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FuelConsumption

Fuel consumption is always a good (but boring) topic of conversation amongst Impreza owners. Depending on how much you enjoy your driving, fuel consumption can vary between about 16 and 30 mpg. Most owners seem to average about 21 - 23 mpg, although on long motorway runs, 30 mpg and above is attainable. Acceleration most hurts economy - steady 90mph driving can easily give better mpg than a 60-70 range involving lots of speeding up and slowing down.

The 60 litre fuel tank (50 litres before 1996) gives a typical range of between 210 and 300 miles between fill-ups. Committed driving, for example on a track day, can see fuel consumption of 9 mpg or less!!!

For the normally aspirated 2.0 models, an average of 30 mpg seems to be the norm, with 35+ on long motorway journeys.

The UK government figures for the 2.0 Turbo models (1997 spec onwards) are:

Modempg
Urban20.5
Extra Urban34.9
Combined29.7

The gauge is not totally linear: "Full" and "3/4" are about right, but when the needle points to the end of the scale you can only get 45 litres in (i.e., it's still got a quarter of a tank). When the needle's on the middle bar of the "E" it'll take 55 litres (bottom bar on '98 models) - and cough around left-handers :-)

Type of Fuel to Use

Many questions are often raised regarding the type of fuel needed for the Impreza. The sticker inside the petrol cap says use Super unleaded only, but this has different meanings in different countries. In the UK, the standard (or premium) unleaded fuel is rated at 95 RON, the same as the Super available in other countries such as Australia and the US. In the UK, Super Unleaded is rated at 97 RON (was 98 until 1999). In Japan, it is 100. Generally, in the UK, normal unleaded is fine. In countries where lower quality fuel is available, then the best advise would be to run on the Super (or Premium) Unleaded fuel. Beware of octane boosters, as many contain lead or lead-like substances which will damage the catalytic convertor and oxygen (Lambda) sensor..

From the knowledge we have so far, it seems that the Engine Management System will react to a lower quality fuel by detecting the onset of knocking. In this case, it will retard the engine timing and reduce the maximum boost available from the turbo. It will then run on these lower settings until such time that the system is reset, though the system should eventually re-adapt to the higher quality fuel. Resetting the ECU simply speeds up this process.

Some owners report gradual improvements by simply running on the higher spec fuel, but most will find that although it is not any faster: the engine will be slightly smoother and the economy increased marginally (by about 5%). This subjective improvement can be hard to justify against the premium in fuel price.

The Fabled ECU Reset

One of the most frequently discussed and debated topics (even among people who have read this description!) is that of running the car on Super Unleaded (97 RON) fuel, and resetting the Engine Management Unit.

When first activated, the engine management system advances the ignition and shuts the wastegate to find the point at which it senses knock. This setting stays in it's memory and it will only alter itself to further retard the map if a fault occurs. This is a safety feature and is common to almost every modern engine. After a fault is repaired (in this case changing to a higher grade fuel), the engine will gradually become better, but unless the memory is wiped, it would take an infinite number of re-sampling cycles for the effect on the average to be overcome.

The knock point of a fuel is generally accepted as the level to which the fuel can be compressed before it self combusts, thanks to the heat generated in compressing it (the universal gas law). This is an over-simplification in an internal combustion engine, as there are many factors that affect this point, most prominent being the retained heat in the chamber from the efficiency of the previous explosions. If these explosions are too efficient, and too much heat is present, a modern ECU can reduce engine efficiency by delaying the spark or reducing the volume of air and fuel going in, by releasing pressure from the turbo, or both. The knock point of 95RON is much lower than 97RON and the engine must run at either lower internal temperatures (not possible without a WRX water spray) or lower boost or retarded timing. i.e. the engine is not running at as efficiently as is desired. As an example, WRX cars are designed to run on 100 RON fuel in Japan, their performance on UK 95RON is in the region of 245 to 265 bhp (various unconfirmed reports), purely from this adjustment in timing & boost.

To overcome the problem of the parameters learnt from running the car on normal unleaded, the ECU can be "re-initialised" with a full tank of pure Super.

The ECU is reset (the dealer uses either a diagnostics computer called a "Select Monitor" to zero all memory settings, or he disconnects the battery for long enough to allow the back up capacitor to drain). After this has happened, the ECU reverts to factory settings and has no previous history of limits learnt from the knock points of previous fuel grades. The car is then driven normally (ie briskly!), and the ECU starts assessing its parameters and adjusts the maximum settings accordingly, such that no engine knockhing is detected. This status will exist until either low grade/defective fuel is used or the ECU detects a fault that it must act upon.

As an example of the difference this makes, a members '96 is running at almost 1.2 bar which it has achieved by 3800 revs. The dealer had found that 95RON would only give 1.0 bar on a '96, as per the manufacturers figures. These figures have also been substantiated by another source in France. An owner in Pretoria reports that the local 91RON fuel allows only 0.5 bar!

The figures for newer cars (post 96) are lower, due to the smaller turbo.

Different people have tried this process, with different degrees of success. Some notice a huge improvement, whilst others report only a minor improvement (if any). The main criteria seems to be the previous performance of the car. If the car has just run for several thousand miles on SUL, then the averaging of its operating conditions will mean that there is an immeasurably small change to what the ECU already has as its acceptable limits. However if the car has run of differing fuel octanes and in particular on the dreaded "supermarket" premium unleaded, then there are potentially significant gains to be found.

The only guaranteed way of resetting the Engine Management System is to get your Subaru dealer to perform the task. They have the suitable diagnostic equipment required to do the job properly, and technicians who were taught how to perform the task during their Subaru training.

Actually getting your Subaru dealer to perform the task can be quite difficult. Some are unaware of the procedure, and lots deny all knowledge that it can be done. Others however, openly admit they perform the required steps, and will gladly perform the task as part of routine servicing, on request.

Comments from an owner ...

"I recently had the blow-off valve (air bypass valve on job card) replaced on my own car (common fault on 97 model cars), and as part of this task, they technician said they would also have to reset the engine management system. Having filled up the three previous times with Super Unleaded, I thought this would be an ideal opportunity to test the results. "Wow" was my first thought, the car picked up much better and pulled more strongly from about 2,500 to 5,500 revs. The boost levels from the turbo seem higher, and the car is now even more devastating in the mid range. I didn't notice any difference in top end power (probably being restricted by turbo output and the standard exhaust and air filter). The car now idles more smoothly, and picks up much cleaner at low revs. I now think it is worth the extra expense of filling up with the more expensive Super Unleaded fuel."

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