> Admittedly, 5 out of 3500 is a fairly small sample set. But I do know
> that many people are perfectly happy running unleaded. I have a GT6
> head about to go to the shop; I'm still waffling about whether or not
> to put hardened seats in (five out of six combustion chambers don't
> have any seat inserts and they all look just fine - I'm guessing that a
> PO burned a valve in #6).
Your machinist may leave you no alternative (especially if you use
Kaeding). When I had the valve job in the B last summer, it needed
at least two exhaust valve seats. The machinist I went to (and I'm
even more pleased with their work after seeing what happens when
you oil-starve the rod bearings -- basically, the bearings wear but
the crank stays true) said that it's all but impossible to get valve
seats in anything *but* hardened steel these days. So if you are
getting new valve seats installed, you automatically get hardened
seats. Hey, it's one less thing to agonize about...
> Maybe I'll do an experiment with the GT6 - not put seats in this time.
> That head will be off again several times in the next few years (it's
> for the autocross engine), so I can do an extended test.
The same machinist told me that most of the valve burning problems
have been solved by fuel composition in any case. For the record,
the machinist here worked for Huffaker in the Seventies and has
therefore built some pretty serious LBCs.
His observation was that the problems with valve seat recession
came about in the mid-Seventies, when lead was being phased out of
fuel. The problem is not simply one of lubrication, it was one
of combustion temperatures. The fuels in the '70s were simply
running higher temps than the unlubricated valve seats could stand,
so you'd get microwelds from the exhaust valves to the seats, and
vice versa. Not only did this remove metal, but it made the seat
and valve interface very rough; couple this with valve rotation,
and you get a continually self-lapping valve.
So in addition to improved materials technology leading to seats
and valves that are much more resistant to such recession, the
modern formulation of fuels is such that combustion temperatures
have dropped, reducing the tendency for such microwelding to take
place. There's still wear, but it's not the rapid (<10,000 miles)
recession you were seeing in the '70s.
He advised against lead additives (which, let me remind everyone
again, are distinct from octane boosters) in general, saying that
removing the lead from gasoline removed a lot of other headaches
than just the pollution benefits, notably lead fouling of plugs,
rings and the like.
So the advice for Chris in all this is that if you don't need
valve seats to make up for an already worn engine (as I did),
then don't worry about accelerated valve wear from unleaded
fuel. If you do need valve seats to make up for an already
worn engine, then don't worry about getting hardened or unhardened
seats because there's only one kind any more anyway.
I've actually seen worn-out valve seats (and I think you have
too, Chris; didn't you oooh and aaah over the great discrepancy
between the valve stem heights on the green car's head last summer?).
It's interesting. You definitely want to catch it before the valve
drills its way into the intake port. On TGC's head, the exhaust
valves in a couple of the cylinders were flush with the combustion
chamber face, while the others were proud about 1/16" or so. One
was actually sunken past the combustion chamber face. Laying a
straight edge across the valve stems was quite educational, like
trying to draw a line across the profile of the Rockies on a
Colorado license plate...
--Scott
|