compression help
Stock LS1 heads will have slightly larger 67cc combustion chambers than stock 243 heads at 64cc, so your compression will go down from 10.9 to about 10.5. Alternatively, 5.3 heads have smaller 61cc chambers, so compression would be pushed up to around 11.3. Those figures will vary depending on gasket thickness, but gives you a ball park idea.
Perhaps more importantly, head flow and chamber design varies amongst all three. On stock castings, you are probably better off with the 243 heads and a thinner .040 gasket. If you'll be porting them, the 5.3 heads with some larger valves can make a big difference on your 4.00" bore.
Of course your camshaft selection might play into your decision as well.
Perhaps more importantly, head flow and chamber design varies amongst all three. On stock castings, you are probably better off with the 243 heads and a thinner .040 gasket. If you'll be porting them, the 5.3 heads with some larger valves can make a big difference on your 4.00" bore.
Of course your camshaft selection might play into your decision as well.
I wouldn't use 5.3s on a 4" bore. I would send the 243's out to be ported and decked and be done with it. If you go to a smaller head gasket you mess with the optimum quench zone. I would use either a .045 or .051 gasket and a 62 cc combustion chamber on the 243s. Talk to Jon or Jason at TSP and they will set you up.
