Thread: compression issues

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I would say if you have any doubts that it won't run on pump gas then go with the 76cc combustion chambers.

I think he is right on the edge for pump gas ,maybe it will run on it with the timing real low or some octane boost ,maybe it wont
The 76 ers would be much better in my opinion,you could back off that abdc number and use more of the stroke of the engine.
I went over board on trying to utilize the entire stroke on my application,I think it was almost a good 3 inches of the 3.48.

The 76ers have been through a recent implosion and most likely have some bent valves. They may clean up with cutting the seats for oversized valves and they may not. If I could spend the same money and do this work on the 64s and lose 3.5 to 5 cc's in the chamber the DCR would fall in line. I would also be using a set of heads that have no impact history on them. The way the rest of the motor went up I am a little shy about using them until I can afford to really have them tested and machined.
I know that as I push the edge for heat and detonation I need to keep this motor cool. What do you think of oversize the exhaust to 1.625 instead of 1.6 or use a 1.6 rocker and use a 160 or 170* thermostat?

A little dremel work to polish/enlarge the chambers a few cc and dremel the piston tops for a additional cc or so........Then I think you may squeeze by with some fine tunning on the timing curve and keeping the intake charge cool etc....you will definatly be pushing the limits.
Maybe some of the groves in the cubustion chamber on the heads just for the sake of gaining a few cc more.........?
I am not sure the exact limit on your combo ,...but you are getting very very close to the limit on pump gas ,I would imagine.

I assembled and measured actual net chamber without the calculators and what not. The net chamber volume is 76.8cc. swept volume is calculated at 727.4cc. this makes the SCR 10.5.
Intake closes at 65*ABDC making the dynamic stroke using 5.700 rods 2.698". Effectively the Dswept volume is 563.95cc and the DCR is 8.34. I am going to run it!

any explanations on the casting numbers?
both are 14102193 but the GM23 is 64cc and the GM17 is 76cc.

threearmsinjune think i found the answer to why they are different some were used on 305 and some on 350 from 87-up both had 1.94"/1.5" valves that is the only explination i can find as to why they might be different