How do I decide whether I should use premium or regular petrol? Is it better for the engine and fuel economy if I use a higher grade? What would be the problem if I use cheaper regular petrol?

There are three grades of petrol sold in Singapore. The classification is based on what the industry refers to as octane rating: 92, 95 or 98.

The octane number defines the petrol's ability to resist pre-ignition or "knocking", which is the phenomenon of fuel igniting before the spark plug can deliver the spark for a smooth and clean combustion.

When this occurs, the fuel is not completely burned, resulting in poor performance, higher fuel consumption, increased emissions and, worst of all, damage to the piston, valves and combustion chamber.

Knocking, as the term implies, is an audible rattle from an engine, especially when accelerating and almost always occurs if it is fueled by a lower octane petrol than the engine was designed for.

Which petrol your car requires is stated in the owner's manual and usually on the fuel filler flap.

Most car engines today are designed to run on 95-octane petrol, although there are also a number of cars that are designed to run on a minimum 92-octane petrol.

Technically, there is no harm in using the higher than recommended-octane fuel since the manufacturers always state what is the "minimum" octane number suitable for the car. However, there are no advantages, either in terms of performance or fuel economy.

Some high-compression engines require 98-octane fuel for optimum performance, such as a few high performance turbocharged engines. But that does not mean that a 92-octane engine would deliver better performance if fed with 98-octane petrol.

Meanwhile, using fuel with lower octane than the minimum specified in the owner's manual could lead to costly repair bills.

As a rule, stick to what the manufacturer recommends and never use a lower octane fuel than specified.

Shreejit Changaroth