To build an engine to run strong on propane you want to pay attention to cylinder pressure. Keeping it high will net you the best results. You can run 11:1 compression with a crappy cam that lets out all the cylinder pressure and end up with an engine that performs worse than a 9:1 engine with a proper cam to keep those pressures up. On your cam choice your exhaust duration should be a little longer than the intake duration,it likes much more advanced timing than gas hence the shitty performance of the old style dual fuel set ups.
Anyways my set up was
A balanced and blue printed 454 30 over 11.5-1 forged pistons,Lunati cam, Duration @ .050 (Int/Exh): 214/224(I think it was a lot bigger ),ported oval port heads, stainless valves,bronze guides, a lot more stuff but hey I know its a chivy
I had a machinist friend make me an aluminum adapter to run twin OHG X-450 mixers on a Holley base plate and 2 X-1 vaporizers.The OHG 450 are about 550 CFM each so with a big block two worked well,I gave the Dyno run info to the new owner but it put out near 600 HP and he was very happy with the performance. As for mileage it had a well built 700R OD with a manual lock up switch so around town it got around 10-12 mpg and 14-16 on the highway 411 gears with a 35 inch tall tire.
The biggest drawback I found was finding auto propane outside major cities and if you did find it they would have no one on staff that was licenced to pump it. Also it was much more expensive in the States at the time but were talking over 15 years ago.
A very smart man sums up much here:
"Propane has a bad rap because of some bad conversions and some knee-jerk conclusions drawn from textbook physics.
First, the dry & boring physics of it all: The exact numbers seem to depend on which authoritative textbook you reference, but on average propane has about 25% less BTUs of energy per gallon than gasoline. So it stands to reason that you'd get 25% less horsepower on propane, all other things being equal. But there's the catchall other things aren't equal. Liquid gasoline has to be atomized and mixed with air before it can be burned effectively in your engine. Engine builders know that in a carbureted engine a portion of that gasoline doesn't fully atomize and falls out of suspension, ending up in a puddle somewhere in the intake manifold. This is why the cylinders downhill from the carburetor tend to run richer than the others. (It's also one of the reasons why fuel-injected engines are more efficient than carbureted engines.)
Propane, on the other hand, is a gas, gas, gas. It doesn't need to atomize, and it doesn't puddle. Propane is only 1.5 times heavier than air, so it can literally hang out all day. And because it stays mixed with the air coming in the intake, all the cylinders get an equal air/fuel mixture. In short, propane makes up for fewer BTUs by burning more efficiently and completely. So the real-world horsepower loss for a propane conversion on an unmodified engine is more like 5%-15%.
Which brings us to the issue of naughty conversions. Propane engines require a different ignition advance curve compared to gasoline, typically advanced 5-15 degrees under 3,000 RPM. Additionally, propane has a higher octane rating than premium gasoline (about 105 compared to 91 or so), so you can build an engine with a higher compression ratio, making the engine even more efficient. The bottom line? An engine built with propane in mind can achieve roughly the same amount of horsepowerin some cases morethan a comparable gasoline carbureted engine."
The biggest drawback I found was finding auto propane outside major cities and if you did find it they would have no one on staff that was licenced to pump it. Also it was much more expensive in the States at the time but were talking about 15 years ago.
