We investigate the differences between 93- and 95-octane unleaded so you can make an informed decision at the forecourt...

The petrol price is at an all-time high and consumers are cash-strapped. You’d therefore assume most motorists are foregoing 95-octane unleaded fuel in favour of 25-cent-a-litre-cheaper 93. However, the latest bulk-fuel-sale data indicates sales between the two octanes split almost equally. Do the people buying the more expensive octane know something you don’t? In this feature, we explain what the octane grade means; what impact it has on your car at the altitude where you live; and, ultimately, which one is best for your car and wallet.
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Background on octane 

The South African National Standard (SANS) 1598 pertaining to fuel specifications make provision for the sale of 91-, 93- and 95-octane petrol to the general public (although only the latter two are sold). Higher octane-rated fuels are permitted only for racing applications to holders of MSA racing licences.

Contrary to popular belief, the fuel-octane number is not linked to petrol’s energy content but rather to its resistance to auto-ignition (also referred to as knock). The higher the number, the higher the resistance.

Petrol engines are called spark-ignition engines because they require a spark (from the spark plug) to start the combustion process of the air-fuel mixture in the combustion chamber at the optimum moment (called spark timing). The normal combustion process is not an explosion but a controlled energy release by the burning of the air-fuel mixture.

Auto-ignition is a rapid combustion process resembling an explosion when some of the air-fuel mixture combusts when the conditions of pressure and temperature inside the combustion chamber is conducive to this happening. The consequences to an engine can be fatal.

Octane and fuel economy

A number of studies link an increase in octane number to slightly better fuel economy but this comes with a caveat: the largest benefit is realised at high-load, wide-open-throttle (WOT) conditions, which is not the desired operating range of an engine if the driver wants to save fuel. The 25 c/L premium of 95 erases any potential benefit from the equation.

Octane and engine performance

To grasp the link between the octane number and engine performance, it is important to understand maximum brake torque (MBT), which is realised at a specific speed and load condition of the engine when the spark timing is optimal. Engine-calibration engineers spend hours on an engine dynamometer to tune the ignition-timing map of the entire grid of speed and load points to realise MBT.

Part-load condition

During day-to-day driving, an engine spends most of its time in a part-load condition, where the driver does not depress the throttle all the way to the stop. With a fixed air-fuel ratio at 14,7:1 to optimise fuel economy and emissions, the MBT curve during part load is illustrated below. Advancing ignition timing any further than MBT timing simply sees a reduction in engine torque. The fuel-octane number has little influence in this condition because in-cylinder pressures are relatively low.
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Full-load condition

When maximum performance is demanded from an engine, the driver depresses the accelerator fully (WOT) to a full-load engine condition. As the in-cylinder pressures and temperatures are now at their highest, it is usually impossible to reach MBT by advancing the ignition timing because auto-ignition (knock) sets in (explained in the graphic below). This is called knock-limited performance. By using a fuel with a higher-octane number, the knock limit is pushed out and more timing advance is allowed, resulting in a gain in performance. In this condition, many manufacturers enrich the fuel mixture to further combat the onset of knock and help cool the exhaust valves, but this obviously has a negative effect on fuel economy.

Octane and altitude

Naturally aspirated engines

Atmospheric pressure has a significant impact on the in-cylinder pressures of naturally aspirated engines during WOT. As Johannesburg, for example, is roughly 1 700 metres above sea level, the atmospheric pressure is generally 17% lower than at the coast, explaining the drop in engine performance. This fact lowers the tendency of the engine to knock, allowing a lower-octane fuel to be used with no loss in performance compared with a higher-octane fuel because the additional benefit cannot be realised.

This was confirmed in a study by Professor Andy Yates, who conducted actual WOT performance testing on a fleet of 18 vehicles in Gauteng using varying octane-rated petrol. He found the following:
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The top graph depicts the probability of maximum performance gain by octane number at altitude. Most manufacturers calibrate their petrol engines to run optimally on 95 octane at the coast, although some gains are still possible. Interestingly, 87-octane petrol appears to satisfy most engines’ maximum performance potential at altitude and is confirmed by the second graph showing no benefit at altitude to use an octane rating higher than 91. Studies show, with each increase of 300 metres in altitude, drops in octane rating of between 1,0 and 1,8 can be safely used with no negative effect on performance from a MBT-timing point of view.

Turbocharged engines

The difference in comparison with naturally aspirated engines is the turbocharger can mostly compensate for the lower atmospheric pressure at altitude (if the impeller is not speed limited for turbo protection). Therefore, the in-cylinder pressures during WOT is similar to the values achieved at the coast. In this case, increasing the octane number does result in higher maximum outputs, as can be seen in the graph below for an Audi 1,8-litre turbopetrol engine. Current turbocharged vehicles have improved knock-control strategies with similar results to be expected.
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Knock control of modern engines

Knock sensors are accelerometers mounted on the block of an engine to “listen” for the onset of knock (or auto-ignition). If detected, the ECU retards the ignition timing to protect the engine, impacting performance. Most modern engines are calibrated to run on 95 unleaded petrol to produce the claimed maximum power outputs and will retard the timing only when a lower-octane fuel is used.

There are, however, advanced knock-control systems able to advance the timing until the onset of knock. The result is a higher power output than claimed at the standard of 95 octane.
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What’s the right fuel for you, then?

To quickly determine which octane number you should choose, follow this flow diagram:
unleadedAuthor: Nicol Louw