I had sent Tony the spread sheet some time ago analyzing the clearance between
the guide and the valve at various temperatures. This is based on the
coefficient of expansion of cast iron (cylinder head, 6.0X10b;b6 In / In /
Deg. F.), manganese bronze (guides, 11.8X10b;b6 In / In / Deg. F.) and
valves (21-4N stainless steel 9.9X10b;b6 In / In / Deg. F.). I think, based
on an internet search, that 21-4N is the common stainless steel used for
exhaust valves.
The model is based on the bore in the cylinder head being smaller than
the
O.D. of the guide at operating temperature due to the differences in their
coefficient of expansion, just as others have recently mentioned. This makes
the bore or I.D. of the guide smaller at operating temperature. The biggest
factor is the coefficient of expansion of the valve being almost double that
of the cast iron, which controls the I.D. of the guide.
I've since refined the model to incorporate different rates of
temperature
rise in the head, guide and valve stem. The next thing to do was to get
better data on the operating temperatures, particularly in a race
environment.
I've been searching the internet trying to get an idea on temperatures
of the
exhaust valve, guide and cylinder head. So far I've found that the surface of
the combustion chamber is in the neighborhood of 266 to 496 degrees F.
Then
I found that exhaust valves run in the area of 1000 degrees F. The
interesting thing was that the point was made that the valve is only "exposed"
to the exhaust gases for 1 out of 4 of the cycles, more or less. The point
being that there is a lot of time to transfer the heat to the seat (which
transfers the most heat) and the guide.
Question: I am aware of the "downside" of not having enough clearance, i.e. a
stuck valve, but what is the "downside" of too much clearance? I think we are
talking about a clearance of 0.004 versus 0.002 on the diameter of the exhaust
valve, cold. That's only a difference of 0.001 on a side, cold! And at
operating temperatures the difference will be quite a bit less.
A stuck valve can cost a race weekend and more, what can a "loose" guide
cost?
Tim Murphy
-----Original Message-----
From: fot-bounces@autox.team.net [mailto:fot-bounces@autox.team.net] On Behalf
Of Tony Drews
Sent: Monday, May 14, 2012 10:09 PM
To: fot@autox.team.net
Subject: Re: [Fot] RE -valve guides revisited
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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