Smarter Engines Protect Themselves
If you're still in doubt about switching to a lower-octane fuel, here's a more in-depth explanation of why the change is unlikely to hurt your car:
First of all, premium gas has a higher octane rating, an important factor in helping prevent engine knock or "pinging." Depending on where you live, this premium-grade fuel could be 90 octane, 91 octane, or even 94 octane.
That's one reason premium costs more. To increase the octane rating and reduce knock, refineries use a more elaborate process that blends an expensive substance called alkylate into the fuel.
What is engine knock? After vaporized fuel mixes with air and fills the combustion chamber, rising pistons compress it. The spark plug then fires, initiating the combustion process. But combustion doesn't happen instantaneously. It takes some time for the flame kernel to develop, grow and then burn the entire contents of the fuel-air mixture in the cylinders. There's a critical period in the middle of this process when the mixture in the unburned region is being heated by the neighboring burned gas while being compressed by the rising piston.
This one-two punch can cause the mixture in the unburned region to self-combust rapidly in an uncontrolled fashion. That's the knock, which hurts power and can damage the engine. Simply put, high-octane gasoline can be compressed and heated to a greater degree without self-igniting. That's why high-performance engines use premium fuel.
In the old days, engines could not deal with fuels of varying octane ratings. Using the wrong fuel would make the engine knock and possibly damage internal engine components. But today, engine control systems can compensate for low octane by adjusting ignition timing to avoid knocking. This sophisticated electronic capability effectively tunes engines on the fly and gives drivers more flexibility in the grade of fuel that they can use safely.
Compared to premium gas, lower-octane fuels don't allow the engine to run as much ignition advance during situations calling for rapid acceleration. More ignition advance causes spark plugs to fire sooner, which (within limits) allows the engine to make more power — and accelerate more quickly — under these conditions. Since the engine doesn't make quite as much power with low-octane fuels, this translates into slower acceleration in cars for which premium fuel is recommended. The performance loss is especially noticeable in turbocharged gasoline engines, which have become increasingly popular in recent years.
The performance loss, however, is something you will only notice if you have a heavy foot and accelerate rapidly from a dead stop or when you change lanes at highway speeds. But if you accelerate moderately, the loss of power may not be noticeable whether you use premium or regular-grade fuel.