Exhaust size. 2.5 or 3" ?

Q&A regarding engines, turbos, and intercoolers and power upgrades

Exhaust size. 2.5 or 3" ?

Postby celigts » Mon May 13, 2013 12:04 am

So I've gone 2.5" turbo back and everyone seems to diss the size choice but I believe if I went 3" I'd lose power bring that I'm mostly stock.

My personal goal was 3-350awhp so I didn't think I'd need bigger than 2.5

Wrong? If like opinions if the technical guys wanna jump in :)
Members don't see the above ad. Register now - it's free!
91 Celica Alltrac - Parts pile pending instal :P
3SGTE project motor very slow
Intake/Heat shield project slowly moving
Winnipegs respected TTI forum. join and love the community will welcome you with open arms and legs ? http://toptierimports.com/
User avatar
celigts
Established Member
 
Posts: 1660
Images: 0
Joined: Fri Feb 11, 2011 3:03 pm

Re: Exhaust size. 2.5 or 3

Postby l0ch0w » Mon May 13, 2013 2:48 am

2.5 should be fine, really where the issue stems from is that often times people will install a 3 in downpipe and keep the stock 2 inch exhaust. This usually causes boost creep issues, dont ask me why cause ill make some shit up. Just try and keep your downpipe the same size as your exhaust, and you should be ok.
Andrew

1991 Red Alltrac
Frankenstein engine, lots of mods :P
User avatar
l0ch0w
Established Member
 
Posts: 1137
Joined: Thu May 31, 2007 7:38 pm
Location: Lansing KS

1991 Toyota Celica

Re: Exhaust size. 2.5 or 3

Postby warracer » Mon May 13, 2013 2:51 am

I ran 3'' dp > 2'' exhaust a whole 6month since I was in the middle of my semester, I experience a bit of boost creep but it wasn't that bad, but I was only running 10psi. Then I got the full 3'' setup, wow the feels great! much better throttle feel, power curve is almost linear compared to the 3<2 setup I had.
____________
ST185H A-Spec:
-IHI VF28 -BC Racing coilover
-Yonaka FMIC -Swift Spring
-ARP head stud -St205 rear end
-Cometic Metal HG -Supra TT Fuel Pump
-3'' SS straight pipe -Poly bushing
User avatar
warracer
Club Member
 
Posts: 525
Joined: Wed Aug 29, 2012 2:43 am

Re: Exhaust size. 2.5 or 3" ?

Postby celigts » Mon May 13, 2013 2:58 am

Is 2" our stock size?! Yikes! Hard to understand that lol. So then what I'm running is 2.5- stock midpipe and 2.5 from cat area back since its catless.

So boost creep is caused by the bottleneck thing or if it doesn't have enough power to flow the 3".
91 Celica Alltrac - Parts pile pending instal :P
3SGTE project motor very slow
Intake/Heat shield project slowly moving
Winnipegs respected TTI forum. join and love the community will welcome you with open arms and legs ? http://toptierimports.com/
User avatar
celigts
Established Member
 
Posts: 1660
Images: 0
Joined: Fri Feb 11, 2011 3:03 pm

Re: Exhaust size. 2.5 or 3

Postby brutekiller787 » Mon May 13, 2013 3:44 pm

the idea that you would lose power due to an oversized exhaust only applies to naturally aspirated cars that rely on cylinder scavenging to draw exhaust out of the cylinders. with a turbo in place, after the turbine wheel starts spinning it creates a vacuum to draw out gasses, but usually pressurized gas WANTS to get out of the cylinder anyway, just as any pressurized anything wants to expand.

turbos "spool up" faster when there is a high pressure differential between pre turbo exhaust and post turbo exhaust. a "low" pressure in the downpipe and a "high" pressure in the exhaust manifold will cause the turbo to spool quickly, while a "low(er)" pressure from an aftermarket custom manifold and a "high(er)" pressure from a too small or stock sized exhaust will cause slow turbo spool.

so to sum up, N/A cars have to worry about exhaust size, turbo cars, go as big as you can, you really won't lose anything. that being said, space pretty much confines an alltrac to 3" pipe. so go 3" if you can. you'll spool faster and will probably make more power than 2.5", but if it's done then it's done don't do it again till you need to.
brutekiller787
Club Member
 
Posts: 371
Joined: Wed Oct 26, 2011 7:27 pm
Location: Lima, oh

Re: Exhaust size. 2.5 or 3

Postby underscore » Mon May 13, 2013 3:57 pm

Changes in pipe size cause turbulence, which messes with the exhaust flow. Whichever size you go with, be consistent, especially from the DP to the MP.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
underscore
GTFour God
 
Posts: 3820
Images: 0
Joined: Mon Apr 09, 2007 11:29 pm
Location: Kelowna, BC

Re: Exhaust size. 2.5 or 3" ?

Postby celigts » Mon May 13, 2013 4:20 pm

That's some great info guys. I did know the turbulence thing was bad going from big to small and vice versa.

The 2.5" will definitely stay then and just possibly get crush bent or DIY midpipe made since mines parked.
91 Celica Alltrac - Parts pile pending instal :P
3SGTE project motor very slow
Intake/Heat shield project slowly moving
Winnipegs respected TTI forum. join and love the community will welcome you with open arms and legs ? http://toptierimports.com/
User avatar
celigts
Established Member
 
Posts: 1660
Images: 0
Joined: Fri Feb 11, 2011 3:03 pm

Re: Exhaust size. 2.5 or 3" ?

