celal

I am planing to rebuilt my cylinder head with 50mm intake 43mm exhaust valves .do I have to change my seats for big valves or re grind the stock?

Steve Weiner-Rennsport Systems

Your stock seats will work.

crg53

A bit OT, but can you use 993 cylinder heads on a 964 engine and what are the Pros & Cons?

Geoffrey

You can go to 993RS valves in the 964 head without seat issues. They are 51.5mm and 43mm.

The 993 cylinder heads are interchangable with the 964, although you'll need to relocate the exhaust studs on one side and you'll really want to use the 1995 intake boots which will match the 43mm intake ports. The 993 cylinder heads have larger valves in the 1996 and later varioram setup and in both the 95 and 96 and later versions, the choke is larger which slows the velocity down. It's been my experience that the 964 cylinder head has better port area than the 993 (which is too large).

silverboy

Geoffrey - do you mean that 964 heads flow better than 993's?

Steve Weiner-Rennsport Systems

Yeah, you can do that on the intake side, however there is damned little seat left and I'm not comfortable doing that unless I install the larger 993 seats for the RS intake valves.

FWIW, I don't see any differences in low & mid-range torque between 964 & 993 heads when used with RS cams. With a 3.8, the 993 heads make more power.

IrocMan

@silverboy:
964 ports have better velocity because of diameter while 993 have a better swirl effect resulting in better mid range power, better efficiency and lower emission. A 964 with bigger valves and higher lift give amazing results.
a quick calculation gets 110 m/s which is ideal for intake flow around 6400 rpm with a 43mm diameter on a 600cc cylinder (i remembered they were 42.5 BTW ?). What is the diameter of 993 intake port please, just to compare ?

It's easy to tune-up 964 intake port, carving it gently on the left in order to accentuate the clockwise swirl. Especially on the turbo