LPG/CNG-specific Timing Advance Processors (TAPs) for older carbureted vehicles are now obsolete. The simplest solution for carbureted dual fuel applications is to determine which fuel is used most often and optimize the timing advance for that fuel. Operation on the lesser-used fuel (ie, gasoline) will have somewhat lower fuel economy, which would be added incentive to return to LPG/CNG operation.
The key getting the best power and fuel economy in any engine is to have the spark occurring at the point in the combustion cycle that generates the maximum torque on the crankshaft at all times and this requires adjusting your initial, centrifugal, and vacuum advance. A good starting point with LPG is the 14-14-14 rule: 14º initial, 14º centrifugal, 14º vacuum. Adjust for best power (initial & centrifugal) and then for best fuel economy (vacuum). Bear in mind that fuel mixture changes can require changes in ignition advance.
LPG/CNG fuel mixtures need a hot strong spark to ignite the fuel mixture. If you're using a points & condenser ignition system, upgrade to electronic ignition. If you already have an electronic ignition system (Ford, Mopar, etc), upgrading to the GM HEI system will give you a hotter spark and many of the necessary components can easily be sourced from scrap vehicles. See HEI Ignition Upgrade
Why convert to GM 4-pin HEI for older vehicles?
- GM introduced the 4-pin HEI system to help it meet emission regulations back in 1974.
- The 4-pin HEI module is compatible with other variable reluctor electronic ignition distributors of other makes.
- The 4-pin HEI module was designed to fully saturate (charge) the coil at normal engine speeds and includes dwell-control and current-limiting circuitry, which avoids the need for a ballast resistor. A fully saturated coil provides a long, hot spark that allows for wide (0.045") spark plug gaps to ignite lean fuel mixtures with EGR.
- HEI modules are very reliable and the elimination of the ballast resistor further increases the reliability of the ignition system. Replacement modules are inexpensive, readily available, and easily replaced. Like ballast resistors, it is still a good idea to keep a spare in the car just in case.
- The 4-pin HEI module will work with ancient points & condenser canister coils but works best with a low-resistance (0.5 ohm) e-core coil. See HEI Coil- E-core vs canister
- Why is the GM 4-pin HEI module so great? See ignition coil, electronic ignition
- How does the GM 4-pin HEI system work? See GM four pin Module and HEI/High Energy Ignition.
There are timing controllers available with dual advance maps but these are more complex to install and configure that the old splice-in controllers. We are aware of 3 timing controller manufacturers with products that could be used for dual fuel applications: