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!

 

[keltalon]

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.

I agree athlete3344 and you are correct but land-speed don't want to accept no way but his. I have read the thread and like your explanations they are very easy to understand.

 

[athlete3344]

The problem I had within this thread (and I believe LS too), was the notion that back pressure and small exhaust = more power than less back pressure and large exhaust.

Its simply not true.

At that point is when this turned into a debate, which quickly turned into name calling.

 

[ceedawg]

The problem I had within this thread (and I believe LS too), was the notion that back pressure and small exhaust = more power than less back pressure and large exhaust.

Its simply not true.

At that point is when this turned into a debate, which quickly turned into name calling.

I think if you go too big or too small depending on application, turbos, turbine housing sizes,intake mani's etc it can have either a positive or a negative affect on torque and usable horsepower per rpm in general. Can we agree?
That has been my experience.


Now to go off topic a little bit.

In cases as posted on this forum there are individuals who have told other individuals how certain things could not be done or should not be done due to mathematical calculations , or scientific postulates and or theorems sometimes unknown variables only to show us all that these things can be naturally defied. Who thought the cyclone manifold could support 700 hp. Who thought small (bc 272)cams could bring a 2.0 L4 cylinder with with supporting mods to almost 800hp. Who thought with a single fuel pump and stock lines and a boost a pump over 700 whp could be achieved without blowing up the fuel pump.

I'm not trying to attack or offend here but a sensible response is welcome.


I have seen on some setups concerning turbine housing swaps too big resulted in loss of usable power where a smaller one made the turbo and response very happy. I would say depending on the turbo and exhaust hp can be effected positively or negatively as well.
There are always variables.

Sometimes bigger is better and sometimes it is not with different setups.

 

[livedsm4g63]

I want to heat things up because it seems as if the MODs dont care.

http://highboostforum.com/forum/showthread.php/24837-3-quot-vs-3-5-quot-exhaust-test-from-2007

Buschur approved. Granted it isnt 2.5''

 

[LandSpeed-DSM]

That's really not conclusive of anything with regard to the downpipe. 660awhp from an old 35r on a Mustang dyno.. compressor is pretty much tapped out at that point.

Both 3.5" runs did churn out more torque amusingly, all of which fell within the margin of error however.

 

[athlete3344]

I think if you go too big or too small depending on application, turbos, turbine housing sizes,intake mani's etc it can have either a positive or a negative affect on torque and usable horsepower per rpm in general. Can we agree?
That has been my experience.


Now to go off topic a little bit.

In cases as posted on this forum there are individuals who have told other individuals how certain things could not be done or should not be done due to mathematical calculations , or scientific postulates and or theorems sometimes unknown variables only to show us all that these things can be naturally defied. Who thought the cyclone manifold could support 700 hp. Who thought small (bc 272)cams could bring a 2.0 L4 cylinder with with supporting mods to almost 800hp. Who thought with a single fuel pump and stock lines and a boost a pump over 700 whp could be achieved without blowing up the fuel pump.

I'm not trying to attack or offend here but a sensible response is welcome.


I have seen on some setups concerning turbine housing swaps too big resulted in loss of usable power where a smaller one made the turbo and response very happy. I would say depending on the turbo and exhaust hp can be effected positively or negatively as well.
There are always variables.

Sometimes bigger is better and sometimes it is not with different setups.

Useable HP is somewhat vague. From what it sounds like, bigger is better only based on a persons expectations. As in, dont get a massive turbo and housing if youre expecting instant spool.

The buschur link was pretty interesting, especially at higher HP. For my own build, Id be curious to see if a cutout made a difference like another poster made note of with a graph.

 

[livedsm4g63]

That's really not conclusive of anything with regard to the downpipe. 660awhp from an old 35r on a Mustang dyno.. compressor is pretty much tapped out at that point.

Both 3.5" runs did churn out more torque amusingly, all of which fell within the margin of error however.

I agree, but I cant help throwing some more wood on the fire.

You already know I agree with you in regards to this thread.

