Ignition Timing For hi-Compression

[Bob Bidonde]

My question is, "How many degrees ATDC at the distributor do you set the ignition timing by changing the distributor cam position clockwise to accommodate hi-compression 5.5:1 & 6:1 cylinder heads?"

My guess is 10 degrees ATDC.

I have 6:1 and 5.5:1 cylinder heads on Model "B" engines equipped with Model "A" distributors and Model "A" timing covers. I use 87 Octane gasoline. The camshafts are stock Model "B." Both engines have stock Model "A" Zenith carburetors.

From experience, I know that the hi-compression cylinder heads require less spark advance than the stock 4.22:1 cylinder head. This becomes apparent when using the spark advance lever with standard timing of zero advance at TDC because only a notch or two of advance can be used without the engine knocking.

[alexiskai]

There is no reason to set initial timing any amount ATDC under normal operation and particularly not on the basis of higher compression. It's true that the timing curve "shrinks" with higher compression, but artificially "restoring" the timing curve by retarding your initial timing beyond TDC has no practical benefit.

[Big hammer]

I set my timing a couple of clicks retard so that when driving the timing lever is closer to normal ! When I use pure 87 octane most of the time I don’t get pinging unless it’s very hot outside. I do run the GAV 1/4 open for a nice color on my plugs, leaning out causes more pinging. With my points closed up closer to .018 I can run the timing lever more advanced, when I set the gap at .022 is when I get pinging with less advance. 6.0 head, stipe 330 cam, larger valves, zenith A carburetor

[alexiskai]

Originally Posted by Big hammer

so that when driving the timing lever is closer to normal

That seems like the only reason to do this – to keep the muscle memory of where the lever is supposed to sit.

Seems not worth it to me unless your habit is to fully advance the timing almost immediately on startup.

[Jim Brierley]

You'll want to set the max advance to about 26* BTDC at full RPM on the 6:1, maybe a little more on the 5.5

[katy]

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[Dick M]

I have the 6.5 Head. I use the usual timing method, test light on point arm until the light goes out.
I then retard the cam a smidge more. Why? Because when the engine is further retarded it starts much easier. I then run the spark advance almost all the way down to compensate and get proper running timing. This works for me on a Burtz engine with a touring cam and a 6.5 head.
JMO

[The Master Cylinder]

Originally Posted by Jim Brierley

You'll want to set the max advance to about 26° BTDC at full RPM on the 6:1, maybe a little more on the 5.5

This makes more sense than adjusting the initial timing ATDC. I set my total advance at 28° BTDC. I have a Snyder 5.5 head.

[nkaminar]

Even two engines with the same compression ratio will require different timing. It could be due to slight differences in cam timing. Anyway, the best method to find the optimum timing is to search for the knee in the timing curve. That is where one click more retarded will slightly slow down the engine and one click more advanced will make no difference. The knee can be found at a fast idle, maybe 1,200 rpm. However it will change with driving conditions, such as pulling a grade or cruising down a level highway at a constant speed. Use the computer under your hat to vary the timing according to conditions.

[The Master Cylinder]

I should have mentioned I have a Nu-Rex Auto-advancer on my engine.

[Hitman]

Always set it for total advance, most A’s would be happy in the 26-28* BFTC.

Don’t set it for initial timing or how many clicks or 9:00 position.

[Bruce of MN]

I find it handy to time for TDC for starting and then use a stop to limit the total advance if that is less than the quadrant provides.

[Bob Bidonde]

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!

[Jim/GA]

Originally Posted by Bob Bidonde

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!

Bob-
The distributor advance is a maximum of 20 distributor shaft degrees and that translates into maximum 40 degrees of crankshaft rotation advance, because that's the way it works.

My cars have 10 clicks from top to bottom across the entire quadrant, and that results in 40 degrees of crankshaft timing change, so that is 4 crankshaft degrees change per click.

Henry said to time them at TDC.

If your engine knocks with 2 clicks of advance (8 crankshaft degrees), you have another problem that is giving you a lot of advance when you think you are at TDC. You're not.

[Jim Brierley]

Originally Posted by katy

26* btdc?

26 degrees BTDC (Before Top Dead Center), at maximum, or near max, RPM.

[Bob Bidonde]

Hi Jim,
I am aware that 20 Degrees of distributor timing = 40 Degrees of crankshaft timing because of the 2:1 gear ratio between the crankshaft & camshaft gears. My math is correct. Each detente of the spark lever quadrant 20/12=1.7 Distributor Degrees = 3.4 Crankshaft Degrees.

[The Master Cylinder]

Originally Posted by katy

26* btdc?

Originally Posted by Jim Brierley

26 degrees BTDC (Before Top Dead Center), at maximum, or near max, RPM.

Originally Posted by Bob Bidonde

Hi Jim,
I am aware that 20 Degrees of distributor timing = 40 Degrees of crankshaft timing because of the 2:1 gear ratio between the crankshaft & camshaft gears. My math is correct. Each detente of the spark lever quadrant 20/12=1.7 Distributor Degrees = 3.4 Crankshaft Degrees.

Bob, I think Jim was responding to Katy, not you. Just to keep thing straight...

[alexiskai]

Originally Posted by Bob Bidonde

this is not acceptable to me because it does not allow enough retard while climbing hills.

I feel like I'm missing something here. You're saying you've got your car timed so that initial timing is TDC, and when you're cruising at high rpm with low load, you get knock at 10° BTDC (crank degrees)?

If you find that you're needing to retard your timing to ATDC while driving to avoid knock, um, something is wrong.

[Bob Bidonde]

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.

[Jim/GA]

Nothing that you report here is consistent with my experience with a 6.0 HC head.

I have 3 cars with 6.0 heads in them. I time every one to exactly TDC. I run all over the place here with the spark lever 1/2 way down (~20 crankshaft degrees of advance BTDC) with no problem. Certainly no knock.

At speeds up around 40-45 MPH I add a click or two of additional timing advance (so now around 24 or 28 crankshaft degrees BTDC) and they run great.

Climbing a steep hill near me at 50 MPH, I retard it back to 1 or 2 clicks less than 1/2 way down. No knock.

So something on your engine is not right (bad bearing?), or the way you are setting your timing is not right, or the sound you are hearing is NOT engine knock. I don't know which it is.

It will really be more clear to others reading this string in the future (especially new owners without much background in the Model A) if you would only talk about crankshaft degrees of advance or retard. That is kind of the industry standard for timing, and is what you would read out with a timing light, if you used one.

Good luck with it.