[Gav]

LPG has a lower calorific value than petrol, so you just require more of it to get the same output.
The same goes for LPG and natural gas, natural gas has a lower calorific value than LPG so the appliance requires more gas flow or put simply larger jets.
The flow rate of the LPG will be governed by the vacume, throttle position, engine temp all of which are controlled via the ECU that gas systems now supplied with.
In the past cars have used a very basic arrangment to control the gas, and have been inefficiant. The latest LPG conversions use control equipment not unlike EFI and thier efficiantcy is now alot better. They start easier and have very little reaction time compared to systems of old.
BUT the system Ford is presently flogging on its falcon is the same as taxi's have been using for years. They are the only mass producer of a dedicated LPG car in Aus and have got an old symple system to comply with ADR's.
I have had LPG cars for years and they are just getting better and better.

Gav