Bellmouth vs Divorced Flow Turbo Elbow?

[NoFlex]

Wondering which O2 housing will get better results on an sr20 with a BB t28 turbo. Bellmouth or divorced flow?

[Mile High Silvia]

I run the bell mouth style, just seems like it would flow better.

I'm running s15 bb turbo with gtx28 billet compressor wheel from Pure.

[NoFlex]

Originally Posted by Mile High Silvia

I run the bell mouth style, just seems like it would flow better.

I'm running s15 bb turbo with gtx28 billet compressor wheel from Pure.

I would think so, but i see a lot of people saying dovorced flow is better

[fatduece]

I think you need divorced for internal waste gate and bell mouth for external waste gate. Hopefully somebody who has experience can chime in

[jedi03]

Yup...that is correct...I use bellmouth regardless tho due to strength of the welds and heat soaking

[Mile High Silvia]

I'd say go with the Bellmouth, because the size of the piping is much bigger and will allow better flow. Some divided designs are better than the pic you posted, that one looks like total crap in my opinion

[jr_ss]

Divorced is better. It keeps the wastegate flow from causing turbulence in the exhaust stream. In an ideal world, wastegate flow would be introduced farther down in the exhaust stream.

[5280VertDET]

Originally Posted by jr_ss

Divorced is better. It keeps the wastegate flow from causing turbulence in the exhaust stream. In an ideal world, wastegate flow would be introduced farther down in the exhaust stream.

The OEM S15 turbo is divorced, if that gives you any help. Since Nissan did this, I would assume that if you are using a internal WG you should keep it as well.

[Mile High Silvia]

I noticed better top end power with my "Bellmouth" style and better spool.

[NoFlex]

Originally Posted by 5280VertDET

The OEM S15 turbo is divorced, if that gives you any help. Since Nissan did this, I would assume that if you are using a internal WG you should keep it as well.

Yeah the s15 turbo has a little divorce plate in tge exhaust housing and yes im using the stock internal wg. Should i get a bellmouth since the turbo already has a divorce plate?

[PoorMans180SX]

Parts Shop MAX has the only proper divided wastegate donwpipe that I've seen.



You need to separate the flow for 10-12in to get a good result. But it WILL improve flow exiting the turbine. I'd say ideally the wastegate would never rejoin the rest of the exhaust.

If you want to buy one of those cheap, shitty, hack parts you posted, get the bell mouth one.

[fatduece]

Tomei makes a very nice bellmouth. Very cheap too!















I dont mean to hijack your thread op, but doesn anyone know if these outlets are okay to use with aftermarket exhaust manifolds? Just wondering if its gonna hit anything.

[yzrider450f]

Do we even bring this up?

[Croustibat]

Internally gated turbo = bellmouth. The divorced design you show is crap anyway, it will leak from each side and it joins too soon. Divorced design on that kind of internally gated turbo is idiotic, it really is an item created to steal money from people.

Use a divorced design when the turbo has a divorced design only (like externally gated turbos), and dump either as a screamer or way further in the exhaust.

[Chernobyl]

Originally Posted by fatduece

Tomei makes a very nice bellmouth. Very cheap too!

Probably not everyone's definition of "cheap", but I really like the Tomei unit I have. High quality part. Recommended.

[Mile High Silvia]

Originally Posted by NoFlex

Yeah the s15 turbo has a little divorce plate in tge exhaust housing and yes im using the stock internal wg. Should i get a bellmouth since the turbo already has a divorce plate?

Get the Bellmouth, look around and find one that has the biggest piping because turbos need room to breath!


The tomei unit pictured on the left is one of the best in my opinion! I have the same one on my coupe, with a Megan down pipe since it has the flex section. The greddy unit I had before was a Bitch to fit with the solid piping

[Croustibat]

Originally Posted by Mile High Silvia

find one that has the biggest piping because turbos need room to breath!

It is a T28, have you seen how small the wheels are ?

[PoorMans180SX]

Originally Posted by Croustibat

It is a T28, have you seen how small the wheels are ?

The best exhaust for a turbo (regardless of turbo size) is the shortest or biggest one. This has been proven like 5 billion times.

Smaller turbine = bigger restriction = even more of a reason to run the biggest exhaust you can.

[Mile High Silvia]

What he ^ said!

[Croustibat]

Originally Posted by PoorMans180SX

The best exhaust for a turbo (regardless of turbo size) is the shortest or biggest one. This has been proven like 5 billion times.

Smaller turbine = bigger restriction = even more of a reason to run the biggest exhaust you can.

hmmmm no. We have seen numerous iterations of "bigger is better". But proof ? You can't prove that with a demonstration. A demonstration can be admissible to show something is wrong.


