Hi Tom,
Glad to see you were paying attention to the two different springs in the
dizzy. A lot of
folks think the one is loose because it stretched or something. You are
absolutely
correct in regard to the stock settings and function of the distributor.
Perhaps I should
have made myself clearer that I was speaking about making settings and
modifying parts
outside the normal parameters as specified by the manufacturer.
As far as changing advance weights, when I was using the word "weights" I was
simplifying
my statement just to make the point that other modifications to the distributor
may be
necessary. You are correct in that you should not have any advance at idle.
If you want
your total timing to be 30 degrees, and your initial timing to be 4 degrees,
you need 26
degrees from the advance weights. While you don't have to change the weights
themselves,
you need to change the springs and that
alone may not give you the results you want. You may also need to alter the
pin that
contacts the upper portion of the dist. shaft - this acts as a stop at full
advance. No,
this is not all textbook material as covered in the manual - find any racer
willing to
divulge all his secrets and buy him a beer - you'll want to be his friend for
sure! ;-)
As far as the chemistry, octane is a compound from the same family as propane,
butane,
hexane, methane, etc. Its chemical formula is C8H18. That is 8 carbon atoms
and 18
hydrogen atoms per molecule. There are many chemical compounds in a gallon of
gasoline,
octane is just one of them. Your local library should have a number of volumes
on the
fundamentals of the internal combustion engine if you’d like to learn more.
Detonation, or pre-ignition as it is sometimes called, occurs when a small area
of the
combustion chamber reaches a temperature sufficient to ignite the air/fuel
mixture.
Usually, it is a carbon deposit on the valve that is what gets hot, but not
always. The
MG’s used a cylinder head with a pointy area between the two valves - on hi-po
motors the
point is ground back to avoid a hot spot which may cause detonation. Running a
lower
thermostat helps to keep the head cooler and reduces the chances of detonation.
That’s
why racers do it. Combustion efficiency actually goes up with heat, so the
engine is more
efficient at higher temperatures. That is why new cars run hotter thermostats,
reverse
flow cooling, and composite intake manifolds. If the head doesn’t detonate and
you can
keep the incoming air cooler, and therefore more dense, run the engine hotter.
21% thermal
efficiency is better than 20%, well, at least a little. On the new Corvettes,
Chevrolet
now pumps the coolant into the heads first rather than into the block first for
this very
reason.
Around here, there are a lot of ads on TV for premium gasolines. Read the fine
print down
at the bottom of the screen. They’ll say something like: "For cars that can
benefit from
higher octane gasoline." Meaning that not all cars will run better with higher
octane.
The oil companies just want you to spend $1.30 instead of $1.10. I doubt you
could ever
remove enough variables to get a dyno to show any difference in a properly
prepared
engine. The difference in milliseconds it takes to complete the combustion
process on one
type of fuel to another is splitting hairs between theoretical and empirical
science.
Bottom line is if you don’t need it, save your pennies. :-)
Regards,
Brian
Tomislav Marincic wrote:
> Brian,
>
> 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
> 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
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