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!

 

[LandSpeed-DSM]

Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

o here, the turbine contributed 19.6 psig of backpressure to the total.

Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).

the above quote was straight from jay kavanaugh. so now he doesnt know what he is talking about?
I only posted the quote because it clearly shows aft turbo pressure effects pre turbo pressure. the part you cant wrap your head around is that raising pre turbine pressure is ever a good thing. and that is where you are wrong. all the other data you posted is nice. but that is all from very big race turbos that have no use for extra response.

Now look again at the graph i posted. This graph is from a BBK turbo. which is similar in size to a red turbo. intake manifold pressure is higher than exhaust manifold pressure all the way to 5000 rpm. there are not yet any reversion issues. if you were to install a smaller exhaust on this car and raise the entire exhaust manifold pressure curve by 10psi between 3500-5000rpm the turbo would be responding faster from those higher pressures, but still not adversely affecting HP since reversion is not yet at high levels.

You are just wrong to think a higher exhaust manifold pressure is always bad. that is what makes a turbo spool. when you post it really just looks like you have no experience with small turbos. and certainly no good back to back data from one.

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No, Jay is correct. You however have applied the quote incorrectly.

He is purely demonstrating how such a small restriction stacks up in the manifold.

You don't want high pressure in the exhaust manifold, you want to produce the least pressure necessary to accelerate the turbine, and from there the biggest drop across the wheel possible.

High pressure in the manifold hurts the engines ability to breathe. Forcing compromises like shorter I/E valve overlap, decreased spark advance, richer mixture, etc. all in the attempt to fight off knock/detonation.

It narrows your powerband.

It slows exhaust flow through the primaries because once again "high pressure = low velocity" when it comes to fluids. Which means: hurts spool and power as a result.

And if you're now interested in what the actual manufacturer's engineers have to say, then you should probably acknowledge that bit from earlier in his quote also.

You have these relationships entirely ass-backwards.

Drive pressure isn't something we'd have a shortage of in the turbine's 98% of this forum will ever be mounting on their cars.

The convenient thing about turbos is.. if you can spool it in the first place you'll have as much drive available as you will need and almost always an excess in practical application. Turning up the boost creams now air info the engine, which you need more fuel to burn, which increases mass flow to the exhaust manifold. That manifold had the same volume it did at lower boost, so in order to fit this increased mass through it, it will have to be at higher pressure.

It's basically a feedback loop, and it is to your advantage to match these upstream components for where you intend to operate. Downstream is a different story.

 

[Seamus]

Thanksy

Great reply. You reinforce almost everything i said whole revealing a more disturbing view into what makes landspeed speed. I really enjoyed the poetic description of your occupation following your subtle dig at mine. You could have been more direct and just saidyou are a cable guy, but that doesn't fit the legend you've built up in your mind. I'm sure it looks great on your resume right below your amazing automobile and ykur important position here at dsmtuners. I guess I was too vague on my occupation, I didn't feel the need to try and brag about what I do for a living. I guess it is important though on a website for such fine automobile like the DSM. Much like the Ferrari club, when we are not talking timing belts we are talking about the Robert graham trunk sale coming next month.

Seriously dude, get a f***ing clue.

But, I am never going to comment on your ridiculous posting habits again. I cannot stop laughing that tou actually did this. Today you have entered that special category of internet clown that is so pathetic it is beyond words. I don't know why you felt the need to post your eharmony.com profile, but that takes the cake. That is a most special brand of pathetic internet jockeys. I will say though that when most of the. Mouth breathers come up with their awesome internet description, they typically portray themselves as something more impressive than the caricature of an obese hillbilly that you use. I hope it was made up, but deep down I fear it wasn't. Regardless, consider me impressed, intimidated, and anything else that you intended in its posting! Now quit posting on dsmtuners and fix my cable!

 

[LandSpeed-DSM]

I am never going to comment on your ridiculous posting habits again.

I'll be sure to quote you on this the next time you're looking for a nut ride, probably before the week is out

I have nothing to do with CATV.

Currently working to expand a regional fiber build-out

 

[boost97gst]

Does anyone notice the bottom of the page?

Here ill quote the part that matters: " Dont spread misinformation or hearsay."

People, albeit the few who argue for the smaller exhaust=better spool(wtf), there ARE rules to this forum.
Attacking LSDSM or others on a personal level for telling the truth is borderline...... Its not ok. What is it? If you cant win an argument just sling mud?

