Knock free

In one of my recent columns I stated that if an engine runs knock free on an 87 octane fuel, using a higher octane fuel will not benefit you in any way. I received several inquiries about whether this was for automobiles or airplanes. Basically it applies to both.

In your automobile, if the owner’s manual calls for an 87 octane fuel, then a 91 octane will not help you at all. In premium fuel requirement vehicles, you can run 87 octane fuel without harming your engine, but your acceleration may be reduced because the knock sensor in the engine can retard the timing to reduce the amount of knock.

In an aircraft, you need to look at the STC or owner’s manual. If the requirement is for an 87 octane fuel, then using a 91 octane fuel will work, but it will not offer any advantage over the lower cost 87 octane fuel.

I also received the same question from several readers: “If the auto industry could solve the problems with the switch from leaded fuel to unleaded fuels, why can’t the piston aviation industry?”

There are several answers to that question. One is the volume of units sold each year. In the automotive world, millions of cars and light trucks are sold each year. That means that the cost of new systems and technology can be spread over a lot of units, which means the unit cost is very low.

The other big difference is that automotive engines are liquid cooled, while almost all aircraft engines are air cooled. Liquid cooling has several advantages, but the main one is even cooling. This means that there are no hot spots and, if there were a rise in temperature in one area, the cooling system can carry the heat away quickly.

This is important because knock will not normally destroy an engine. The knock will raise the temperature in an area of the affected engine cylinder. Then if the cooling system does not carry the heat away quickly enough, the surface temperature can rise to a point that it will ignite the incoming charge during the compression stroke. This is called pre-ignition, which can destroy an engine in very short order.

Another advantage of a liquid cooled automotive engine is that the cylinders have a common cylinder head. This is important because the sound you hear when an engine knocks is the vibration in the cylinder caused by the end gas auto igniting.

If you install a knock sensor, which is just a small accelerometer, on the automotive engine cylinder head, it will sense when any of the cylinders knock and retard the timing for the entire engine. On an aircraft with individual cylinders, the vibration does not transmit well from one cylinder to another, so you would need a sensor on each cylinder.

However, the biggest problem with knock sensors is that they retard the timing to prevent knock. Say you are a normal pilot who has flown your aircraft for many years. You replace the engine with a new one that is of the same horsepower, but is equipped with a knock sensor system. You take your plane on a trip and are ready to take off fully loaded on a hot day from a short field you have flown out of many times. But this time you are flying on a lower octane fuel, which is now legal for your aircraft because of the knock sensors. You take off and all goes well until you leave ground effect, when all of a sudden your engine knocks and the system retards the timing to eliminate the knock. You now have less horsepower than you expected as you head for the trees. You can see that this may not end well.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at


  1. Jack Thompson says

    It is a sad commentary given that the IC engine is such an intergral part of avaition (and everything else in modern life) that the user community is so ignorant of its operation and engineering issues. What percentage of the users in the automotive world and aviation world understand the impact of fuel detonation resistance and the if-then relationships of fuel suitability for a given service? I don’t know, but I would wager that it is a small percentage. Even the aviation media struggles with getting these issues explained correctly and fully.

  2. Jeff Scott says

    The other issue is the failure mode of such devices in an aircraft. 1). How do you know if a knock sensor has failed or not working correctly. and 2.) How is the failure handled safely in an aircraft. Those are big obstacles to overcome in the world of aviation. The automotive world addressed it with electronic fuel injection and knock sensors. On an aircraft, the FADEC system can’t simply go into a “limp home mode” like a car, or default to a worst case sea level limp home fuel mapping mode as that could present quite a problem in an aircraft while cruising at 12,000 feet as there would be a significant loss of power and an unplanned increase in fuel burn.

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