Unleaded v Super Unleaded

When you fill up with Unleaded at your local forecourt you are usually offered a choice of 2 kinds of petrol, premium or super unleaded. These super petrol’s are sold at most petrol stations and come under different names, Tesco has Super Unleaded 99, BP- Ultimate Unleaded and Shell – V-Power. The price difference between the premium and super unleaded ranges from 5p to 10p per litre depending on where in the country you fill up. The average car will have a fuel tank of between 40 litres (ford KA) to 70 litres (Audi A4 ). If you take a mid range car of 55 litres you are looking at around £2.75 extra to fill up your tank with Super unleaded over standard unleaded. This doesn’t sound much but if like me (who incidentally has a car with a 55 litre tank) you use a full tank of petrol each week then you are talking about spending an extra £143 a year just for switching petrol.

With money being tight for everyone at the moment that’s quite a lot extra per year, so what benefits can you expect to get for your money from super unleaded? The Petrol manufacturers claim that the benefits of using their super unleaded petrol’s are:

Less Pollution and a Cleaner engine – Exhaust emissions include nasty gasses such as, carbon monoxide, nitrogen oxides, hydrocarbons and particulate matter which are all harmful pollutants. Super unleaded has an advanced engine cleaning formula that is said to prevents deposits forming on your engine whilst producing fewer harmful emissions in the process.

More Performance – Super unleaded is designed to enhance your engines performance which means you get more mileage per tank and an added performance boost.

Ordinary unleaded is known as 95 RON and super unleaded 98 RON. The term “RON” refers to the petrol’s Research Octane Number; the measurement taken to determine a fuels resistance to detonation. This also tells us how good a fuel is at resisting engine knock. In a modern engine we get our cars power by a mixture of air/fuel being compressed then ignited by a spark plug at exactly the right moment. Although the ignition is meant to be caused by the spark plug, sometimes it’s ignited prematurely because of pressure. This kind of ignition results in an uneven explosion which causes knocking in the engine and can result in damage occurring.

Super unleaded petrol’s with a higher octane number are more resistant to engine knocking because they are able to withstand a higher pressure. It’s for this reason that super petrol’s are used in high performance cars that have engines with a higher compression ratio. Most modern car engines have management systems which are capable of adjusting the engines timing at the first sign of knocking to prevent damage.

If lower octane petrol is used in a high performance car, clever sensors in the engine can prevent engine knocking by adjusting the engines settings. Although no damage may occur the car will likely suffer from a reduced performance and lower fuel efficiency. It’s important to note that just because a car experiences engine knocking doesn’t mean that a higher octane fuel is required; the car may just need to have its engine tuned to fix the problem.

For a car to take advantage of a super unleaded fuel it needs to be able to adjust the timing and firing of its engine via an electronic control unit (ECU). For many mainstream petrol-powered cars using high performance petrol will result in little or no improvement to the cars fuel efficiency or power output because their engine just isn’t equipped to take advantage of the higher octane fuel. Some people still feel that a higher octane fuel has better detergents included for keeping your engine healthy. This is not necessarily the case as almost all modern unleaded fuels already contain the detergents required for keeping your engine healthy.

Unless the manufacturer of your vehicle recommends you use high octane petrol for your car you are just as well sticking to standard unleaded petrol which will saving money and more than likely give your vehicle the same level of performance as a higher octane fuel.

Which magazine recently did a comparison test between premium and super unleaded petrol use in both a Ford Focus 1.6 petrol (98bhp) and VW Golf 1.4 TSI (168bhp)