91 octane….or 87 octane. Hmm…..91….or 87…… We’ve all been here, standing at the pump with a few crumpled American dollars in your pocket and a car that you know is a high performance beast (under the right conditions of course). The owners manual in your 22 year old Volvo recommends 87, but you decide to drop the extra 10 cents per gallon and go with the 89. It is after all, 2 more than 87, a job well done. But really, what does this all mean in a practical sense?
Putting in the more expensive liquid definitely FEELS faster, because more expensive always means better, but does it? Surprisingly few people understand what octane means in the real world, but it is a critical number in defining the way your engine runs. TL:DR use the fuel octane outlined in your owners manual.
What Does Octane Realistically Mean?
Octane, in a real world sense pertaining to our internal combustion engines, basically describes the volatility of the fuel we are putting in. This is where a lot of people get kind of hung up, as HIGH OCTANE is usually used by event marketers and toy manufacturers to describe something exciting and explosive. In reality, higher octane fuel has slightly LESS usable energy, but much more importantly lower volatility. I am going to skip the science portion of this because it is a little drawn out, but it includes lots of chemistry and terms like 2,2,4-trimethylpentane. Basically the higher the octane, the lower the susceptibility is to exploding unprovoked.
I do want to quickly mention that there are a few ways octane is measured, so numbers may slightly vary. US fuel pumps state octane numbers using the (R+M)/2 method, also known as AKI or Anti Knock Index. This is an average of both the RON (primarily used in Europe) and MON octane ratings, which use the results of a very controlled test featuring variable compression engines.
When does octane matter?
To describe when octane matters, you must first understand the two basic conditions inside the cylinder of an internal combustion engine.
- Compression and compression ratio – All engines are rated with a specific compression ratio as you have probably seen. The compression ratio is a main influence to determine what kind of fuel octane an engine will run at it’s best, and describes how compressed the air-fuel mixture is when the piston is at top dead center. For instance, a Volkswagen ABA engine has a compression ratio around 10:1, meaning at full compression, the air fuel mixture is squeezed down to around a tenth of what it was when the piston was bottom dead center.
- Heat – If you paid attention in the thermodynamics portion of physics class, you would know that as you compress a gas it heats up, and as it expands it cools down. As the air/fuel mixture is compressed it gets HOT. The more compressed it gets, the hotter it gets.
Detonation and pre-ignition
With a high compression ratio or a lot of boost pressure, you add a significant amount of heat to the cylinder. Heat ignites fuel, this is where octane comes in. If you try and put 87 octane in that S54 engine, the mixture will be so volatile that it will explode without being set off by the spark plugs. The result is a sudden, hot, and very uneven detonation of the mixture. If this happens well before the piston gets to the top of the cylinder, it’s called pre-ignition and it’s very bad. It can be from a glowing hot spark plug, or some burning carbon (sometimes as a result of detonation).
Also called knocking or pinging, if the mixture explodes substantially before the piston hits TDC, the explosion violently and suddenly pushes back on the piston as it still travels upwards, robbing horsepower and potentially damaging engine components. Often pre-ignition can occur in a single cylinder. In the worst cases, I have seen pre-ignition bend connecting rods, crack pistons, and damage crankshafts.
When computers started being installed in cars, knock sensors were used to inform the brain box that detonation was occurring, and react by retarding the timing. This allowed the spark plug to take back control of the ignition, but firing the spark later robs horsepower and fuel mileage as the piston has already started its way on the downstroke. More advanced sequential systems can control the timing of spark and fuel to actively adapt to whatever octane you put in.
Naturally Aspirated Engines – In a normally aspirated or naturally aspirated (non turbo) engine, power is a function of compression ratio and displacement. Sport and racing engine builders often raise the compression of an engine at the sacrifice of displacement to make more usable power, which is exactly what BMW did with their M54 engine when preparing it for the E46 M3. To get more power, the compression ratio was raised from around 10.5:1 to a fairly high 11.5:1 ratio. Ways to raise the compression ratio would be to make a flat or slightly domed piston top, flat valves, use a thinner headgasket, and/or shave the cylinder head, lengthen connecting rods, and use a different crankshaft to reduce the volume of the combustion chamber as much as possible.
Forced Induction Engines – Because a turbocharger or supercharger crams air into the combustion at high pressure, it acts in basically the same fashion as a high compression ratio and introduces the same problem. Consequently, if you want to turbocharge an engine, you want a lower compression ratio. You can lower the compression ratio by having a deeply dished piston, dished valves, thicker head gaskets, shorter connecting rods, and a different crankshaft. If not, when you add boost you are effectively adding onto the compression ratio, and can run straight into knocking and detonation. Supercharged top fuel drag race cars have a ridiculously low compression ratio around 6.5:1 to cope with the amount of air being forced into the combustion chamber.
The Fix: Higher Octane and Colder Plugs
By putting in a 91 or 93 octane fuel, the less volatile mixture stays calm all the way though the cycle, allowing the ignition computer to dial in the perfect spark timing. Generally, that involves firing the spark plug right before the piston reaches TDC, so the slower moving and evenly formed flame envelope pushes the piston down just as it begins its downward stroke.
Often, a colder plug is also added to reduce the likelihood of a spicey hot electrode from setting the mixture off before the spark plug actually ignites. This is why a stock Saab 9-5 uses an NGK plug with a heat range of 6, but specifies a spark plug with a heat range of 7 for the “hard driving option”, assuming that your cylinder temperatures would be higher because of “hard driving”.
Race gas is generally around 100 octane or more, which allows significantly higher turbo pressure and/or compression ratios. Our Audi RS 3 LMS is able to reliably make around 350hp out of the 2.0l TFSI turbocharged engine without knocking, due to the use of 100 octane VP racing fuel.
When doesn’t octane matter?
Using a higher octane fuel does NOT make any difference in engines that are tuned to use regular fuel. That means in general, normally aspirated engines with a low compression ratio or smaller cylinders/pistons. My Honda VFR800 motorcycle has nearly a 12:1 compression ratio and makes over 100hp out of 782cc, but because the pistons and cylinders are so small it only requires 87 octane fuel. My grandmother used to only put 93 octane fuel in her Mercury Gran Marquis, but in reality all benefit realized by spending the extra money on a modular 4.6l was strictly placebo.
In fact, an 87 octane fuel actually has a slightly higher usable energy content, so you may even be making less power on an engine that’s expecting 87 by putting in 93. The only time you would raise the octane is if you happen to encounter the knocking and pinging, such as in higher altitudes or extreme temperatures. After all is said and done, always use the lower octane fuel when you can, otherwise you are just flushing money down the drain (or in reality sending it to our friends in the world’s oil producing nations).