Running divided turbo on stock (open) flange?

[kenikh]

Is there any downside divided turbo on stock (open) flange? The reasoning being one can run the divided turbo on a factory exhaust until one can afford to fabricate a divided exhaust to suit.

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[Tippy]

I did it. Honestly, when I finally divided the header, it didn't make a lot of difference.

But, I did make the divider about 3/8" tick instead of the 1/4" of the turbine housing possibly choking a little and causing turbulence. It's on my list of things to address while I have the motor down.

[copbait73]

Originally Posted by kenikh

Is there any downside divided turbo on stock (open) flange? The reasoning being one can run the divided turbo on a factory exhaust until one can afford to fabricate a divided exhaust to suit.

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No problems. Now, if you ran the full 24 hrs of Le mans you may have heat checking/erosion at the flange surface.

[kenikh]

Thanks guys!

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[MikeD930]

My 930 is set up that way using divided turbo with custom headers that are not divided. It was already that way when I bought this car. I don't know why whoever did this mod did not divide the headers. Anyway the divided housing will be fine as mine were good shape when I removed the turbo to add new compressor seal.

[RarlyL8]

If you were going to leave it that way (divided turbo only) I would file the turbo plenum to a sharp edge to improve flow. When you get your split plenum headers (all of ours are split by the way) you should match the dividers so flow is smooth. Every little bit helps.

[911nut]

Originally Posted by kenikh

Is there any downside divided turbo on stock (open) flange? The reasoning being one can run the divided turbo on a factory exhaust until one can afford to fabricate a divided exhaust to suit.

As it was explained to me by the exhaust guys I work with, when an exhaust pulse from one bank flows to the turbo, there's low pressure in the opposite bank. The divider forces flow into the turbo instead of having the exhasut flow into the low pressure area. Without the divider, you lose some low speed response in the form of backflow into the low pressure in the opposite bank.
I'm in the opposite situation; I added the divider to my collector to stop the backflow and improve boost onset but I don't have a twin scroll turbo. I did gain a small improvement in boost onset.

[Ken911]

So with a twin scroll equal length primaries are supposed to help a lot. Does the Secondary need to be equal length also? Or is the fact that they are divided enough?

[kenikh]

Everything should be equal length. There's some debate that you can essentially get the same effect in unequal exhausts by timing the length of one set of headers to be in phase, but one cycle off, but the maths hurt my head.

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[RarlyL8]

Everything needs to be equal length for a divided plenum to be optimized. Each pulse needs to hit the turbine at the same unique time and sequence as the exhaust valves open. The complexity to do this in an unequal system based on individual tube length would not only be a mathematical challenge but a packaging nightmare. Any compromise would ruin the effect. Our headers are equal length both on the primaries and secondaries with a divided plenum. Each bank gets a smooth transition bend waste gate outlet. The result is a minimum of back fill when using a single waste gate and open plenum turbo such as the K27. The tuned sound is completely different than short tube systems, off boost torque is greater, turbo lag is essentially the same as the short tube systems and HP is greater once on boost. All of this is enhanced further when using twin waste gates and a twin scroll turbo. Very quick response, big off-boost torque and big HP capacity. Tubing sizing can be kept conservative as well which enhances velocity for quick response without reducing power capacity at full boost redline.