Measuring spark timing

JoeCB

JoeCB

Senior Member
Reading the current thread on point setting and spark advance, Raised a question... how to measure the actual advance on the car with engine running, like using a timing light as would be done on a modern car. This would require a degree wheel on the crankshaft, or at least marking the crank pulley rim in degrees.
Is the starting point at Zero advance when following the normal point setting instructions (using the cam gear pin) ?
And what is the design full advance at leaver full down and the distributor plate arm full stop in the distributor "window" ?
And finally, with a stock motor and modern fuel how much advance is optimal for high speed (50 MPH) driving.

Thanks

Joe B
 
You can buy a degree wheel and a indicator such as the one Bruce showed from Brattons. But the timing should be changed depending on conditions. It is not a set and forget.

A modern car has a computer to do the thinking for you. But in a Model A the thinking is up to you. The spark lever is there for you to adjust the timing depending on whether you are lugging the engine or cruising down a level highway at half throttle. In general the timing should be set at the knee, where advancing one notch will not change the engine but retarding one notch will. I usually try to be one notch on the retard side of the knee because I am trying to be kind to my engine, but retarding too much will burn your exhaust valves and cause other problems. I have a high performance engine but if you have a stock engine you can run right at the knee. After driving the car for a while you will get a feeling for where the ignition lever should be set for different conditions and it will become second nature.
 
First, full advance is supposed to be 40*. Personally I think thats too much.

Full retard should 0*/TDC or thereabouts.

Normal driving should be fine with lever at 8:30/9:00 assuming the lever moves from 10:00 to 8:00.

Just drive down a nice level road at 40-45mph and play with the lever. You'll find the sweet spot. Each car is different.

If you want to use a timing light with piece of timing tape, thats fine. I've not found the need.
 
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daren007 said:
Stop making a monster out of this. Use the Ford method and drive by feel and ear. Keep it simple.
Click to expand...



I agree. Keep it simple stupid. That’s the way it was designed and has allowed it to last 90 plus years.

Happy Holidays.
 
Patrick L. said:
First, full advance is supposed to be 40*. Personally I think thats too much.

Full retard should 0*/TDC or thereabouts.

Normal driving should be fine with lever at 8:30/9:00 assuming the lever moves from 10:00 to 8:00.

Just drive down a nice lever at 40-45mph and play with the lever. You'll find the sweet spot. Each car is different.

If you want to use a timing light with piece of timing tape, thats fine. I've not found the need.
Click to expand...

I agree with this if you're running a stock head. If you're running a HC head you may need to retard more than the standard timing will allow. I have been advised not to adjust the distributor cam more than 25 degrees BTDC. The problem with the timing strip being sold is that it only shows 10 degrees BTDC. My car with a 5.5 head ran fine up to a little less than 40 mph full retard. Beyond that speed it started hammering (pinging??). I tweaked the cam for more retard and it runs quieter at higher speed. I try not to go over 45. I'm not sure how much it is retarded but a timing light shows more than 10 degrees. I will further add that with all the hype about HC heads, I have not read too much about HC heads vs timing. I welcome further input on this topic.
 
I use the timing strip and set initial timing at 5* ATDC and use that for starting a idle for warm up. It is also useful a bit of extra engine braking going down hills. I run a Snyders 6-1 head and set the timing lever at about 9 oclock which I have set at 25* BTDC. (I have turned the steering column cover a bit so the lever will not pass 25* so I can just put it "all" the way down and forget about it.,With the timing light this comes out to about 23* BTDC and runs fine. Timing should be a less advanced for an HC head to avoid detonation. Under 25*BTDC seems common a popular with guys using HC heads.
 
Goodcar, Here is the scoop on timing and heads. The burning rate is determined by how tight the air and gasoline is packed together and how much turbulence there is. The combustion chamber design (squish area) and the engine speed determines how much turbulence there is and the compression ratio determines how packed together everything is. So a higher compression head with a good squish area will need much less advance. Also, as the engine speed increases the burning rate increases so that the needed ignition advance remains fairly constant (within limits) with increasing rpm.

There is another factor and that is the distance the flame front has to travel. So a small combustion chamber will need less advance. Or two spark plugs placed on opposite sides of the combustion chamber will require less advance. Old T head engines where the intake and exhaust valves were placed on opposite sides of the cylinder required two spark plugs because the combustion chamber was so large.
 
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Patrick L. said:
First, full advance is supposed to be 40*. Personally I think thats too much.

Full retard should 0*/TDC or thereabouts.

Normal driving should be fine with lever at 8:30/9:00 assuming the lever moves from 10:00 to 8:00.

Just drive down a nice lever at 40-45mph and play with the lever. You'll find the sweet spot. Each car is different.

If you want to use a timing light with piece of timing tape, thats fine. I've not found the need.
Click to expand...

This pretty much answers my questions, full retard (lever up) is 0/TDC. That is achieved by following the "Ford manual" timing procedure.
Full advance (lever full down) is supposed to be 40*/BTDC

Got it !

Thanks

Joe B
 
FWIW, I recently took the manual advance dist out of my coupe and put in a later model Mallory cent advance.
I set it up with 20° crank advance at 2000 rpm on external machine.

It works pretty good, I set initial at 2-3° and that was too much, backed it down to about 0°.

This motor has Lion I head and a touring cam among some other tricks.

John
 
nkaminar said:
You can buy a degree wheel and a indicator such as the one Bruce showed from Brattons. But the timing should be changed depending on conditions. It is not a set and forget.

A modern car has a computer to do the thinking for you. But in a Model A the thinking is up to you. The spark lever is there for you to adjust the timing depending on whether you are lugging the engine or cruising down a level highway at half throttle. In general the timing should be set at the knee, where advancing one notch will not change the engine but retarding one notch will. I usually try to be one notch on the retard side of the knee because I am trying to be kind to my engine, but retarding too much will burn your exhaust valves and cause other problems. I have a high performance engine but if you have a stock engine you can run right at the knee. After driving the car for a while you will get a feeling for where the ignition lever should be set for different conditions and it will become second nature.
Click to expand...

Thanks for the input, also thanks to Dave in MN for earlier PM advice. I'm surprised that vendors selling HC heads don't mention changing timing to reduce knock. Imagine a lot of babbitt has ended up in the oil pan of those who are unaware of the need to retard the timing--they enjoyed the extra power as long as it lasted.
 
Listen to your engine. Advance the timing just until it starts to ping, then back up a click.
 
Eagle yes it is so easy. Just listen to your engine and get a feel for how it’s running. No given spec will work. Each engine will have it’s sweet spot.
 
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