Blending Valve Guides

Have an interesting technique, tool or just anything related to porting? Post it here . . . please no bashing anyone for the model of engine they are working, we're here to share ideas . . .

Postby hollywood63 » Wed Sep 07, 2005 9:42 pm

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
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Postby 84-1074663779 » Wed Sep 07, 2005 11:18 pm

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.
84-1074663779
 

Postby hollywood63 » Thu Sep 08, 2005 6:33 am

Thanks Tony that pretty much answers my questions. I quess I should have sat back and thought alittle harder
Thanks again
Art
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Postby cboggs » Thu Sep 08, 2005 9:30 am

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
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Racing Flow Development.
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Postby SWB » Fri Sep 09, 2005 12:12 am

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
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Postby cboggs » Sat Sep 10, 2005 10:18 am

SWB wrote:I agree with Curtis about 99%.
SWB

[color=#000000]Shouldn't you agree with me 100% of the time?? :p

Yes, it doesn't work all the time, .. but in cases where you need some
area to slow the airspeed over the short turn, .. sometimes it's the
only place to get it. Most of my ports are largest at the short turn
and is often nessesary to keep the airspeed from going over mach .55
at the short turn. Newer style "oval" ports with high airspeeds through
the entire port you really have to keep and eye on short turn speed.

In many of the aftermarket or race type castings it isn't needed as
you can get the area & proper airspeed needed. I just assumed we where
talking about "stock" style castings.

The real point I wanted to make is the trailing edge of the guide boss
is more important then the leading edge.

What heads have you found a longer leading edge to work better in?
I tend to leave a long leading edge in my 15
Curtis Boggs
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Postby SWB » Sun Sep 11, 2005 10:12 pm

Curtis,

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

SWB
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