924Board.org

[MikeJinCO]

Has any put the large 44mm intake valves in a 931 head? If so does it work well?

From my initial inspection with no good measurements it looks like the valve would be badly shrouded by the combustion chamber(the 40mm doesn't look any too great) until it clears the head or about .220 lift. It might necessitate slightly enlarging the combustion chamber, plus making the problem of the turn in the floor of the port even worse unless the throat was not enlarged very much.

This would be on a NA(special piston) application.

I am not committed to using the 931 head, just looking at the possibility as I have a standard NA head also. Having a combustion chamber and the 36mm exhaust vs 33mm on the NA is appealing.
_________________
Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[gegge]

IMHO, a 44mm inlet is too much. About 10% of the valve is almost touching the cylinder wall resulting in zero increase in curtain area or flow compared to a 40mm valve.

I got 173,5cfm@0.55" lift with a 42mm valve. But the head was moved 1mm and the bore was 87mm for better distance valve to cylinder wall.

The shrouding effect decrease with high lift and the port area at the valve seat is the bottleneck for high flow.

IIRC, the 933 and GTS/GTR used 40mm inlets.

By the way, you want a trumpet shaped combustion chamber for pressure recovery. That´s one reason why the 924 head never will show large flow numbers. The 44x37mm EURORACE 924 head used excentric valveguides in order to move the intake away from the cylinderwall. The valves are almost touching each other.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[MikeJinCO]

When I measured out to out of the valves while in the head I got 83.5mm so adding larger valve would like a bigger bore. Hadn't thought as much about the cylinder as the head.

Valve shrouding might be the reason that putting in the larger valves is no guarantee of a good performance increase. Also increasing the throat size makes the problem of the port floor radius a bit worse.

I think I will first do a multi-angle valve job on the 931, test, then add to the port floor and D shape the intake. If I can get a 15% improvement which should be equivalent t to about 160cfm.

My testing method only allows for percentage changes. Making a precise calibration plate is quite an undertaking.
_________________
Mike


'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

[gegge]

It is my belief that those improvements will get you more than 15% over stock.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs