Octane 101 December 17 2013

We've seen people put 87 octane gas in their premium-fuel-only $70,000 car to save a buck. We've seen people pour octane booster into their car that only calls for 87 octane. What are the facts about octane? Is it safe to save a couple of bucks by switching to a octane level lower than the factory recommended level? Do you get more power out of higher octane gas?

To understand why octane matters, we have to explain a little about how engine design works. We'll assume that you're familiar with the strokes of the four-cycle engine - intake, compression, power and exhaust. During the compression stroke, the amount the fuel/air mixture is compressed is expressed as the compression ratio.

Designers can get more power and efficiency out of the engine by increasing the compression ratio. We won't bore you with the thermodynamics behind this. So what stops engine designers from designing an engine with a ridiculously high compression ratio? The limiting factor is that the harder you compress the fuel/air mixture, the hotter it becomes. If the fuel air mixture gets compressed hard enough, it combusts before it is supposed to. This is called auto-ignition. The piston is still try to go up, while the fuel/air mixture has already ignited and is pushing extremely hard against the piston out of sync. This is extremely bad for the engine and is called engine knock.

Fueling E46 M3

Enter high octane fuels. Fuels with a higher octane rating are more resistant to auto-ignition. This means that if high-octane fuels are used, engine designers can increase the compression ratio safely and get more power and efficiency out of the engine.

So what happens when you put low octane gas in a car that specifies high octane gas? One of two things could happen: 1) The designers usually design a small safety margin into the engine. This safety margin could be large enough that reducing the octane rating might be okay. Keep in mind that high rpm and high load increases the likeliness of knock, so while your engine might tolerate loafing around town, if you step on it, you could cause knock to occur. 2) The engine starts knocking. Most cars from the 90s and on have knock sensors built into them, so the engine will detect the knock and immediately go into self-preservation mode. Sometimes, you can't even tell that the engine is doing anything differently. While in self-preservation mode, the engine will generally dump a bunch more fuel in, since that prevents knock, and it will also delay the spark plug firing. Both of these things are bad for engine efficiency. This decrease in fuel economy might offset any savings made at the pump, and is certainly bad for your engine. There is also a small chance that your knock sensor goes bad and your engine self-destructs in a matter of seconds.

And what happens when you put high-octane gas in your car, even though the owner's manual says that low octane is good enough? High octane fuel isn't any more explosive or energy dense. It is just more resistant to auto-ignition. There really aren't any gains here unless you modify the engine or engine software to take advantage of the higher octane fuel. As an example, our E36 M3 project car specifies 91 octane gas. Since we get 93 octane gas in Texas, we reprogrammed the ECU software to take advantage of the higher octane fuel. It safely advanced the spark timing to create more power and efficiency.

So basically do what your owner's manual tells you to do. The engineers know what they're talking about.

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