1SxyRXy

i was just wondering if there is a big difference in 3 compared to 2.5 because ive been thinking about it and if its that much better than i want to get my whole exhaust redone.

dvls-7

yes there is a difference.....bigger is better.....

Mr. Gadget

No, not true, infact at somepoint you will have diminished returns. In theory engines will have a optium exhaust size for best tuning perfomrance. Things like location of collector will effect the car's performance at different spots in the RPM band. I believe the optium size for an NA is 2.5". I am not sure on TII's

Big is good, but tuning your car intails more than size.

dvls-7

Well being that he has a T2, that was the basis for my answer.

sub9lulu

iTrader: (7)

i'd get 3" for turbo

Mr. Gadget

Your point has no limit. My remarks were intended to show him that you can't go nuts with exhaust size.

mazdaspeed7

For turbos, bigger IS always better. Less backpressure lets the turbo spool faster. Thats all that matters.

Fingers

Right but when the turbo outlet is only 2.5", why would you get anything bigger, if you stick with the stock turbo? The flows won't be increased, thats for sure, and the velocity will be decreased. So what's the point?

Mr. Gadget

Screw it, I have a 5" exhaust off of a Cummins, I'll just slap that on. Turbo will love that...

NZConvertible

The law of diminishing returns applies to almost anything, but that does not make a 3" exhuast unnecessary. The point is a 3" exhaust will make more power than a 2.5" one. Period. The size of the turbo's outlet also doesn't matter. The bigger the exhaust pipe, the easier it is for the engine to push it's exhaust gases down the length of the system. That's simple fluid dynamics. The less work the engine has to do pushing gases out, the faster the turbo can spool and the more power that is available at the crank.

This applies equally to turbos and NA's, but for NA's the larger pipe must come after the tuned section of the exhaust.

Note that the only disadvantage a 3" exhaust will have on FC's is boost creep. You must have made the necessary mods to keep boost down.

chuch

From what i know
go the 3".

Just make sure that you watch out for boost creep. With a flow through exhaust system, you should expect to see up to 14psi on your boost gauge.

This is a no-no as motor/turbo might go "pop"

Port your waste gate, get a fcd/boost controler and you should be fine


You'll probably regret it when you want more power in the long run. I don't know about you, but thats my goal for sure!!!
Chuch

$150FC

when you get into big-boy power ranges (450+ crank hp) then a 3.5" pipe is optimum. There's no reason not get 3" pipe when you redo your exhaust.

silverrotor

iTrader: (1)

Your latter point has piqued my Interest. Details would be great. I have a full ss 3" Brullen Exhaust coming up In the next week or so. I want to make sure this Is point Is addressed.

$150FC

easiest way:
port the **** out of the wastegate. that's the easiest way. you can port it to the size of the puck or you can have bnr port it and put a bigger puck on it. i drove a t2 with a gigantic port on the wastegate and full 3" exhaust and there was no boost creep.

silverrotor

iTrader: (1)

I've already ported the snot out of the s5 Wastegate. Primarily the secondary Port, no room at all to port on the other wastegate port.

importboi22

get 3 inch!!!! i got 2.5 and i can feel it choking the motor after 5500....and no its not my turbo!! you can just tell by the exhaust note.... you'll know what im talking about if you get 2.5 exhuast... but none the less... WAY better than stock

Fingers

What exact difference are we talking about when we're comparing the 2.5" RacingBeat exhaust and the 3" RacingBeat exaust. I know that 3" will flow better regardless, but it will impede velocity.

So what HP differences are we talkin about here?

rotary>piston

the boost increase with the 3 in will make all the difference in hp, so it depends how you set the controller.
The lost velocity will only take effect at low rpm, and since TII's have crap low end anyway, it doesn't matter. On an N/A you have to decide, larger pipes will raise the high end power, but smaller pipes will help low-mid range

Fingers

So if i get a 2.5" turboback by RB, and just up the boost to where it would normally creep with a 3" exhaust (lol), I would get similar power as I would from a full three incher?

I'll eventually be going to a BNR stage 3... but it doesn't really matter as the hot side of the turbo remains stock :P

Makenzie71

^...no

With a turbo car if you want 5" plumbing under there then by all means get it. With a turbo charger there are no pulses, no delta waves, just smooth and constant flow. You want the freest flowing possible exhaust. 3" would be good.

minclo

everyone says that with a 3" exhaust you will need a FCD...but won't that lean the crap outa the A/F ratio? wouldn't it be better to get a boost controller and just keep the pressure to 8 lbs. I mean you won't be seeing the gains of 13 lbs of boost but would this not be better for the life of the engine?

ponykiller

exactly, but that is why there are piggyback fuel controllers (SAFCs, etc)

Rizla+

Could someone explain me sonething about Boost Controllers ? For exemple, the simple ball & spring design manual one. If I understand it right, it is used to fool the wastegate, letting it see less boost than the actual value, like I want 10 psi so I adjust my BC for it to show 8 psi at the wastegate when the real pressure is at 10 psi. Am I right ? So if the wastegate cannot flow more (without porting it of course), even with a MBC you would still be experiencing boost creep right ? This is my first turbo car so it is new for me.

minclo

well i'm not really familar with those as of yet, do they also just trick the ecu or do they actually replace the fuel maps or whatever in the ecu?

Rizla+

S-afc and such piggyback fuel controllers simply trick the ECU, letting you adjust fuel delivery in a certain range. It keeps the stock fuel/ignition maps and does not change anything in the ECU itself.