Motor Life Feb 1954
Spark Advance Hop-up Trouble Spot
By Barney Navarro
Among the mistakes made in hopping up engines, few
exceed in number the misapplication of spark advancing principles. The
chief source of error is the limited information available on the subject of
spark lead. That which is distributed, unfortunately, fails to cover some
essential factors and very often is no more than a comment to the effect that fuel
charges take a certain amount of time to burn so spark must be advanced enough
to compensate for the time lapse.
Well informed engineers wish that the problem
really was that simple. Most ignition system purchasers overlook every factor
except the amount of spark produced. The wrong system can cause plenty of
trouble: plug fouling, poor gas mileage (even though the engine has no tendency to
misfire), overheating in slow traffic, and other maladies. Basically,
engines require some means of advancing spark timing as rpm increases
since the pistons, in effect, try to get
ahead of the burning speed of fuel charges.
Combustion, witch takes a definite length of time, must occur when
pistons are at the top dead center before the start of the downward power stroke. If
burning finishes too early, energy is wasted because the resultant pressure rise
produces a force opposition to rotation. This is readily apparent when
starting an engine that has too much
spark lead; it will actually kick back
against the starter’s efforts. Modern high compression engines, while under full
load, audibly indicate spark that is too far advanced by pinging. So the
popular method of setting spark timing for maximum horsepower is to set it just
below the ping point under full throttle operation. Distributors that employ
flyweight governor advance mechanisms use a spark advance curve that
conforms to the engine’s requirements under full throttle at any point within the rpm
range. At low rpm a lesser spark lead is required so the governor advances a
small amount. As speed picks up it advances more and more, always conforming to
the full throttle full load requirements. On a drag machine, where full
throttle and full load conditions are maintained, the flyweight governor is
required. But for ordinary driving, which consists mainly of partial throttle
operations with Very light loads, it is not enough. Some other means of
compensating for varying loads must be provided. The load compensator is necessary
because a light fuel mixture burns more slowly than a heavy charge since the
concentration is less and flame takes longer to travel from one fuel
particle to the other. If the utmost energy is to be obtained from light charges, their
burning should be completed at the same point that the heavy charges
finish. So if they take longer, the only way to make them finish at the same point is
to start them earlier.
Consequently, partial throttle partial load
operation requires more spark lead at any given speed than is required at full
throttle full load. Load compensation is the most commonly achieved by using
intake manifold vacuum to actuate a diaphragm. This diaphragm advances and
retards the distributor breaker plate and
in some cases the whole distributor case.
When the engine is operated with Very light throttle pressure, the manifold
vacuum is high, so the diaphragm advances the spark timing to produce the
most efficient combustion possible. As the throttle is depressed, the vacuum drops
of and the diaphragm produces less advance until it reaches a point of
being completely ineffective at wide open throttle. Thus the ideal
load compensation is always maintained and
results in more power from every drop of
fuel.
The second most popular method of obtaining load
compensation, though further from perfection is that employed in Ford V-8
distributors from 1932 trough 1948. Instead of a diaphragm, there is
piston brake actuated by manifold vacuum, The flyweight governor mechanism is
equipped with a breaking disk which cancels five degrees of the governor’s
advance when pressure is brought to bear on its edge. At this edge a spring-loaded
piston is located in a small cylinder. The spring is on the side of the
piston opposite the disc so it causes
the piston to be pushed against the disc.
Vacuum is introduced on the spring side to oppose its action and lift the
piston off the disc. In action, the high vacuum produced by operation with small
throttle openings lifts the piston of the disk, allows the full action of
governor weights to take effect and gives the Ford engine five degrees more
spark advance. By depressing the throttle further, the manifold vacuum drops
off and the spring again pushes the piston against the disc to retard the spark.
The flaw in the operation of this mechanism lies in the fact that it is either
“full on or full off” and permits no gradual compensation like the
diaphragm.
