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High Octane Fuel

With the availability of high octane unleaded fuel in Australia, Prado enthusiasts have invariably tried it at some point. There is however varied feedback ranging from quite positive in terms of performance and improved fuel economy to no change and a waste of money.

Let’s look at why this is - in an effort to help members understand how their petrol Prado adapts to varying fuel quality.

Briefly, a Prado petrol engine requires fuel to be injected at the right time and a spark to be generated at the spark plug at a precise point in the compression cycle. Fuel then burns in a controlled fashion generating lots of good energy to push the piston and ultimately drive the car.

If that spark is generated a little bit later in the compression cycle (retarded ignition timing), combustion is less efficient resulting in less power and torque.

Conversely, if that spark is generated a little bit earlier in the compression cycle (ie advanced ignition timing), fuel combustion occurs in an explosive fashion commonly referred to as detonation or pinging. This can lead to engine damage.

Generally the optimum ignition timing is advanced to a point just before the onset of detonation (in most engines). This results in the highest efficiency and results in the maximum amount of power and torque.

The higher the fuel octane rating, the less prone it is to detonation, hence the greater potential for ignition timing to be advanced further. – Again resulting in higher efficiency. However, we have no control over the ignition timing advance as it is adjusted by the engine management system.

In recent years, Toyota had been quite proactive in adopting advances in engine management and implementing them on the Toyota 4WD range. One of these advances was an ignition advance system that slowly advanced ignition timing to the point where minor detonation would occur and was registered by the engine knock sensors. The engine management system would then retard the ignition timing slightly and after a period of time without further detonation, slowly advance timing again. This ensures that the engine runs as efficiently as possible given the fuel quality available. The tuning industry’s term for this adaptive system is one that “runs on the knock sensors”.

This is not to be confused with less advanced systems that run a predetermined ignition advance map and use the knock sensors as a safety precaution to retard only when serious detonation occurred.

Prado petrol engines are equipped with adaptive ignition timing however the rate at which the timing advances is very slow indeed. In addition, if even minor detonation is registered at one part of the RPM range, the entire ignition map is retarded across the entire RPM range.

When high octane fuel is used, the likelihood of detonation is diminished and the engine management system can advance the ignition timing further than that with standard fuel. Resulting in higher efficiency, greater power and torque as well as reduced fuel consumption.

However, in the range of Prado petrol engines, the rate of ignition advance is very slow indeed. This means that one needs to run high octane fuel for many km before any change in performance or economy is fully realised. In the case of the 3.4L V6 in the 90 Series, 5,000 – 7,000 km and for the 4.0L V6 in the 120 Series, 3,000 km – 5,000 km.

Generally after one or two tanks full, the owner will give up and return back to standard fuel because he notices no difference – and does not realise that it takes so long. However if you persist, the benefits are real and quantifiable – more so in terms of fuel economy.

A direct example that I can quote is my wife’s 120 Series 4.0L V6 petrol Grande. She is a most consistent driver in that she travels the same roads and highway day in day out. In addition, her driving habits do not change over time.

Fuel economy with standard unleaded was consistently 15.5 l/100km. However, with 98 RON Shell Optimax, fuel economy is consistently 13.1 l/100km – a reduction of 2.4 l/100km or in percentage terms, a reduction of 15.5% in fuel consumption. This incidentally is greater than the price difference between standard unleaded and Optimax, so in her case, running the better quality fuel results in an overall saving in fuel bills, extended range – and of course the additional engine performance is free!

The secret is though to persist with the better quality fuel for a long period of time.




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