Question: How do you know what fuel to put in your car?

Usually the manufacturer puts a sticker near the filler opening on the vehicle that will tell you the minimum octane rating of the fuel to be used and or it will be listed in the owners manual. That's for current vehicles. For older cars or cars with custom engines its a different story, the answer lies in the engines dimensional specifications.  To interpret the information one has to have a little understanding of what goes on inside the internal combustion engine.

An engine is effectively one or more tubes that are called cylinders with precisely fit slugs inside that slide up and down the tubes these slugs are called pistons. There is a collection of springs and levers etc. that move valves open and closed in time with the piston movement that allows air to enter the cylinder as the piston moves down the bore and allows exhaust to exit the cylinder as the piston moves back up the bore. The pistons are connected to a rod shaped piece, appropriately called a connecting rod that has its other end secured to a crank just like the pedals of a bicycle, called a crankshaft -- here is where the linear motion of the piston is changed to rotary motion and through various shafts and gears. It is this rotary motion that turns the driving wheels of our vehicle. Now back to our pistons sliding up and down the bore of the cylinder.

There is a cycle that takes place between the opening and closing of the valves and the entering of the air and the exiting of the exhaust.  The Intake valve is opened as the piston moves down the bore allowing air to enter. When the piston is at the bottom of the bore the intake valve closes now the piston moves up the bore squeezing the air that filled the bore into a very small space that is left at the top of the cylinder when the piston is at the top of the bore. This is the key to the whole process. Because we are not just squeezing air we are squeezing a mixture of air and fuel.

The ratio of squeezing or compressing of the air fuel mix is called the compression ratio. When you mix air and fuel in a particular ratio of so much air with so much fuel  (14 pounds of air to every 1 pound of fuel) and squeeze it you increase the volatility of the mixture and if you supply a spark and ignite the mixture you get  "controlled explosion " or rapid burn. This explosive expansion that takes place is what pushes the piston back down the bore of the cylinder pushing the crank around just like when we push the pedals down on the bicycle. When the piston gets to the bottom of the bore and starts back up the exhaust valve is opened and the burned mixture is pushed out by the piston so that the cylinder is empty and ready to take on another charge of air and fuel on its next trip down the bore. Now that this is clear as mud -- lets go back to the air mixing with the fuel and squeezing it.  When you squeeze any molecules they don't much like it in fact they get pretty hot under the collar and when you have a perfect ratio of air and fuel to support combustion if you squeeze to much to the point of the ignition temperature of your mixture, the mixture will go off all by it's self without the help of our spark and ignition system. This could be bad if the piston is not in a position in the bore where it can take advantage of the explosion and really bad if the piston has just started up the bore because the explosion is now applying a force opposite to the pistons direction of travel instead of complimenting the piston travel.  And you won't just hurt their feelings because they didn't get something that is complimentary this is more like hitting them over the head with a steel bar and they may not live through the experience.

How this all relates to fuel is we put stuff in the fuel to make it resist the compression ignition effect and even give it a rating called octane. Basically it keeps the fuel from going off except when we want it to. It therefore follows that if you raise the compression ratio then you need more octane to control when the fuel is ignited. So octane of the fuel is directly tied to the compression ratio of the engine, which is available from the engine builder whether the builder is a custom one or a manufacturer and if the car is so rare that the information is not available the compression ratio can be calculated from making some measurements of the cylinder and associated components (some disassembly required).

Fuel continued.....Why do race cars run on different fuel?