I have been thinking about the benefits of twin vs single for some time.
Because there was no hard proof, no dyno figures or any real evidence of one being better than the other I thought it might be worth looking at some compressor flow maps and plotting air flow requirements.
I used a spreadsheet to calculate the VR-4s air flow at pressure ratio. this allows me to plot the RPM line demands on the compressor flow maps.
According to a quick sanity check using this calculator, my projected air flow should be fairly accurate, if not a little pessimistic due to the calculated figures being 15PSI at 5500RPM.
As a third check, the power output figures approximately match my dyno figures (with exhaust + intercooler + MBC @ 15.2PSI) however my dyno figures were not at 15PSI as do to the turbos inability to hold 15PSI... more like 12PSI. (which is why the plot should be a touch pessimistic)
I have plotted on 2 compressor maps, both Garret BB turbos... The theory being that most of us want good response with good power potential. Personally I am willing to sacrifice top end performance for good response.
In this vein, I selected the GT25R for the twin turbos. Garret rate them at up to 270HP each, giving a total of 540HP total using 2.
This selection was based on the maximum response attainable from a good BB turbo.
For the mapping I based the calculations on 1250CC (half engine displacement)
For the single I used the GT35R. Garret rates this turbo at up to 600HP. Looking quickly at the compressor map it looked like it could be usable.
And here are the compressor maps with the RPM lines marked in. Obviously I cant say for certain that these are going to be accurate, but interesting nevertheless.