Anyone know how far ATDC 15 degs is. Measuring piston position with a dial gauge?
Go to Tulsa club website this link should take you there
Larry Young shows 0.0873" = 15 degrees ATDC
Dave, Here is another way to accurately determine 15 degrees ATDC based on crank shaft rotation. See pages 5-7
(Message edited by mkossor on June 28, 2018)
Timing is only set for starting. Put the piston on the firing stroke so it's just over TDC and set the timer so the points just start to fire with the lever up. After that it just does not matter because you are adjusting for best running and that depends on YOUR motor etc. By the way, for 99.9% of the T's on the road, that would be, the crank pulley pin just below the 3 o'clock position looking at it.
If you think of the crank pulley pin as the hour hand of a clock, TDC is 3 O'clock; 15º is halfway between 3 and 4.
The above are all great ways to determine crankshaft position. If you do have a drop gage similar to this, 15 deg. ATDC would be as Mike Bender shows is 0.0873”. This number was sourced from the chart Mike provided.
(Message edited by AzBob on June 29, 2018)
I'm with Mark on how critically important this isn't. I engaged the crank and turned it until the pulley pin indicated TDC and marked with tape on the front apron where the crank was. I then marked about fifteen degrees past that with another piece of tape and moved the crank to that spot. I turned the key to BAT and rotated the timer to where a coil just started buzzing, switched off, put the timing lever all the way up and bent the timing rod to fit.
Mark & Tim have pretty much explained my long-standing thought that ignition timing does really not have to be all that precise. Think of it this way,.....for any given rpm and particular load requirement, there is an ideal "sweet spot" when the spark should occur for maximum efficiency. If the ignition timing is anywhere close at all, that "sweet spot" will exist somewhere within the range from maximum retard to maximum advance. When you go to all kinds of lengths to set the timing very precisely, the only thing you do is move that ideal "sweet spot" a tiny bit toward maximum retard or maximum advance. That very same sweet spot still exists somewhere within the maximum retard and maximum advance range. In other words, all that precision when setting timing merely means that to cruise along with maximum efficiency, your spark advance lever might be at a tiny bit different position on the quadrant. And my opinion is,....SO WHAT! Who cares,....it doesn't really matter, right? Again,....just my opinion,.....for what its worth,.....harold
Got to thinking a bit more about this ignition timing thing, and, there's one more way to look at it that sort of backs up what Mark and Tim and I are saying:
Starting around the late '30's and all thru' the '40's, '50's and well into the '60's, most American cars had the same basic systems for "automatic" ignition timing and I guess what you'd call "automatic spark advance". Visualize if you will, the common "low-priced-three" with their inline 6-cylinder engines. When the engine was not running, either the whole distributor body, or, at least the plate upon which the points were mounted could rotate a few degrees, but was in the retarded position which was for first starting the engine. As soon as the engine started, intake manifold vacuum instantly advanced the spark timing by rotating the distributor (or plate the points were mounted upon) a few degrees by means of a vacuum diaphragm that did the INITIAL spark advance automatically, similar to the way we might slightly advance the spark lever a bit as soon as the Model T engine starts.
In what we'll call "the mid-century modern" systems, all additional spark advance, after the initial vacuum advance, was accomplished automatically by means of centrifugal weights attached to the spinning distributor shaft that advanced the spark in direct proportion to increased rpm of the engine and thus, also the rpm of the distributor shaft.
Now then, here's my main point,.....all of this "automatic spark advance" was not very precise, as it did not fully compensate for the amount of "load" on the engine, because the centrifugal weights only sensed distributor shaft rpm. In other words, most of this spark advance was merely a compromise as designed by the Detroit engineers, and was no more precise than our "seat-of-the-pants" approximation with our manual spark advance Model T Fords. Again, all this to say that finding that perfect "sweet spot" is not only nearly impossible, but really, not all that important, because the Detroit engineers didn't think so either!
Okay,....I'll shut up now, except to repeat once again that I think all of this very "precise" setting of Model T (and for that matter) Model A as well) spark timing is a waste of time! FWIW,....harold
As soon as those cars started the vacuum advance did NOT advance the timing.
Most of those cars , Dodge, Plymouth, chrysler, Studebaker, Chevrolet, Ford, etc., had a a static and idle timing of one to five degrees.
When the engine was started the timing did not change until the accelerater pedal was pushed. At that point the vacuum advance took over.
Some also had centrifugal advance that advanced the timing when the engine speed was increased past an idle speed.
The ‘57 Chev V8 was an exception, it got full vacuum to the distributor as soon as the motor started.
A fifties Ford had only vacuum advance.
Timing was set at tdc or one before tdc. as soon as the accelerater pedal was pushed the vacuum advance pulled the timing up to a maximum of 11 degrees BTDC.
There were after market distributors that had centrifugal advance.
If you look at the distributor on a forties or fifties Chevy 6 you will see the whole distributor turns advancing the timing when, and only when the accelerater pedal is pushed.
If you hand crank your T you should set the timing so the coils don’t buzz until the piston is 5/16 of an inch down after TDC to be sure it will not fire before top center and kick back.
That will allow for a little slop in the linkage and error on the safe side.
If you set the timing too much retarded with the spark lever way up you will not be able to advance it enough for high speed economy and power.
Also if the spark timing is not advanced enough at high speed the engine usually runs too hot.
Since cars of the forties, fifties and sixties were not hand cranked the static timing was not required to be after top dead center.
Most were a few degrees before TDC. Some were five before, some as much as 10.