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Exhaust valves

Posted: Sat Nov 05, 2011 3:53 pm
by 75evo
I Just got plain steel valves for my 3.0 V7. The heads are going to be shaved so I will end up with 11:1 compression.

The valve stem looks so damn wide, can I trim it down a little below the guides to improve flow and lose some weight?


Re: Exhaust valves

Posted: Tue Nov 08, 2011 2:33 pm
by scott.venables
You could convert to 8mm or 5/16" stems and then getting guides to suit. 9mm Is a strange size that's for sure. There might be a little machine work involved if you can't find some with the groove in the right place though.

Re: Exhaust valves

Posted: Sat Nov 12, 2011 12:14 am
by WhizzMan
Trimming down the guides depends on your cam profile, valve spring strength and max RPM you'll be going. Imagine the force on the valve if you'd be getting a little float on closing at 7800 rpm and the extra strong springs whacking it onto the seat. That's what would cause deformation and eventually, breaking of the stem.

From what I hear from Guy Croft, the flow of a well ported head will hardly be influenced with thicker/thinner valve stems, it's mainly weight loss that is the benefit of trimmed down valve stems. At 9mm there may be a bit more influence, but the money spent on machining down the stems would probably be more beneficially used on a radiused 3 angle seat job or something like that.

Re: Exhaust valves

Posted: Sat Nov 12, 2011 7:10 pm
by MR2 Zig
Adding to that is that the real restriction in the 12v is the size of the intake runners (according to Jim K.).

hope that helps,