Postby celigts » Mon May 13, 2013 4:24 pm

brutekiller787 wrote:the idea that you would lose power due to an oversized exhaust only applies to naturally aspirated cars that rely on cylinder scavenging to draw exhaust out of the cylinders. with a turbo in place, after the turbine wheel starts spinning it creates a vacuum to draw out gasses, but usually pressurized gas WANTS to get out of the cylinder anyway, just as any pressurized anything wants to expand.

turbos "spool up" faster when there is a high pressure differential between pre turbo exhaust and post turbo exhaust. a "low" pressure in the downpipe and a "high" pressure in the exhaust manifold will cause the turbo to spool quickly, while a "low(er)" pressure from an aftermarket custom manifold and a "high(er)" pressure from a too small or stock sized exhaust will cause slow turbo spool.

so to sum up, N/A cars have to worry about exhaust size, turbo cars, go as big as you can, you really won't lose anything. that being said, space pretty much confines an alltrac to 3" pipe. so go 3" if you can. you'll spool faster and will probably make more power than 2.5", but if it's done then it's done don't do it again till you need to.



So reading this, I have a tubular manifold I was going to put on. eBay one and just had the tubes to flanges re welded to help prevent future breaking between pipe and such. Will I lose alot installing that? Can't say off hand what the pipe size is :/

Same ct26 but plan to get the ct27 from ats in future. If slower spool up is all ill see with the tubular then that's fine. I can deal with that for now.
91 Celica Alltrac - Parts pile pending instal :P
3SGTE project motor very slow
Intake/Heat shield project slowly moving
Winnipegs respected TTI forum. join and love the community will welcome you with open arms and legs ? http://toptierimports.com/
User avatar
celigts
Established Member
 
Posts: 1660
Images: 0
Joined: Fri Feb 11, 2011 3:03 pm

Re: Exhaust size. 2.5 or 3

Postby brutekiller787 » Mon May 13, 2013 5:34 pm

if it's decently made, and pipe length and bends are equal, you won't lose anything, you will gain. what i was talking about was like a ridiculously oversized manifold, like 2" runners and stuff. if you have an st185, you have a twin scroll turbo as well, which will help spool anyway. st165, you are out of luck lol
brutekiller787
Club Member
 
Posts: 371
Joined: Wed Oct 26, 2011 7:27 pm
Location: Lima, oh

Re: Exhaust size. 2.5 or 3

Postby Hotrodhendrix » Mon May 13, 2013 5:46 pm

My experience based on 2 dyno runs about a year apart.

1st dyno had a crappy 2.5" short mid to fart can under front seats.

2nd dyno only thing that changed as far as the engine went was 3" mandrel bent exhaust with flow through style mufflers run all the way back. 72' glass pack style bullet muffler and dyno max style in the rear.

"NOTE I DO NOT HAVE DYNO PULLS IN FRONT OF ME"
Between the 2 setups the 1st pull spooled a good bit quicker. 2nd increased max hp by 60. Hard to believe I know. Boost was run off spring both times so it did not change. As for temp outside I dont know how different it was. Both pulls were on the same dyno.

Take from that what you will but at your goal range I would go ahead and do 3" as you will want more power eventually.... When that happens you will be ready for it.

~James
"The more you let life's obstacles slow you down the longer it will take to defeat them."
~Yours truly
User avatar
Hotrodhendrix
GTFour God
 
Posts: 3480
Joined: Thu Sep 16, 2004 5:45 pm
Location: DFW Texas

Re: Exhaust size. 2.5 or 3

Postby underscore » Mon May 13, 2013 6:21 pm

Why bother with a tubular manifold? I'm pretty sure the stock one is good til above your power goals.
★ 1991 GTFour RC ~ "Rebel Scum" ★
It's for sale! http://www.alltrac.net/phpBB2/viewtopic.php?f=64&t=52181
Build thread http://www.alltrac.net/phpBB2/viewtopic.php?f=44&t=44216
underscore
GTFour God
 
Posts: 3820
Images: 0
Joined: Mon Apr 09, 2007 11:29 pm
Location: Kelowna, BC

Re: Exhaust size. 2.5 or 3

Postby Greg_Canada » Mon May 13, 2013 7:17 pm

When i put a 3" MP and catback on my car, it made a noticeable improvement in power. This was an all-stock car. Just the catback did nothing but sound nice, with the MP, it "woke it up" and felt like the restriction was taken away. Did not get to the DP before i sold the car. I felt spool was similar to stock but with much more top end.
1989 4runner lifted, way too many mods
1999 4runner limited, IPF lights, JVC screen
Greg_Canada
Club Member
 
Posts: 318
Joined: Thu Aug 27, 2009 5:39 pm
Location: Toronto

Re: Exhaust size. 2.5 or 3

Postby zero06 » Mon May 13, 2013 7:33 pm

Stay 2.5" if the turbo you want is internal wastegate. Boost creep is not an issue you want. If you are going external then go 3" for better response.
1991 White st185 eternal project
2006 Silver Metallic RX8 6 Speed Daily (until alltrac is done if ever)
1976 Copper Metallic Ford Gran Torino Restomod Muscle-Boat
zero06
Club Member
 
Posts: 562
Joined: Sun May 18, 2008 3:47 pm
Location: Milwaukee, Wisconsin


Return to Performance and Power

Who is online

Users browsing this forum: No registered users and 4 guests