Barry wrote:
>
> With all the talk about whether or not to keep the vacuum advance units
> on distributors, and the lack of understanding of what exactly the
> function of these are to some individuals, I thought I might try to
> enlighten a few of you as to what the function of the vacuum unit is.
> This is no means definitive and I don't purport to be an expert, but I
> have learned a few things over the years (ahem, your kidding, Paul
> McCartney had a band before wings?) and thought I would share this
> information for those interested. Also for the purposes of this
> discussion the following only applies to maximum efficiently or power,
> and not the control of harmful emissions. That said, and all
> disclaimers applied, a little simplified internal combustion basics -
>
> Engines, to operate efficiently, require the spark to fire at some point
> BEFORE the piston reaches TDC. This is to allow the explosion to build
> enough pressure (push) on the top of the piston, at just the right time,
> to provide optimum power. If it is started too soon (advanced) then
> this explosion reaches piston while it's still traveling upward and you
> lose power, (trying to push the piston the wrong way) waste energy, and
> create heat in the combustion chamber area (and usually knocking or
> detonation from an explosion instead of a nice smooth flame traveling
> from the upper cylinder to the piston top). If started too late
> (retarded) then you loose power because the piston is already traveling
> downward, before the flame explosion can "push" it. This also creates
> heat in the surrounding combustion chamber because remember, heat is
> energy. This energy, if not used to push the piston, is released either
> into the surrounding water jacket or the exhaust manifold instead of
> powering your vehicle. Both are inefficient as far as maximum power is
> concerned, but it makes an effective heater! As the engine RPM's
> increase, given that the flame propagation speed remains the SAME, then
> the combustion cycle needs to be started earlier to achieve the desired
> "push" on the top of the piston. Also, as the pressure (more fuel/air)
> inside the cylinder increases, then the less advance the engine can
> handle at a lower RPM (bigger explosion). So as you can see it depends
> upon the speed (RPM) of the engine, AND the amount of air/fuel mixture
> (throttle position) that the engine is operating at. OK, internal
> combustion 101 out of the way -
>
> Now that we understand (I hope) why we advance our timing it should be
> clear that as the engine speed (RPM), and combustion pressure (amount of
> air/fuel mixture, or volumetric efficiently) increases then the timing
> has to advance or retard accordingly. (there is a maximum amount but
> that depends on many variables, that we needn't go into for the purpose
> of this discussion). So, say the engine is idling. Very low cylinder
> pressure (load), very low speed. Since there is relatively little
> fuel/air mixture inside the cylinder, then we need to advance the timing
> quite a bit (say 30 deg before TDC for this discussion) to start the
> flame at the proper time. As the RPM's increase and more fuel/air is
> introduced BUT, no or very little load is applied, then the cylinder
> PRESSURE remains fairly constant and therefore we can use the same
> amount of advance (30 deg). A purely mechanical advance wouldn't
> achieve the same advance at idle as it would at 3000 or 4000 RPM (unless
> of course it had almost no spring pressure retarding the centrifugal
> weights controlling the advance mechanism). However, the intake
> manifold pressure is very low (high vacuum) so we can use this to
> advance the timing, via one side of a diaphragm connected to this
> source, and the other to the breaker, or sensor plate in the
> distributor. Now say your cruising at 2000 RPM little load, again low
> cylinder pressure, optimum advance (30 deg) engines happy. Suddenly you
> snap open the throttle. Now you have maximum cylinder pressure, low
> engine speed and advance needs to be at say 12 deg to prevent
> detonation. If the advance were purely mechanical again, and set for
> optimum advance (30) at the no/low load condition, then we would have
> too much advance for this high load condition, and one unhappy engine
> because of detonation. However, during high load conditions, the intake
> manifold pressure drops to zero (equals outside manifold pressure or no
> vacuum). IF the mechanical timing were now optimized for high load, low
> speed conditions (12 deg@2000 RPM), then the vacuum unit can optimize
> the timing at light or no load conditions (30 deg) because it is in
> effect not operating at high load conditions, and the mechanical advance
> can be optimized for high cylinder pressure or maximum load conditions.
> So in this case, when you stomp on the pedal, the timing (at 30 deg
> light load, relatively high vacuum) would drop back to 12 deg, because
> the vacuum is now not operating, as stated before, the manifold pressure
> increased (vacuum dropped to zero) and the diaphragm returned to it's no
> vacuum position. In this way, timing can be optimized for all engine
> conditions. For racing, and max power applications, you don't really
> need a system for controlling advance at low or no load conditions
> because these engine are operating at maximum power most if not all the
> time. (and is one reason why some tend to overheat at idle) Also,
> another reason that early emission systems with idle retard, or advance
> cutouts have a provision that during extended idle periods, when the
> engine begins to overheat, it restores PROPER advance to prevent that
> overheating!
Barry, I am about to swap my Lucas Electronic distributor with vac
advance to a Mallory dual point, all mechanical.Based on your note, does
this mean the engine will be a dog at low RPMs?
Stu
74TR6 80TR8
|