wrote:
On Feb 9, 2:10 pm, Anthony W wrote:

I was wondering about that too. The compression ratio doesn't seem high
enough to require that. Perhaps a thicker head gasket could be used to
lower the CR so it would run on auto-premium to make it cheaper to operate.



I've had very little to do with Franklins, but if they're built
like most other opposed aircraft engines, there is no head gasket. The
steel cylinder screws into the aluminum head, more or less
permanently, using a shrink fit. The only way to lower the compression
ratio in such engines is to shim the cylinder base/crankcase
junction.
I once owned a McCulloch two-stroke four-cylinder drone
engine that had been modified this way to derate it from 72 hp to 50
and to allow it to run smoother at a lower RPM. Terrible engine,
commonly used years ago in Bensen's gyrocopters. They'd been built by
McCulloch for the military, who used them in target drones for anti-
aircraft gunners to practice shooting down, so the engine was designed
for a 15-minute service life or something like that. In homebuilts,
they vibrated like mad, and the mag would get hot and quit. The
carburetor throttle shaft would wear the thin carb barrel walls out in
no time flat and leak air. But cheap they were, light, and plentiful.
Couple pictures of one, and one of a Franklin drone engine, too:
http://www.barnstormers.com/EVENTS/0...museum-21.html

Dan



Perhaps, like teh VW, a spacer at teh bottom of the cylinder can be
used to adjust compression?