Propane engines make higher demands on the ignition system compared with that required of a gasoline engine.  This higher demand is due to the characteristics of the fuel which requires a higher ionization energy.  While it might seem that simply creating a voltage across a gap results in a spark, there few things that happen before a spark jumps across the spark plug electrodes.

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Spark Plugs

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Recurving Distributor Ignition Advance

Hot Rod Magazine has an excellent on-line article about recurving GM HEI distributors.  However, the ignition timing requirements of propane-fuelled engines are slightly different that of gasoline engines.  Jay Storer recommends that maximum mechanical (centrifugal) ignition advance not exceed 28°-30°.  Full advance should come in by 2250 to 2500 RPM.  Initial advance should be 10° BTDC.  Total advance (mechanical + vacuum) should not exceed 40° to 46°.  Similarly, I have seen other recommendations for 14° initial, 14° mechanical advance (total mechanical = 28°) and 14° vacuum advance (mechanical + vacuum = 42°).

This recommendation is a guide for the starting point for relatively stock gasoline engines converted to propane.  Modified engines (such as higher compression) will likely require timing curves different than those provided above.

To check whether my own timing is within range, I painted timing marks on my damper for 28°-30° and for 40°-46°.  I replaced my centrifugal timing weights with a Moroso HEI Advance Curve Kit (PN 72300) and my vacuum advance pod with a Proform PN 66952C adjustable vacuum advance.

Although the Moroso instruction sheet shows 24° crankshaft degrees all in by 5000 RPM with the lightest springs, my observation shows that it actually comes in fully around 2500 RPM.  My measurements show that 12° initial advance provides about 30° maximum mechanical advance using the lightest springs of the Moroso kit.  Other manufacturers centrifugal advance kits should be similar.

Vacuum Advance

Because less dense fuel mixtures (as you would find under high vacuum cruising conditions) burn at a slower rate than denser fuel mixtures (as you would find at wide open throttle), the spark needs to occur sooner in the combustion cycle.  Engines were provided with vacuum advance to provide this additional advance at high manifold vacuum levels.  Vacuum advance was simply a vacuum-operated piston that pulled the breaker plate or magnetic pickup assembly around the distributor shaft.  Engines came with a variety of vacuum advance units from the factory satisfy the sometimes conflicting requirements of emissions and fuel economy.

The specifications of the various OEM vacuum advance units may be found in Motor or Alldata manuals.  However, these are now often considered obsolete by OEMs so exact replacements can only be found by searching junkyards.  Failing that, aftermarket suppliers still offer replacement units but they often consolidate their parts so that one part fits a wide variety of applications.  For Delco HEI ignition systems, Blue Streak lists the following vacuum advance units suitable for propane applications:

Another alternative is to try an adjustable vacuum advance for performance applications.  I have tried to find out adjustable vacuum advance specifications from a number of manufacturers but not one has given me a good reply.  From the similar appearance of the various brands' vacuum advance units, it would not surprise me that they all buy their parts from one manufacturer and put their own name on it.

For those who are not familiar with these devices, the adjustment simply limits the maximum advance. There is no way to adjust the vacuum level at which advance starts.  However, I was able to measure the Proform adjustable vacuum advance and came up with the following specifications:

Total Number of Turns	11
Vacuum Advance Starts	1" Hg
Vacuum Full Advance	9" Hg (with 11 turns)
Advance per Turn	3° (measured from 12° @ 4 turns)
Maximum Advance	33° (estimated)

Obviously, with the Proform unit, you would have maximum advance under most normal operating conditions.  From the crude measurements that could make from my timing light, full vacuum advance seemed to be closer to 20° than 33° when the advance was turned out completely.  The centrifugal advance, which is of course RPM-dependant, may have had an effect on my measurements.  Use timing tape, painted-on damper markings, or an adjustable timing light for verification on your own vehicle.

Making changes to the Proform vacuum advance is quite easy since Proform has provided a vacuum can with six sides.  Lining up the included Allen key with one of the corners of the can would allow you to make changes in 1/2° increments.  Turning the Allen key clockwise increases the maximum advance.

Start off by gently turning the adjustment counter-clockwise until it stops.  This is 0° of vacuum advance.  Readjust your Allen key so that the tip is pointing up.  As a starting point, turn the key 3 full turns.  Remember that vacuum advance is a device for maximizing your fuel economy and that your engine will likely never produce audible knock from excessive advance with propane due to its high octane rating.  Drive you car through several tanks of fuel and note the fuel consumption.  Increase the vacuum advance by 1 turn (clockwise) and drive for several more tanks of fuel.  Repeat this procedure until your fuel economy stops improving.  Reduce your timing (turn counter-clockwise) in 1/4 turn increments to the position where your fuel economy is best.

The calculation for the painted-on timing marks is quite straight forward:

Damper Diameter		6.75"
Advance		28°
Advance Length	= 28° / 360° x 2 x PI	0.489 radians
Segment Length	= 6.75" / 2 x  0.489 radians	1.649"

For those of you who aren't interested in doing the math and making a template, I can make you a paper template for a small fee.  I will need your damper diameter and advance ranges in degrees.

Dual Fuel Advance Curve

When running a dual-fuel setup, ideally you should have both a gasoline advance curve and a propane advance curve.  For older vehicles, Dual Curve makes a dual advance curve ECU for these applications under PN 40622.  I contacted Dual Curve about the operation of their 40622 ECUs and they gave me the following response:

Regarding timing advance, the PN40622 is designed to give an additional 8-10 degrees of advance, after the distributor is recurved.

Please refer to the PN40622 instructions pg. 2:

“The Timing Recurve will electronically delay the factory set timing signal 15 degrees. The user must mechanically move the distributor timing ahead or advanced 15 degrees. Add 15 degrees to the original factory gasoline set point. With the 15 degree delay for gasoline, the Recurve can then provide timing advance for the higher octane LPG/CNG fuel. At idle on LPG fuel, only 10 degrees (CNG fuel, only 13 degrees) additional advance will appear on your timing device. The other degrees (5 LPG or 2 CNG) of timing are used to compute a smooth, accurate timing for RPM changes. After the (gray wire loop) fuel timing program is set-up and selected. The fuel selector switch will select the proper timing program via the yellow wire.”

You can use this ECU for straight LPG or CNG applications too.  I believe the reduced timing advance when on LPG at idle would make the engine easier to start, which would be the main advantage for installing it on a straight (or dedicated) propane vehicle.  However, I found that I have not had any starting problems with the advance curve used at the top of this page.

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