 

[mercaholik]

I agree with this, but I am trying to account for some people's real world experience, which seem to defy the norm. I could say it is their perception, and it very well could be but I'm not gonna assume anything. As far as I know, the turbine wheel effectively cancels out the pulses of sound that can be used to scavenge the cylinders, so unless the smaller exhaust somehow sped up the flow of post turbine exhaust gases, it should not have lost any power anywhere. The results from buschur's test are interesting, I wonder about going to 4"? Either way going to big for your power level is better than too small, and if 3" will support 700+ hp, I think most of us should be just fine. Someone needs to do a back to back dyno of no exhaust vs 3". What would cause the loss of low end power from going to a bigger exhaust?

 

[keltalon]

[QUOTEhlete3344, post: 153444724, member: 118380"]The problem I had within this thread (and I believe LS too), was the notion that back pressure and small exhaust = more power than less back pressure and large exhaust.

Its simply not true.

At that point is when this turned into a debate, which quickly turned into name calling.[/QUOTE]
I saw thst

I agree with this, but I am trying to account for some people's real world experience, which seem to defy the norm. I could say it is their perception, and it very well could be but I'm not gonna assume anything. As far as I know, the turbine wheel effectively cancels out the pulses of sound that can be used to scavenge the cylinders, so unless the smaller exhaust somehow sped up the flow of post turbine exhaust gases, it should not have lost any power anywhere. The results from buschur's test are interesting, I wonder about going to 4"? Either way going to big for your power level is better than too small, and if 3" will support 700+ hp, I think most of us should be just fine. Someone needs to do a back to back dyno of no exhaust vs 3". What would cause the loss of low end power from going to a bigger exhaust?

I did a 3" turbo back run and then opened my cutout the car picked up 70whp and 50lbs of torque, 573 with cut out closed and 641 with cutout open. I can see if I can get an overlay from the dyno its going to take lots of digging because it been a while ago.

 

[lil corb]

I agree with this, but I am trying to account for some people's real world experience, which seem to defy the norm. I could say it is their perception, and it very well could be but I'm not gonna assume anything. As far as I know, the turbine wheel effectively cancels out the pulses of sound that can be used to scavenge the cylinders, so unless the smaller exhaust somehow sped up the flow of post turbine exhaust gases, it should not have lost any power anywhere. The results from buschur's test are interesting, I wonder about going to 4"? Either way going to big for your power level is better than too small, and if 3" will support 700+ hp, I think most of us should be just fine. Someone needs to do a back to back dyno of no exhaust vs 3". What would cause the loss of low end power from going to a bigger exhaust?

Probably the savaging effect pre spool up. There are so many different variables here with in different set ups.

Example: would you "feel" loss of torque in a late spooling t42 turbine or other big frame turbo by going 3.5" or would you think you are gaining torque by going with a 2.5" exhaust on same turbine? I bet one party would say they increases the torque with smaller exhaust even though dyno number wouldn't show that and the other party would say they loss torque even though dyno numbers prove they gained it. It's all perception and to some people peak numbers isn't worth the "loss of drive ability" in their eyes.

Do what's best for your goals and also be open minded and take bits and pieces from everyone's own experience.

 

[LandSpeed-DSM]

mercaholik^

If the compressor is maxed, you won't be able to increase power but you can alter spool/response (torque)

I did a 3" turbo back run and then opened my cutout the car picked up 70whp and 50lbs of torque, 573 with cut out closed and 641 with cutout open.

Gee, so what are we arguing about in here again?

Imagine that.. picked up 70whp/50wtq by reducing backpressure!

Sounds an awful lot like what I've been trying to explain this entire time.

 

[mercaholik]

Keltalon, did you have any changes or did you make any, a/f ratio, timing, after opening the cut out. Thanks for the info, that is still the greatest thing about this community is sharing of knowledge. It's why limits are being pushed higher and higher.

 

[livedsm4g63]

It's why limits are being pushed higher and higher.

More like tolerance limit..

 

[ceedawg]

We can also throw in the idea of efficiency! Bigger or smaller depending on setup can have a big effect on the efficiency of a particular setup which would also have an effect on torque and hp numbers per rpms or should I say area under the curve.

Craziest turbo I have ever seen that has made obscene power with a tiny little exhaust housing is an HX40 with a tiny little Mitsubishi style or Garrett style housing. How come this turbo makes so much power with this little turbine housing an such a big exhaust wheel. Too me made no sense should be choking the turbo!! Don't know but it is doing it. Is it efficient I have heard folks say yes and other say no due to the amount of back pressure the housing creates. Would a .70ar housing make it run more efficiently I would say yes however with the .55ar housing this turbo is putting out the same hp and gobs of torque just at different rpms . Cams, intake mani's, exhaust pipes or exh runner diameters all will have different effects. In some cases bigger has worked at better and in some cases not. And we can say this can be applied to exhaust pipe size.

