Compression ratio choice on modded RB26?

[Chris Wilson]

I am slowly collecting parts for my RB26 engine in my R33 GTS-t chassis job. I have moved away from modding the RB25DET engine due to the better, bigger valved head on the RB26 and superior parts availability for the RB26 engine here in the UK. I now need to address compression ratio choice. Engine spec is as follows:

RB26DETT block bored 87mm. Billet steel crank at stock stroke. Tomei Carrillo pattern "H" section steel rods. Tomei 87 mm forged pistons, but NOT the oil gallery type. These pistons run a slightly lower crown height than stock, so I guess out of the box, with a stock head and stock gasket thickness they are about 8.5 to 1 ? Brand new RB26 DETT head casting, which I will mildly port and manifold match, on stock size valves with multi angle seats. Modded RB26DETT throttle bodies with 50mm plates in them. Stock plenum. 700 ish cc injectors. Tomei 270 degree duration, 10.85 mm lift cams in inlet and exhaust positions. Tomei springs. Single Apexi RX-6 turbo on Apexi cast iron manifold, with remote wastegate discharging back into main 3 inch system. Intake will have single large AFM, hot wire type.

ECU will be RB25DET Power FC, as it will best match engine loom.

I will use the car virtually solely for track days, road manners are secondary, as are emissions et cetera. Fuel will be 98 RON super unleaded. I may do a second map on race fuel, probably Elf turbo, about 102 RON unleaded.

What should I configure CR at? I don't want to lose squish by adjusting head gasket thickness, I want to run that quite tight, around 40 thou max, for better det control. I will machine the piston crowns to adjust, and run a 1.2 mm steel head gasket (Cometic). I was thinking that some of you engine builders would have good experience of what CR works. Thanks!

[Cubes]

The stock 8.6:1 works well.

I wouldn't go any lower especially if you are considering a set of cams.