I should know this, but don't. When everything is properly adjusted and the engine is running well with the spark lever fully advanced do the plugs fire before TDC, at TDC, or after TDC?
Should be slightly before TDC.
This diagram assumes you have the initial timing set as Ford recommended and running on the magneto.
Magneto Fig8.pdf (48.5 k)
Come on guys, Henry is asking.. with the spark lever fully advanced...., sure, it's before TDC.... but what is the total advance on magneto ???, and the total advance on battery ???
Very interesting diagram. One question; it is mentioned that the magnets have been advanced 7 degrees ?
I had been lead to believe that the 41 degrees of total spark advance was normal for the T magneto ignition
My understanding is that a optimal total spark advance for virtually all gas engines is around 32 degrees.
SO does this lend credence to a theory that retarding the T flywheel about 9 degrees would be beneficial???
Of course it leads me to see a advantage in doubling the number of magnets and cutting the space in half between the magneto coils might be beneficial on something like a Montana 500 car!!!
I have found that my T pulls highway hills much better with the spark set back 1/2 way. I can pass quite a few cars on hills that way
Ron, thanks for the timing diagram. Let me see if I understand it correctly.
-This diagram is for one spark plug/coil (the diagram is identical for the other plugs when it's their turn to fire).
-The crankshaft is turning counterclockwise.
-Top dead center is at the 12:00 position so the degree markings to the left of TDC are after TDC (retarded) and degrees to the right are before TDC (advanced).
-The band labeled "timer segment fully retarded" represents the degrees of crank rotation that the roller is in contact with the timer segment for one spark plug/coil. The same is true for the "fully advanced" band when the timing lever is moved to fully advanced.
-When fully retarded: even though the roller is in contact with the timer segment for the whole 87º there are only four sparks that take place because those are the times when the magneto has provided sufficient voltage to make the coil operate.(In other words, the magnetic field was strong enough to pull the coil contact points open).
-If the engine were poorly tuned such as carburetor trouble, then the cylinder could misfire when provided with the spark and then we have to wait 22.5° of crank revolution for the next available spark to occur. It's like having a second chance to get the cylinder to fire, although it will be late and power will suffer.
-When fully retarded: If this Model T was switched over to battery the plug/coil would be firing continuously (buzzing) for the full 87° of crankshaft rotation. If the engine was in good tune and running properly we would expect the fuel/air mixture to ignite at the moment the timer roller first touched the segment - 15.5° after TDC - and then the rest of the continuous sparking is redundant and doesn't really matter.
Please correct me if I got it wrong.
If you study the diagram I posted you will clearly see the answer to your questions.
He is the same diagram showing how it works on battery.
Magneto Fig 7 R.pdf (28.4 k)
We conducted some tests moving the flywheel position in an effort to find a better spot for the 18.5 BTDC spark location, but the results were inconclusive. Afterwards I commented to you that there was something we did not understand about the optimal spark advance curve for the Model T engine.
The two diagrams above are part of an article we published to explain in simple terms how the Model ignition system worked.
The Model T Ford Ignition System and Spark Timing
-Do not think about cranksahft travel, only BTDC and ATDC.
-Yes. The fourth spark likely does not occur.
Thank you for all your hard work. Lots to think about!!!
There are two complete answers to Henry's original question. One for running on battery and one for running on magneto. They are not ALWAYS BTDC or ATDC, but normally BTDC.
OK, I think we are over thinking this whole issue. The gas/air mixture burns at a fixed rate. Pressurization in the cylinder makes a minute change in this rate. As the engine speeds up, that set burn time necessitates that the burn starts sooner so that the peak burn occurs just past TDC. The faster the engine turns, the sooner the burn needs to initiate. Therefore, a high RPM engine will have a greater degree of advance needed. Remember now, the Mosel T is a relatively low RPM engine so the degree advance needed is a little less. On startup, the e gone is turning very slow. That is why we need to retard the spark to avoid firing prior to TDC and suffering a kick back.
As far as the coil buzzing continuously on battery. It is not continuously buzzing. The electricity flowing through the coil causes the field to build to a high voltage followed by a collapse in the voltage. This occurs in rapid cycles of a relatively set frequency dependent on the structure of the windings. As the field builds to a certain strength adequate to pull the contacts down against the spring action, the points close and the current is directed to the plugs. The difference between battery and mag is that the battery supplies DC continuous power to the coils at a constant voltage and the mag supplies AC voltage to the coils at a strength dependent on engine speed.
All in all, sounds complicated, but it really isn't, just remember, it is like hunting ducks, or shooting down planes. You have to lead the target, and the faster that target is going, the more you need to lead it. And if it is a sitting duck, just aim right at it.
Oh, forgot one thing, because of that building abd collapsing field, the advance works in stepwise jumps along the course of the lever movement. And the battery and mags are a little different due to differences in the electrical properties.
Perhaps my choice of the word "continuous" was ambiguous. "Vibrate continually" would have been a better choice. About 200-400 vibrations per second based on how the points are adjust.
You used the word "step" which is how by moving the spark lever it selects the magneto current pulse to fire the cylinder charge.
Here is another article I wrote expanding upon the nuances of the Model T Ford ignition system.
More on Model T Spark Timing
As you can see from the attributions near the end these articles were extensively peer reviewed by Model T experts in several fields prior to their publication in the Vintage Ford and Model T Times magazines.
