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

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Boostin18

15+ Year Contributor
53
1
May 5, 2004
Chicago, Illinois
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!
 
I thought you were joking for a sec 94awd. SMH. This is not a fight worth fighting. I wouldnt argue with someone who doesnt believe in gravity so i can not argue with you. Do whatever you guys want with your car, its not mine.
 
The red pill...or the blue pill?:argue:

Humor break! Its too early for all this negativity.
 
Turbo generators in sulfuric acid plants use superheated steam at 600 psi and make power from it. The discharge of the turbo is atmospheric. Any increase in pressure decreases power production from the turbo.

The turbos in our cars are no different, but instead of high pressure steam we use air.
 
Correct^

This applies to all turbines.. any downstream increase in pressure (restriction or "backpressure") between the turbine exducer and atmosphere or any subsequent turbine stages is going to hurt performance.

There's no circumstances where backpressure is beneficial to turbine operation.
 
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just got off the phone with mike. very knowledgeable guy. I told him ten years ago I was running a 16g with a 2.5 inch exhaust. On upgrade day when I installed a 3.0 exhaust I was very disappointed in the low end loss from the freer flowing exhaust. for three weeks i retuned retweeked retuned but to no avail. I couldnt get the torque loss back. I am 100% certain it was from the exhaust change. and then i told mike i was arguing this point on the internet with some who say the torque loss is impossible from such a change. he said and i quote. "well he doesnt know what he is talking about. the smaller exhaust creates back pressure before turbine which spools the turbo faster"
 
^^^ LandSpeed, remember, you can't fix stupid!

LandSpeed is 100% correct in everything he's said thus far.... I don't understand why you guys are arguing with the laws of physics. Landspeed is not giving you opinions, he's giving you mother effin FACTS!
 
Ok, lets put some math to this instead of relying on opinions of those who really have no clue what they are talking about. Odds are, the decrease in low end power was due to the decrease in velocity in the larger exhaust.

I ran a few numbers in a program called AFT Fathom - we use this program at my firm to size hydraulic systems.

I used 40 lb/min at atmospheric pressure as a flow from a 16g (close enough for this exercise), and used both 2.5" and 3" stainles steel lines (ID of 2.635" and 3.26" respectively). The velocities for each size are:

2.5" @ 40 lb/min = 270 ft/s
3" @ 40 lb/min = 176 ft/s

The pressure drops in the pipe are below:

2.5" @ 40 lb/min = ~ 0.7 psi
3" @ 40 lb/min = ~ 1.9 psi

The drop in velocity from the larger pipe may be the effect in increase in lag because of the low flow. However, once the turbo is spooling, the amount of air being pushed is now under pressure, and moving much faster. At this moment, spooling up the turbo is not the intent - the intent is to now make power.


Now, let me direct your attention to steam turbines (which is applicable in our discussion)
http://en.wikipedia.org/wiki/Steam_turbine#Principle_of_operation_and_design

And scroll down to the section titled: Thermodynamics of steam turbines

Here you will see an equation:

W/m = h3 - h4
W = work
m = mass flow
h3 = inlet enthalpy
h4 = outlet enthalpy

W is what we are solving for, since work is power.
mass flow is held constant @ 40 lb/min
h3 varies depending on the pressure inlet of the turbo
h4 is based on the outlet pressure of the turbo

Next to this equation is this nifty graph:
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Now, lets say these to horizontal red lines were very close, meaning the delta h (h3-h4) is very small. Well, work would be equal to massflow * a small number. As the red lines grow farther apart, so does the delta h, or difference in enthalpy. Since the equation remains the same, work is equal to mass flow times a larger number.

The only two ways to make more power from the turbo is to increase the pressure you run at, and/or decrease the outlet pressure of the turbo. There are no arguments here. These are governing laws of thermodynamics which can not be broken. They exist in every system on earth, and are the principles of any engineering firm or industrial plant. Our cars are no different. And a guy isnt going to disprove this with a faulty opinion.
 
"the smaller exhaust creates back pressure before turbine which spools the turbo faster"

So you believe that by making it harder for the turbine wheel to spin, you're increasing spool?

bahahahahahaha
 
First off I am no expert, just generically interested on the subject.

I think the problem here is actually the word 'backpressure' not being used correctly. More specifically I believe that the lower spooling RPMs seen by 94AWDCoupe (that I have personally no reason to doubt as most of the DSMers that have lived in the Tampa area) are due to the higher exhaust gas velocity created by a smaller diameter pipe, which is the case at lower RPMs until the smaller diameter effectively become a restriction thus creating backpressure. The higher velocity would in fact help reduce backpressure at low RPMs rather than rise it (think of the narrower 2g intake manifold runners as opposed to 1G larger ones, if I can mention that without starting another flaming war).

TSIMonsterR, it's not about arguing with the laws of physics as much as remember that they ALL apply at the same time, not just the ones that you remember...

