This discussion is persistent which shows both that people are
interested in the subject, and it is not a settled matter.
Some contributors to the discussion are time-served veteran racers who
have learned the hard way what works for them.
Less-experienced people may find great value in a study of the
fundamentals, which are expressed most readably in the old classic
(updated to the fairly recent 6th edition, which is in print) "Tuning
for Speed" by the late P.E. Irving, one of the greatest engineers of the
old (pre-1975) British motorcycle industry. Mr. Irving was best known
as the designer of the immortal Vincent motorcycles which were both
tremendously successful road bikes, and in racing form held speed
records in many countries, as well as the World Motorcycle Speed Record
in the mid-1950s.
Whether air-cooled or water-cooled, car engine or motorcycle engine, the
problem for the racing four-stroke engine is the same as it has been the
whole of the 20th Century. Hence the value of the study of "Tuning for
Speed" for the speed trials competitor.
In a nutshell, Irving tells us that piston trouble (holes) arises from
excessive internal temperature, which in turn arises from the
relationship between the fuel qualities (whether gas, alcohol, or nitro
blends) and the compression ratio. He assumes that mixture is correct
for the fuel, and spark lead and all the other factors are as they
should be. And there are many factors that bear on the subject.
The crown of the piston is probably always the weakest part of the
combustion chamber, and the part that fails first when there is
"excessive internal temperature."
It's probably fair to say the only difference between air-cooled and
water-cooled racing four-stroke engines is that the latter run around a
200 degrees F. cylinder head temperature while the former easily run 400
degrees and over. This has a lot to do with the said "internal
temperature," the immediate practical effect being that air-cooled
racing engines must use substantially lower compression ratios than
otherwise-equivalent water-cooled units.
A recent catalog of Classic Motorbooks lists the above book as item
107755D and you can access them at 800-826-6600.
I've been riding and building motorcycles, road and racing jobs (many
Vincents) for more than 40 years. But all with carburetors. So this
Hilborn on my Maxton Ardun threw me, at first. But I believe I've got
it figgered out now. We'll see. Regards to all, ArdunBill Hoddinott
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