Spark Advance Hop-up Trouble Spot
By Barney Navarro
Motor Life Feb 1954
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 starters 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 engines 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 governors 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.
Fords 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 butterflys 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 isnt 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 wasnt 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 chargesat 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 Mercurys 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 doesnt do the job correctly, can prove to be the most costly. The bestway 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.
Cool stuff huh? Did ya get all that? ?
Before seeing that essay by Mr. Navarro, I wrote this, on another message board:
....There are good reasons to use ported vacuum distributor advance and good reasons to use manifold vacuum distributor advance.
If you want to progressively add advance as RPMs increase in addition to the mechanical advance which also does that, then you want ported advance. I can see wanting this kind of setup for drag racing to get an unfluctuating reliable advance throughout the 1/4 mile. his would definitely need to be dialed in on a Dynamometer every tune up. I have an old Ford flat head inline 6 that has to have ported advance because it doesn't have any mechanical advance at all.
If you want to have stop and go traffic driveability and a high advance for cruising at highway speed with good mileage. and enough advance at idle to not be running retarded and over heating, then you want manifold vacuum.
If you set your initial timing that high the engine will be hard to start or wont start at all, so you set it at 12º± with the vacuum disconnected, then hook it back in to manifold vacuum which is high at idle, and the V/advance advances it up to the 25º±.
Then you drive on down the road, your RPM goes up to 2200-2500 RPM and all your mechanical advance is in so that's around 36º-40º advance, plus the vacuum advance brings it up to around 50º± advance.
Why isn't it pinging? it has a light load. If you had 50º advance with ported vacuum you'd have to run racing or aviation fuel to keep from detonating. but with manifold vacuum, if you punch it to pass or whatever, the manifold vacuum drops the advance retards to a safe 38º± and you don't detonate even on regular gas.
Anyone, if you heat up in traffic and are running ported vacuum, switch it to manifold vacuum after setting your initial timing to whatever degree gives you 36º-40º all in mechanical advance, with the vacuum advance not hooked up at all. then hook up to manifold vacuum distributor advance.
I set up my new 262" SBC engine last month by making an extra timing mark at 38º BTDC on the dampener, then with the vacuum disconnected and plugged, I brought the Revs up to around 2500-3000 RPM, and then moved the distributor so that my timing light showed this new timing mark at 0º on the indicator. At idle that gives me around 13º BTDC initial advance then I hooked up the manifold vacuum advance. I can let it sit and idle all day in gear at 181º F, with a 180º thermostat. and a couple of weeks ago, with only 200 miles on the new engine it might have got up to 183º pulling a hill, with no detonation on the cheapest regular. (timing retards pulling hills, so no pinging)
So it's your choice. Driveability or maximum racing power.
Now on that old Cadillac, it may have a vacuum advance/mechanical advance setup designed to work with a specific carburetor. like the Ford Mr. Navarro described. So when you change cams and carbs you throw the system all out of specification. The best thing to do in that case is determine what the distributor actually does, probably on a distributor machine so you can check and adjust/modify the mechanical advance to what you need and the same with the vacuum advance. My Motors repair manual says to disconnect the vacuum advance to set the timing which leads me to believe it used manifold vacuum, or had a dual source setup IN the carburetor similar to Fords in which case it has to work with that one specific carb if it uses the "ported" source.
Another note, I have three different "stock" Chevy vacuum advance units and they each will pull different amounts of degrees, ranging from 12º to 25 degrees! so they aren't all equal either. you have to check what you are dealing with in your application, and after all these years, the "stock" parts you have may not be as stock as you think.