Thus is all real world ish.

 

[mercaholik]

Lol, I guess so but I think everyone is on the same page now. No matter what the proof is in the pudding, and real world results matter. I think what we can take from this is that going too big on your exhaust is better than too small. So to the o.p., I would say stick with the 3". At the least you have some head room.

 

[keltalon]

was never arguing about the subject in discussion I was arguing your tackless demeaning

Approach! I consider you a leader therfore its pointless to demean people.

 

[ceedawg]

maybe like e hem (clearing throat) ,someone mentioned metaphysics since they threw that in there!!! Lol But real live results cannot be denied.

 

[ceedawg]

mercaholik^

If the compressor is maxed, you won't be able to increase power but you can alter spool/response (torque)



Gee, so what are we arguing about in here again?

Imagine that.. picked up 70whp/50wtq by reducing backpressure!

Sounds an awful lot like what I've been trying to explain this entire time.

Based on Kels setup or in every case?

 

[keltalon]

I stopped by the dyno lab today and it took us a minute to find what we were looking for but we found it and here it is! I was wrong about the numbers but I at least got the 573 right. Here is the dyno sheet blue lines cut out open red lines cut out closed. Dyno pulls was done 8 minutes apart. This was my dyno dated 9/2013

You must be logged in to view this image or video.

 

[mercaholik]

Nice, that's a significant enough jump in power that it had to be due to the cut out. Awesome numbers too, thanks for posting them and they agree with what we were saying. I'm not as seasoned as you guys, some of my knowledge comes from tuning books and U.T.I., so I appreciate the info. If I or anyone else that would be happy with 500+ hp can get the same parts and know they will work together when tuned.

 

[keltalon]

Nice, that's a significant enough jump in power that it had to be due to the cut out. Awesome numbers too, thanks for posting them and they agree with what we were saying. I'm not as seasoned as you guys, some of my knowledge comes from tuning books and U.T.I., so I appreciate the info. If I or anyone else that would be happy with 500+ hp can get the same parts and know they will work together when tuned.

No problem I try to help when I can

 

[ceedawg]

I stopped by the dyno lab today and it took us a minute to find what we were looking for but we found it and here it is! I was wrong about the numbers but I at least got the 573 right. Here is the dyno sheet blue lines cut out open red lines cut out closed. Dyno pulls was done 8 minutes apart. This was my dyno dated 9/2013

You must be logged in to view this image or video.

Kels where exactly do you have the cut out located?

 

[keltalon]

Kels where exactly do you have the cut out located?

just right before the cat.

 

[LandSpeed-DSM]

was never arguing about the subject in discussion I was arguing your tackless demeaning

Approach! I consider you a leader therfore its pointless to demean people.

I definitely don't hold myself to be a leader here, but I appreciate the vote of confidence.

Either way, I'm not going to sit here and take shots without returning them. Check out my first contributions on page one, before we went off the deep end thanks to our good buddy Seamus.

I can be blunt, and not shy of confrontation when it occurs which in person nearly everyone in my personal and professional circles appreciates. This doesn't always translate well through text however.. not much of a PR guy.

My primary focus in the tech section is technical accuracy. I make an effort to explicitly qualify what's my opinion vs. what's objective with the language I use.

The mailman's attitude doesn't change the message he's delivering anyways.

 

[keltalon]

Okay now we are at the heart of the problem we see you as a leader but you don't consider yourself a leader! So you definitely have a modest side to yourself and that's good! Whether you want to accept it or not you are a leader here on tuners because people listen to you whether wrong or right its up to you to use that influence for the benefit of the forum or not! All I am saying when you disagree with someone don't try and make them feel less than human with big words and math but instead pm Them and reason with them your point then come back to the open forum you and them a united front. If you do this these forums will be awesome because I think you can be a powerful leader on tuners if you get you act together. I know we disagree most of the times but a can't imagine the boards without a land speed dsm there to give the scientific explanation of the matter! With the benefits of the forum in mind that is!