One of the FOT shared some calculations with me concerning the
shrinking of clearances when hot...
With Bronze guides with 0.004" clearance and Stainless Steel valves
with 5/16" stems, when the exhaust valve hits 1000 degrees there will
be 0" of clearance.
I have no way to check the math and it's way above my pay grade, but
what Michael is describing is exactly the issue in my mind. You want
to run a clearance just big enough to avoid sticking the valves.
I can't argue that Bronze is better than Iron for the guides, but a
number of have found that if you don't run "enough" clearance, bad
things can occur - and they get infinitely worse if you have enough
lift that you're running an interference valve train.
The conventional wisdom seemed to be that the iron guides had higher
wear or were somehow inferior to Bronze but I'm really unclear where
that notion came from.
Tony
At 07:26 PM 5/14/2012, Michael Porter wrote:
On 5/14/2012 4:13 PM, Enquiries Road & Track wrote:
i think you guys are missing the point. bronze guides were intended to be
able to run *smaller* clearances for better heat transfer. there
is absolutely no point in increasing clearances unless you have chromed
stem valves (very rare nowadays). you want the absolute minimum for best
heat transfer.
Umm, the issue is not the cold clearance, but the hot running
clearance. Since the thermal coefficient of expansion of bronze is,
depending upon type, about 2-1/2 to 3 times that of cast iron, their
use in cast iron heads requires additional cold clearance, so that
the hot running clearance is adequate. When the guide grows with
heat, the actual running bore gets smaller, because the mechanical
strength of the cast iron is higher and the coefficient of expansion
is lower than that of the bronze.
This is a matter rarely of concern when using bronze guides in an
aluminum head, since the coefficients of expansion are closer to the
same (a ratio of perhaps 3 to 4), but with cast iron, a much bigger
deal because the ratio is more like 3 to 1. This is why so many
racers in this country have experienced valves sticking when using
the recommended factory cold clearance (appropriate only for cast-iron guides).
Cheers.
--
Michael Porter
Roswell, NM
Never let anyone drive you crazy when you know it's within walking
distance....
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