I have itr rods with ctr pistons on them . i know there is a slight difference in the wrist pin height between the ctr and gsr piston as well as different rod lengths and stroke of the crank. I have a hondata and below 4500 rpms i can run 0 degrees of timing or it will detonate on pump gas. i have a race chip that i burned that i use 110 octane with and run 13.2 -13.6 a/f ratio and produce 205 whp at 8900 with skunk stage one cams. The cylinder head is done by dpr to stage 5 specs. the chambers have been welded and reshaped but lost some cc's the chambers are 48cc's know! I never did all the measuring to see what my cpr is since at the end of the job i was getting impatient!

 

the GSR rods are the same length as the ITR rods, 137.9mm.
and the crank has the same stroke as well as deck height.

the only variables to change are the head cc's and the C/R from the CTR pistons.

if the GSR head was stock with CTR pistons then you'd have a hair over 12:1 C/R.
but with material removed from the head, 41.6cc up to 48cc, then the C/R falls to 10.5:1. that's what your compression is if the head is really 48cc, but maybe that figure is wrong. if you're detonating on pump gas then i'm sure the C/R is higher than 10.5:1.

you can go to c-spreedracing.com and adjust things yourself.