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The commercial grade of propane for automotive use is known as HD-5 in North America. “HD-5” stands for Heavy Duty (propane) containing a maximum of 5% propylene (also called propene) and a maximum of 2.5% butanes and heavier hydrocarbons (also shown as C4+). This grade was developed back in the 1970's to establish a grade of propane, based on composition, that would be suitable for automotive use. That is, if the composition of propane met the HD-5 requirement, then the fuel would be suitable for automotive engines both for stability and for octane quality, without having to actually test every batch for octane or stability.

Propane fuel also goes by the name of LPG (Liquefied Petroleum Gas) in many parts of the world. LPG does not refer to a specific type of gaseous hydrocarbon compound but instead refers to a mixture of liquefiable gaseous hydrocarbons that includes propane, butane, isobutane, propylene, and butylene. Commercially, LPG consists mainly of propane or butane or a mixture of the two.

While it might seem that there would be other grades of propane in addition to HD-5, there never has been an HD-1, -2, -3, or -4. However, some states (such as possibly California) have allowed an HD-10 (meaning 10% propylene) as a means of extending the fuel supply and/or reducing production costs, but there has not been any move within the industry to follow this lead. As I am not familiar with the market outside of Ontario, I don’t know if any product is actually being sold to the HD-10 specification.

Generally, there are no blends of propane and butane sold at retail in Canada or the US. However, such “PB” blends are apparently sold in many tropical and sub-tropical countries around the world – mostly for heating purposes. Butane has a higher heating value than propane, so a tank of PB fuel would have more energy than the same size tank of propane. Propane fuel composition varies in other parts of the world and may well not be called HD-5 but some local name or grade.

There can be a wide range of composition of commercial propane-propylene mixtures and PB mixtures, and the composition of these mixtures are normally agreed between seller and buyer (for large accounts). However, there are no distinct ‘grades’ or ‘types’ as such. Such mixtures can be used as petrochemical feedstocks or as fuel for heating purposes. The only widely available (in Canada and the US) specific product is HD-5 propane. Where ‘commercial propane’ is sold, it is usually a ‘waste product’ or byproduct that a producer wants to get rid of.

While HD-5 is the common grade of propane (LPG) available in Canada and the US, some marketers sell ‘commercial propane’, which can have a wide range of propylene (generally 10% – 50+ %). This product is satisfactory for most heating applications but might potentially cause a problem as a motor fuel.

The issue around propylene is that propylene is much less stable than propane and can polymerize (under hot conditions, as occur in an engine compartment) to form gums and varnishes – which interfere with engine performance. Just remember that propylene is the base for polypropylene plastic. The 30+ years of experience with HD-5 has shown that limiting the propylene concentration to 5% results in an acceptable product.


Cars are converted to propane or Liquefied Petroleum Gas (LPG) generally because their operating costs are much lower than for gasoline. Nowadays, automobile conversions are much more popular in Europe than in North America. Conversions were quite popular in Canada in the 1980s because of government incentive programs. In North America, the best candidates are vehicles in fleets driven long distances because the lower operating cost makes a faster payback in recovering the capital cost of conversion. The main concern in this type of conversion is cost and performance is a minor issue.

Although propane has a higher energy content (J/kg or BTU/lb) than gasoline, it has a lower density (kg/litre or lb/gallon). Because of its lower density, a car running on propane will have a higher fuel consumption than one running on gasoline but its higher energy content will compensate somewhat for the higher fuel consumption. When the fuel price is factored into consumption, the driving cost ($/km or $/mile), propane is significantly cheaper to use.

An additional benefit of propane is that, as a cleaner burning fuel, fleet vehicles last much longer between overhauls. Propane does not produce carbon deposits in the combustion chamber no matter how long it is used. Without the carbon deposits, engines do not require higher octane fuels to compensate for the gradually increasing compression ratio or the hot spots that cause preignition. There is no carbon to blow by the rings to contaminate the oil, which allows the oil to remain clean. Engine oil at 10,000 km will look as clean as newly changed oil. The main problem with the oil is that extended distances between oil changes cause the lighter parts of the oil to boil off and the oil gradually becomes more viscous. Modern engine oils are formulated for longer oil change intervals (compared with engines of the 60s and 70s) so it's wise to follow the oil change intervals specified in your owners manual. For extended oil change intervals, it is useful to do Used Oil Analysis to verify that the extended interval is safe.

A good place to start research for a propane conversion is Jay Storer's book: "Economy or Performance Propane Fuel Conversions for Automotive Engines (1986-01-24)". It was published by S-A Design (ISBN 0-931472-12-1) and is still available on Amazon. A few book sellers still seem have it stock however. Another useful book to read is Larry W. Carley's Propane Conversion of Cars, Trucks and Rvs. It is published by TAB Books (ISBN 0-8306-3103-8) and this book is also available on Amazon. You may also have luck finding these books in your local library.

An excellent source of information is Franz Hofmann's publication: Diagnostic Guide to Alternative Fuels. Besides great general information about LPG-fuelled engines, it also has a lot of LPG engine-building and repair advice. His very reasonably priced book contains the most current and comprehensive information about North American alternative fuel systems available anywhere. To order a copy of the Diagnotic Guilde, contact Franz Hofmann directly.