Im debasing myself posting in here at this point. This thread needs to die

OP: Everyone got way off track but basically, get a 3" TB and call it a day. Its ok not to run a side exit, you dont need it for the gains you would see. Thats for big boy snails or people looking for the best spool/performance possible and since the 16 is so tiny you'll be just fine with a 2.5" flaring to 3" at the flex. Thats what most people run even though they call it a "3 inch Turbo back exhaust".
Everyone will agree with what i just suggested. If you want the very best performance then dump the shortest exhaust you can possibly fit. Fact. Thats how turbos work, on a motorcycle, semi truck, lawn mower(? ), and yes, even the 4g63 as hard as that may be for some to grapple with. So flame away, I couldnt care less. Pick on me all ya want for agreeing with the factual statement.

 

[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 also had similar experience working on a car. with just 14b. I needed to drive the car but there was no exhaust. I drove it with just 02 housing. I then came back and installed a stock downpipe. I was afraid I might catch something on fire. With the downpipe installed it made far more torque.

 

[GRAVEDIGGER]

And round and round we go...

 

[TurboLaserRS]

I also had similar experience working on a car. with just 14b. I needed to drive the car but there was no exhaust. I drove it with just 02 housing. I then came back and installed a stock downpipe. I was afraid I might catch something on fire. With the downpipe installed it made far more torque.

Did you dyno the car to see if it made less torque? Because the 14B was basically insta-spool when I ran my car without exhaust on it, and I'm pretty damned sure that added up to more torque production and not less. Maybe your car lost torque because you were dumping from the o2 housing, and the 02 sensor was getting funky readings and causing your fuel trims to go fubar. I had about 12 inches of downpipe left on my car when I ran it this way..

And round and round we go...

Rofl. I'm pretty sure Landspeed has offered up the most technical and accurate explanation when it comes to exhaust size. If people want to argue with mathematics then screw it - let them run their 2.5 inch exhaust, hell put a 2.25 inch on there instead.. it might be even better.

 

[RamenPride]

hell put a 2.25 inch on there instead.. it might be even better.

My next mod will be removing the 3" exhaust and putting the stock one back on for more power

 

[Black95TSIawd]

I think most of you are getting confused with peak power on a dyno graph and area under the curve and throttle response. I agree, a car with an exhaust will produce more backpressure than one without. It will also make LESS torque and horsepower. However, that first few percents on the TPS and the threshold between vacuum and boost will feel better/smoother and produce a better powerband.

 

[lil corb]

I think most of you are getting confused with peak power on a dyno graph and area under the curve and throttle response. I agree, a car with an exhaust will produce more backpressure than one without. It will also make LESS torque and horsepower. However, that first few percents on the TPS and the threshold between vacuum and boost will feel better/smoother and produce a better powerband.

This post could of ended the thread days ago. Although all the technical answers are correct sometimes a KISS method is all we need.

 

[YZFR1]

Installed a full 3" exhaust on my brother's 2012 Jetta GLE over the weekend. He drove the car for a whole day or about 100 miles. Than he calls me up to say that he loves his new exhaust, the only thing he said that the car now feels a little less torquey. He was never aware of this thread or the pissing competition.

 

[jakelandry]

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"'
QUOTE]

Also to recap: the discussion went from the OPs 16G to exhaust systems in general when folks kept banging the "backpressure is gud!I hurr durr" drum and making statements like "X inch pipe is good for 'Y' horsepower"

There is actually validity to both sides of the argument. Increasing the drive pressure is beneficial for certain applications. Decreasing the exit pressure is beneficial for MORE applications. What becomes the point of diminishing returns though? The vast majority of thermodynamic formulas do not take into consideration EVERY variable. If they did, anyone with access to windows 98 and excel could design the perfect motor with the help of a 15 dollar thermo book from 2001.

 

[mercaholik]

It may have something to do with off boost/ transient response? But I have read in several turbo books like maximum boost, that a short diffuser cone that tapers out at a 10-15° angle is the optimal turbo exhaust. In fact full race incorporated one into their custom 4" exhaust on a turbo civic making like 700+ hp.

 

[donniekak]

I think most of you are getting confused with peak power on a dyno graph and area under the curve and throttle response. I agree, a car with an exhaust will produce more backpressure than one without. It will also make LESS torque and horsepower. However, that first few percents on the TPS and the threshold between vacuum and boost will feel better/smoother and produce a better powerband.

A faster spooling turbo, due to less exhaust restriction produces more torque, sooner.

This is what happens. A stock exhaust car may make 200ft lbs at 3000 Rpms, and tapers off. A modified exhaust car, that is tuned may make 210 ft/lbs at 3000 Rpms, but quickly ramps up afterward, making more peak power. Which car feels torquier, the one with the smooth torque at low Rpms, that falls off, or the one that hits real hard later in the rpm band, while making more actual torque down low? It's all about perception. The dyno doesn't lie. Look at the chart I posted with an exhaust cutout in the down pipe, with the cutout open, it makes more power everywhere.

With a stock exhaust, stock intake, e85 and a 16g I trapped 107. Going to a full 3" and an intake, I trapped almost 120. I also gained about 3-400 Rpms of spool.

