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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