J (915) vs. Magnum heads

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DaveBonds said:
Open chambers with metal shim gaskets give it an optimistic 8.5:1. Closer to 8.0
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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.
 
Thanks for all the technical advice with all the differences between both heads. I enjoy reading them all. Im sure this gives other people that are uneducated about heads some good info. I know I appreciate it.

I can also tell you that Im just going to stick with my magnum heads. Even though I have everything to do the swap with the exception of the new valves. I'll save my money for a new torque converter!
 
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.
 
True, in regards to compression ratio the math isn't all in the chamber volume...but if you're considering swapping one head to another on an existing short motor. the chamber volume is the only thing that changes (assuming the same head gasket is used)

All I'm saying is that once fitted with large 2.02 valves, a J head is only about 3-4cc's bigger than a magnum.

When changing from magnum to 2.02 J heads on a 360 motor you're not going to drop from 9.0:1 to 8.0:1 with 4cc or even 7cc.

A quick mathematical check tells me that would require a massive 12-13cc increase in chamber volume. To my knowlege, this is unlikely to be the case.

A drop from 9.0:1 to 8.7:1 is a more realistic number to expect and that's with the same head gaskets and no surface grinding on the J heads.
 
You would have to go to the outer edge of the seat taper, to check diameter.

However, I'm not sure how much taper and diameter increase there is with the valve margin, over the head's actual valve seat, but I think you could rule one or the other valve out, on a bare head. It's probably very close to actual measurement, on the outer edge of the valve seat taper.
 
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