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Understanding Crankcase Compression
Hi Jay:
Have you done any testing with varying two-cycle outboard crankcase
compression ratios? I'm sure you're aware crankcase compression ratios are
different than the combustion chamber ratios that most of us are familiar
with. I've read in some engine design books where lots of testing was done
with varying crankcase compression ratios on a sample test base of two cycle
engines, but after testing was said and done, a ratio of around 1.5:1 was
about optimum for high rpm (10,000+) or low rpm (below 6000). Any thoughts
on this?
Thanks.
Boatnik
Boatnik,
We have done a lot of work with crankcase compression with the Mercury's. We
have found that the amount of pressure is limited by the "weak link" in the
chain, which is the reed itself. The reed sealing on a slight ledge with a
slight amount of sealing area verses a large area of "masking" area is
susceptible to damage and failure at it best, and when one tries to over
pressurize the reed beyond service ability and port reed cages beyond
sensibility, a failure is inevitable.
When crankcase compression becomes too high, the reed fails and allows the
incoming intake charge that was to be used in the cycle of the motor to be
"back" forced the other way. This has a detrimental effect on several
aspects on the motor. If this is a plenum motor, the normally
negative-charged plenum will become positively-charged around the suspect
reed, and the one above and below it will suffer a loss of performance
because of the positive pressure emitting from the leaking intake valve.
Hopefully, you have taken note as to what I called the reed - an intake
valve, because that's essentially what it is. With this loss of intake
charge which is the 2-cycle's lifeblood it looses ability for cooling,
lubrication, and resistance to detonation and with this charge going the
other way the cylinder is domed. We have found by lowering the reed into the
crankcase a nominal number and trying to stuff the block to demising the
volume which has been created by porting and directional rod slotting we
have increased torque and air speed. I have no scientific numbers just
through doing dozens of them we have found the right combo. I am sort of
like the old lady that bakes bread I can make it work but I can't tell you
the exact amount of flour and baking power I use I just know how to do it
and it works.
Hopefully I have answered your question and thanks for asking JSRE.
Jay @ JSRE
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