 

[ForcePower]

To the OP....

We don't need five pages of so called "experts" getting overly technical to determine what has been tried and proved over and over again.

It's simple BIGGER is better, when it comes to the exhaust.

Will you reach your goal on a 2.5" exhaust (350 - 400) perhaps. But it's going to take much more than an exhaust to get a 16G to this power level reliably.

Stock to 300 hp, 2.5" is fine.
300 - 550 hp, 3" is fine.
300 - 800 hp, 4" is best.
800 + go side exit and big.
* other things to keep in mind, minimize bends and remove your cat when racing.

To the people that say they lose power or response when going with a larger exhaust. "Get your car returned", you can't just remove back pressure / improve flow and think all is good to go.

I've never put much stock in "dyno numbers", they mean little to nothing. It's not how much power you make, it's how well you use the power you have. I have customers with 350 hp cars that can run down 500 hp cars without breaking a sweat.

Also, if you have your car dyno tuned, make sure it is finished or refined for your application, "street or track". A dyno tune will not maximize your cars performance in real time conditions.

Example: We now have a Top Mount EVO III 16G with a 4" exhaust on our car for Time Attack and the response and power is amazing, making approx 450 whp at 27 psi and running 11 seconds flat on the same tune at the drag strip. Tuning is everything at the end of the day.

 

[LandSpeed-DSM]

"Bigger is better" is what I said in post #8 of this sh!t show.

It's not the "experts" you should be taking issue with.. None of which have called ourselves that, by the way.

But at least it seems we're on the same page here. It's not a debate, fact is: a less restrictive exhaust will allow for quicker spool/response, the sooner you're making positive pressure the more torque you make.

It will also improve top end performance, so again to the "confused" the bigger exhaust is better under all conditions here.

If we're going to talk about area under the curve, by the way, that's (basic) calculus. Literally the first derivative of the function, be it power or torque that we are taking the area from.

Sooo, again it all comes back to that 'faggy science' so many of you clowns seem to want to sh!t all over.

 

[keltalon]

Interesting reading!

Threads like this are a god send when you are waiting in-line somewhere or just plain bored.

Also, i don't understand why it's so important to try and change someone's mind. Just share your experiences and/or opinions and leave it at that.

With that said, i'm all for math and science but about 99% of scientific "theories" don't hold up in the real world because of other factors. Just because it's in the science article you read on Google doesn't mean that's how it's going to be in real life.

To the OP, 2.5" exhaust is plenty for 350-400 hp on a max effort 16g. You might benefit some with a bigger exhaust but it will certainly not prevent you from making power.

Read the whole thread and dude you have my vote on best answer! I been preaching this from day one!

 

[mercaholik]

That is true though, you see it all the time. A car that is less powerful and not a whole lot lighter, beats the more powerful "faster on paper" car. So many factors are involved that sometimes things are counter intuitive. I'm just trying to keep the peace and keep an open mind. Everything I've read about turbo cars points towards the more free flowing the better.

 

[keltalon]

To the OP....


I've never put much stock in "dyno numbers", they mean little to nothing. It's not how much power you make, it's how well you use the power you have. I have customers with 350 hp cars that can run down 500 hp cars without breaking a sweat.

You wouldn't happen to have any videos of this happening would you, I would love to see! thanks

 

[NHerron]

No I did not participate in this thread, I'm sorry but I read this entire thing I feel like I can leave a couple notes.
I forgot about the OP after page one. Then someone pulled the classic *to the OP* "sumup paragraph" on page friggin 5 and I remembered where I was. Like the OP is reading through all the pages waiting for someone to give him that green light and his head shrinks back to normal. If not for the act of valor, his head would surely blow by page 6.
No offense to anyone.

Holy crap seamus. Golf clap to the wordplay on #106. I understand your angle but damn, just damn.

94awdcoupe you are one patient person. I have to reread this thread a few more times since I have no knowledge on some of the things you guys mentioned. Point is usually the guy everyone shits on (sorry, unfortunately its how a forum works sometimes, see:bandwagon) is starting to loose it and on the verge on being banned. I don't mean to upset you or anything hopefully that makes sense, it's a compliment of sorts, you know, cool calm collected, etc.

One more thing. You guys who have thrown around the 'stupid', 'idiot' and otherwise emotionally provoked names in this thread. No one's dumb in this thread. Everyone here is like a genius compared to the common 'mechanically inclined' person you meet everday. I haven't met many smart people outside this forum. But seriously, banging out a post with formulas to calculate blah blah blah and then calling someone an idiot? For god sakes auto zone jagoffs would freak out and scream witch if they heard the intelligence they wouldn't understand in here. It's great your mechanically smart but c'mon have some respect to a respectable argument and don't act like a jerk.

