Nearly all the conventional wisdom we think we know from hotrodding with liquid fuel is reversed with gaseous fuel, like:
A rich mixture with gasoline is considered "safer" for a turbocharged motor because it produces lower EGT's and helps cool valves. On the other hand, a rich mixture with gaseous fuel produces higher EGT's and burnt valves!
Lean mixtures with liquid fuel puts holes in pistons, while lean mixtures with gaseous fuel simply lowers power and gives cooler EGT's.
You can ask two different LPG "experts" about rich vs lean mixtures, and you'll probably get two different answers. A guy in the business of doing LPG conversions and selling Impco gear in Canada maintains that running lean LPG mixtures is what causes valve recession and "torched" exhaust valves, when it's just the opposite that is true.
There is no evaporative cooling effect with LPG vapor injection, because it is already a gas when it enters the manifold & chamber. There "could" be some cooling effect with LPG liquid injection, but these systems are so new they really haven't hit the mainstream yet.
An LPG mixture burns at a slower rate than a gasoline mix, so it requires more timing advance to get the best power out of it. Fortunately its octane equivalency is much higher than gasoline, so the motor will tolerate the higher advance just fine. Running normal gasoline advance with LPG and a slightly richer mixture than optimum will just about guarantee burnt exhaust valves, because the mixture is still burning when it's exiting on the exhaust stroke.
Here in the US it is expressly forbidden for anyone to do a DIY LPG install for a street driven car. Doesn't mean it doesn't happen, it just means that if you're caught, you face hefty fines, and possibly a federally imposed vacation.
To legally put an LPG system on a street driven vehicle in the US, the "system" must go through an approval process similar to CARB approval, and in fact it is the EPA who oversee this. Hence, "approved" LPG systems in the USA are few & far between, which is a real pity, because LPG is such a great fuel. It's cheap & readily available, distribution is already in place, storage is no problem, retrofitting is not that expensive, and normal gasoline engines can be fitted without any major changes or problems.
About the piston coatings - regardless of the fuel, I would always coat pistons for any FI'd engine I'd build. But that's just me trying to keep as much heat as possible in the combustion chamber, rather than letting it spread to the rest of the engine.
About the valves and valve seats - I'm trying something a little different with my valve seats versus conventional wisdom. I've gone with beryllium seats instead of stellite to try to improve the heat conductivity between the valve and seat. I also had the valves ground with very flat angles to increase the width of the seating surface. They certainly won't flow as well as the nice 3 angle valve jobs, but they should cool better. Remains to be seen if this will work or not. Beryllium is certainly a lot softer than stellite, so I'll have to be watching closely for valve seat recession.
Sorry for the longwinded post, but I'm a real fan of gaseous fuels and hope they'll catch on better here in the US, soon. Perhaps with a new administration in 2008, the restrictions on fitting aftermarket LPG systems will be relaxed......
