Does Size Matter? 2.5" vs. 3" Exhaust discussions. [merged]

[Boostin18]

2.5" vs. 3" exhaust discussions are merged here.

I have a 90 GSX and when i bought it the guy said it was full 3" turbo back with a EVO3 O2 housing (ported), now here is my question. When I took the exhaust down, it had a 2.5" exhaust flange at the top of the down pipe and then flares for about a half inch out to 3", if that makes any sense. This is my first 3" exhaust and I was wondering if this is right? I was thinking of porting the o2 housing because it is 2.5", making it 3". Then bringing the exhaust to a shop and having them weld a 3" flange to about a half inch long peice of 3" ehxaust pipe and then welding it on to the whole exhaust. So basically getting rid of the 2.5" flange and the half inch long flare and just welding on the 3" flange with the new piece. Would this make a diffrence if i do this or not. I don't know if this would make any diffrence or not, but when i put a boost controller on, i would try and get it to set at 15psi, but never would it would just spike and go down to like 12 or 13, but never above 15, but never at 15. I also have a ported 2G manifold (havent had it off to make sure it really is ported). Any help would be great!

 

[Talyn]

You want to use 2.5'' to 3''inch for the most street power from a 16g MHI turbo.
3" inch is more likely to rattle then 2.5", but some tuners have used/custom made 3" that does not rattle, it just takes a lot of time test fitting, and mounting it all on and off a few times, to get it just right.

Your 2.5" setup, is at least better then the flow limited, stock pea shooter setup.
I think since you already have 2.5" setup stick with it, and raise the boost [after you do a boost leak test] and see if you can reach your target of 350hp, because buying/making a 3" inch setup is gonna cost you more money$$$, and if you can do it with 2.5" it wont cost you anything you haven't already spent.

There is no need for a 3" exhaust on his car, it would be overkill on 95% of the cars on this board, and certainly overkill for 300hp as Awd-laser pointed out.

 

[bradcurtis15]

If you want to make power you better upgrade your clutch first and your fuel system. Do you have head studs? Spend the money on those things not an exhaust.

 

[Pipnorcali]

I run a 3" DP and qtp cutout and i pick up almost 2mph in the 1/4 with the cutout open vs closed with a t28 and 280-300hp

But on your setup i would just stick with the 2.5 you got

 

[boost97gst]

2.5" is fine. However, remember this isn't NA land anymore, backpressure doesnt matter as much as the turbine creates its own backpressure. A 3" pipe dumped straight through the bumper would net the best performance. On a 16g though? Not much extra to be had with full 3", thats why i say for you, 2.5" turbo back is just fine.

I run 2.5" to the flex which bells out to 3" and pumped 52lbs/min of air through. The system was still bottlenecked at the 2.5" section. Ported 2g o2 housing as well, not an evo 3 housing. Just some info for ya.

 

[LandSpeed-DSM]

No one in this thread understands how turbines work, apparently. Google "Expansion Ratio" If you want to learn.

You want ZERO backpressure above atmospheric after the turbine wheel.

The biggest/shortest pipe you can fit and stay legal with is what you want.

Even for NA, you don't want backpressure.. you want to maintain sufficiently high exhaust gas velocity through the system so you can use pressure differentials across each cylinder's pulse to pull the next cylinder along with it. The term for that is "Scavenging."

 

[boosted1stgen]

Remember also that theres a big difference between mandrel bent and crush bent. The 2.5 crush bent will neck down to 2 inch on the bends. The mandrel will stay 2.5 the whole way, also carries a bigger price tag.

 

[jakelandry]

3" becomes a restriction around 500hp. A big restriction? NO... but its still a restriction. Backpressure can force any quantity of air out of any size pipe but the easier it does it, the less power wasted. Get the biggest you can afford.

 

[NHerron]

You cannot go too big with turbo exhaust. From an efficiency standpoint go with the biggest pipe while maintaining peak velocity, because there is minimal space under the car and bigger gets complicated to route.

 

[werx]

As several members have said 2.5 is probably big enough and I agree with bradcurtis15 you might wanna focus your attention on the clutch and fuel system now...way before 2.5" exhaust ever becomes a restriction.

