Converting to LPG

Can you give me your thoughts on converting a direct-injection Mercedes-benz AMG engine to run on LPG? I would like to eventually buy an E63 AMG fitted with the current 5.5-litre twin-turbo direct-injection petrol engine. I know that this engine is more ‘economical’ than the earlier 6.3-litre atmospheric engine, but this is relative and any big AMG will be hard on fuel, so if the running costs can be reduced by converting to LPG without causing any damage, then I would be very interested!

I have read a lot of comments online concerning potential problems with exhaust valves and valve seats burning after some engines were converted. Presumably a port injection engine would be worst affected by LPG as the valves and seats would no longer get a blast of cooling and lubricating petrol on each induction stroke? However, as the 5.5-litre AMG is a direct-injection engine, it should already be fitted with hardened valves and valve seats as these components wouldn’t normally be cooled or lubricated by petrol anyway. As this is obviously a high-performance engine, I would assume that all the components exposed to the heat of combustion would already be very tough and made from the highest quality materials and unlikely to be damaged by LPG. Would a flash-lube kit be a good idea to help to ensure that both the valves and valve seats are properly protected when burning LPG?

Another matter of which I’m unsure is the economics of an LPG conversion. We all know that LPG is cheaper than petrol, but it also has a lower calorific value than petrol. This means that, journey for journey, an Lpg-burning engine will always use more fuel than the same engine using petrol. Then there is the question of how many miles would have be driven before breaking even on the installation costs and making any savings.

Last, do you know if this engine suffers from carbon build-up in the inlet valves, as do some direct-injection petrol engines? Danny Gillis

A direct-injection petrol engine is not ideally suited for LPG conversion. Unlike conventional modern petrol engines, the injectors are installed directly into the cylinder. The injectors are subject to intense heat from the combustion and, during normal use, are then cooled as they inject the next measure of fuel. If the engine is converted to LPG, the injector is not in use and does not get cooled by the incoming fuel, allowing it to overheat.

It is possible to fit a system where the injectors and the LPG work in tandem, but the software is extremely complex and, given the rising cost of LPG and all the associated fitting costs, no financial benefit would be seen for many miles.

With regard to flash lube kits, these can also cause problems with the catalytic convertor, and careful finetuning of the system would be required to ensure the best set-up.

My personal opinion is that Mercedesbenz spent a lot of production time to develop the AMG unit to run at its best. Any adaptation to LPG would only be a detrimental move, and if running costs are a concern, I would not choose this vehicle. Some conversion specialists might disagree with me about this, but they’re entitled to their opinion.

Finally, I have not heard of any problems with carbon build-up on the inlet valves on these vehicles.