Anyway, I am not saying that I am right but just thought I would share my thoughts on the matter.
 
No^ even at low rpm the nominal pressure change is more important.

Per Jay Kavanaugh, Garrett Turbocharger Development Engineer:

'"Downstream of the turbine, you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine

Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.

The "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. [Approaching] 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side"'

Just for fun, because I know net to none of you measure drive pressure, or even think about it.. on a 14B, even in a 7cm housing, at 14psi boost (2:1PR) you're very likely dealing with drive head requirements upwards of 45psi in the exhaust manifold. This is an awful Drive:Boost (by modern standards) of around 2.5-3:1. But the turbine's Expansion Ratio is between Drive and atmosphere, 45psi Drive is an ER of 4:1.

If you have 3psi backpressure from your 2.5" pipe and catback, that 3psi is multiplied by the ER (4:1) becoming ADDED to the required drive of ~38-45psi for another 12psi.. so now 50+ psi is fighting every cylinder trying to exhaust is burned charge, polluting the new charge during the overlap period and pushing back into the intake ports in extreme cases.

Significantly less power potential, richer mixture, higher EGTs, lost margin of error in the tune, poorer response and recovery on shifts, etc.

High pressure = low velocity in fluids, so the one place where exhaust velocity is important (upstream) you are compounding all these problems with any downstream restriction by being a stubborn, ignorant f"ck.
 
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^^^ LandSpeed, remember, you can't fix stupid!

LandSpeed is 100% correct in everything he's said thus far.... I don't understand why you guys are arguing with the laws of physics. Landspeed is not giving you opinions, he's giving you mother effin FACTS!

no he is not correct. and that makes both of you stupid. stupid being someone with a lack of knowledge.

back pressure spools turbos. without it you wont make any power. if you didnt need back pressure you could just run a huge turbo and suffer no ill effects. and we all know that not to be true.

I have never said the 3.0 didnt make more power, it did. it just lost a bunch of torque up to about 4000rpm. and for a street car its not always fun to loose power below 4000 rpm no matter what the gains above 4000 might be. everything I observed switching exhausts was 100% fact. there where no theories spouted. just first hand experience thats not going against science or physics. those who thing it is just dont have the whole picture.
 
No, Ignorance is not knowing. Stupidity is knowing the facts/being presented with them and simply refusing to believe them........ I'll leave you to yourself. Also stupidity denotes a lack of intelligence.
Know the definition of the words you're using in the future
 
no he is not correct. and that makes both of you stupid. stupid being someone with a lack of knowledge.

back pressure spools turbos. without it you wont make any power. if you didnt need back pressure you could just run a huge turbo and suffer no ill effects. and we all know that not to be true.

I have never said the 3.0 didnt make more power, it did. it just lost a bunch of torque up to about 4000rpm. and for a street car its not always fun to loose power below 4000 rpm no matter what the gains above 4000 might be. everything I observed switching exhausts was 100% fact. there where no theories spouted. just first hand experience thats not going against science or physics. those who thing it is just dont have the whole picture.

Did you read either of the links I posted which explains how turbos work, or even think about the math of how turbos create power? To reiterate what I said about your notice in loss of low end power - it was probably related to velocities and negligible pressure loss at low rpms and low boost. In your mid/high range, you are hurting yourself with the smaller exhaust.

If you are claiming losses upwards of 4k rpms with a 16g, Im thinking you have other problems such as boost leaks or maybe a bad tune. Especially since landspeed is hitting 30psi at 4700 rpms on a much larger turbo.
 
Both of these guys had issues with their setup.

On my S16G I switched directly to the custom 3" Dump O2 housing and open downpipe leftover from my last Laser after pulling the 2.5" that came on this chassis.

On that Laser, I switched from the stock exhaust to that full 3" system w/ O2 dump when it was originally fabbed up and the overall improvement in response and 3.x lb/min increase at the same boost was a dramatic illustration of the effect.
 
Did you read either of the links I posted which explains how turbos work, or even think about the math of how turbos create power? To reiterate what I said about your notice in loss of low end power - it was probably related to velocities and negligible pressure loss at low rpms and low boost. In your mid/high range, you are hurting yourself with the smaller exhaust.

If you are claiming losses upwards of 4k rpms with a 16g, Im thinking you have other problems such as boost leaks or maybe a bad tune. Especially since landspeed is hitting 30psi at 4700 rpms on a much larger turbo.

no I am not going to waste my time. and the numbers you entered are pretty far off in your example. the 16g was making around 265whp. no where near 40lbs per minute. maybe 10lb per minute on the exhaust side. but I dont care to try to correct you. you are dense and have all the answers.
 
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no he is not correct. and that makes both of you stupid. stupid being someone with a lack of knowledge.

back pressure spools turbos. without it you wont make any power. if you didnt need back pressure you could just run a huge turbo and suffer no ill effects. and we all know that not to be true.