 

[LandSpeed-DSM]

Okay, so there's still a lot of misconception floating around in here.

For this discussion to make sense, you guys need to understand that ANY backpressure is indicating restriction.

So if we're going to start throwing out phrases like 'well 3" becomes a restriction at XXX whp' it's time to start quantifying things.

On a turbo, you want the shortest and most direct route to atmosphere possible. Period.

A bigger diameter/shorter path out is every bit as important on a small turbo (T25/14B) as it is on larger units. For spool, overall power.. everything.

In fact if you could fit a cone that tapered out to even 4" from the wheel on a T25 at a shallow angle over the course of 18-24", only just shallow enough to keep the flow coherent, there would be tangible benefit.

A full turbo to bumper exhaust, even straight pipe, will have measurable restriction well before you're trying to pull 400whp worth of hot exhaust. This is why so many folks on little DSM flange turbos who switch to dumped O2 housings pick up so much power and response.

 

[91fwdrst]

Ill be upgrading the clutch and my fuel system here soon. Have to move first then come the goodies for the car. Im not going to be upgrading the exhaust for awhile if i needed to but i wanted to make sure that the 2.5" was going to be big enough for my goals.

 

[Seamus]

Okay, so there's still a lot of misconception floating around in here.
e.

Your no compromises view on you car doesn't make sense to me. I would much rather have the benefits of added compression the option of more displacement from a built motor than driving around with a giant pipe sticking out of my hood. Then again, Ilike nice fat ppowerbands, so I dont see the point of running very large turbochargers at 50% of the capability on stock short blocks either.

 

[LandSpeed-DSM]

I'm making 30-31psi on a 67mm T4 as early as 47xx rpm in 3rd and wind out almost 5000rpm from there. Tell me more about your response and wide powerband

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I'm very likely making more power than 95% of this forum on this 1G Rods/Piston block, and pushing this turbo well past "50%" to boot.

I don't make many comprises because I understand the physics behind these things and the resulting performance can't be argued.

If you have something of value to contribute, please do. Otherwise you'll need to find someone who cares about your opinions and general bitterness.

Unless somethings changed, I'm running more boost than you on a turbo twice the size and winding out higher.. so maybe, just maybe you should try to improve yourself before calling out others

 

[94awdcoupe]

landspeed, I dont agree with your understanding of exhausts either. the whole bigger is better for turbo exhaust has some snags. I ran 16g turbos for years. My experience is a 2.5 exhaust on the 16g makes more low end torque and is far more fun to drive. I have seen this a few times personally . and there is no shortage on the internet of bigger downpipe swaps loosing low end torque. dont want to get in a debate about it with you because your mind is already made up. but you might want to cut people some slack.

 

[habitatguy187]

I too have felt the lag/loss of low end power when I ran an open 3" down pipe on my 14b with 7cm housing. First drive after bolting up a dual chamber muffler directly to the downpipe I noticed a HUGE improvement in drivability/low end response.

 

[athlete3344]

No, landspeed is 100% correct in his understanding. First, do some reading on the basics:

http://auto.howstuffworks.com/turbo1.htm

Specifically,

"One cause of the inefficiency comes from the fact that the power to spin the turbine is not free. Having a turbine in the exhaust flow increases the restriction in the exhaust. This means that on the exhaust stroke, the engine has to push against a higher back-pressure. This subtracts a little bit of power from the cylinders that are firing at the same time."


A turbo is nothing more than a pump. On a typical pump curve, you see head (pressure) and flow. As head increases, flow decreases. If you wanted to max out the flow of any pump, the line should be as large as possible and completely horizontal. The turbo is the exact same way, if there is any back pressure, it will restrict the flow of exhaust, which will in turn reduce power created by the turbo.

One of the main reasons to run an external wastegate off the manifold is to reduce turbulence the exhaust and turbo will see. This allows for more flow since turbulence creates eddies that contribute to pressure increases.

 

[livedsm4g63]

The best exhaust is no exhaust ..... Side exit ftw.

