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.
|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.
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