Blending Valve Guides

[hollywood63]

What is the general opinion on blending valve guides pro's and cons? Is there some sort of formula for shortening the guides? IE if the valve stem is 6 in. long and the guide is 4 in long you should keep at least 50% of the valve stem length (3 .in) of guide for support. The heads that I'm starting to play with seem to have alot of the guide and head material around the guide blocking the port. So before I destroy a head I was looking for some input.
Thanks Art

[84-1074663779]

I doubt if there is one golden rule to cover all situations, but I would look at a few factors.

Is the engine going to be a short life competition engine, or a street engine that is expected to last a considerable time between rebuilds ?

Does the design of the valve gear create high side loadings on the valve stem ? For instance, is it a rocker system requiring high valve spring pressure and lift with extreme angularity ? Or is it a DOHC head with direct in line bucket actuation and comparatively light valve springs ?

With exhaust valves, how easily will the heat escape from the valve head ? If a very long duration exhaust cam is used there will be less time spent on the seat, and more time sitting out in hot gas. This is especially true in supercharged or turbocharged engines. Shortening the guide is not going to help valve cooling, even if it is mechanically acceptable.

[hollywood63]

Thanks Tony that pretty much answers my questions. I quess I should have sat back and thought alittle harder
Thanks again
Art

[cboggs]

I'm totally against removing valve guides in the port like seen in some
Honda drag race heads, .. it's just wrong and doesn't gain much if anything.
The valve has to be in there, .. ;-P

try to narrow the guide, with a round front and a "pointed" back or trailing
edge, .. like an air plane wing. Make it narrow but leave enough to
support the valve or guide insert. Almost never shorten.

Also on some heads the guide boss goes into the port fairly far towards the
manifold side, .. the leading edge on the roof can be cut back to gain
area at the part of the port where it's needed, .. the short turn apex.

That's all you need, ..

Curtis

[SWB]

I agree with Curtis about 99%.

I do like to see a longer taper up to the guide and less of one after it in some circumstances however. I feel that this can effect a more consistent cross sectional area along the length of the port. My main concern is to not have the area expand or contract dramatically during the time the air is turning over the SST. Obviously the cross sectional area needs to be large enough (and the port contured properly) to eliminate extremely high speeds or turbulance over the SST, prior to the valve seat throat.

I'm not saying Curtis is wrong, but I am saying that while his advice holds true in some situations, I have found others where it does not.

I also agree that shortening the guides often decreases guide life significantly and as a rule, should be avoided.

SWB

[cboggs]

[SWB]

Curtis,

Certain Chevy heads (as you stated) and also some of my aftermarket 4-cylinder stuff.

SWB