Need For Manual Spark Advance ?
 

Group,

My 1933 Chev. had an automatic spark advance with a fine adjustment at the distributor for different values of fuel octane.

Did Ford use a manual spark advance to save money, or did he use it because the fuels were so poor in the 1920's that it was impossible to develop one advance curve for all types of gas ?

As a side note, is there a book that details "why" Ford designed the model A the way he did ?

Marc

Re: Need For Manual Spark Advance ?
 

The 32 ford had an automatic spark advance. Perhaps Mr Ford was just progressing slowly. Remember the model T had the obsolete low tension mag with 4 vibrator coils and a timer. The model A battery ignition was a big step for Mr Ford, who was not often accused of being modern, or up to date.

Re: Need For Manual Spark Advance ?
 

And he stayed with updraft carbs for quite a while (the story went that he liked that they didn't flood the engine), demanded the distributor and oil pump run off the center of the cam (again, a story), and thought people could sent their own advance without fancy magnets or centrifugal force.

I've always thought my A was an elegantly built, but simple car with superior materials (roller vs. ball bearings, no wood in the wheels, welded spokes, etc).

This is in stark contrast to later years when Ford, and all of Detroit, looked for wacko answers to Government demands, and used substandard materials (how many A starters have you replaced vs. the 3 in 5 years on my '73 GTO)?

Diesels built on Chevy small blocks set diesels back 30 years and counting in the US. Throttle Body Injection (just port or rail the injection). I remember the first time I saw an electrical wire running into the side of a Chrysler Carb! Plastic body panels on Caddys-you could make a living supplying replacements.

Novel (?) answers with cruddy materials at lowest price, but we bought them anyway because Furrin' cars were so weird (but reliable), and who cares if your body panel gaps change 1/4 an inch to the foot!

Maybe the Hammer had is right! Simple, elegant, chop your own wood to warm yourself twice, set your own timing and advance, set your own charging rate.

Re: Need For Manual Spark Advance ?
 

The use of automatic spark advance was a step forward, but still did not meet all the requirements for optimum combustion. A short review:

Combustion starts with a spark at the plug electrodes, which if not extinguished, develops into what we call a "flame kernel". This then grows expanding outward in a roughly spherical wrinkled "flame front". The speed at which it expands to consume the unburned mixture is a function of density, air-fuel ratio (A/F), turbulence and a number of other parameters. The flame speed is generally not twice as fast at 2000 rpm as it is at 1000 rpm so the spark has to be initiated earlier (relative to piston TDC) at the higher engine speeds. This was accomplished with "centrifugal advance" in traditional distributors (after the Model A for Ford). However, this "advance curve" could be developed for full throttle (or WOT, wide open throttle as we call it in the engine testing business) but did not take into account the fact that at part throttle the burn rate is slower. For that, "vacuum advance" was added. A spring-loaded diaphragm was linked to the movable distributor plate and was also connected to the intake manifold to sense vacuum. Driving at part throttle increases the vacuum which pulled the movable plate (carrying the points) to a position which advanced the spark timing even more.
The operator of a stock Model A sort of mentally performs both these functions simultaneously in order to adjust the spark lever at the position where the engine runs best. The operator is in fact performing what we call "MBT" (mean best timing) in dyno testing.

In modern engines we perform exhaustive testing of the engines at all kinds of speeds, loads, EGR rates, etc to establish "timing maps". These are essentially "look-up tables" loaded into the engine control computer which are used to set optimum spark timing with fewer or no moving parts.

Ken

Re: Need For Manual Spark Advance ?
 

Given the same circumstances of mixture strength, cylinder filling and compression ratio, the time taken for the mixture to fully ignite and burn is the same regardless of engine speed. At increasingly higher RPM the time available for this burn to take place is correspondingly less, so it follows that you have to start burning the mixture earlier in order for it to push on the piston at the right time. This is the basis for increasing the ignition advance.

A normally aspirated engine makes its best power when peak cylinder pressues occur between 14 and 18 degrees after top dead center.


Larry B.

Re: Need For Manual Spark Advance ?
 

With regard to the engineering answers...very interesting but don't you think that a car operator would not be able to do as good a job as a mechanical distributor system ?

When I'm driving the Model A, I'm engaged in watching the road and looking for the "trouble locations" around me. I try to set the advance for best operation, but I it's a basic "seat of my paints" if it's O.K.

Marc

Re: Need For Manual Spark Advance ?
 

Henry looked at the Model A as an evolution of basic car design, and also as a car to replace the Model T.

Just like the Model T, the Model A had to be economical to purchase, easy to use, realibilable and simple to maintain. Model T owners were very fimiliar with manually adjusting the timing as were most other car owners of the day. In 1928, Automatic spark advance was fairly new and mostly found on the more expensive cars.

Most mechanics understood the manual timing mechanism and most mechanics could set the basic timing of an engine, but a lot did not understand the automatic advance mechanism and if they did, did not necessarially have the tools to check it for proper operation and repair it when necessary.

By 1932, the public was demanding more features in their transportation and also there were more women drivers that wanted an easier car to drive. I think by then, most manufacturers had gone to a centrifical advance mechanism for their passanger cars.

