What would be the (ballpark) compression ratio if I put a set of GT40 (non-P) heads onto a stock 289 motor?

 

The compression ratio for a stock 289 C code is 9.3:1 while the compression ratio for an A code is 10.1:1. The gt40 heads certainly allow the engine to breathe better, but I doubt it will affect the compression ratio very much.

 

just find out chamber size, and compare to chamber size of the 289 heads.... i dunno honestly

 

it wil take you down to about 8.5 or so caise you are going from 54cc heads to 64cc heads you would have to get them milled down