I'm moving forward with my plan for an LPG powered Stovebolt - my '55 1st series 3600 (3/4 ton) had a 250 engine which has been recently rebuilt, and I stopped at the point before I bought carbs and intake/exhaust to start a family...
Now getting back to this project, and with gas prices above $2 a gallon - and California's SB 708 potentially reinstating smog for cars older than 30 years (though my '55 should still be exempt) - I thought I'd give this a try. Did some research, and here's the plan so far....
The truck will be LPG only, not a dual gas/LPG. It's difficult to get optimal mileage with a dual system, and filling stations in California are close enough that maybe a 300 mile range is sufficient. Shooting for about 20mpg.
Will be utilizing a dual intake with two Impco 425 "mixers" - carburetion not required as LPG is already converted to a gaseous state. These will be controlled by a closed-loop feedback system that monitors the O2 levels in the exhaust and adjusts the fuel-air mixture appropriately. With this kind of system the fuel mileage should be better than a carbureted gas system, but not quite as good as fuel injected. Close though!
The 250 head was mildly ported and polished, and will be exhausting through some nice cast iron manifolds. Two O2 sensors will be plugged into the exhaust pipes just south of the manifolds to measure and control the fuel mixture.
One of the problems on a gas engine using these manifolds is the lack of exhaust heat to the carbs - with LPG this is not a problem! So no manifold heat plate required. [Back in high school I had a combo LPG/gas pickem-up-truck and on cold days it'd take several minutes to start on gas. With LPG it'd start up and run smooth immediately.]
However - the regulator/vaporizer which lets the liquid LPG expand into gas gets very cold, and can freeze up as moisture condenses and solidifies. To avoid this a hot water circuit is run to this unit to keep it warmed.
The remaining pieces then are the fuel tank and a fuel lockoff wired to the key. The fuel tank will go under the bed between the frame rails, probably 2 small long tanks straddling the driveshaft. Since it's a 3/4 ton the 2 piece driveshaft is carried in the middle and has little chance of becoming a projectile and puncturing a tank. I may decide to add another guard around the tailshaft end to prevent any escape of driveshaft problems.
Still working out if it's possible to run the filler up through the cab to use the standard filling location. Would probably have to weld an external filler pipe inside the cab to run the fill hose through, to prevent the possibility of LPG leaking into the cab.
The one "unknown" for me right now is the compression ratio. Using the standard 250 pistons it's pretty low (8:1?) - but LPG likes a higher ratio (13:1 to 14:1 optimal). I'm thinking to replace my pistons (unused - any takers?) with a higher compression piston to at least achieve 10:1 ratio. This should improve performance somewhat, but raise the cost.
That's about it for now - looking to spend about $1K on everything including the tank, which will probably have to be a used LPG tank to fit this budget. But over the next 2 years that should balance out the difference between gas and LPG, even considering the potential reduced fuel mileage.
Looking to finish up by fall. Keep ya posted!
-Boyo
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