The flange on that turbo has like a 1.6" exhaust wheel diameter. The wastegate flapper is like 1" in diameter. That is 2.8 square inches.

A 2.5" downpipe is 4.9 square inches. Please, explain to me (with numbers) how a 3, 4, or 5" is going to be better, because i am so downright stupid i don't get it.

[KAT-PWR]

Originally Posted by Croustibat

hmmmm no. We have seen numerous iterations of "bigger is better". But proof ? You can't prove that with a demonstration. A demonstration can be admissible to show something is wrong.


The flange on that turbo has like a 1.6" exhaust wheel diameter. The wastegate flapper is like 1" in diameter. That is 2.8 square inches.

A 2.5" downpipe is 4.9 square inches. Please, explain to me (with numbers) how a 3, 4, or 5" is going to be better, because i am so downright stupid i don't get it.

I am in for this info.
The only reasoning I see to support bigger is better is that less pressure behind the turbine and naturally a greater pressure in front of the turbine, the least amount of resistance behind the turbine wheel would lead to faster spool.
In King talon terms,
From the existence of expanding gasses due to the internal combustion process exhaust gas ideally maintains velocity with appropriate sized exhaust runners before the turbo. As the gas is violently trying to escape the exhaust ports it generates a lot of heat and pressure which must be controlled with the proper air fuel ratio and header pipe diameter. On a stock SR I prefer 12.5:1 under full boost of 12psi. There becomes a pressure differential between pre turbine (high pressure) and after the turbine (lower pressure). The pressure pre turbine must be greater than post turbine otherwise the turbine wheel would not spin and it must always be greater pre turbine. The greater the pressure post turbine the less effect the expanding exhaust gasses have in imparting energy on the turbine wheel. Theoretically as you would increase the exhaust diameter to infinity the pressure is decreased towards 14.7 (atmospheric pressure) . This means the pre turbine pressure would impart all of its energy (none being lost to back pressure) to the turbine wheel, spooling it faster

[Mile High Silvia]

There ya go crustybat! Some people's children.....

[dorkidori_s13]

That divorced outlet shown in the original photo is shit anyways just looking at how it is designed to dump exhaust gasses into the piping itself. The wastegate side enters at a hard angle and would divert wastegate pressure to where it actually creates MORE turbulence due to the outlet being pointed directly at the opposing wall of the piping (causing a direct flow interference).

The Parts Shop Max design keeps the exhaust gasses flowing evenly at the same angle and same direction (basically a proper 2:1 design).

But if you're worried about getting the most out of your T2 series turbo, you're better off upgrading to a smaller T3x series turbo... Pushing 300+whp out of a larger turbo is much easier than pushing it out of a T2x series turbo... Better CFM at lower boost pressure with less wear to the turbo yada yada yada

[PoorMans180SX]

Originally Posted by KAT-PWR

I am in for this info.
The only reasoning I see to support bigger is better is that less pressure behind the turbine and naturally a greater pressure in front of the turbine, the least amount of resistance behind the turbine wheel would lead to faster spool.

This is exactly the reason.

You can see this everywhere on turbo cars. Stock turbo evo's gain 30whp from externally dumping the wastegate, and 50whp on big turbo bolt on cars.

Take off your exhaust after the downpipe. The turbo will react faster.

Switch to a hood exit and it will be even faster.

The turbine wheel is always the restriction in the system, so any reduction in restriction after the turbine helps the turbo make boost faster and restricts the entire system less.

Now of course there are diminishing returns. A 3.5" exhaust on a gt2560r car probably adds more weight than it makes in more power. A bigger bellmouth can't hurt though, especially if you feel like porting the outlet.

[Kingtal0n]

Originally Posted by KAT-PWR

I am in for this info.
The only reasoning I see to support bigger is better is that less pressure behind the turbine and naturally a greater pressure in front of the turbine, the least amount of resistance behind the turbine wheel would lead to faster spool.
In King talon terms,
From the existence of expanding gasses due to the internal combustion process exhaust gas ideally maintains velocity with appropriate sized exhaust runners before the turbo. As the gas is violently trying to escape the exhaust ports it generates a lot of heat and pressure which must be controlled with the proper air fuel ratio and header pipe diameter. On a stock SR I prefer 12.5:1 under full boost of 12psi. There becomes a pressure differential between pre turbine (high pressure) and after the turbine (lower pressure). The pressure pre turbine must be greater than post turbine otherwise the turbine wheel would not spin and it must always be greater pre turbine. The greater the pressure post turbine the less effect the expanding exhaust gasses have in imparting energy on the turbine wheel. Theoretically as you would increase the exhaust diameter to infinity the pressure is decreased towards 14.7 (atmospheric pressure) . This means the pre turbine pressure would impart all of its energy (none being lost to back pressure) to the turbine wheel, spooling it faster

I didnt say any of that. I cant even read it.

