I know of quite a few large BMWs running on LPG. If you do a reasonable
annual mileage in the car it's well worth it.

Griff, I can't comment on BMW's running on LPG, although I'm seriously
considering it for my '92 730. I do, however, have a 3.9V8 Discovery
which was converted a couple of years ago ... and it's brilliant. This
vehicle currently does about 800 miles per week with about 100 of those
off-tarmac.

My observations are:
1. There is no apparent loss of power on LPG. This may be because the
Disco engine is only mildly tuned anyway and not as "highly strung" as a
BMW engine.
2. I get almost exactly the same fuel consumption on LPG and petrol (16
vs 17 mpg) and at just £25 for 80litres I can live with the difference ;-)
3. The engine is notably smoother on LPG. Don't know why but it is.
Acceleration is smoother with no flat spots - it just pulls like a train
all the way to the red line :-)
4. Running low on gas can promote backfiring in the air intake. This
would probably wipe out the air flow meter but you should install a
simple flap valve to prevent this from happening. When we first
converted the Disco I had a blow-out valve put in the air box (the afm
is pretty bullet-proof) but after the first blow-back found that it
couldn't handle the volume of air involved. Fitting another one has
cured the problem but if I was doing it again I'd go for the simpler
flap valve.
5. You need to have the engine running almost perfectly on petrol before
doing the conversion. If there are any weaknesses in the ignition system
they will be more apparent with LPG, which needs a stronger spark to
ignite. Make sure the plugs, HT leads and distributor are all A1. On the
Discovery I fitted Magnecor leads which have better insulation and
conductivity properties than the standard Land Rover ones. Poor HT leads
can aggravate the backfiring problem mentioned in 4 above.
6. You need to run on petrol ocassionally to stop the fuel injectors
getting all gunged up. I usually start mine on petrol and let it switch
to LPG automatically when the revs go above about 1800 (the switch point
can be adjusted via a laptop). Even in winter you can start up and blip
the throttle straight away and it'll run fine. I never get a power drop
at the change-over point.
7. Choose your installer with care. Make sure he is LPGA approved and
can give you a CofC for the installation because your insurance company
will want to see it. The most expensive is not necessarily the best.
Talk over the installation with the installer and get agreement about
how things will be done - remember it's your car and your money so get
the job done that you want. Also, get agreement up front that they will
tweak the system for free over the first few weeks of use.

HTH

If you have any specific questions then feel free to ask.

Got my 1998  E36 318iSE converted over 12 months ago. 60 litre tank and gas
injection. Only problem was when i disconected the battery to replace it and
the LPG ECU lost all its settings and defaulted back to original settings.
Quick trip back to installer and all's been well ever since.
The only difference I notice is in my wallet each time i fill up. Currently
saving around £35 BPS each time i fill up.

Go for it.

JohnP