The choice of turbo in a build like this is quite important so I've spent some time on Excel today calculating everything out and plotting some points on the compressor maps on the Garrett website. Super helpful that they offer this level of detailed information, the only one that does i believe! So i went about doing all of the required calculations and got an Airflow value for peak torque and maximum RPM, then plotted them on the graphs for the three turbo's i was looking at. The G30-660, G30-770 and G30-990. Excusing the appalling colour of the points (can't seem to change them so thank Microsoft for Grey and Orange) you can see that the 660 would be inefficient at peak RPM whereas the other two would be at peak efficiency at peak RPM and working well at peak torque. In the areas of the chart that the RB26 will be operating the two turbo's compare very closely and ultimately comes down to running either the G30-770 with a larger rear housing (1.01AR) or the G30-900 with a smaller rear housing (0.83AR). I think the 770 with the larger rear housing is going to be the way to go as it's going to utilise the efficiency of the compressor well and spool up a bit later which will help with keeping traction. Obviously the calculations assume a few things and they're only an indicative finger in the air but it's really good of Garrett to offer these tools with the explanations of how to get there. If anyone's interested in doing this themselves then there's loads of tools on the Garrett website and you can even use their Boost Advisor which will do most of the calculations for you. Engine build and mapping is all being done by Mark Gillam over at Abbey Motorsport.