John,
You should use the lowest octane fuel you can provided the engine shows no
signs of
detonation. Octane actually makes the fuel harder to ignite, hence it's use in
very high
performance engines. These engines make their power from the compression, not
the octane
in the fuel. An engine's power output is a funtion of the pressure created in
the
combustion chamber above the piston. The more compression, the higher the
pressure at the
moment of ignition and the more "push" down on the piston.
As the air/fuel mixture is compressed, the temperature goes up. The more
compression, the
higher the temp. At some point, the fuel may spontaneously ignite prior to the
spark plug
firing. This is detonation and it is very bad for an engine because it often
occurs just
before the plug fires. You then end up with two flame fronts in the cylinder
that crash
into each other which can crack pistons and rings, etc. Most often in TR
engines, I find
cracked ring lands (the area between the topmost two rings) to be the most
likely victim
of detonation damage.
Octane is added to the fuel to reduce the chances of detonation. In doing so
it actually
slows the combustion process. The lower the octane, the faster the flame front
spreads
thoughout the combustion chamber - this yeilds higher cylinder pressure just
after the
piston passes TDC, and theoretically more power for any given compression ratio
provided
there is no detonation. Bottom line - for best performance, run the lowest
octane you can
that does not cause detonation.
As far as timing goes, you must do some experimentation to find the best
combination for
your engine. Most motors will make good, reliable power with a full advance of
28 - 34
degrees total timing. Double your distributor advance, then subtract from your
desired
total timing to find your initial timing. (e.g. If your distributor weights
are stamed
with a 12, then that is 12 degrees distributor advance. 12 degrees x 2 = 24
28 - 24 = 4
degrees before top dead center initial timing.) You must make timed runs and
compare
results to find what is the best for you. The amount of advance the engine
will tolerate
depends on the quality of fuel you're using, the general condition of the
engine, and the
atmospheric conditions in which you live. First, make sure your carbs are
tuned properly
and that your car is fully warmed up by driving for a few miles. Then find a
flat length
of secluded roadway to make some tests. Start on the low end of the scale, set
your
timing, and make a timed acceleration run in top gear between 30 - 60 mph
(helps to have
an assistant in the car with you to record times with a stopwatch). Advance
the timing a
few degrees (try 2 degrees to start) and retest. If at any point, you hear
detonation,
stop the run and retard the timing until the detonation goes away. You will
find your
runs get faster to a point and then fall off again. If you continue advancing,
detonation
will set in. Once you find your fastest timed run, set the timing to that
figure provided
it is at least 4 degrees away from the point at which you heard detonation.
I bet if you spend an afternoon experimenting, you'll be surprised at how much
better your
car will run afterward!
BTW if you find that the car idles too high after setting to your best
performance, you'll
need advance weights with more timing built in to bring the idle back down
again.
Regards,
Brian Schlorff
Power British
http://www.powerbritish.com/~britcars
John Cowan wrote:
> Spec for timing of Triumph four bangers is 4 degrees before true dead
> center. But this was written with 96 octane gas in mind and required. Is
> anyone retarding spark to two or zero degrees as a matter of course in
> order to deal with the the 92 octane gas that is the best grade available?
> This was recommended to me.
> Thanks.
> John Cowan
>
> BTW, I toy with the idea of using a mixture of 92 octane and 100 octane
> Aviation gasoline.
>
> Visit - In Their Own Words Website - an on-line non-commercial publishing
> experiment, plus movie reviews, articles and Left/Progressive links at:
> http://home.earthlink.net/~jfcowan
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