Since the introduction of E10 petrol across the UK, motorcyclists have faced a new complexity at the pump. The days of simply choosing ‘unleaded’ are over. Now, every rider must understand the critical difference between E10 and E5 fuel – a choice that profoundly impacts everything from engine performance to the lifespan of classic rubber components.
The choice isn’t just about price; it’s about protection. This detailed guide breaks down exactly what ethanol does inside your fuel system and why choosing the right grade is essential for your bike’s health and longevity.
The terms E10 and E5 simply refer to the maximum volume of bio-ethanol blended into the standard unleaded petrol.
The government mandated the switch to E10 to help reduce carbon emissions. While noble in intention, this increased ethanol content introduces significant risks to motorcycle and scooter fuel systems, particularly in models manufactured before the year 2000, and even some newer bikes.
Ethanol is hygroscopic, meaning it readily absorbs and holds onto atmospheric moisture.
Many older motorcycles and even some early fuel-injected bikes were built before high-ethanol blends were common.
While the corrosion risk is the biggest concern, the increased ethanol content can also subtly affect running characteristics, particularly in high-performance engines.
The additional oxygen molecules in ethanol change the air-fuel ratio. On some bikes – especially those that cannot automatically adjust the air-fuel mix (i.e., older carburettors or early closed-loop injection systems) – running E10 can cause the engine to run slightly lean.
Running lean causes the engine to run hotter and can increase the risk of detonation or damage over the long term, particularly under heavy load.
In the UK, E5 fuel is readily available as the ‘Super Unleaded’ or ‘Premium Unleaded’ grade at most service stations (often 97, 98, or 99 RON). It is clearly labelled with the E5 symbol, which features a square box with E5 inside.
By understanding the chemical differences and making the simple choice of premium E5 fuel for your classic or stored bike, you are actively safeguarding your machine against corrosion and ensuring that every ride is as smooth and reliable as the manufacturer intended.
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