Hot Rod Magazine has an excellent on-line article about recurving GM HEI distributors. However, the ignition timing requirements of propane-fueled engines are slightly different that of gasoline engines. Jay Storer (Economy or Performance Propane Fuel Conversions for Automotive Engines (1986-01-24)) 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, another rule of thumb is 14-14-14, which is 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.

Manufacturer Device Part Number
Moroso Mechanical Advance Kit 72300
Moroso Adjustable Vacuum Advance 72315
Mr Gasket Mechanical Advance Kit 929
Mr Gasket Adjustable Vacuum Advance 31075
Mr Gasket Mech & Vac Advance Kit 6011
Pertronix Mechanical Advance Kit D9001
Pertronix Adjustable Vacuum Advance D9006
Proform Mechanical Advance Kit 66948C
Proform Adjustable Vacuum Advance 66952C

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.



Custom Marks


If timing tape is not available for your engine, cut a thin strip of paper to the segment length (as calculated below) and tape it to the damper with one end in the 0° TDC mark. The strip of paper should then be placed in the direction of rotation to show the advance timing mark lining up with the stationary TDC mark on the engine.

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

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"


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:

Blue Streak PN
Advance Starts
(" Hg)
Maximum Advance
(Distributor Degrees)
Maximum Advance
(@ "Hg)
VC-24A 8-11 6-18
VC-171 7-9 14-16
VC-207 5-7 9-11


DIY Advance

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.




Programmable Advance

Vacuum advance for engines other than small block Chevys is a bit more challenging due to the lack of available parts. One alternative is to use a fully programmable ignition system like a MegaJolt Light, which is integrated with the MegaSquirt injection system. It is also available as a stand-alone ignition system from AutoSport Labs.

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