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A guide to LPG

Fuel For Thought Published: 14th Jun 2011 - 0 Comments - Be the first, contribute now!

A guide to LPG
A guide to LPG Here’s a typical set up and while not an easy DIY conversion it is possible. But best left to the pros we reckon
A guide to LPG The ‘filler’ is neat and unobtrusive (always have it sited for easy access). There’s plenty of filling stations
A guide to LPG Here’s a typical diagram showing how system works. It’s been around for a long time and most are dead reliable
A guide to LPG Dash lights signify the system in use
A guide to LPG Tanks are usually boot mounted but can be fitted wherever practical.
A guide to LPG Ok it’s an old sign (below) but savings are same
A guide to LPG
A guide to LPG Gas guzzlers ripe for LPG fit and it’s as easy as putting in petrol.
A guide to LPG
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If running a classic is an absolute gas then why not run yours on it to save money – and the planet? We have all the answers!

If there’s one problem with our hobby it’s the cost of using our classics. With petrol nudging £5 a gallon, this sheer expenditure can limit the amount of fun and relaxation we yearn for during the show season. Believe it or not but we all waste petrol! Ensuring your classic car is in tip-top tune can save a bundle but nothing can beat buying fuel at half price, can it? That’s just what LPG offers but is it for you? Read on.

Just what is LPG?

Liquid Petroleum Gas (Propane) is essentially petrol in gas form and has been around for decades, first coming to prominence during WW2 where the ‘tank’ was an inflatable bag fitted to a roof rack! It’s a lot more sophisticated these days and installations can be virtually undetectable. It’s a cleaner fuel than petrol and so cuts emissions plus is a lot kinder to the engine, especially the fuel lines and exhaust system.

How does LPG work?

It works roughly in the same way as petrol power; fuel from the dedicated tank feeds a vapouriser situated in the engine compartment heated by the engine’s cooling system to aid vapourisation. A connector feeds the venturi(s) where it feeds the inlet manifold in the usual manner. A simple dash-mounted switch controls the choice of either petrol or gas power.

How does it save me money?

LPG cuts fuel costs because it’s cheaper at the pumps – approximately half the price of unleaded. Also the Government is keen to promote cleaner fuels such as LPG and so it’s very unlikely that its price will rise to unleaded levels.

What’s the cost of a typical LPG conversion for my car?

Typically the cost of converting a carb-fed classic is anything from £1000 to almost £3000 depending upon car and set up. This sounds a lot perhaps but not when dialled into a longterm car restoration and ownership package.


Simple… it costs less to fill your car’s tank! It has to be admitted that most classic car engines were never that efficient even when new. A flip through some road test results in the back of old Autocar and Motor magazines provides sobering reading on overall fuel economy. For example, a 4.2 XJ6 returned just 15.3mpg, a Stag 20.9mpg, a Ford Zodiac 3.0 V6 18mpg and a V8 Jensen FF 11.4 (remember that’s one of the chief reasons Jensen prices have been kept so low for so long). Even a humble Herald 13/60 returned just 25.5mpg according to Motor’s figures! LPG won’t improve on those returns – indeed economy slightly dips as the fuel is not as efficient. Instead the savings come from using the cheaper fuel as much as possible. Say your classic barely struggles to make 20mpg. Well according to today’s fuel prices it would take roughly 10,000miles to recoup the cost of a £1500 conversion. Okay so most of us don’t cover such big mileages in an oldie – but spread over the cost of ownership of say threefive years and it’s more attractive plus encourages you more to use your classic more… simplybecause it’s more affordable to do so!

Will the conversion make my classic car run better?

Probably not. As LPG is not so efficient as petrol, they’ll be a drop off in performance as a consequence, especially on older carb-fed engines. Experts reckon around 20 per cent power loss is typical although things are improved greatly if you use a special ‘gas’ carb to make the most of the conversion. This is particularly desirable on higher-powered classics where engines produce 180bhp or more. Fuel injected engines fare better and the difference is minimal: rather like driving a car with a dirty set of plugs where the engine loses it’s edge rather than power, for example. It depends on how hard you drive your classic. For many, who don’t use full power, the performance tail off won’t be too much of a bind but for other it could well spoil the car and the driving experience.