Gasoline still varied a lot but by this time, "Regular" Gas was consistant enough that a basic advance curve could be built into the distributor that generally worked well for most situations. Actually, the curve in the early Flat Head V-8s was quite good, even by todays standards. The dealer or a good mechanic could adjust the initial timing a bit to compensate for really poor fuel or high altitude operation.

As far as operating a Model A, when the initial timing is properly set, you push the spark lever all the way UP for starting, about half to two thirds way down for stop and go traffic and slow running around town and all the way down for "the open road". This may not always be optimum but the engine will usually run just fine.

One of the main reasons Henry came out with the V-8 was Chevrolet already had a 6 Cyl. which was very smooth and "civilized". The '32 "Model B" was originally just supposed to be an evolution of the Model A with a "refined" 4 Cyl. Engine but in order to offer a competative product, Henry had to do something to counter the 6 Cyl. Chevrolet.

Just some thoughts on the subject.

Chris

Re: Need For Manual Spark Advance ?
 

Hi Marc ....

This may sound strange coming from an engineer, but as slowly as things happen with a Model A (most of the time), and given the fact that you can adjust the spark lever without taking your eyes off the road, I wonder if an experienced A driver who is sensitive to his engine sounds couldn't do just as good a job as we did in "mapping" engines. With just centrifugal and vacuum advance we still were compromising (compared to today's extensive maps).

At 2000 rpm you don't need twice as much advance as at 1000 rpm since turbulence is increasing (thus flame speed increases), but to predict the optimum spark timing was sufficiently difficult that we always developed the curves with dyno time. We still "validate" predicted maps on the dyno.

Ken

Re: Need For Manual Spark Advance ?
 

This may also sound strange coming from an Engineer but a Model A Ford is a simple car and I like to not make things all complicated.

Yes, theroletically, you can improve performance and fuel economy by always keeping the spark advance adjusted exactly for optimum engine operation, HOWEVER, any improvement you may experience when driving a Model A will be off set by constantly having to change the timing and also wearing out the spark advance mechanism.

I am a Mechanical Engineer having spent years in engine development and many many hours in the Dynamometer Cell. I am also a Race Car driver and feel I have a farily good ear when it comes to engine performance. I drive my Model As with the spark lever set as I have outlined in my previous post. I taylor the advance curves on my Race Engines for what I feel is optimum engine performance.

If someone wants to try to optimize the operation of their Model A by constantly adjusting the spark, that is fine with me BUT I think he is missing the enjoyment and simplisity of driving a car that has been around for more then three quarters of a century.

By the way, WHO SAID an engine should have twice the advance at 2000 RPM then 1000 RPM? Anyone with basic knowledge of spark ignition internal combustion engines would know this is not the case. :confused:

A Model A is a very simple and straight forward machine, Lets keep it and its operation that way.:)

Chris

Re: Need For Manual Spark Advance ?
 

Marc: it's worked for 80 + years. JMO
Paul in CT

Re: Need For Manual Spark Advance ?
 

Group,

Good comments so far. By the way, I'm not sure if anyone stated that you needed twice as much advance at 2000 RPM as at 1000 RPM.

It seems that with all the factors in play, that the advance needed is quite non-linear.

So, in summary, Ford used the manual system because it was the cheapest and most reliable.

Marc

Re: Need For Manual Spark Advance ?
 

Yes, that is my understanding.

Chris

Re: Need For Manual Spark Advance ?
 

I agree that the main reason was for the T owners changing from the T to the A.

Re: Need For Manual Spark Advance ?
 

I agree.

Simplicity is why many people are drawn to the Model A even today.

Controlling the timing manually is not some complicated science; it's basically just common sense. The best place for the timing is always just short of detonation.

If you hear any pings then you have the timing too advanced for the particular speed and load. Retard just enough to make the noise stop. At this point you will have the best power or peak cylinder pressure which occurs between 14 and 18 degrees after TDC on a normally aspirated engine.

I might add that anything that affects the temperature of the combustion chamber (even hot or cold weather) will affect the correct timing setting for a particular speed and load.

Heat increases as compared to cooler temperatures make the fuel mixture burn faster just like increases in turbulence and compression. This is why the timing sometimes has to be retarded when climbing a hill for example.

When climbing a hill the extra load put on the engine generates more heat which causes the flame to burn faster AUTOMATICALLY. If too much of the burn occurs too close or slightly before TDC the chambers get hot fast because the piston is harder to push down (in fact temporarily static for an instant and basically locked until the crankshaft rotates) or it is still coming up to the top. If the temperature is greater than what the octane will stand then here come the pings or knocks. Retarding the timing moves the best part of the burn beyond TDC and the piston is easier to push down so the heat becomes less and the knocks stop.

Simple.


Larry B.

Re: Need For Manual Spark Advance ?
 

No doubt excess heat causes knocking, but my understanding is that the main cause of the knock-inducing excess heat when pulling a hill is greater compression heating due to wide open throttle bringing in a larger air/fuel charge and thereby raising the pressure and compression heating in the combustion chamber. That's why knocking will occur in a poorly tuned car as soon as you step on the gas, even at the bottom of the hill and long before the motor has had time to overheat. As the motor warms up from laboring up the hill, a bad situation becomes worse.

Steve

Re: Need For Manual Spark Advance ?
 

Well your understanding is wrong.

What you describe as the cause of heat increase happens with wide open throttle whether on a hill or not.

On a hill, the resistance to the greater load is the main factor for the heat increase.


Larry B.