Different Sized Scrolls in Divided Turbos

[talonDSMerr]

I posted this question in another DSM forum but haven't gotten many replies so I figured maybe a site which sees more traffic such as this one would yield some more response, opinions, thoughts, etc.

There have been a couple questions on my mind that I just can't figure out. I noticed there are two different kinds of turbine housings used for turbos labelled as "twinscroll" or "divided." The first kind is more common, especially on aftermarket turbos. The 2 divided scrolls are the same size, as seen in the cross-section and outside of the casting:

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The second kind is where the 2 scrolls are differently sized, one scroll having a larger A/R than the other scroll. We can also see it on the outside of the casting on the stock evo8 turbo:

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And I'm just having the hardest time figuring out why and how the different sized scrolls work. Intuitively, the smaller scroll will help turbo response in low rpms while the larger scroll keeps top end from building up too much backpressure. But with a different A/R per scroll, there is going to be different backpressure between the paired 1-4 cylinders and 2-3 cylinders. Won't this wreak havoc on cylinder to cylinder air/fuel variations? How is this accounted for? Is it even accounted for? Obviously, it works on the evo8, is it just not enough to make a difference? Why do aftermarket companies decide to go with scrolls that are the same size?

Thanks guys.

 

[dsm-onster]

It WILL reak havoc on betweein cylinder VE variations, unless the cylinders are already at different VE amounts in the first place and the scroll sizes are offsetting it . . . . The evo8 turbo is very specific to the platform; or at least the turbine housing can be built to be, like any other turbine housing.


It makes a difference with a 4 banger with a small bay but not so much with a different platform where better even cylinder filling can be employed.

After market upgrading assumes that the rest of your setup is already optimized. That's the value in optimizing things as you go along. . . Better even flow between runners on the intake manifold should already be tackled when you're looking at big honkin' twinscroll manifolds. . .

 

[talonDSMerr]

That sounds like a design/tuning nightmare haha. I would agree with you that the evo8 turbo is application specific and takes advantage of cylinder to cylinder air/fuel variations, but the last picture posted above is of BorgWarner's new EFR turbo. Its turbine housing cross-section shows two volutes of different A/R. I looked up a picture of a real-life EFR turbo, and sure enough, it's visible on the outside of the casting as well. Using this turbo on an engine with supporting mods, such as an intake manifold that allows equal cylinder filling and an equal length exhaust manifold, would cause significant air/fuel variations between cylinders?

My head is about to explode...

 

[dsm-onster]

Yes it would cause significant cylinder VE variations, and even at different rpms. . . You are right to make it even fill. It's obvious.

. . .But a monky wrench. I havn't researched this, but the flow through the blades from the volute, how is it different from one scroll to the next especially since the pulses are divided well. . . That COULD be an issue. I have neither the time nor the resources to experiment. . .

 

[tkelly27]

I think it is just the angle you're looking at it. It looks like they twist them for packaging purposes and that they are equal in size. Just because the one is further out on the casting doesn't mean it is bigger, the other one is at an angle so it won't come out as far.

They sell equal length manifolds that pick up power on the evo. The fuel/air distribution issues should be between the cylinder that is closest to the throttle body and the one that is furthest away, those (1&4) are paired into one scroll. To me, the signs point to them being equal in size.

The only way you'd know for sure is to chop one up and measure it. I think you'd find a small variation, but nothing you wouldn't also find on a straight TS turbine.

 

[dsm-onster]

Makes sense too.

Bottem line is that you just need to know the concept and purpose, then you can make things work for you the way you want them to. No need to look at it as black magic or market hype.

 

[talonDSMerr]

They do look like they might be the same size, but while the cross-sectional area might be the same, one of the volutes is closer - overall - to the center of the turbine. Is this not the "R" in "A/R"? Thus we would have 2 different A/R right? The volute who's center of cross-sectional area is closer to the turbine's center would make the airflow velocity faster than the other volute, even if they are the same size.

I don't question that it works or not, and definitely do not see it as market hype. I was just trying to understand how it works and what difference each of these styles have on performance.

I guess if anything, this is food for thought? I know I've been pondering this for awhile and even tried contacting somebody at BorgWarner about the design, but haven't gotten through.

EDIT: If the volute closer to the center of the turbine has a slightly smaller cross-sectional area, flow rate would be the same between the two volutes.

Since velocity increases as you get closer to the center of the turbine, a decrease in the cross-sectional area would help keep flowrate the same.

Equation: Q (flowrate) = A (area) x v (velocity)

Just a thought!

 

[dsm-onster]

They CANT be closer- reducing the R- since the turbine inducer is consistant in depth. They both meet at the same diameter.

But besides that, the R means nothing for flow any how. All that matters is the critical area. That's why some turbo specs only reference the critical area: 7cm^2, 18cm^2, etc.