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What are the benefits of running a vehicle on Superunleaded or Superdiesel?


The basic advantage of Superunleaded is it has a higher Research Octane Number than ordinary so-called 'Premium' unleaded. Superunleaded is at least 97-99 RON, whereas 'Premium' unleaded is at least 95 RON.

The higher octane gives more torque at lower RPM, which allows an engine to run safely at lower RPM than lower octane petrol, as well as giving higher power at high RPM. In testing, Ford found the increase in power at 6,000rpm to be as much as 10%.

The 'better burn' itself helps to keep the fuel system and valves of an engine clean:

Branded Super also contains a package of additives including detergents and lubricants to keep the fuel system of an engine additionally clean.

Similarly, Superdiesel has higher a Cetane rating than ordinary diesel, providing better performance and a cleaner burn at low revs, while branded Superdiesel contains a package of detergents and lubricity enhancers to extend the life of the injection pump, as well as keep the injectors and valves clean.

Emissions testing of all vehicles has forced manufacturers to achieve the best results they possibly can in test laboratories (NEDC, WLTP) and, from September 2019, in Real Driving Conditions (RDE).

So manufacturers 'optimise' their engines to give the best possible performance in the tests and, to achieve this, they optimise the engines to run on Super.

Engines will still run on lower Octane and lower Cetane fuel, but not as efficiently, not as cleanly and will be more prone to damage from deposits such as olefins and carbon left inside the engines.

If vehicles are driven to take best advantage of Super, which means changing up earlier (at low throttle openings a properly programmed automatic will do this for you), then the improvement in fuel economy is usually between 4% - 6%. 

Where Super is sold at up to 8p more than ordinary 'Premium' fuels, the improvement in economy usually covers the increased cost. Where Super is sold at up to 15p more it doesn't, but the benefit remains one of keeping the engine in peak condition.

In some other countries, a 'Motor Octane Number' (MON) is used to rate the octane of petrol. The testing is completely different than for RON. Depending on the composition of the fuel, the MON will be about 8 to 12 Octane lower than the RON, but there is no direct link between RON and MON. In countries using the MON rating system, 'Superunleaded' is 95MON.

See: https://www.cnet.com/roadshow/news/aaa-top-tier-fuel-study/#ftag=CAD0610abe0f 



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