ZT: Running cost comparison: 91 and 95 Octane

ID就是个符号

http://www.aa.co.nz/motoring/owning-a-car/petrolwatch/running-cost-comparison-91-and-95-octane/

---

Running cost comparison: 91 and 95 Octane

With 91 Octane fuel being the cheapest option at the pump, motorists looking for savings have been tempted to make the change from 95 Octane. Often this decision is made contrary to the manufacturer's recommendation for a particular make and model.

So which one should you use if you want to save money? Even after the AA tested consumption between the two fuels, the decision whether to use 91 in preference to the higher 95 octane was not as simple as it seemed.

---

Using 91 Octane instead of 95 Octane

For short distances and one off runs in urban driving conditions, there were minor differences recorded in the consumption figures between the two fuels. Where 91 Octane was added to a vehicle recommended to run on 95 Octane, the consumption figures rose slightly.

On longer trips (in excess of 400km) the differences become more apparent with fuel consumption on 95 octane (where recommended) having a benefit over the 91 octane of up to one litre per 100km.

Using 95 Octane, when 91 Octane was recommended by the manufacturer, made no difference at all in testing.

95 Octane and the modern engine

The modern fuel injected engine is programmed with detailed on-board computer fuel mapping systems. When a 95 octane fuel is recommended the engine electronics will deliver the best performance possible based on that rating over a long period. Where a lower octane is used, the system will often compensate for that situation and allow the engine to run without inflicting any damage.

One of the biggest factors in selecting the appropriate fuel is the engine's compression ratio, which in simple terms is how much pressure the fuel and air mixture can tolerate before it is ignited by the spark plug. A low octane fuel will tolerate less pressure and is inclined to self ignite before the spark plug has fired when added to a high compression engine. This can have a damaging effect on the engine long term and is sometimes audible by a pinking or rattling noise when the vehicle is placed under load.

While the modern engine will compensate for the lower octane fuel and the occasional top up with a lower octane fuel is not going to do any harm, we recommend motorists stick to the manufacturer's recommendations to ensure they enjoy the overall driving experience the engine was designed to produce.