results. No matter what the cost.

My post is not meant to insult. Just point out fact.


Actually thinking more about it.... in the last 12 months i've had fitted...

4 intercoolers (2x TM 2x FM)
2 turbos
2 sets of injectors
3 air filters
2 fuel pumps
3 exhausts
2 sets of headers
3 up pipes
4 remaps with Bob Rawle...
....
2 front ends
2 sets of alloys

I consider myself to be a hobbyist!! Imagine what the pro's have gone through???

Im with nyscooby on this one,it would be interesting to see some real world tests on this.
I had been told that a three inch system would give me more BHP,wich was proved on the dyno,also that a large front mount intercooler would increase BHP,also proved on the dyno.
But now the car seems to be slower on acceleration,this might be due to the extra lag from the fmic,but what about the bigger bore of the exhaust?
Obviously the bigger exhuast expells gasses quicker but what about the lack of back pressure,will that affect performance?
Does anybody know?

A smaller exhaust has a faster gas flow, which has an effect on pulling gases through, this is alot more imortant on NA than forced induction, but as a jist, smaller exhausts bring peak torque down the rev range, the torque curve would be more interesting to see on a back to back test than the power curve. For road manners, torque is the new power! :-)

Cheers for the replys, ill stick with the 2.5 then but change the backbox to an after burner or something similar.

the prodrive box runs and sounds sweet