Just guessing but I suspect that the "bowtie" heads are produced outside the normal production scheme (read low volume casting house and low volume machining house...both outside vendors) so it is easier to obtain the bronze valve guides compared to any production PM guides that might work. And I have no idea whether there are any production PM guides that might work in those heads. There might not be any at all so the bronze works for low volume. Then the PR guy gets the sript and starts writing....LOL. Sounds good. The bowtie heads are produced same as any aftermarket head might be produced (very low volume) and are just marketed thru GM Performance Parts. The head design might (or might not) come from GM sources but the actual parts are produced outside and purchased.
Note that the 505 HP LS7 small block in the Z06 Corvette has powdered metal valve guides.... not bronze guides.
PM guides have one thing going for them that bronze guides do not. PM guides are porous (to some extent) by design so they absorb oil and retain it and provide excellent lubrication even with the dryest of seals....as long as SOME oil gets to the guide. Even PM guides need some oil. The PM guides are formed via powdered metal that is literally powdered metal of different types blended together and then compressed into shape and heated or sintered for strength. Part of the metal powder used is copper and bronze along with graphite and ferrous alloys for strength. That way the PM material resulting has many desireable material characteristics along with the inherent porosity that is allowed by the process to retain oil.
The valve stem diameter really shouldn't be an issue. The smaller the stem the better generally...until the valve head falls off. Heat rejection is not the problem. While the smaller stem has less contact area in the guide it also has less cross sectional area to carry heat so less heat gets transfered into the guide. Most of the heat is transfered from the valve head into the seat...which is why the valve starts to "burn" when it doesn't seat squarely.
Note that the 505 HP LS7 small block in the Z06 Corvette has powdered metal valve guides.... not bronze guides.
PM guides have one thing going for them that bronze guides do not. PM guides are porous (to some extent) by design so they absorb oil and retain it and provide excellent lubrication even with the dryest of seals....as long as SOME oil gets to the guide. Even PM guides need some oil. The PM guides are formed via powdered metal that is literally powdered metal of different types blended together and then compressed into shape and heated or sintered for strength. Part of the metal powder used is copper and bronze along with graphite and ferrous alloys for strength. That way the PM material resulting has many desireable material characteristics along with the inherent porosity that is allowed by the process to retain oil.
The valve stem diameter really shouldn't be an issue. The smaller the stem the better generally...until the valve head falls off. Heat rejection is not the problem. While the smaller stem has less contact area in the guide it also has less cross sectional area to carry heat so less heat gets transfered into the guide. Most of the heat is transfered from the valve head into the seat...which is why the valve starts to "burn" when it doesn't seat squarely.
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hosphor Bronze
