On Wed, 11 Nov 2009 16:51:06 -0800, Oren wrote:

On Wed, 11 Nov 2009 19:27:54 -0500, wrote:

What's wrong with 92 octane? Just make sure it is ethanol free.

What small engine book suggests a higher octane?

It will burn hot (92), damage the spark plug and maybe internal parts
like rings or pistons or warp a valve. (prolonged use)

87 octane for a small engines.

I put 87 octane in my truck. My mower can run on the same - it's not
special, nor deserves 92 octane.


Are you a mechanic? A small engine mechanic? or an engine designer?

I'm not the latter, but both of the first two. High octane is not
required by most small engines, but is NOT damaging to use. The only
reason I recommend premium fuel, and in particular, in Canada, SHELL
premium, is because SHELL has gone on record in Canada as guaranteeing
there is NO ETHANOL in their premium fuel.

Years ago, with leaded fuel, there WAS an issue with using premium
leaded fuel in small engines (and air cooled engines in general) due
to lead accumulation on valves and valve stems causing valves to
stick. Sticking valves overheat. In aircraft engines (which I am also
familiar with) designed for the old 87 octane fuel (no longer made)
agressive leaning (lean of peak EGT) is required to keep lead buildup
from causing "morning sickness" - or sticking valves on startup when
using LL100, the universally available AvGas of today.

Lead buildup was the only thing that would damage spark plugs due to
running premium fuel - and again, that is no longer an issue..

The other advantage of using higher octane fuel in air-cooled engines
is you are MUCH less likely to destroy a piston due to detonation in
the event you do overheat the engine under load. Not usually an issue
with "L" head engines, but possible with today's higher compression
OHV engines.