Cars converted to propane generally suffered a performance loss in the bad old carbureted days. The problem was that propane was metered (or fumigated) into the engine in a gaseous form. The fuel gas displaced air that the engine was drawing which caused a slight loss in volumetric efficiency. Gasoline is metered into the engine as an atomized liquid. The density of a liquid is much greater that that of a gas and so the volumetric efficiency loss compared to pure air is negligible. The other contributor to lower volumetric efficiency is the somewhat restrictive mixer.


In North America, a common option for a dedicated or straight propane option on older cars was the Impco CA425 mixer. A mixer becomes a carburetor when it is fitted with throttle valves. For the larger American cars that could benefit from propane operation, Impco recommends this mixer for up to 450 CID engines. This mixer is rated for 460 CFM @1.5" Hg but this may in practice be restrictive for big block engines. It should be quite suitable for engines such as the Chevrolet 350 CID (5.7 liter) engine. The shape of the gas jet in Impco mixers controls the fuel mixture so that it becomes richer as air flow increases. A different gas jet is available for larger engines (over 370 CID), which is leaner at their higher air flows. Even though the fuel mixture may be leaner at higher flows, it really means that the fuel mixture is closer to the ideal fuel mixture required to achieve the best power and economy. Rich fuel mixtures in gaseous fuels, unlike with gasoline, result in detonation and burned valves.

From the information we have been able to find by searching the internet, propane-carbureted race cars are very uncommon. Richard is one racer we have been able to find. You may find more information on his web site: As a racer, sponsorship is always welcome so don't be shy to put your name on his car.

However, one great way to take advantage of the potential of propane is through turbocharging. The turbocharger overcomes the loss in volumetric efficiency and the higher octane rating of 104 allows higher boost pressures. Ak Miller of California was the one most known in doing a lot of development of turbocharged propane engines in the sixties and seventies and Jay Storer features him prominently in the turbocharger chapter of his book.

Nowadays, with modern vehicles being equipped with digital fuel and ignition control and port fuel injection, sequential fuel injection of propane overcomes the volumetric efficiency disadvantages of the propane mixer. Digital fuel and ignition control is a feedback control system. The engine's computer has a preprogrammed plan for controlling the fuel mixture and spark advance as a function of various parameters measured by sensors in the engine. The O2 or Oxygen sensor in the exhaust, for example, tells the computer how much to correct the fuel mixture to the target mixture. A far better means than the best guess provided by the manufacturer in the design of the shape of the gas valve or the size of the metering jets in the carburetor. Impco and Dual Curve both offer feedback systems for controlling propane fumigation carburetors.

As there is no longer a carburetor on modern engines, propane fuel injection would be the logical option for conversions. There is a great deal of development work currently going on in the area of sequential propane injection. There are two means of injecting propane: gaseous and liquid. Each has its advantages but liquid injection has the potential for higher performance . The reason for this is that as a liquid, propane must absorb heat from the air to evaporate which causes the air to cool and become denser. The increased air density effectively increases the volumetric efficiency of the engine and increased volumetric efficiency is the goal of every engine performance modification.

There are several companies world-wide developing systems for gaseous propane injection and these systems are designed to seamlessly integrate with the engine's control system. Either the injectors replace the gasoline injectors for dedicated operation or the propane injectors supplement the gasoline injectors for dual fuel operation. These systems are the natural evolution of the fumigation system to allow the computer to precisely control the fuel mixture.

Gas Injection Technologies Pty Ltd of Australia had done a great deal of work with gaseous propane fuel injection. They were also one of the few companies that have already built propane-fuelled race cars. GIT has provided us with some additional information than what is available on their web site. Unfortunately, their system was not yet available in North America and seem to gone out of business.

In Canada, EDPRO and Technocarb are companies specializing in gaseous propane injection. While both are focused on fleet conversions, Technocarb has a much wider product range. The systems of each manufacturer uses the vehicle's on-board computer (PCM) to drive a second set of injectors but alterative fuel system's controller intercepts the signal and modifies the injector pulse width to suit the alternative fuel. In this manner they "mirror" the manufacturers fueling strategy so the check engine light does not come on when the computer performs a system check.

EDPRO is an alternate fuel designer, manufacturer, supplier and installer of both LPG (Propane) and CNG (Natural Gas). They are also starting to get involved in hydrogen. Their proprietary system is known as SEQUIN and the name SEQUIN is an acronym for SEQUential INjection. The SEQUIN system uses a heat exchanger to heat the fuel in the LPG tank, which allows their system to operate in extreme cold conditions. By withdrawing only vapour from the fuel tank, EDPRO also claims this system eliminates the potential for ‘heavy ends’ contamination of control components. However, the SEQUIN system is only available for Ford 4.6L and 5.4L vehicle platforms.

Technocarb has many conversion systems for North American domestic vehicles and a number of universal systems. The majority of their systems are for LPG but they have a few natural gas applications as well. To meet current EPA certification requirements, Technocarb has developed their own Sequential Vapour Injection (SVI) LPG injection system, which available for North American domestic applications as well as in universal kits for other fuel injected vehicles. Rather than circulating warmed LPG fuel through the fuel tank, Technocarb simply allows the engine to remain in gasoline operation mode if the tank pressure (due to low ambient temperature) is too low. Technocarb addresses the 'heavy ends' issue by providing an optional vapour temperature controller if warranted by poor fuel quality.

Car and Driver magazine ran an article about a propane injected police car in their July 2002 issue. This car used a liquid propane injection system, which made the transplanted V10 truck engine much quicker than the stock 4.6 litre gasoline engine.