27 replies to this topic

[yel327]

This is true. All 150 production versions of them are listed in the GMP&A bulletin dated 28/9/73. Did they publish a revised compression ratio for these engines though? They did publish that the pistons for these engines had a shallower bowl (previously 0.050-0.055, now 0.01-0.018 (assume ci)) but I can't recall seeing a revised compression ratio, which should mean legally it had to stay as per the prior specs? Is the shallower bowl piston to offset the ci lost in the head combustion chamber by fitment of larger valves and/or the valve cuts in the top of the cylinder so that compression remains the same?

[S pack]

That's why GMH homologated a completely new engine............

 

Looks like a 202 to me.

[S pack]

This is true. All 150 production versions of them are listed in the GMP&A bulletin dated 28/9/73. Did they publish a revised compression ratio for these engines though? They did publish that the pistons for these engines had a shallower bowl (previously 0.050-0.055, now 0.01-0.018 (assume ci)) but I can't recall seeing a revised compression ratio, which should mean legally it had to stay as per the prior specs? Is the shallower bowl piston to offset the ci lost in the head combustion chamber by fitment of larger valves and/or the valve cuts in the top of the cylinder so that compression remains the same?

Exactly. The fly cuts and opening up the combustion chamber around the valves would have lowered the comp ratio.

Shallower bowl pistons and shaving 30 thou off the head would have brought the comp ratio back up to spec.