Kas, I don't think my experience is completely contradictory to yours.
First, my pistons were custom made by me with a die grinder to get a good
fit into the combustion chamber. As a result, the pop-up part of the
piston was more like a dome which projected into the combustion chamber. I
don't remember the exact height, but I think they were about .140" above
the normal height of a flat top piston.
Our (I have to give credit for the idea to Hardy, who suggested I try
pop-ups in about 1983) objective was not to create a better flame pattern,
or more efficient combustion. At least in my case, the sole objective was
to get the higher compression without having to mill the heads any further.
With flat tops, I was milling almost .200", and most head castings
wouldn't last long with this much milled off them. After investing all the
money to port and polish a head, I needed for it to last a few years. By
using the pop-ups, I was able to get the 13:1 compression and only had to
mill the head .150". The heads lasted forever.
I also found a considerable improvement in power with the higher
compression. With everything the same except the cylinder head and
pistons, I went from 155 HP with flat tops to 167 HP with the pop ups.
The new head may have been better, and the dynos were different, but the
order of magnitude increase we saw on the dyno related directly to improved
times on the track. My conclusion is that the increase in compression more
than offset and 'confusion' caused by the pop-ups.
This analysis does not follow the scientific method completely, but it made
sense to me at the time and, as we said, "the proof of the pudding is in
the lap times". Does this make sense?
-----Original Message-----
From: Kas Kastner [SMTP:kaskas@earthlink.net]
Sent: Monday, October 11, 1999 12:42 PM
To: Bob Kramer; Jack Wheeler
Cc: 'FOT'
Subject: Re: TR compression vs. HP
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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