The issue of pop-up pistons has come to the fore several times over the
years and the information never seems to be saved or passed. To wit: I made
a series of engines for exp. purposes in the early 60's to view the effects
of compression ratio increases. In a nut shell here is what I found on an
honest engine dyno with an operator that didn't lie to himself (me). The
13:1 pop-up piston that entered the combustion chamber confused the flame
pattern in the combustion chamber so much that the measured BHP was the same
as an engine with flat top pistons and 10.25 ratio. The slightly domed
pistons were better than the pop-up by a considerable measure but NONE were
as good as the flat top. The combustion chamber in the wet sleeve engine
series is so ancient that there is not room for a decent flame front to
progress. Look at the tops of your pistons if you have the pop-up or domed
type. See if the carbon is a reddish or off color brown instead of black
and that there are swirl type patterns on the piston top as opposed to a
flat covering of carbon. Generally you will find exactly that as I have
described. The flat top piston allows the flame front to progress evenly to
the back of the combustion chamber and reach there before the end gas has
time to heat up and explode on its own (detonation). On the flat top pistons
you should see that the carbon is thinnest or maybe even non-existence on
the squish area side of the piston and closest to the exhaust valve because
this is where the detonation is going to occur. I ran cast pistons for years
and years in these engines both the stock 83mm,86mm and the Hepolite 87mm.
The Hepolite in my opinion was a great deal better as it was a lot lighter
than the stock castings. When we ran forged pistons they indeed had the
longevity but did not produce the power as the bore clearance was twice that
of the cast pistons and they were quite a bit heavier.. The thing here being
that if you control the detonation you can run a cast piston. Of course I
am speaking from efforts 30 years ago but gasoline still is supposed to burn
not explode so maybe some of this is still true. The best and most powerful
engine I made were done with 87mm cast pistons, chrome compression ring and
.003" bore clearance. To eliminate the run in time and additional wear on
the engine I lapped all the rings into the individual mating sleeve and had
the highest effective compression immediately and it showed on the
racetrack. With all my work in these areas I have a small caveat that must
be remembered, and this is that I was only interested in something to last
the weekend not the year and there is the "wolf in the bush" Kas.
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