I think you don't understand your own terminology. You've now switched to arguing before the turbine, in which case, backpressue is not what "spools" turbos. Its the volume, heat, and velocity of exhaust being produced by the motor. Anyway, I'll let the guys that know way more than me continue the conversation, but on the whole, I think you're confusing a few things.

For some reason, it won't let me edit my earlier post. I was half kidding when I called you all stupid. So, you'll just have to deal with that, LOL
 
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no I am not going to waste my time. and the numbers you entered are pretty far off in your example. the 16g was making around 265whp. no where near 40lbs per minute. maybe 10lb per minute on the exhaust side. but I dont care to try to correct you. you are dense and have all the answers.

Its more of a waste of time writing a response filled with fluff, instead of admitting you cant argue against basic laws of thermodynamics. Your post basically said, Im not going to educate myself, but Im going to try to pick apart a constant that in the end doesnt matter.

I pulled the numbers from this thread, http://www.dsmtuners.com/threads/16g-flow-rates.375030/

"38/40/42 in lbs/min for small/big/EvoIII respectively, IIRC."

I used 40 as a round number, but the mass flow rate really doesnt matter when you look at how turbos produce power. A larger mass flow will just be a larger increase in power. The term which you need to learn about is the difference in enthalpies and what an enthalpy is.

In reference to the post with the scary math - Inlet enthalpy (h3) is the enthalpy of air at the inlet pressure and temperature. Outlet enthalpy (h4) is the same thing but at outlet temperature and pressure. You missed this completely, and its very important. The difference in enthalpies multiplied by the massflow rate equals work (or power). Since mass flow is typically held constant (unless you swap turbos), the only way to push a turbo is by changing the inlet and outlet conditions. This is the reason for an intercooler (to cool the mixture, making it more dense which equals more flow), and to run a short run of pipe (to decrease the outlet pressure as much as possible).

I think that fact that you think you were pushing 10 lbs/min out of a 16g even further shows you have serious boost leaks, a terrible tune, or clearly you have no idea what you are talking about.

I work as a Process Engineer and have designed systems much larger than the ones we have in our cars.
 
Even a process engineer should be able to follow the scary math ;)
Just messing with you.

10lbs out of a 16g is just screaming bad things. Normally you will see that when the compressor o-ring fails. aka large boost leak. My 2 cents.

As an added noted:

If by back-pressure you're referring to the pressure between the cylinders and the turbo, yes, that should be high. But if by back-pressure you're referring to the pressure behind the turbo, it should be low.
 
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There are a few things i am certain of. I saw a torque loss with a larger freer flowing . thats not up for debate. I am sure this happen. It was not a tuning issue or a mechanical malfunction of some sort. I am also certain this is repeatable phenomenon. as indicated my my conversation with mike, he instantly got the it. "he doesnt know what he is talking about" pretty much sums it up. I am also very certain if you called any other turbo shop or garret engineer they would explain the exact same thing mike did. and i didnt even need mike assurance or anyone elses for that matter. smaller downpipes can create more low end torque on setups. its end of discussion for me. because I only see a bunch of stupid people arguing for no good reason. human nature I guess.
 
Keep the lower diameter. Is it hurting me... Nope. Just another turd on the road.

You can't teach people that dont want to learn. So Im done here.

Owning a 9sec car. I vote 3 inch with cutout. The op can do as he pleases.
 
Im not going to argue with you on the topic, its a gang bang at this point. What i would like to ask is the "torque loss" you experienced a butt-dyno thing or did you strap it up to a dyno to confirm it? A "feeling" is accurate to a degree but when on a dyno you can see exactly whats going on. I cant explain why you felt a low end loss in power but i would suspect(rather, im 100% sure) that a back to back dyno would have set things straight for you.
 
OMG Wow this escalated pretty quickly... I don't see any downsides to the op using the 2.5 inch he already has bolted on there. [:idontknow:it certainly wont cost him anything to try it out]

But that said, others have proven and stated a 3" exhaust is gonna produce more power [and I agree], but will add I have seen an instances where a 4G63T circuit track type car lost some usable response, in some areas of the power-band and the track running 3", but you can make up for this with fine tuning and careful parts selection.

If you open up the exhaust to as big as you dare, you really better have other parts to help manage this upgrade, don't get me wrong some people have gotten away with just exhaust, but to take full advantage of it you got to think of modifying a car as changing an equilibrium, if you... open the intake, exhaust,more fuel, better spark,all these things need to be fine tuned to perform/work with each other, the way you want them to.

a few ill working parts [or good missing ones] might just ruin the feel of the car.

I think It largely depends on what you plan on doing with the car and what type of track [street?] driving you intend to do with it and how good the tune is setup, along with all the parts.

I feel some good points were made in this thread overall.
 
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