Performance Part Dealers

LPG Conversion Guide – LPG Information

LPG Conversion Guide – What is LPG?

A mixture of around 20% butane and 80% propane although this can vary



It has a stenching agent added otherwise you would not be able to smell it.

LPG filler adapters. There are 3 main types in use in Europe. Image Source: http://www.tinleytech.co.uk

LPG Conversion Guide – Is LPG different in Europe?

It is possible that the mixture is different but the biggest difference is the filling nozzles, there are 3 different types in use.

LPG Conversion Guide - Does LPG have a higher octane than petrol?

Yes, 108 vs 95 for unleaded.

LPG Conversion Guide - Will modern turbo cars take advantage of the higher octane?

Yes, over time they will continue to advance timing until knock is detected. As the engine is increasing ignition advance the engine will be producing more and more power.

LPG Conversion Guide - Does LPG hurt performance?

Gas injection is the best method and with this you can expect to lose around 10% peak power all other things being equal.

LPG Venturi Mixer - Image Source: http://www.iwemalpg.com

Venturi mixers are very restrictive and liquid injection can cause problems so with these 2 you can expect to lose more power.

If you tune the engine for LPG, ie advance ignition you can expect a 10% gain in power with gas injection.

Gas injectors are the only method worth considering. LPG injectors use rubber to seal and for this reason they do not last as long petrol injectors. LPG injectors can be expected to last around 100,000 miles. Keihin make LPG injectors for Prins LPG systems, Romano and BRC.

LPG Conversion Guide - Does LPG cause the engine to run hotter?

If timing is not advanced it is possible because more of the combustion will happen when the exhaust valve is open. Advancing the ignition reduces stress on the exhaust valve as the combustion has already taken place by the time the valve opens.

Generally, LPG runs at a slighter lower temperature than petrol.

LPG Conversion Guide - Does LPG destroy the rings?

LPG gas injectors - Image Source: http://www.megagas.com.au

No. Because LPG is a gas is does not run down the cylinder walls and wipe of the oil like a petrol engine.

For this reason it is very difficult to run in an engine on LPG due to the reduced wear.

LPG Conversion Guide - Why does LPG burn so cleanly?

Because it is a gas. Petrol is a liquid which means droplets are injected in tot he engine. The se droplets have a much lower surface area in contact with oxygen than gas which means the petrol does not burn up as completely. LPG is not perfect but it is better than petrol.

As a side not, this droplet issue is why cars are now running direct injection. By running higher fuel line pressures the petrol atomists more.

To give an example of the effect of better atomisation. By giving fuel a longer time to mix with the air power can be increased by 5%, this can be done by locating the injector further away from inlet valve. Although when you do this you cause all sorts of low-speed drivability issues.

LPG Conversion Guide - Is LPG harder on exhaust valves?

Yes. Some people say they can run out LPG in a car that supposedly has a problem with no issues. Ford Zetec for example. But this is more down to luck than engineering.

If you don’t rev the car then the exhaust valve takes less of a battering which is why there is no problem, drive the car harder, the exhaust valve gets beaten and that is when you get the problems.

LPG Conversion Guide - LPG tanks, the differences, pro and cons

LPG donut tank - Image Source: http://www.arunautogas.co.uk

There are 2 basic types of tank, single hole and four hole.

The are four hoses connected to an LPG tank. One big one from the filler, one to feed the engine, an emergency vent to stop the tank exploding in a fire and one for a fuel level gauge.

4 hole tanks have individual fittings for the hoses. The biggest benefit of this is that it is faster to fill up.

Single holes tanks have a combination valve which are much cheaper but they are slower to fill.

There are two type of shapes available. Donut and cylinder.

Cylinder tanks have a capacity of 10-230 litres. Donut tanks 35-95 litres.

Donut LPG tanks cost around 30% more.

What happens when the relief valve does not operate

LPG tanks run at around 5 bar, although in hot temperatures they will run with more. The pressure realease valve should activate at around 25psi but in reality should only open in a fire. The vent pipe should obviously vent outside of the car.

LPG multivalve tank fitting. Cheaper tank but slower filling. Image Source: http://www.landyzone.co.uk/lz/f8/lpg-tank-wont-fill-100082.html

All LPG tanks should come with a test certificate. All tanks must have a unique serial number and the manufacturers stamp. It should also indicate that the tank complies with R115 and EC 67-01 European directives.

LPG Conversion Guide - Will LPG taxes increase?

Probably not, LPG will only ever be in limited supply so the government will never get that much revenue from it. Plus LPG is classed as a low CO2 fuel meaning it cannot be taxed at the same rate as petrol according to some international treaties.

LPG Conversion Guide - Conclusion

Dont buy a second-hand kit, things wear out and it is safety critical.

The only exception to this rule would be if you are buying a car that already has a kit installed that way you know it works.

If you see parts that are too cheap to be true then it is best to avoid. Some parts even have fake certificates.

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