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RoyJ

wrote:

Needs to be corrected for "standard atmosphere" which is 70F at sea
level. At 70F and sea level that 9.4HP "10HP" engine would quite
likely excede its rated power.

Doubt it. The test cell is room temp, elevation is 835 feet.

We didn't try any of the 84 to 85 octane gas we get in Colorado, we
figure that might have gained a solid 10% right there. The lower octane
has slightly more fuel value per pound plus it doesn't have any alcohol
to lower the fuel value.

Actually, premium gas goes not NECESSARILY have lower power density -
and up here if you want to guarantee no Ethanol you buy Shell PREMIUM.

MOST fuel vendors in Canada lace the low octane stuff with up to 10%
Ethanol. Mixed with 0% premium, you get 5% mid-grade.

Not here, many states mandate 10% ethyl alcohol. The Colorado fuel
mentioned is only sold at altitudes above 4000 feet, the thin air allows
the lower octane in a carburated engine. Doesn't fool a good computer
controlled engine with O2 sensors, hence the car mfg recommendations to
not use the stuff. But it does improve the power output in low
compression, lean jetted small engines.

MOST small OHV engines actually prefer high-test unleaded fuel,
possibly for that reason.

But the fuel BTU density is such that lower octane fuels have more BTU's
per volume, hence make a richer air:fuel mix. The small engines we are
talking about have compression ratios around 7.5:1 or 8:1, more than low
enough to handle the low octane fuel.

Most 2 stroke (like snowmobile) engines also prefer premium.

Most snowmobile engines are much higher performance than the typical one
lunger 4 cycle lawn or small industrial engine, like or need the high
octane.

Jim Wilkins wrote:
On Apr 4, 10:02 pm, RoyJ wrote:
...
Not really, we have run these small engines on a legitimate dyno at one
of the major lawnmower mfg facilities. (They paint their stuff red) The
engines tend to put out less than the engine mfg lists: something like
95% of the nameplate number down to not much over 50%
...
Do you know why? Compression, carburation, ignition?