Quote:
Originally Posted by steve25
I have that same chart and it’s wrong as hell!
It’s showing the 75.7 cc head as making a 10.25 compression, and then the casting 12 head at 9.43 CCs less ( 66.27 CCs ) increasing the compression by only .25 of a point!
And if that is not enough of a red flag the chart shows the RA4 motor with its 69 CC head as only being a 1/4 point higher in compression then the 75.7 cc number 13 casting head used on the 350 hp motors!
When I run the numbers I calculate the 75.7 cc head as making for a 9.8 to 1 compression.
The 66.27 head would make for a 10.8 to 1 compression, when I run the numbers, so in other words your asking me to believe that Pontiac let the RA4 motor out of the factory doors with less compression then the RAIII motor!
I don’t believe that for even half a split second!
So here we go again , anyone have a close to stock number 12 casting sitting around they can CC for us ?
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Keep in mind that the RA IV used aftermarket forged pistons. Have you ever compared any Pontiac cast pistons to the RA IV forged piston. Could easily be differences in pin height or even deck height. From what I have seen, if the magazines are showing an actual RA IV piston, the tops do have a different valve relief configuration. Maybe a true side by side, measured, comparison between the cast and forged piston would settle it. Maybe you could find an NOS RA IV piston and provide the comparative specs, and then we would have some facts to fall back on for the sake of argument.