Can i fit it myself?

Yes but it’s not for the novice! Remember, you are dealing with high-pressure gas and the installation is like plumbing in a gas cooker. Now, for many car restorers who have tacked all sorts of jobs it may not be a problem – but don’t risk it! You can have the system checked by an LPGAapproved agent and that’s very wise for insurance purposes. Indeed a conversion certificate may prove essential for cover anyway (more on this later). One LPG expert we spoke to said he has seen DIY conversion jobs so bad that he has drained the system there and then to prevent amobile bomb travelling around! You sometimes see cheapo LPG systems advertised on the likes of eBay for a few hundred quid and claimed for DIY fitting but beware… they can be unreliable and spare parts for the kits are virtually unobtainable. A proper pro job is money well spent as well as a darn sight safer…

Any running problems?

A quality LPG conversion is usually very reliable, although some earlier set-ups were blighted by unwanted traits such as backfiring under load, caused by poor carburation. It can still happen and is due to the simple ‘mixer system’ employed for basic carb set ups where the mixture strength can vary. The aforementioned ‘gas’ carb goes a long way to eradicate this; it looks like an Stromberg carburettor and works on roughly the same principle plus is coupled to a modern exhaust (Lambda) sensor to constantly monitor and control mixture strength. Leading LPG specialist HKL Gaspower of Birmingham (0121 359 6131) imports the US Impco products and has gas carb adaptors to suit most oldies from Hillman Imps to big Vauxhalls. Typically a set up for a twin carb XK engine is just over £300. According to HKL, the biggest point overlooked when setting up an LPG conversion is retiming the ignition. This is critical and as much as 20 per cent advance may be required, according to engine. Also many older units may well require having hardened valve seats if not yet installed as gas burns hotter and so compounds the problem. Servicing usually only requires an annual examination, replacing the filters and checking out the system. It usually costs around £70 although a thorough service of the tank is advised every 15 years and it’s usually as cheap to renew it outright. Systems can be transferred from car to car, if suitably modified, although it’s not as easy as you’d think as designs frequently change and the cost easily parallels that of an all new installation.

What about driving and servicing an LPG classic?

It’s generally agreed that an engine should be started on petrol as it’s hard to get going on LPG. Also periodical running on petrol is advised to help lubricate and keep the system clean (LPG is a dirty gas and UK spec fuel is one of the dirtiest in Europe!). Finding the fuel when out and about is easier than ever and most motorway service stations have filling points – it’s as easy as petrol, too. Go on the internet and you can obtain a thick 40 pager giving the exact locations ( If there is a point to watch then it’s hot running engines. This is not a good situation for classic power units which are prone to overheating at the best of times – especially if the valve seats have not been converted to unleaded types. As one Jensen owners club put it to us: “the last thing you want is an Interceptor running even hotter”.

And the MOT test?

Unlike certain European countries there’s no checks for an LPG installation (for how much longer-ed) and the vehicle can have its emission test conducted on either fuel although HC (Hydrocarbons) readings will differ.

Are there any legal points?

When checking out a conversion (or even buying a classic already converted), look for proper installation receipts, valid service records, a guarantee (if applicable) and most importantly a certificate of conformity (installation quality). If a car or conversion lacks this later slip of paper then you may find it very hard find an insurer. One such specialist who has no qualms covering an LPG classic is Footman James but it has to be a professional job that’s properly certified. Check with your insurer first and above all else, always let the company knowof the conversion. Failing to do so will probably invalidate your cover.

Ok but will it hurt my classic car’s value?

For many of us this is the most important question – and rightly so. And yes an LPG conversion will devalue a classic… that is if you are a stickler for originality and insist that your car is kept in the same state it left the factory – right down to how the wheel hub split pins faced! But for the more pragmatic owner it’s a different story and certainly the bulk of the numerous car clubs we approached said there’s no ill feeling towards a conversion. Graham Searle of JEC was one who took a positive stance and is in fact considering having his MkVIII so converted as part of its restoration and sees many later cars – especially the V12s – converted.


With almost 150,000 LPG-powered vehicles on our roads, it’s a growing trend and although classic car take up is minimal so far, we can predict this changing over the years. Do your own sums and see if it works for you, too.

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