There is more to it than just having an open dump after a turbine. If you remove the downpipe completely so the turbine wheel can directly "see" the atmosphere you will lose performance, despite that simulating the largest exhaust possible (none). There is some gain to be had with the coordination of exhaust gas flow as it exits the turbine. An elegant design would change shape (of both turbine wheel and exhaust system) as the turbine and engine requires more flow, but it would never "disappear"

Originally Posted by dorkidori_s13

But if you're worried about getting the most out of your T2 series turbo, you're better off upgrading to a smaller T3x series turbo... Pushing 300+whp out of a larger turbo is much easier than pushing it out of a T2x series turbo... Better CFM at lower boost pressure with less wear to the turbo yada yada yada

This is where you all need to be these days. It is 2016 guys, top mount manifolds are affordable and reliable. We all know brake boosters are the number one cause of 240sx engine bay fires. Buy a blanket and a heat shield, and Get With The Program.

[Croustibat]

Originally Posted by PoorMans180SX

This is exactly the reason.

You can see this everywhere on turbo cars. Stock turbo evo's gain 30whp from externally dumping the wastegate, and 50whp on big turbo bolt on cars.

Take off your exhaust after the downpipe. The turbo will react faster.

Switch to a hood exit and it will be even faster.

The turbine wheel is always the restriction in the system, so any reduction in restriction after the turbine helps the turbo make boost faster and restricts the entire system less.

Now of course there are diminishing returns. A 3.5" exhaust on a gt2560r car probably adds more weight than it makes in more power. A bigger bellmouth can't hurt though, especially if you feel like porting the outlet.

I am genuinely interested in understanding how anything that could flow more than what can flow through the turbo is going to be less restricting.

The elbow and downpipe have bends, so this limits the flow, but in what proportions ? I can't see it creating a restriction when the surface is already twice the turbo exhaust surface, but i may be wrong. I'd just like to understand why, not just "because you say so" or "because everyone else is doing it".

Why would anyone use anything bigger than a 2.5" elbow with a gt2560r? Isn't twice the area the turbo has already overkill ?

[KAT-PWR]

Originally Posted by Kingtal0n

I didnt say any of that. I cant even read it.

There is more to it than just having an open dump after a turbine. If you remove the downpipe completely so the turbine wheel can directly "see" the atmosphere you will lose performance, despite that simulating the largest exhaust possible (none). There is some gain to be had with the coordination of exhaust gas flow as it exits the turbine. An elegant design would change shape (of both turbine wheel and exhaust system) as the turbine and engine requires more flow, but it would never "disappear"




This is where you all need to be these days. It is 2016 guys, top mount manifolds are affordable and reliable. We all know brake boosters are the number one cause of 240sx engine bay fires. Buy a blanket and a heat shield, and Get With The Program.

It was satire talon.

[Mile High Silvia]

Originally Posted by Croustibat

Why would anyone use anything bigger than a 2.5" elbow with a gt2560r? Isn't twice the area the turbo has already overkill ?

You can't have too big of an exhaust with a turbo, especially with a big turbo. Bunch of vr6 guys run 4" systems to breath upwards of 700hp. Back in my vw days I ran 6766's with the largest t4 housing they had which was .94 and I still hit full boost at 4K. I switched my style tho and now I love
Fast spool and small turbo efficiency.

I run the s15bb t25 with pure billet wheel, tomei outlet, Megan downpipe(bcuz flex section), and apex'i N1 exhaust. I love my setup, it performs

[Croustibat]

Yeah, big turbos, losts of boost, i get it. gt2560r ? Nope. I run one, i also have a free flowing 3" exhaust, a shiney elbow and a 3" flex downpipe, but it just is overkill. And it takes room; i bet a 2.5" one would flow better as the bends could be smoother...

[Mile High Silvia]

Originally Posted by Croustibat

Yeah, big turbos, losts of boost, i get it. gt2560r ? Nope. I run one, i also have a free flowing 3" exhaust, a shiney elbow and a 3" flex downpipe, but it just is overkill. And it takes room; i bet a 2.5" one would flow better as the bends could be smoother...

My system is super smooth, not too many bends after the downpipe and there's no way 2.5" is better than 3". Just not even a question in my mind. I've owned a few turbo cars that I built myself and I can assure you that the larger piping is the way to go. I'm sure I can find some dyno videos on YouTube somewhere comparing down pipes or exhaust systems. I don't have any dyno comparisons myself but it's just a matter of efficiency. The easier a turbo can breath The better it will perform with less stress from back pressure