On Fri, 3 Jul 2009 19:11:24 +0200, "Steve Lusardi"
wrote:

Bruce,
You are a nice guy, but you are not correct here. Please pay close
attention, heavy lube oil contamination will create detonation in spark
ignition engines. It will melt pistons, spark plugs and heads, especially in
outboard motors. The percentage of oil in the fuel that creates this damage
is a matter of heat.The higher the heat of the combustion chamber, the lower
the oil percentage threshold that diesels the fuel mix . The reason this is
more dangerous in outboards is because the noise of detonation is muffled by
the water jacket and masked by excess ambient noise in the boat.
Additionally the severity of this condition is greatly amplified by throttle
position and the general mode of operation is WOT. If you think for one
second that flash point of fuel is not significant, place a drop of gasoline
down the barrel of a daisy BB gun and fire it. No change. Place of drop of
oil in the barrel and repeat the test. It easily detonates when the spring
is released. I used to this all the time as a kid.
Steve

I suspect that you are correct when you use the words "heavy lube oil
contamination will create detonation" however whether this is a factor
in outboard motors in normal use is questionable. In the old days
outboard used gas oil mixes of 10::1 with no apparent damage. Then
mixes went to the 20 - 25::1 range and more recently to the 50::1
range.

While I do not doubt that there is some level of oil mix that will
cause detonation but I doubt very strongly that it is a factor in
actual use given that oil mixes were, in the early days, recommended
that were 3 times richer then today's rather anemic mixtures.

In addition premixed gasoline is used in chain saws, motorcycles, weed
whackers, small generators, lawn mowers and even model airplanes. All
without noticeable detonation problems.

Another point is that I have seen 4 stroke engines that used so much
oil that the spark plugs fouled within hours of use, but were not
damaged by detonation. Most 2-stroke outboard engines, on the other
hand, do not contaminate their plugs in months/years of use.

While I would not argue with your assertion I do not believe that
"heavy lube oil contamination of the fuel" is ever going to be a
problem in even poorly serviced outboard engines to a degree necessary
to produce detonation.


"Bruce In Bangkok" wrote in message
.. .
On Sun, 14 Jun 2009 13:03:58 -0500, Brian Whatcott
wrote:

Not so sure. Gasoline flashes fairly low. But oil has lower octane than
any gas. So pinking or detonation could still be an issue, no doubt.
That's if the compression still makes it up there.....

Brian W

Steve Lusardi wrote:
50 to 1 Remember, too much oil will reduce the flash point of the fuel
and
create detonation, which will melt piston and head.
Steve

"rckchp" wrote in message
...
OT. I'm reactivating a 1997 Evinrude 9.9 hp outboard and I've
misplaced the owner manual, and I don't remember the proper oil mix
ratio. Is it 50 to 1, or 100 to 1? Thanks.

Flash point has nothing to do with detonation which is controlled by
octane rating and compression.

Flash point is the temperature at which vapors ignite and since
kerosene (with its higher flash point) will burn in most internal
combustion engines it seems that flash point is of only minor
interest.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)