340 Clean Slate - what to do

The picture below is pretty close to what a factory 340 piston will look like. Actually, this is a JE piston that has shorter skirts and is made of different material. The difference between high compression 68-71 340 and the low compression 72-73 is the pin placement in the piston. There also seems to be some difference of the height of the piston cap (top ring to top of piston) between the low and high compression piston. The high compression piston sits .045 above the deck at TDC, while the low compression piston sits .054 below the deck at TDC.

Granted aftermarket pistons will be all over the place. 9.5:1 seems like a reasonable ratio on a street car to shoot for.

Given a 9.5:1 compression ratio, I recommend a cam that ranges 195º-214º duration @ .050". Couple that with no more than a 3.73:1 rear end. This is higher than I have ever recommended for a street car (3.55:1). If you want to keep the 3.92:1 rear gear, you're heading for race car territory; 10.5-11.0:1 compression, camshaft from 225º-240º duration @ .050". One of the problems IMO is that a 10.5:1 iron head LA won't run well on currently available pump gas.

View attachment 340 piston.jpg