[T3] Does anyone know the difference in a U case and X case?

[William Jahn]

I only have the Bentely to go by and the timing listed for a U is TDC and
an X 5 BTDC. The U has a higher compression 7.7 and X 7.3 .

 What was different between the two?

 The 72 case I rebuilt is a Late U and the X was what was in my T-3 yet
both looked the same. When I rebuilt the U case I used a flat top rather
than the dished pistons. That would increase the compression ratio. I know
the 72 had a dual chamber vacuum can yet was still set at 5 BTDC it was the
retard of the vacuum can that retarded the spark at Idle some sort of smog
device.

 Reason I ask is when I went back to points and was setting the timing it
seemed to have better pick up at TDC. And if I connect the vacuum can on
the stock AH distributor it will ping. The original X case with the dished
pistons didn't ping @ 5 BTDC.

 Since I went to points it pings @5 BTDC even with the can plugged did the
same with the pertronix. I also know each engine is a bit different as far
as the timing goes.
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