The article said these were "typical" or base numbers with timing set at
34 degrees full advance. Spark plug placement, piston design, chamber design,
cam and ignition timing all have a part to play. There test octane engine was
a pretty basic design.
NOTE:
Automotive octane ratings are determined in a special single-cylinder engine
with a variable compression ratio ( CR 4:1 to 18:1 ) known as a Cooperative
Fuels Research ( CFR ) engine. The cylinder bore is 82.5mm, the stroke is
114.3mm, giving a displacement of 612 cm3. The piston has four compression
rings, and one oil control ring. The intake valve is shrouded. The head and
cylinder are one piece, and can be moved up and down to obtain the desired
compression ratio. The engines have a special four-bowl carburettor that
can adjust individual bowl air-fuel ratios. This facilitates rapid switching
between reference fuels and samples. A magnetorestrictive detonation sensor
in the combustion chamber measures the rapid changes in combustion chamber
pressure caused by knock, and the amplified signal is measured on a
"knockmeter" with a 0-100 scale [104,105]. A complete Octane Rating engine
system costs about $200,000 with all the services installed. Only one
company manufactures these engines, the Waukesha Engine Division of Dresser
Industries, Waukesha. WI 53186.
Doug
James Barrett wrote:
> At 07:58 AM 4/12/99 -0400, you wrote:
> >I don't have the figures in front of me but but if I remember correctly the
> >100LL
> >av. gas octane is about the same as the 92 M*R/2 pump gas. The ratings for
> >the two different. Pump gas is rated by taking the Motor rating and the
> >research
> >rating and averaging the two. Aviation gas just uses the higher rating. Are
> >you sure
> >you need 100+ octane? If doesn't ping or knock on pump gas higher octane
> >will not help you HP. If you do need the higher octane I would use the pump
> >gas
> >and add 10% Toluene (124 Research octane if I remember correctly) from a
> >paint
> >supply house. The higher the octane generally speaking the energy in the
> >fuel goes
> >down and you have to burn more of it to get the same HP.
> >
> > Compression Octane Number Brake Thermal Efficiency
> > Ratio Requirement ( Full Throttle )
> > 5:1 72 -
> > 6:1 81 25 %
> > 7:1 87 28 %
> > 8:1 92 30 %
> > 9:1 96 32 %
> > 10:1 100 33 %
> > 11:1 104 34 %
> > 12:1 108 35 %
> >
> >Avgas is less dense than most racing gasolines. Instead of weighing about
> >6.l to 6.3
> >pounds per gallon like racing gasoline, it weighs 5.8 to 5.9 pounds per
> >gallon. The racer
> >must compensate for this by changing to richer [larger] jets in the
> >carburetor when
> >changing from racing gasolines to Avgas.
>
> Doug,
> I am not sure I agree with your table. I previously ran
> the original 289 Tiger motor with modified 351 W heads and pop up
> pistons that had a very carefully calculated compression ratio of 10.5.
> It worked fine on 93 octane. My 351C with flat top pistons and 9.5
> compression also does not ping on 93 octane. In both cases the timing
> was advanced a couple of degrees from factory stock and there was a non
> stock camshaft ( but not outrageous in lift or timimg.)
> What gives?
> James Barrett Tiger II 351C and others
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