Ignition Timing For hi-Compression

[40 Deluxe]

I tried timing the ignition with the spark lever down 6 notches that equates to 10.7 Degrees Retard at the distributor. During a road test today (Oct 3, 2024) spark knock occurred with the lever halfway down (6 notches down). Next I will retard the timing 4 more notches for a total of 10 (17 degrees ignition retarded from the standard timing). My goal is get full use of the quadrant with the 6:1 hi-compression head. More rocket science to follow.

There seems to be an uh-oh here. You mention that "6 notches equates to 10.7 degrees retard.'' Actually, pulling the timing lever down ADVANCES the timing. Moving the lever all the way up is full retard for starting. As I remember, Ford specs are for ignition to occur at TDC with the lever completely up to prevent kickback when hand cranking. Then after the engine is running the timing lever is to be pulled down to advance the timing to the desired setting.
If you want "full use of the quadrant", yet not advance the timing too much, you will end up with excessive retard with the timing lever up.

 

[Bill G]

I try not to over think it. I time my 5.5 just like normal, and as such it takes a little less advance at full speed to get the full effect.

 

[Bob Bidonde]

Here is my understanding of ignition timing relative to a stock Model A.
The ignition timing is set at TDC of the Compression Stroke of Cylinder 1 in a stock Model A. The time it takes to reach Maximum Combustion Pressure is a constant but slow enough to require a spark BFTCD of the Compression Stroke.
When the Compression Ratio is increased, the time it takes for Maximum Combustion Pressure decreases but remains constant, so less Advance is required.
I am finding that the 6:1 Compression Ratio increases the combustion speed so much so that it is necessary to have "x" degrees spark Advance before TDC of the Compression Stroke for low speed acceleration & hill climbing, and "y" degrees Retard after TDC of the Compression Stroke to recover the full range of the spark lever quadrant cruising speeds. Thus at 30 to 55 MPH Cruising Speeds, the Maximum Combustion Pressure is being reached when the Ignition Timing is Retarded after TDC.
When the Maximum Combustion Pressure occurs before TDC of the Compression Stroke, the engine suffers spark knock.
Is my understanding theoretically correct?

 

[Jim/GA]

No, it is not.

Start here for a good compilation of work done on spark knock:
https://saemobilus.sae.org/articles/knock-a-century-research-03-15-01-0004

Most research is done on test engines at Wide Open Throttle (WOT). Most of us do not drive our Model A at WOT. So you have to be careful about translating the test results to your everyday driver.

When thinking about ignition timing, you need to consider flame front speed, which is a function of several things (compression ratio [CR], temperature, pressure [both impacted by CR], Air-Fuel Ratio [mixture], fuel being used, cylinder head design, and probably a few things I am forgetting because I'm doing this from memory). Directionally how each of these affects flame speed is known. For example, a lean mixture burns much faster than a rich one. Hot air-fuel mixtures tend to burn faster than cool ones. There is thermodynamics behind this.

Flame front speed dictates how soon before TDC you want to fire the spark plug, in milliseconds (ms). Depending on RPM, the number of degrees of crankshaft rotation that those ms represent gets larger and larger at higher and higher RPM. You can sit down and do some basic dimensional analysis and back into that. So even if the flame front speed did not change, the degrees before TDC that you want your spark to fire will increase with increasing RPM. But the flame front speed DOES change, as various conditions change, so it is not just about RPM.

You want peak combustion pressure a few degrees after TDC. To get that, you have to start the flame front with a spark some time (ms) before that. There is a delay until the flame front really gets going. That time combined with RPM converts to degrees BTDC. The flame front speed is not fixed, it varies (see above). In conditions that give a very fast flame front speed, you do not want to have the spark too early or you will hit peak pressure too soon. But if your flame front is slower, then you do want to advance the spark timing more, to get peak pressure at the optimum point in piston/crank position (not too late).

The attached advance curves include the stock Model B, which illustrates the point. This curve was determined empirically by Ford engineers in the lab and on the road, based on previous experience and an understanding of the basics. It is not ideal, it is an acceptable engineering solution.

