Ive run propane engines for years and recently built another engine up (my third for LPG). If you build it specifically for LPG you wont see any power loss over a petrol engine (92-96 octane).
They run and sound just like a petrol engine.
Cold starts are a breeze because of the homogenous nature of the fuel and the top end is only restricted by the size of the mixer, same as a under carbed pertol engine.
You will see power loss if you run a dual fuel setup because of the mixer having to be placed (usually) above the carb causing a constriction. I wont run a dual fuel set-up, I have in the past and recomend them to very few people.
Some standard setup points for a straight propane engine:
Compression: up to 10.5:1 (depends on the application and heads ie alloy heads vs iron and trail truck vs heavy hauler)
Timing: 10 deg at crank, Full adv by 2250 - 2500, If you tow max total adv should not exceed 30 deg.
Ignition: GM HEI considered a minimum standard, good leads and plugs also required.
Plugs: at range colder, maybe 2 depending on where and how you drive
Heat: Get rid of any heat risers etc that transfer heat to the fuel, LPG is gas and does not need it. This is a big thing, cool the intake charge and you will see a good change in power and economy.
Intake: Any single plane manifold with raised runners will give good service. It will idle like petrol factory manifold and run to what ever RPM you want. The raised runners help reduce intake charge temps.
Heads and Valves: Hardend seats and a good valve job, concentric seats, 45 deg face angles, no interference angle. Valve seats between 1/16 - 3/32 depending on the application
Rings and pistons: Whatever pistons you want for your application and if you want chrome rings break them in on petrol for a 1000 miles first otherwise it will take anywhere up to 60K to bed them on propane.
Cam: whatever suits you application really, you can get LPG specific cams here but they cost $$$ and the gains are almost negligible.
My K20 gives 11mpg on the highway with 4.56 and no OD. Unleaded is $1.30 AUD a litre at the moment and LPG is 42c....
HTH If you want any specific questions answered I'm more than happy to advise.
Jason
