Ashland Automotive – Premium Fuel

Customers often have questions about fuel. At the top of the list is this: Do I need to use premium fuel? The short answer is all vehicles have a label next to the gas cap (inside the flap) stating which type of fuel is required. Premium fuel is only required if the label says so. Cars which don’t require premium fuel won’t benefit from using it, assuming the engine is running properly. If the label says to use premium fuel, use it.

Here’s the science behind the answer. As the engine pistons compress the air and fuel mixtures in the cylinders (herein referred to as compression), friction between the molecules results in heat. Engines requiring premium fuel, or higher octane fuel, are engines which have high compression ratios. Turbocharged or super-charged engines are usually of this design, as are many European cars which are normally aspirated. Engine knock, or pinging, is more likely in a high-compression application. Because high octane fuel burns more slowly, it has a greater ability to resist engine knock. Lower octane fuel, which burns faster, can self-ignite in a high-compression application. This pre-ignition, which happens before the spark jumps the gap in the spark plug, causes a two separate flame-fronts to collide resulting in a “pinging” noise (this sounds like marbles in a coffee can). Not only is pinging annoying, but it also creates strain on the combustion chamber and reduces engine performance. The vehicle’s powertrain control module utilizes a detonation sensor to detect pinging, and is able to self-adjust, or retard, ignition timing to correct for the pre-ignition. A retarded ignition timing results in reduced engine performance. Furthermore, this same powertrain management computer is also adaptive, meaning it recognizes past patterns and adapts to them. If your car is regularly fed low octane fuel, but should be fed premium, the computer will “relearn” how to drive without the premium fuel, thus creating a “new normal.” Over time, the new normal is noticed by the driver. Finally, the car is brought to the shop and causes your favorite mechanic to lose sleep trying to diagnose symptoms (this never happens to us-yeah right!)

Some drivers are on the other end of the spectrum and are unnecessarily using premium fuel to help their older car stop pinging. In these cars several problems can result in pinging, the most common of which we see is carbon build-up on the surface of the piston. The carbon build-up takes up space, thereby increasing the compression ratio in the combustion chamber. The result is pre-ignition, or pinging. Other causes of the pinging can be mis-adjusted ignition timing, or a faulty detonation sensor.

We’ll address Gasoline Direct Injection, or GDI, later in the year. For now, as a general rule, use of a quality, name-brand fuel with additives/detergents will help reduce the carbon buildup on pistons and valves allowing systems to operate as designed. Use premium fuel if it’s recommended, and enjoy the performance which results!

Ashland Automotive
280 E Hersey St #15
Ashland, OR 97520
(541) 482-4042

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