LPG

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Running an E30 as a daily driver isn't cheap. The E30 was never designed as an economy car, and with fuel prices constantly increasing the thirst of the E30 becomes increasingly hard on your wallet.

It's usually the fuel costs that force people to buy some revolting modern diesel, relegating the E30 to a weekend toy. But it doesn't have to be this way. You could always convert to LPG.

Learn more about installing LPG.

Overview

LPG is an acronym for Liquefied Petroleum Gas, which you'll probably know as either propane or butane, and the rest of the world calls autogas. It's in the petrol family, but is a by-product of the extraction and refining processes rather than a "desired" fuel. Fuel producers end up with tankloads of LPG whether they want it or not, which is what makes it so cheap. More and more petrol stations these days have a big white cylinder on their forecourt, allowing you to top up your tank for half the price of standard unleaded.

LPG works exactly the same as petrol - mix it with air and provide a spark and you'll get an explosion, so converting your car is very simple; most of the equipment is already fitted. You just need to install a tank, a vaporiser to convert the liquid to a gas, and find a way of feeding that gas into your engine. There are a number of systems to achieve that.

With a properly installed system, you can swap from running on petrol to LPG at the flick of a switch, and when your tank runs low just switch back again. You won't have to make any critical changes to your existing fuel system, and you're not committed to using LPG exclusively. The systems are relatively easy to install and are available from a number of reputable suppliers. We've outlined the different systems for you to get your head around.

We know what you're thinking. If LPG is some incredible wonder-fuel, why isn't everyone using it? The simplest explanation is lethargy; car manufacturers have been designing petrol-based systems for so long that there's little incentive to switch to LPG. There are some other downsides too. LPG really needs a warm engine to run at its best so petrol systems are still needed for cold starts, and the most practical downside is finding space for the tank which needs to go in the boot. But if you're willing to cope with two filler nozzles and the loss of your spare wheel well, you'll see your motoring costs plummet immediately.

We've outlined the pros and cons for you in one handy table, and if you're convinced then you can learn more about installing LPG.

Pros and Cons

Positives Negatives
  • Simple installation
  • Very cheap to run
  • Pays for itself in a very short term
  • Kinder to the engine than running petrol
  • Extended driving range from running both petrol and LPG
  • Loss of boot space or spare wheel
  • Modifications to the shell to install
  • Lack of convenient refill stations in some areas
  • Limited LPG range due to available tank sizes
  • Poor cold starting

Systems

LPG has been around for a surprisingly long time; the history goes back over 100 years, and has been a practical petrol alternative since the 1940s. However, in all those years there's only really been three commercially viable systems. These are:

All three of these systems achieve the same goal, which is to inject LPG into the Intake of your engine. That way, the fuel enters the engine via the air flow, completely bypassing the Fuel system of the car. The differences between the systems is how they go about getting the LPG into your air intake.

Gas Carburettor

The Gas Carburettor, sometimes confusingly called Single Point, is the simplest form of LPG system. With this system a mixer plate is fitted into the intake, and on E30 engines it bolts directly between the air flow meter and the intake boot. LPG from the tank is fed into a vaporiser where it is warmed by a hot water feed from the coolant system, and from there it flows naturally to the mixer plate. When you press your accelerator pedal and open the throttle body, air is sucked into the engine as usual, only this time it draws LPG from the mixer plate with it, just like an old school carburettor.

The beauty of this system is its simplicity. There's no need for electronic management or special engine sensors; you simply cut the wire that powers your fuel injectors, and wire in a relay to your LPG tank. With the flick of a switch you turn off your injectors and allow LPG to flow through the mixer at the same time. It's not the most fuel-efficient system, but it's perfectly usable. If you want better efficiency and cleaner exhaust gases, the fitting of a simple ECU and lambda sensor will improve the system considerably.

The downside of the gas carburettor system is the risk of backfire. By mixing LPG and air before it enters the cylinder, you're filling the intake with a flammable mix which can (and does) pre-ignite occasionally. The resulting blast can destroy your AFM, which is why a one-way flap is added to the mixer to protect your fragile air flow meter. However, this has the effect of slightly limiting the performance of the car by blocking the air flow.

Single Point Injection

Does exactly what it says on the tin. With SPI systems, a single injector is mounted into the intake system, which squirts the required amount of LPG depending on the engine load. These systems are more complex than the Gas Carburettor systems, but they work on the same principle, with the freezing liquid fuel being vaporised before being fed into the intake. On E30 engines the injector that supplies the LPG is commonly mounted on the end of the intake manifold, opposite the air feed for the FPR. The ECU that controls it has access to the throttle position switch so that it knows how much power is required, and monitors the exhaust gases via a lambda sensor.

The advantages are clear. By incorporating an ECU, much more precise fuelling can be achieved. This not only improves your economy but makes sure you get the most power from the engine. And with an injector instead of a mixer plate, there's a lot less interruption to the airflow, so the LPG won't strangling your car. Of course, a flap is still needed to protect the AFM from backfires, but these don't happen as often with injector-based systems.

SPI is the precursor to Multi Point Injection. Because of that, you don't see the systems for sale that often any more as they're considered outdated, so new systems, or spares for old ones, may be hard to come by.

Multi Point Injection

Multi-point LPG systems introduce a direct copy of your car's existing Fuel system. Every cylinder has its own injector, and their fuelling is digitally controlled by a dedicated ECU that is separate from the standard engine management. These injectors are usually mounted on the "fingers" of the intake manifold, just before they reach the cylinder head, and therefore require patient drilling and fitting to install.

Multi-point offers the optimum fuelling for your car by giving you total control over the LPG fuel, and it's pretty much mandatory if fitting LPG to a modern engine. However, on the dinosaur technology that E30s use it's a bit of a waste - there are no appreciable gains from fitting multi-point unless you're already running a custom engine management system... or you've undertake an engine swap.

Installation

Main article: Installing LPG

A competent home mechanic can install an LPG themselves in a weekend if they have all the right parts. Of course, LPG is a pressurised explosive liquid so needs to be treated with care. If you're unsure, follow the guidelines or pay a professional to install the system for you.

The main considerations are what sort of system you want, and how big a tank do you need. The bigger the tank the better, but it comes at the cost of boot space. Many choose to fit a tank in the spare wheel well, but this adds a new dilemma - where do you keep your spare wheel?

Once you've weighed up the pros and cons of the system and found a system within your budget, crack on with Installing LPG.

Which Kit?