Hello, Upgrading a 1971 904 that is behind a 350+ HP /6. Bored 0.060, milled 0.020 off block and 0.100 off head. Back yard port job, MPFI, and twin 2.0L turbos. Stock cam (not too bad in that year)

I milled the pressure plate on the front clutch set 0.050 to accomodate 4 race clutches in place of the 3 that were stock. I have 0.070 clearance there and think I'm OK. I would like to upgrade the rear clutch drum as well to accomodate 4 frictions in place of the 3 stock but I would have to mill 0.125 off the pressure plate. That seems like a bit much. I went to my bone yard and found out that I cant just put a 4 disk rear drum in as that would require new shaft and pump assy. Does anyone make a thinner/stronger pressure plate? Can I mill that much off w/o major warpage issues? Is it a big deal to leave 3 clutches in the rear drum? Some say its about line pressure not # of clutches, but I'm not so sure. Looking for advice!

Thanks,

The best way would be to put v8 parts in it. It has to be the correct year range and or get pump with both drums out of a v8 unit so you don't have to worry about mixing and matching stuff.

A 4 clutch rear drum- 4 will be fine
A 4 clutch front drum- you can use forward (rear clutch) discs to accommodate 5 discs with little machining required.

Good luck!

How much line pressure you plan on using ?

Be careful, mix and match in a 904 is tricky business. The stack up between the pump, high gear clutch an the splines on the input shaft can result the pump hitting the drum and the last friction in the high clutch jumping off the splines burning up the high gear clutch pack. Don't ask how we know.
Doug