Well, the $3500 system mentioned by bfhoon was clearly from many years ago, my system cost $2250 in March 2013, less the $1000 gov rebate = $1250 out of pocket conversion cost (which also included plugs, leads & air filter which were all due to be changed in any case, so another $80 saving for me)
Unleaded. 17,000km/yr @ 12L/100km = 2040 l/year. @$1.40/lt = $2856/yr
LPG. 17000km/yr @ 14.5L/100 km = 2465l/year @0.65/lt = $1602/yr
Out of pocket expense of $1250 payed back in 1 year based on annual fuel expense saving of $1254/yr.
Re Valve cooling or more correctly "engine valve seat recession protection mode", it's in the GMH factory service manual for the factory LPG system fitted to the VT Commodore, I expect Holden know something about how their factory fitted system works and why they had a certain function built into the system. Under high load conditions it injects a small amount of petrol, Petrol burns cooler than LPG, resulting in cooler combustion temperature which equals a cooler exhaust temp as it passes the exhaust valve, it does not cool the valve as such but rather avoids heating it to the same level as would be the case without the added petrol. Is it essential to have? Probably not, especially If just putting around town, but for a small extra outlay why not have it setup the same as was developed between Impco and Holden? Sure fitting an injected LPG system would be ideal but not really required on a 2001 model nor as cost effective as a mixer system.
Quote from GMH Factory Service Manual for VT Commodore LPG section:
Injector Pulse Width
The injector pulse width is set to zero in all LPG operating modes, except during engine cranking or, on when the vehicle runs in “engine valve recession protection mode”.
During engine cranking the amount of petrol delivered is determined by the engine coolant temperature and the engine crank time. The injection of petrol during engine cranking is to aid engine starting.
When operating in the LPG mode, and under conditions of high speed / high load, a small amount of petrol is injected into the engine to protect the engine from engine valve seat recession. When petrol is injected under these conditions, it is referred to as the engine valve seat recession protection mode.
I have had ICOM LPG Liquid Injection on my LS2 for 4 years now, 160,000km, never misses a beat. Payback on that $5500 system (less $2000 rebate that was available in 2009 = $3500 out of pocket) was less than 2 years (I did about 55000km/year for the first 2 years). Picked up 5 RWKW over premium unleaded (Tune was optimised for both fuels separately) uses about 15% more LPG than Petrol per KM.
Note: Taxi's in Australia (nor company vehicles) don't get the rebate and they are also driving late model cars that likely need a much more expensive Injected LPG to be compliant, their out of pocket expenses are much higher than a mixer system on a 2001 model private vehicle that also can claim the $1000 rebate.
Cheers
Jamie