Running on Petrol
An optimised conversion using carefully selected components (probably not the cheapest) and being skillfully installed will make no difference to engine performance when running on petrol.
Running on LPG
The top end of engine performance may be 'clipped' by a small amount, typically 5% (but not with every engine / gas system combination). In reality, no-one drives in the top-end range very often so this reduction is hardly significant. Also consider that most cars converted to LPG have the largest engines ( cc ). Reduction of performance at the top end is least noticeable with a larger engine and so it becomes less of an issue. (If this does become an issue at some point (i.e when towing) remember that you always have the ability to revert to petrol at the flick of a switch).*
* Most drivers of Go LPG! conversions report that they cannot find any difference in the overall performance of their vehicle when running on LPG.
LPG has a lower calorific value than petrol. (It produces less total heat and therefore less peak pressure in the cylinder.) Simply put, LPG gives slightly less 'bang' per charge. LPG is slightly less potent as an internal combustion engine fuel. The driver will instinctively react to this by opening the throttle more to achieve the desired speed or rate of speed increase (often called acceleration). Thus, more liquid fuel (LPG) will be used than when running on petrol.
The actual increase in liquid volume consumed (or, if you like to see it in this way, decrease in fuel endurance) varies between 10 and 20%. A mean figure of 15% increase in fuel volume consumed is a good figure to rely upon.
How is this relevant?
Notice that LPG is LESS than half the price of petrol per litre or gallon. It is reasonable to assume a fuel cost reduction of at least 50% as the reduction in MPG has been allowed for in the price differential of more than 50% less. LPG currently costs between 55% (at 36p / litre) and 60% (at 32p / litre) less than petrol.
Other differences
Although LPG produces less total heat ( it has a lower calorific value) that heat is released in a shorter time. The reason for this is that LPG is a 'perfect' gas at ordinary atmospheric temperatures whilst petrol vapour is liquid that has been forced to vapourise. The cooling effect of liquid petrol droplets is not present with LPG and the total heat is released more quickly. A greater heat 'spike' is the result. The engine will run slightly hotter, although the difference may not be large enough to show on the temperature gauge. Only the cylinder head temperature (CHT) will increase significantly (although not enough to cause any problems). Most cars do not have a device for monitoring CHT (unlike aircraft). A typical car temp. gauge measures coolant temperature only, which is much more of a general picture.
Leading on from that, it follows that an engine running on LPG will ' warm up ' more quickly than it would on petrol. Petrol consumption is extremely high when the engine is cold as a choke or excess fuel device (both giving a very rich mixture) must be used. LPG scores over petrol again, as this is not necessary. LPG does not need an artificially rich mixture and the engine warms (to its normal operating temperature, where it is most efficient) even more quickly as a result.
LPG has a high octane rating (it tends to give less 'knock') and is more easily and thoroughly mixed with air. One of the results of this can be a quieter and smoother running engine.
In cold or damp ambient conditions, rapidly vaporising LPG can cause refridgeration icing around the regulator and gas mixer unit. The result can be narrowing of the induction system (at the venturi) by deposited ice. A temporary reduction of performance may be experienced (manifested as 'flat' spots, backfiring, an inability to increase speed or even an 'engine cut'). This is why you are well advised to run your engine up to temperature when such conditions exist.
