2.5" vs 3" exhaust systems
05 May 2013, 06:18 PM
#1
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
cheers
Rich
07 May 2013, 02:14 PM
#4
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.
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.
07 May 2013, 02:36 PM
#6
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.
07 May 2013, 02:38 PM
#7
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"
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07 May 2013, 02:44 PM
#8
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"
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"
07 May 2013, 03:31 PM
#9
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07 May 2013, 03:32 PM
#10
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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
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
07 May 2013, 04:31 PM
#11
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.
08 May 2013, 06:52 PM
#14
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.
08 May 2013, 08:22 PM
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Cheers
08 May 2013, 11:59 PM
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Cheers mate, thats very helpfull of you, are you going for a sports cat too?
09 May 2013, 12:01 AM
#21
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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.
09 May 2013, 05:43 AM
#23
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.
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.
09 May 2013, 05:47 AM
#24
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???
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???
Last edited by nyscooby; 09 May 2013 at 05:54 AM.
09 May 2013, 06:58 PM
#25
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?
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?
Last edited by stu turbo 98; 09 May 2013 at 06:59 PM.
09 May 2013, 09:34 PM
#26
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! :-)
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