Hey Listers,
In my never-ending quest to get my Spitfire back on the road after a
2-year-long 6-month engine rebuild, I'm now trying to sort out the timing
problems. It will run, but at low rpms or with timing set to about 10-12
degrees BTDC, the timing seems to bounce around quite a bit. In fact, it
has a hard time running at that timing. My Dad sent me the following from
an E-Type list, which seems to imply that mechanical advance only on a
street engine won't really work. I should say that before the engine
rebuild, it ran fairly well, just no real power. Idled fine, didn't stall
(after warm-up). And it always only had mechanical advance (the vacuum
retard was never hooked up). Here's what he sent:
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<< Can anyone explain the rationale and the effect
of vacuum advance Vs vacuum retard? If any of us should be inclined to
purchase a Mallory unilite, it would be nice to know the differences
between the vacuum advance and the mechanical advance units that are
available. >>
Ok... Ignition 101: class is in session
For every given design of engine, there is an optimum time to fire the spark
plug. This optimum time produces the most efficient combustion, and the best
power, while maintaining correct conditions to maximize engine life. (little
or no engine knock)
As RPM increases, this optimum point must occur earlier for the combustion to
reach its full potential. It is a form of inertia in the combustion process.
A centrifugal, or mechanical advance works strictly by RPM. the faster the
engine turns, the more the spark is advanced by mechanical means... up to the
limits built into the advance mechanism. Also, the design and weight of the
advance flyweights and their springs determine the exact relationship of RPM
to engine timing. This is called the advance curve.
This type of advance works great for engines that have a given, fixed load,
such as race engines, where they run flat out most of the time.
The problem is that cars driven under normal road conditions see widely
different loads, at all ranges of rpm. To maximize performance, and also
economy, a method of optimizing engine spark advance in relationship to load
as well as rpm is needed. This is where the vacuum advance comes in.
engine intake manifold vacuum varies directly with engine load. When the
engine is under full throttle, the carb butterflies are wide open, and as a
result, manifold vacuum is low. When the car is coasting down a hill, with
your foot off the gas, the carb butterflies are closed and manifold vacuum is
high. Under normal cruise conditions, the engine vacuum is somewhere in
between. Now if we hook up a line to the manifold, and connect that line to
one side of a rubber diaphragm, and hook the other side of the diaphragm to
the distributor, we now have a method of varying the advance in relationship
to engine load. Below is the simplistic explanation of how it all works:
Light load = high vacuum = more advance = greater economy
heavy load = low vacuum = less advance = no destructive engine knock
Now some newer engines use a combination of vacuum advance and vacuum retard,
mainly for emissions control.
Some Turbo charged engines use a pressure retard mechanism to retard the
ignition timing at full manifold boost.
The aim of all these systems is to have a method of varying ignition timing
according to load.
Modern computer controlled ignition systems measure engine vacuum, throttle
position, engine rpm, engine and outside temperature and engine knock
detectors, to then compute the optimum advance to use.
To put it briefly, the purpose of mechanical, (centrifugal) is to increase
the
spark advance as rpm increases. The reason for this is that as rpm increases
the time that piston spends near top dead center becomes shorter. The speed
of
the combustion also increases with rpm but not rapidly enough to keep up with
the faster moving piston. As a result, in order to make sure that maximum
combustion pressure is developed at about 15 degrees after top dead center,
the charge must be ignited earlier on the compression stroke. That's what
centrifugal advance accomplishes.
Vacuum advance compensates for a different combustion phenomenon. At part
throttle, there is less charge in each cylinder than at full throttle. This
less dense charge burns more slowly. Therefore, in order to compensate for
this slower combustion, the spark must also be fired earlier on the
compression stroke. This advance is achieved with a diaphragm that response
to
manifold vacuum. Remember that at full throttle there is very little vacuum.
As you back away from full throttle, intake manifold vacuum increases. This
increase in vacuum moves the diaphragm and advances the spark to compensate.
Vacuum retard moves the spark int he opposite direction and it was employed
as
an emissions control device to sacrifice power and efficiency for reduced
emissions at certain speed-load points.
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Someone not long ago asked about Spitfire advance/retard, but I don't
recall seeing any answer. Can anyone comment on the above in general, or
my problem specifically (at the top)??
Thanks,
George Parker
George R. Parker | THINK -
AHNTECH, Inc. | It's good practice for when
(540)663-5936 | the computer goes down.
|