We talked before about whether or not using high-octane premium gas gives you better mileage if your vehicle doesn't specifically call for it (we don't believe that it does). But what about the reverse question? If your car needs an 89 octane gasoline and you put in 87 octane, will your gas mileage suffer?
What does octane do?
For a brief primer, octane is a measure or an index of gasoline's ability to resist the temptation to ignite too early in the engine. The gasoline gets injected into a hot combustion chamber, with the piston heading up on its way to the top peak or top dead center position. If the gas ignites at the right moment, the piston is where it should be and the engine gets maximum work out of the fuel's ignition value as the explosion pushes or drives the piston back down (turning the wheels). If the fuel ignites too soon, the piston isn't up where it needs to ideally be, and you get a pre-detonation situation with the corresponding "knocking" sound.
"Retarding The Spark"
In order for your vehicle to operate at its best, everything needs to be working in sync and working at its best. This means the fuel needs to be igniting and exploding when the piston is in the right place. But as we just noted, if the gas octane rating is too low, the piston isn't going to be high enough when the explosion happens. And this can cause serious damage to the engine if it happens for a long enough time.
So the car's computer is programmed to watch out for the sounds of pre-detonation knocking, and what it does to correct the problem is to slow down the firing of the spark plug. Some mechanics refer to this as "advancing the timing" or "retarding the spark". The computer knows that the fuel is igniting too quickly in the cylinder. So it wants to slow down the process to give the piston enough time to get to the proper position. Since it can't do anything about how long it takes for the fuel to ignite once it is injected, what the computer does is wait a little bit longer in the process before it tells the spark plug to fire.
But what can happen at this is that instead of the fuel now igniting too early or even at the exact right time, many times the fuel will ignite after the piston has passed top dead center. This means the piston is on its way down when the fuel ignites.
Low octane's effect on mileage
If the piston is on its way down when the explosion happens, you won't get a pre-detonation knock. That's good. But you'll get less power and less mileage, and that's bad. The engine gets the maximum "push" when the explosion happens with the piston at its highest point. Now the piston is further down and heading away from the explosion. There will still be a push, but it won't be as strong as ideal, plus not all of the fuel will burn completely. Both of these factors cause the vehicle to get less mileage and have less power. A result of the computer's efforts to prevent engine damage caused by low-octane knocking.