X-heads vs J-heads: what's the difference??

69 Cuda 440 said

Baxter,

One of our Car Club Members had a 1968 340 GTS Dart with Factory Stock 'X-Heads'.

Basically 'stock', with Headers and 3.91 Gears and a few Bolt-Ons.
It ran 13.85's @ 101 MPH.

He did 'swap' on a set of 1972 'J-Heads' with 1.88" Intakes. He lost 2/10's
at the Drag Strip and ran in the 14.00's.

But, the 340 was more responsive at lower RPM's {for Street Driving}
and with improved Gas Mileage.

Click to expand...

Could have been related to production variances other than valve size.

I bet if you took a 340 short block and put it on a dyno, then, tested 10 sets of stock 2.02 "X" heads and 10 sets of stock 1.88 "J" heads, your numbers would be all over the place. Some of the 1.88s would beat some of the 2.02s and the other way around.