Prior to THIS engineering solution, Ford's solution for the Model A was "TDC to start the engine, part way down once it starts (so fixed, not varying with RPM) and then advance it some more at higher RPM (again, fixed at the new, more advanced, position -- not varying with RPM). Instead of the Model B line, you get a Model A stair-step with a couple of flat spots. An engineering approximation that is "good enough". Of course, the stock compression head is very forgiving to having the timing be not quite optimum. Higher compression heads a little less so. But 6.0:1 compression ratio is not really "high".

Sorry this is so long. I hope it helps.

 

[alexiskai]

I am finding that the 6:1 Compression Ratio increases the combustion speed so much so that it is necessary to have "x" degrees spark Advance before TDC of the Compression Stroke for low speed acceleration & hill climbing, and "y" degrees Retard after TDC of the Compression Stroke to recover the full range of the spark lever quadrant cruising speeds. Thus at 30 to 55 MPH Cruising Speeds, the Maximum Combustion Pressure is being reached when the Ignition Timing is Retarded after TDC.
When the Maximum Combustion Pressure occurs before TDC of the Compression Stroke, the engine suffers spark knock.
Is my understanding theoretically correct?

There's a lot to unpack here and a lot of it is not correct.

You want peak cylinder pressure to occur at 20° ATDC, but it should never be necessary to have the spark occur ATDC in order to achieve this.

Increasing the compression ratio of the cylinder decreases the optimal degree of spark advance because the fuel-air mixture burns faster when the molecules are compressed closer together. However, the amount of decrease is relatively small, e.g., a stock engine might have optimal advance when cruising of 30° BTDC, while a 7:1 head might decrease that advance to 22°.

Spark knock is another term for detonation. The Wikipedia article on ignition timing is confused on this topic and should not be trusted. It writes, "If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly combusting mixture can actually push against the piston still moving up in its compression stroke, causing knocking (pinking or pinging) and possible engine damage, this usually occurs at low RPM and is known as pre-ignition or in severe cases detonation." This is wrong.

Pre-ignition is when the fuel-air mixture ignites before the spark occurs. This can be very bad for engines but it's also very rare. You wouldn't get pre-ignition just from messing with the advance lever.

Detonation is "when unburned fuel–air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used)... Detonation is characterized by an almost instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each pocket, and the cylinder pressure will rise sharply – and possibly beyond its design limits – causing damage." (also Wikipedia, but from the article on engine knock)

So when detonation is happening, it's not the case that the peak cylinder pressure is occurring before the piston reaches TDC, but rather that peak cylinder pressure is occurring in an explosive, disorderly fashion.

 

[GPierce]

My experience with the 6.0 head is the same as Jim Cannon’s above. Spark lever full retard timed to TDC. I have stop on the spark advance lever half way down that limits my advance to 26 degrees.
I have never experienced engine ping

 

[Synchro909]

My experience with the 6.0 head is the same as Jim’s above. Spark lever full retard timed to TDC. I have stop on the advance lever half way down. I have never experienced engine ping

I also run a 6:1 head and I made an adjustable stop that screwed tot he side of the distributor and had a screw like a throttle stop to stop the advance at 22°. I took that off because the engine was far too sensitive to timing as I drove through the rev range. As I took off in 1st gear, I'd have to advance the timing as I accelerated, then retard it again as I changed to 2nd and advance as the speed increased. Repeat for 3rd gear. It was worse than a PIA so I went tot he local ignition shop and had them make me a distributor with centrifugal advance to 22°. They also put vacuum advance on it. The car was transformed once I got it right.
My first attempt (a guess) was a little too advanced so I retarded it a little, drove on and tried to made it ping. After a few adjustments, I now have it right. The car starts on the first compression every morning if I have driven it the day before, 3rd compression if it hasn't been driven for a week.
BTW, I use 91 octane fuel - the lowest available here without ethanol.
On another engine, I run a 5.5:1 head and simply don't move the lever quite as far. The sensitivity of the engine to pinging seems to increase markedly at about 6:1. That's my experience, anyway. I realise cam timing, mixture etc all have an affect.

 

[Hitman]

Bob, you’re all over the place here. Trying to set the timing based on variables the differ from car to car.

For what you’re trying, you need a timing light and the crank degree indicator which will help with the settings.

