T4 'head flow characteristics/valve sizes
 

I have two pairs of T4 'heads. A pair of 021 101 371 S with 41*33mm valves. Also a pair of 071 101 371 A with valve sizes 40*33mm.

I may consider resizing the valves to suit my plans for a 2056 revving to little over 6krpm using a single TB and plenum behind a 911 fan.

Am I right in thinking that the exhaust ports on 021 heads are less restrictive than those on the 071, which look deeper?

What would be good valve sizes for my application?

I would go with 44/36 valves, use 8mm stems on the exhaust. I would port the heads anyway so it doesn't really matter which casting you use. That's just me though....
ken

I just read an 1974 article on the T4 heads on a racing Formula Vee. They were running a 1700 T4 and stated that the biggest problem with the heads was the exhaust port. It is too restrictive. I don't remember the valve sizes, but they went through some major work to get bigger exhaust ports. I think the engine ended up with 153 hp @ 6500 rpm. I had to give the magizines back so I can't look it up for you. You should see some of the prices for the products back then.