LPG conversion for 645ci (or an A8 3.0)

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Joined: Sat Mar 15, 2008 6:35 pm

LPG conversion for 645ci (or an A8 3.0)

Post by drocter » Sat Mar 15, 2008 7:06 pm

Hi all

I'm new to this forum and am in need of some advice.

My new job means I will be travelling approx 50-60k every year and my search for a comfortable/stylish and reasonably practical 4ish seater has led me to either a 645ci or an A8 3.0 V6. I had considered opting for a nearly new BMW 520d Sport, but even 50mpg at todays diesel prices has made me lean towards the 'big engined LPG conversions' I realise they are quite different, but both have managed to fulfil the brief in different ways.

Given I intend to keep the car for 2 years and will probably cover around 120k in that time, it is becoming a tough decision. I understand that the Prins VSI system works well with these larger engined cars, but can you advise me of a suitable installer up here in the West of Scotland and whether this should be my kit of choice?

Also I've read a bit on the forum about removing inlet manifolds and single versus four hole fillers - does this tech bit come into play with the conversions I'm looking at.

I have read from petrol using 645 owners that they average around 25mpg if mainly driven on motorways. So I would expect the LPG'd cars to give an equivalent of 45-50mpg when driven gently - is this unrealistic and have any of you converted a 6 series?

The 645 has a battery and various electrics plumbed into the spare wheel well, can these be repositioned as with X5s or must I opt for the space-stealing torpedo tank in the boot? I would like as close to a 100L tank as possible, as I shall be covering over 200 miles a day and don't want to spend half my life filling up.

The A8 seems to avoid this problem and although they are meant to be remarkably frugal on petrol (~32mpg on the run), I have to admit, I'm a little more seduced by the 6.

Last, but not least, what do you think the latest Budget spels for LPG drivers, will they face an equivalent rise in cost in October and are there any savings on the road tax?

Sorry for this exhaustive new post, but I really could do with some help.

Thanks in advance

Posts: 288
Joined: Mon Sep 03, 2007 8:48 pm
LPG System: Prins VSI
Car: BMW 540iA Sport
Location: All over the UK

Re: LPG conversion for 645ci (or an A8 3.0)

Post by mak-uk » Sat Mar 15, 2008 10:36 pm


In regards to a suitable installer near you, look up Alternatech Fuel Systems Ltd - they are the importers of the system (as well doing installations) and they can advise you.

Your engine is the modified version of mine so removing the inlet manifold is key to getting the gas injectors as close to the petrol injectors as is possible. In regards to gas tanks - a single hole will work fine.. it will just takes ages to fill. a 4 hole is more expensive but when used with a flexible filler pipe, is almost as quick as petrol for filling time.

My 4.4 can get 26 mpg on good run on petrol. On gas I am averaging around 18 MPG.. I expect yours will be similar or slightly more if driven gently (19-20 MPG).

In regards to tank fitment, once again, speak to Alternatech Fuel Systems Ltd - ask for David King and say that Mak from this site sent you. The reason I say this is that I know that David converted his fathers own 6-series!

Hope this helps.
Site Owner
2001 BMW 540i Auto Sport Saloon
~300 BHP; ~320 ft/lbs, Prins VSI system

Posts: 4
Joined: Sat Mar 15, 2008 6:35 pm

Re: LPG conversion for 645ci (or an A8 3.0)

Post by drocter » Sun Mar 16, 2008 6:21 pm

Hi Mak

Thanks for the info - very helpful indeed. I have found Alteratechs distributer in Glasgow and will contact David next week.

Thanks again and I will keep the forum updated of how I get along.

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