J (915) vs. Magnum heads

frosty_the_punk said

8.5 sounds about right to me for 1.88"/1.6" valve J heads without metal shim gaskets if the motor was about 9.25:1 with magnum heads.

Most people fit the J heads with bigger 2.02 valves which makes the difference much less. In the above example it would be about 9:1 with standard gaskets and 2.02" valves.

My J chambers come in at 66.5cc with 2.02 valves, with 1.88 they came in at 70cc

Magnums are about 63cc with a 1.92 and 1.625 valve.

Do the math, it's pretty surprising.

Click to expand...

I agree that the valves having a flat face are tighter, when you increase the surface to 2.02" diameter. What you've got makes sense with the volume of the flat area on the valves, but what are you running for pistons? The math isn't all in the chamber volume.

By '73, Chrysler's 340, like the 318 and 360, was not a zero deck or even close.

That's where everything started going south in the engine (literally, down). They ditched the notch pistons, went to flat tops and dropped them in the hole. They got rid of quench and had to get rid of ping, so they dropped the compression distance.

Still, you can make a set of J heads run awesome, no doubt. I prefer the casting quality to the Magnum, while I like the magnum chambers, the J runners are a better bet for modding.