If I set timing as you describe, I’d blow a hole in the piston or cause other damage.

 

[nkaminar]

Modern engines use a knock sensor to tell the engine control computer where to set the ignition advance. If your hearing is good enough you can hear the knocking when the ignition is too advanced. If not, use the knee in the advance curve. If you are using an distributor with automatic advance, temporally install a manual advance distributor and use that to find the knee in the advance curve. Then set your automatic advance distributor to that degree of advance. My contention is that the automatic advance using centrifugal weights will not account for whether the engine is under load or not. And that the manual advance, with an experienced driver, is better.

Except for starting, there is no occasion when the ignition should be after top dead center. In all cases the ignition should be advanced. The only exception is when you want to slow down the idle to a crawl to impress your buddies.

 

[Big hammer]

I am going to iterate the timing by adjusting the distributor cam with a goal of getting the full travel of the spark lever without spark knocking at full throttle over 1000 RPM running with a 6:1 Cylinder Head & 87 Octane Gasoline.

At the current standard timing of Zero Degrees at TDC of the Compression Stroke, I can only use 3 Detentes down of the spark lever (~-5 Degrees Retard), and this is not acceptable to me because it does not allow enough retard while climbing hills.

There are 12 Detentes on the spark lever quadrant of the Gemmer steering column in my Victoria. So the spark lever, with its limit of 20 Degrees in spark timing variation, varies the spark 1.7 Degrees per Detente (AKA Notch; 20/12=1.7). I will start my timing iteration with the spark lever down 6 Detentes at TDC of the Compression Stroke. This timing will enable a variation of +10 Degrees ATDC (Retard) & -10 Degrees BTDC (Advance).

I have installed an APCO temperature gauge to monitor the coolant temperature going into the radiator. My expectation is that the engine will run with a full throttle load without exceeding 190 Degrees F.

More to follow!

What can cause spark knock ? Wrong fuel, to hot spark plugs, carbon in combustion chamber, wrong timing.
I’am wondering if the internal timing is off , timing gear detent is off, possibly the fiber gear has slipped on the center part, or cam gear is advanced on the crank gear ?
3 clicks down on the hand lever is not right before spark knocking !
Bringing this back to the top, just trying to help Bob .

 

[nkaminar]

If setting the timing with the spark lever down a touch works for you then do it that way. I set mine up with the spark lever 3 notches down.

As the Big Hammer says, knocking, or pre ignition can be caused by a number of reasons. I little bit of glowing carbon can cause it.

Ignition advance is necessary because it takes some time for the fuel/air mixture to burn. If you look at the tests with transparent heads you can see that the mixture is still burning when the exhaust valve opens. The time it takes for the mixture to burn is mostly determined by the amount of turbulence in the combustion chamber. That is why, with the Ricardo head design used in the Model A the ignition should not have to be advanced very much or not at all with increased engine speed. The turbulence increases with engine speed.

 

[nkaminar]

Bob Bidonde, I just re read the entire thread. I think there is something funny with your engine. It may be, as someone suggested, that there is a misplacement in the dimple in the cam gear or the placement of the pin. There is no way that any engine will knock or even run correctly with a retarded ignition. The pin for a Model B engine indicates an advanced ignition position and the problem may be just this simple.

Here is my suggestion: Set the engine on TDC, #1 cylinder, using the pin. Take the #1 spark plug out and insert a stick or piece of wire to touch the piston and rotate the engine forward. The piston should start down a after a few degrees of rotation. If it is still coming up, rotate the engine until the piston stops moving. You should be able to rock the engine back and forth with the fan by pressing on the fan belt, or use a wrench on the crankshaft nut that holds on the pulley, to get an accurate crank location of TDC. If the dimple does not line up with the pin at TDC, then mark the location with a small drill with a shallow hole. Then take the cover off and drill a new dimple using a vacuum cleaner to collect all the chips. You could fill in the other dimple with JB weld after cleaning all the oil off with acetone.

Or use a Model A front cover, the cover with the pin in it.

After that you can time your engine with the advance lever 3 notches down, or any amount you want, as long as you can get the the knee in the advance curve with the lever about mid way.

Please report back what you find.