From Forcedperformance.net

Q. What are twin scroll or divided inlet turbine housings? What are the benefits of using a twin scroll turbo?

A. A "twin scroll" or "divided inlet" means that there are two separate volutes within the turbine housing. The main reason for doing this is to isolate the pulses coming from each exhaust port and maintain more of the pulse energy from each cylinder all the way down to the turbine wheel. There are no differences between the turbine wheels used in open or single inlet turbines compared to those used in twin or divided inlet turbines.
Generally speaking, a divided inlet turbine setup will respond faster and produce boost quicker than single or open design of the same nozzle area, of course this is dependent upon proper execution. The simple fact that a divided housing is used does not guarantee these results.

While it does not cause any problems or harm to run a divided inlet turbine housing on a manifold that is an "open" design, none of the benefits of the twin inlet will be seen.

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Top end power is more to do with the Mass or Energy of the exhaust gas flows into and out of the turbine and exhaust housing of the turbo. The greater the gasses that can be flowed up to a point, the greater the HP output for the same sized turbo. Well until you end up outside the compressors efficiency range. One would think that with virtually nill pulse energy interferance that the turbine wheel would work more efficiently generating more HP at any given RPM.

On the flip side maybe twin scroll headers may have a slightly reduced internal overall diameter piping into the turbo to speed up the exhaust gas flows and promote quicker turbo spool up at the expense of the mass of gasses that can be flowed at high RPM's.

Anyone got some piping internal OD measurements ?

A VF34 / VF36 comparison would be interesting. Same compressor housings, but again different exhaust housings. PZ18 compared to PZ25. I would be putting my money on the VF36 for outright HP as well due to virtually nill exhaust pulse interferance.

J