55MPG Amazon
by Nigel Robinson
It was the final straw - £1.40 per litre! I had been thinking about an LPG conversion of my Amazon for a while, but the oil companies and the Chancellor enriching themselves at my expense just had to stop. There are a number of LPG conversion companies close to home, but in the end I decide to ask Dai Brace to do the work as he had experience converting Amazons.
The conversion adds an LPG tank or tanks to the boot - I opted for two tanks as I was happy to trade boot space for extra time between fill ups. My Amazon has a range of about 425 miles on LPG. The conversion is a simple open-loop system, i.e. it doesn't use a lambda sensor and electronics to control the mixture. It also uses mixers plates between the air filters and carbs to feed the LPG to the engine rather than electronic gas injectors.
As an LPG conversion had been in the back of my mind when I fitted a 123 distributor to the car some time ago, I had opted for their 123Tune model rather than the slightly less expensive 123 Volvo-specific unit. The 123Tune allows its advance curves to programmed from a laptop via a USB cable. It also has a wire that allows you to switch between two different advance curves programmed into the distributor as the car is being driven. The result is I have one advance curve set up for use with 95RON unleaded petrol and one for LPG. When the car is switched to run on LPG, the distributor switches to use the LPG curve, which is significantly different to the 95RON curve. My car running on LPG needs more advance than 95RON at low rpm and less advance than 95RON at high rpm. My LPG curve was worked out one happy Sunday morning with a friend and a stop watch blasting up and down a local main road that was temporarily shut to non-local traffic.
The Amazon hasn't missed a beat since conversion, i.e. business as usual for an Amazon. The power and throttle response are better on petrol. The engine runs more smoothly and MUCH more economically on LPG. The car starts hot or cold on LPG and so I have no real need to run on petrol at all. Dai's advice was to run on petrol every now and again to check the petrol system is still working just in case it is needed. Switching from petrol to LPG takes a while as you switch off the petrol flow to the carbs and have to wait for the float chambers to empty and the power to falter before switching on the LPG. When switching the other way, it takes a few moments to fill the carbs and so I find it necessary to switch the car back to LPG for a few seconds so as to keep the engine and fuel pump turning until the petrol starts to reach the engine. In practice, it is easy just to run full time on LPG.
My Amazon (twin carb B18D) gives 31mpg on petrol. Running on LPG it gives 26mpg - LPG contains less energy per litre. These figures are based on multiple averaged full-to-full measurements over my daily commute that is a mix of fast main road, motorway and congested city driving. As of today, my local price of LPG is £0.65 per litre and 95RON unleaded is £1.37 per litre. This equates to a cost equivalent of 55mpg running LPG compared with petrol. As diesel is more expensive per litre than petrol, the comparison with diesel would give a still higher mpg for LPG.
As the reason I had the car converted was cost, the financials are important. The basic cost of Dai's conversion was £875 in August 2012 for a twin carb 122S. I added £200 to this for the second LPG tank. At todays prices, petrol to cover 1000 miles would cost me £201. LPG to cover the same 1000 miles would cost £114, i.e saving £87 per 1000 miles. The LPG conversion with one tank should pay for itself in 10000 miles of use. My conversion with two tanks should pay for itself in 12500 miles. I cover about 14000 miles per year in my Amazon and so should reach break even on the conversion cost in 11 months and thereafter should save £1200 per year in fuel costs.
It will be interesting to see if running on LPG has any knock-on effects on the long term reliability of the car. Dai warned that LPG is dry relative to petrol in the induction system and can burn hot. Twin downpipes and a sports exhaust are recommended to help the hot exhaust gases leave the engine. Luckily my car already had these. Dai warned that valve guide wear could be a problem after 30-50 thousand miles and that using a different alloy for any replacements fitted would fix this problem. If this happens, then my current thinking is that I would take the chance to fit a B20 built to take full advantage of LPG, e.g. it would be possible to run a high compression head with LPG as it has a much higher octane rating than petrol. I can use some of my £1200 per year fuel saving to pay for this.
What would I do differently if I did it again? One thing I hadn't expected was that filling up would change from being an exercise in trying not to notice how much it was costing to one of enjoying the feeling that somehow you are beating the system. A single LPG tank would reduce the cost of conversion and save boot space. The single tank positioned over the rear axle fits very well and you would get twice as many chances to feel smug!
Nigel Robinson
(editors note) I think we can safely say the conversion has proved to be 100% after being tested to the extremes
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