 

[LandSpeed-DSM]

"about 99% of scientific "theories" don't hold up in the real world because of other factors"

Simply put - this statement could not be father from the truth.

It's a pretty appalling reminder of just how poor our education system, even at the primary levels, has gotten for so many to have come to such a conclusion.

Because an article linked through Google by what passes for a "science correspondent" is the same as the peer-scrutinized and repeatedly field tested post-doc research report that it was skimmed from, right?

No wonder the rest of the Western world and developing countries are crushing us in terms of grads in the STEM fields.

If any one of you have veritable evidence of an actual Theory not holding up as described, you stand to become a legitimate historical figure and fairly rich.

Surely I'll be seeing you all on the next list of Nobel Laureates!

 

[keltalon]

land speed , Admit it the YZFR1 is 100% correct. And I must add the problem is not the the lack of education its the arrogance of those who think they are educated and this arrogance makes it hard for them to accept the failure of their so called scientific methods. Landspeed whenever you start throwing out all these big words and mathematical formulas the first thing the common dsmer looks at is your profile. They reason the way he talk he should be up there with shepherd, twick, boostin performance, ect. but your are not up there with them for this reason "about 99% of scientific "theories" don't hold up in the real world because of other factors". in fact you are not even above some of the average joe blows on tuners and this must be very frustrating to you, so you attack the education system. most of us here are happy to be here and are very satisfied with our results , but you my friend roam the forum looking for an opportunity not to help but to attack I don't know why you are even here you are only creating self inflicted frustration. 99% of your post are demeaning and negative, your mental state must be at its limits all the time you are here. You did some scientific math on trying to predict my dyno here recently but you were way off, why is that I will answer ""about 99% of scientific "theories" don't hold up in the real world because of other factors" plain and simple. 728whp@6500rpm I achieved these numbers not with calculated science but with the common man's approach, real world logic and reason based on years of personal experience as this is the approach of most of us here, this is what you have a hard time with, when any common dsmer achieve similar results without all the science and big words. Oh by the way I did with a 3" ebay cat back exhaust that has been on the car for 3 years now

 

[LandSpeed-DSM]

Hypotheses failing, that's not uncommon.

When Theories fail however, they usher in new eras.

How about you go find and link the exct quote of this "scientific math" I did for you.

You could have saved yourself at least 3 aborted trips to the dyno had you just done the math in the first place instead of continually running out of fuel. That you've gotten this far at all is seemingly in spite of yourself, and is largely thanks to the 20+ years of development in this community that let you start on third base thinking you hit a triple.

Neither one of us is a pioneer blazing a new trail here.

"Members of the general public (along with people with an ideological axe to grind) hear the word "theory" and equate it with "idea" or "supposition." We know better.

Scientific theories are entire systems of testable ideas which are potentially refutable either by the evidence at hand or an experiment that somebody could perform. The best theories (in which I include special relativity, quantum mechanics, and evolution) have withstood a hundred years or more of challenges, either from people who want to prove themselves smarter than Einstein, or from people who don't like metaphysical challenges to their world view.

Finally, theories are malleable, but not infinitely so. Theories can be found to be incomplete or wrong in some particular detail without the entire edifice being torn down. Evolution has, itself, adapted a lot over the years, but not so much that it wouldn't still be recognize it.

The problem with the phrase "just a theory," is that it implies a real scientific theory is a small thing, and it isn't." - Astrophysicist Dave Goldberg

 

[keltalon]

The new era that should be ushered in is landspeed should see scientific failure and respect other's successes!

 

[athlete3344]

The whole problem here is understanding pressure in hydraulic systems. Pressure loss is very easy to calculate and becomes very simple to understand once you work with it enough.

Chemical plants deal with pressure every day from steam systems to distillation columns to condensate systems. For utility systems, understanding pressure and accounting for the correct losses results in either a work/fail situation. Either you get the pressure drops right, or you dont. If using a turbo with a smaller exhaust and a lower pressure drop resulted in more power, every power plant and sulfuric acid plant would use that system. They dont. Most have a few giant turbo generators with enormous pressure drops and large piping.

Pressure drop across a turbo is no different, it operates at a rate based on an inlet and outlet temperature and pressure, and a work efficiency. The turbo will have heat losses, since nothing is perfect, but the losses will be so insignificant it will not affect the overall result by much. As long as youre close with the efficiency, you will be in the ballpark for power.

Whether youre someone who goes by working designs, or checks systems by calculation, the results will be the same. Problems can be diagnosed by 20+ years of experience or by calculation. It just depends on the type of person you are along with your background.

Everything related to these cars can be checked by experience or calculation. The benefit of understanding thermodynamics and math is that they can get you in a ballpark for sizing injectors, turbos, exhausts, etc. Attitudes aside, we are just trying to spread useful information.