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Re: TR-2,3,4A - Retard timimg for use with 92 octane gas?

To: Triumph list <triumphs@Autox.Team.Net>
Subject: Re: TR-2,3,4A - Retard timimg for use with 92 octane gas?
From: Tomislav Marincic <>
Date: Thu, 29 Oct 1998 22:00:33 -0500
        RE:" If your distributor weights are stamped
with a 12, then that is 12 degrees distributor advance."

        Not exactly. That means 12 degrees is physically
possible, but most year TR6 distributors were equipped with
weights stamped "13" (26 deg advance at crank) but were 
limited to less mechanical advance at redline by the larger of
the two springs on the weights. Yeah, you might get 26 deg at
8000 RPM, but if you check the specs in the Bentley manual
you'll usually see a lower figure is usually quoted.

        RE:" 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."

        I disagree. There should be zero mechanical
advance at idle RPM with any distributor I've ever heard of. 
Again I'd refer you to the Bentley manual. Also, you don't
recurve a distributor's low-RPM performance by getting 
"new weights with more timing built in" but rather by
getting new springs with more or less resistance. 

        As for your chemistry, well...
Is octane really "added" to fuel? Does low octane fuel
really boost performance in *any* situation? Is detonation
really what happens when two flame fronts "collide"?
        May I ask for a source or reference?

        Best Regards,

        Tom Marincic

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
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


Brian Schlorff
Power British

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