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Dynamohum Dynamohum is offline
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Join Date: Jan 2008
Location: Midland Ontario Canada
Posts: 173
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Good question, hopefully members will share their experiences and we can all learn.
I’m new here and no expert, but I’m willing to share my thoughts, in hopes that we might help each other.

As you know Heat is the big enemy, the longevity of valves and guides is directly dependent on cooling the valves.

The exhaust valve in particular, while open and off its seat is exposed to greater heat load from the passing Combustion gases. This means that the exhaust valve needs to transfer this heat primarily through the valve guide interface for cooling. The sodium in the exhaust valves stem turns to liquid and helps transfer the heat from the valve face up through the stem as it is transferred from the valve to the guide on its was to being dissipated through the cyl head. The proper valve stem to guide clearance is critical in this operation, the larger the gap the less surface contact and less heat transfer.

So what can we do to Optimize heat dissipation?

First lets look at our approach. The big picture, everything is interrelated, dependent on each other and needs to be designed as a whole. One component or modification will not compensate for a week link in the chain.

Take advantage of each and every opportunity to contribute to the whole

1: Clean: Everything inside and out. All engine components must be clean to shed heat. Any built covering of external grim or internal pantin acts as an insulating material that will reduce cooling and create potential hot spots.

2: External surface preparation. For a performance motor or one that will be driven hard, I would reframe from top coating with paint or any materials that will encapsulate and retain heat.
Exploring the potential of Increased surface area. The surface area of certain components such as the head fins, engine case, rocker covers etc. can be increased by abrade blasting for further benefits.

3: Insuring minimal and yet proper valve stem clearance when fitting valves to new guides: This is often an issue when reusing old valves as stems tend to wear more from the ends than the middle leaving them barrel shaped. The question of “ what is acceptable” can be debated to application.
For example on my non-racing street motor build, I could not justify replacing all the valves. As A Extreme DYI…. I R&R my own guides, purchased the proper reamer to finish fit them to spec. using my old valves.

4: Choice of Valve Stem Seals: The Pros and Cons of using Teflon seals can be debated.
The extra oiling to the guides might provide extra life and additional heat transfer but at the cost of adding oil to the combustion process and potential increasing detonation.

5: Modifying Cylinder Head : There is an interesting photo on pg 73, 1978 printing of Bruce Andersons Book. It shows a 935 head with an oil passage machined to about half way down exhaust valve guide to supply a constant flow of oil for cooling extra cooling.
Not sure if they supply a return path for the oil or how beneficial this modification might be, but would love to learn more about it.
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Thank You for your time, Paul. We do because we can.
87 911 3.2 (Turbo conversion, build in progress, Thermal Barrier Coatings, High Pressure Dry film coatings) Modified heads, boat-tailed case, ARP hardware, OBX Header, 930 clutch disk, G50 Trans
89 5.0 Mustang convertible (For Sale)
Old 01-25-2009, 09:15 AM
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