 

[athlete3344]

The low end gains noticed are a result of backpressure allowing the turbo to spool up quicker, but in contrast, mid/top end power will be lost.

 

[94awdcoupe]

I too have felt the lag/loss of low end power when I ran an open 3" down pipe on my 14b with 7cm housing. First drive after bolting up a dual chamber muffler directly to the downpipe I noticed a HUGE improvement in drivability/low end response.

I like the way you make the "huge" "HUGE". i ran the 16g with 2.5 exhaust for two years. it was a lot of work to get the 3 inch on the car. (89 mirage turbo). I was absolutely flabbergasted at the loss of low end pulling away from the muffler shop. while the losses where everywhere the most noticeable was on the highway in 5th cruising at 3000rpm. with the 2.5 exhaust there was MASSIVE torque hit instantly. with the 3.0 it was a torque whimp. really required a downshift to 4th which was no where near as satisfying.

 

[boost97gst]

I just basically reiterated what LSDSM said in a more articulate manner. So i deleted it. But what the f*** people? Let no one on here spread misinformation that a small exhaust on ANY turbo can be a good thing. It cant. Simple. Im sorry and you can run whatever size you want, but facts are facts.

 

[LandSpeed-DSM]

Backpressure after the turbo is a bad thing thing.

A physical fact of how turbines work. Response, efficiency and overall power WILL increase with less restriction after the turbine. Period.

Your lag situation literally cannot have improved by adding restrictions downstream.

Backpressure only hurts.

This isn't a topic subject to opinion.

If you think you saw an increase in lag when going to an open downpipe and then it was remedied by adding a muffler you need to put the crack pipe down, figure out what issues you inadvertently fixed while adding the muffler and start taking better data.

Especially on a smaller turbine with higher expansion ratios for the same intake side pressure ratio. That is a physical impossibility.

Backpressure is multiplied by the expansion ratio and additive to the required drive pressure.

You are not some unique snowflake that is exempt from the laws of thermodynamics.

You don't even want backpressure on an NA car, it's only incidental to scavenging.. not the goal or intent.

How does this sh!t stay gospel to so many people?

 

[94awdcoupe]

I just basically reiterated what LSDSM said in a more articulate manner. So i deleted it. But what the f*** people? Let no one on here spread misinformation that a small exhaust on ANY turbo can be a good thing. It cant. Simple. Im sorry and you can run whatever size you want, but facts are facts.

seriously dude calm down. you will learn more. the theories you guys spout are only about power production. so for a drag car they are 100% correct. they do not address spool up area which can be a significant part of a street cars powerband. I assure you if i ever build a 16g car for myself again it will have a 2.5 exhaust. If you havent actually felt the difference between the two you might want to open your eyes to what some are trying to get across.

 

[LandSpeed-DSM]

It's not a theory, it's not an opinion, it's not about "drag only"

It's a fundamental principle of how turbomachinery works. Pressure differential

The biggest you can get your delta P, the faster you can accelerate that wheel and the more shaft torque you can produce. Period

End of discussion.

 

[94awdcoupe]

Backpressure after the turbo is a bad thing thing.

A physical fact of how turbines work. Response, efficiency and overall power WILL increase with less restriction after the turbine. Period.

Your lag situation literally cannot have improved by adding restrictions downstream.

Backpressure only hurts.

This isn't a topic subject to opinion.

If you think you saw an increase in lag when going to an open downpipe and then it was remedied by adding a muffler you need to put the crack pipe down, figure out what issues you inadvertently fixed while adding the muffler and start taking better data.

Especially on a smaller turbine with higher expansion ratios for the same intake side pressure ratio. That is a physical impossibility.

Backpressure is multiplied by the expansion ratio and additive to the required drive pressure.

You are not some unique snowflake that is exempt from the laws of thermodynamics.

You don't even want backpressure on an NA car, it's only incidental to scavenging.. not the goal or intent.

How does this sh!t stay gospel to so many people?

classic response I knew was coming. the 16g with 2.5 inch exhaust defies physics. its more fun than the 3.0 exhaust

 

[LandSpeed-DSM]

God forbid someone knows what they're talking about interrupted the rice-tech shadetree circle jerk.