You guys are amazing. I thought it was a simple question. Thanks for all the responses!
Ron, thank you for sharing your article. I am sure you know much more about electrical properties than I have ever known and forgotten based on what Uncle Sam taught me over 40 years ago at avionics school in the Air Force.
It amazes me what the designers of Henry's electrical system knew in order to design it. Or were they just good at playing with it until it was at its best?
I hope you didn't think I was criticizing your comments, Ron. I just thought a little understanding of how it works would better explain the initial question.
No offense taken, my wording was at best imprecise.
It is apparent to several of us who studied the design of the Model T Ford ignition system that in some areas the designers knew exactly what they were doing and in others it appears to be trial and error. We will never know because the designers notebooks have never been found.
The big difference between running on battery and on mag is that on battery the spark will continue to advance with each notch the lever is advanced. On Mag, it depends on which peak or magneto output is reached at any particular point on the spark lever . So with battery, the spark comes as soon as the rotor makes contact with the segment and on magneto it will come a little later than on battery. When you advance the spark lever on magneto, you will notice that the spark does not advance with every notch but you move a few notches and then it advances noticeably.
As far as where to run the spark it depends on engine speed. As the engine speeds up it likes a more advanced position. As it slows down it will run smoother as the spark is retarded a few notches. So with experience, you will find the best position for the spark lever for your driving conditions.
I am still amazed of the ingeniuity of the Model T ignition system given that it was designed in 1908.
And in respect, I do what I can to make my T's run on the original system.
They deserve that.
Excellent good simple explanation.
I've seen some T owners that were afraid to mess with the spark. Someone had told 'em back when, that you retard to start and advance to drive and that's all they do. Some are so afraid to mess with it that they want to convert to a system that does it for you and I'm not just talking about T's here. A's too. Don't be afraid of the spark lever. Mess around with it under various conditions and see what it can do for you. Try to get to highway speed with it fully retarded and see why you need advance. Retard it a little on a steep hill and feel that little extra burst of energy. Embrace it, don't be afraid of it. It's not so mysterious after all.
Henry: The are told to fire in the firing order as needed.
They are ordered to fire 1243 and no changes are allowed, do it in this order or else!!!!
Just kidding Henry, and have a great day and go out and enjoy the ride
They say timing is everything!
My question is very simplistic and pragmatic compared to the information you've given here on this thread. But, my car was running about 30, 20, 20, 0 pounds of compression (wet and dry) so I needed to have the valves ground. The restorer suggested I put on an aluminum head and a 7.5 degree timing gear.
Now. the car starts brilliantly and has a lot of power, but it sounds like someone is taking a tire iron and banging it on a metal garbage can. If i retard the advance almost all the way to fully retarded, it sounds better, although still noisier than it used to be. of course, it used to be a bear to start.
(Surprisingly, I always had a fair amount of power.)
Is there anything normal- or harmful- about running with the advance so near the top?
Sounds like the timing is off on your engine if it runs best retarded and perhaps knocks = preignites if you advance the timing lever.
The sound of the engine does get somewhat harder with more compression, so some of it may be the new nature of the beast.
Running actually retarded tends to overheat the engine, so if it doesn't glow red in the exhaust manifold after a short while, it can't be all retarded with the lever up - but then there's a problem with starting if you can't retard it properly - you may get kickbacks that bends the starter axle or break your arm, so the timing has to be set right.
Here's a guide by Steve Jelf: http://dauntlessgeezer.com/DG97.html
What kind of Aluminium head did you purchase?
Good question by Ron
There have been cases where Z heads hasn't been just bolt on - they needed some grinding for clearance of the pistons. Pistons making contact with the head makes just this kind of noise you're describing.
There have been other cases where rod bearings that were about to be loose started to sound with just the increase of compression from a high compression head. Some adjusting of the rod bearings was all that was needed.
A new thread may give more answers, though you have some ideas on what to check now
(don't run it much more with such noises)
Thanks, Roger. This is very helpful.
Ron, I'm not sure. I'll ask him today to see which kind it was. I think I'm getting 70 pounds of compression which is great when you come to the big hill, but the sound is really awful.
I love this car so much. But, I enjoy the variations- it might start great today or it might be a bit reluctant. Today the sweet spot for the advance and carb adjustment is here and tomorrow it might be a little different.
Weird, I know.
Thanks again, Roger. The guy who put the head on was just here and he said he thinks it's rod bearings as well.
I just put a Prus head on my T and had to remove a small amount of material in every combustion chamber. It was right where the squish area started at both sides of the combustion chamber. That was without the head gasket in place. Works great! Also I get no knock at all using a 7 1/2 degree advance gear and full advance. PK
A little off topic perhaps, but amplifies the comment made about what they did back then. The fire control system used in the South Dakota and Iowa class battle ships (designed by Ford) was an electro-mechanical system. Tons of gears and knobs. When they brought the Iowa ships back into service in the 1980's they spent a ton of money trying to figure out how to make the 16 inch guns more accurate with a digital fire control system. In the end all they could do was add a doppler radar to measure velocity on a round out of the number one turret, and apply that to the rest of the rounds fired with that powder lot number. The system was designed in the 30's.
We aren't perhaps as smart as we like to think these days. Or they were not an ignorant in their day.