LPG - Less Point Guys

Combustion is a funny old thing. I was talking to a mate of mine this week who has just changed his V8 3.9 litre Land Rover Discovery for the diesel variety. His old V8 had an after market LPG conversion but it was still too expensive.

Matt explained that the octane rating of LPG is 112 but the flame front is so slow he had to advance the ignition by 12-15% to get the bang in the right place. Any later and by the time the piston was going down the bore the exploding air/fuel mixture was stumbling down the bore after it when the fireball should have been forcing the piston down there.

Despite advancing the ignition, his Disco was noticeably more sluggish and returned no more than 15 mpg. It seems that LPG doesn't burn very well. Maybe a more sophisticated engine management system would improve performance and economy but it's unlikely anyone will engineer bespoke systems for what are often quite old engine designs.

Since LPG can cost up to 63p/litre and his new diesel Disco does at least 25 miles to the gallon (if we still have gallons) there wasn't much saving. And that's before taking the cost of conversion into account.

Coincidentally, another mate of mine, John, told me of his neighbours who have a Subaru Imprezza. They love this car. When they first got it they explored its full performance potential - and got 4 mpg. They have now shelled out on an LPG conversion, which makes the car much slower. That's a judgement call, though. A slow Imprezza is still fast.

The conversion has also de-valued the car for re-sale.

(Matt tells me that Discoveries like his are so cheap nowadays they are unlikely to depreciate any further.)

John and I reckon his neighbours should have kept their Scooby stock and spent the cost of conversion on a Nissan Micra or something.