Ford’s latest method of controlling spark
advance employees an ingenious system utilizing manifold vacuum and venturi
vacuum. With this system the flyweight governor is eliminated and in its place
is nothing but a diaphragm. This diaphragm not only advances the spark to conform
to rpm changes but is also makes load compensation adjustments. All ´49
through´54 Ford and Mercury carburetors have in addition to the conventional
manifold vacuum takeoff, such as is found in the throttle body of most passenger
car carburetors, a connecting venturi vacuum passage. The manifold vacuum, as
usual, is obtained from a small port in the throttle body located
slightly above the butterfly’s closed, position, on the side where the butterfly swings
upward to open. When the throttle is closed at idling, the vacuum port does
not receive vacuum because it is on the opposite side of the butterfly. As the
throttle is opened slightly,
this port is uncovered and a vacuum is applied to
the distributor diaphragm to advance the spark. If the throttle is
fully depressed, the manifold vacuum is destroyed and no advance takes place. As
speed increases, however, the venturi vacuum increases gradually and
advances the spark to conform to the rpm. Letting up on the throttle increases
the manifold vacuum (Provided it isn’t let up all the way) and the spark receives
load compensation. A balance is always maintained so that the correct amount
of spark advance is supplied for all speed and load conditions.
The greatest installation errors center around the
misunderstanding of the late Ford distributors. A distressingly large
number of mechanics are unaware of the difference between manifold vacuum
and venturi vacuum. In fact many attempt to operate Ford and Mercury
distributors by connecting the vacuum line to the windshield wiper connection on dual
intake manifolds. This sometimes results from a desire to use the old
Stromberg carburetors, which are not equipped with vacuum takeoff. So the simple
solution seems to connecting the
distributor vacuum line to the handiest
apparent source of vacuum. Such practice is worse than having no spark control at all
for when the engines idles the spark advances fully and retards as throttle
is depressed. There is no venturi vacuum available to advance the spark as the
speed picks up and it remains retarded until the throttle is let up. So if
the old style carburetors are preferred, the stock Ford distributors must
be discarded on the late models. However, Stromberg has resumed production of
the old “97” and is now fitting it with a venturi vacuum takeoff to make its
use feasible. Four throat carburetion installations also have
had their share of improper distributors. Early articles in certain
publications gave the impression that no vacuum control whatsoever could be
tolerated. It wasn’t pointed out that the only forbidden type is that of the stock
`49 through `54 Ford and Mercury distributor. This caused many to purchase
distributors and magnetos that were equipped with flyweight governors only. Such
installations get very poor gas mileage, so the car owners blame the four-throat
carburetor. Even more irritating, is the tendency for spark plugs to
foul. Having no load compensation, the spark is never far enough advanced under
partial throttle to fire the fuel mixture charges at the most opportune time.
In effect, the engine is being operated with a lower effective compression ratio
because burning is completed as the pistons travel down the cylinder
bores. And since the plugs never
receive a hot flame, soot collects on them.
Furthermore, the condition cannot be remedied by using hotter plugs because they
will burn up under full throttle operation of flyweight governor distributor with vacuum-operated load compensation device.
In practice, the installation of dual intake
manifold on Fords and Mercury’s of the ’49 trough ’54 series should be
accompanied by a change in distributors such as prescribed in the
preceding paragraph. The addition of two carburetors divides the airflow so only half
as much airflows through one carburetor as previously at normal operating speeds.
Venturi vacuum is dependent upon the air velocity through the venturi so any
reduction in velocity will result in less spark advance. And connecting a line
to each venturi vacuum takeoff
of a dual set up will not increase the
vacuum---such a practice is just a waste of copper tubing. The best advice to
keep in mind when purchasing a distributor is not to pinch pennies. An
inexpensive unit, if it doesn’t do the job correctly, can prove to be the most costly.
The best way to avoid mistakes is to study the problems involved and learn
enough about them so that you can select a distributor that matches your
engine requirements.