Join the Car Talk Community!

Being one of those who marvels everytime I turn the...

RSS
Dear Tom and Ray:



Being one of those who marvels everytime I turn the switch and the car
starts, I would like to know why some vehicles need a higher octane
gasoline. -- Jeffrey

TOM: The simple answer is "to stop them from pinging," Jeffrey. But since
we get paid by the word, we'll give you the complicated answer, too.

RAY: You might be surprised to learn that there is no octane in gasoline.
The octane rating is just a measure of how a fuel matches up to a test fuel
called iso-octane.

TOM: And the most relevant thing they're comparing is the ignition point of
the fuel.

RAY: Why is the ignition point so important? Well, historically, to get
more power out of an engine, you increased the "compression" or pre-
ignition pressure in the cylinders. But you could increase it only so far,
because if you increased the compression too much, some of that fuel would
ignite under the high pressure before the spark plug had a chance to fire.
That's called "pinging" or "detonation," and it's not good for the engine.

TOM: So how do you avoid pinging? You increase the ignition point of the
fuel, so that it doesn't ignite too early. And those gasolines with higher
octane ratings have higher ignition points.

RAY: So cars that have high compression/high performance engines require
high octane fuel primarily so they don't ping.

TOM: As for the marvel of why the car starts when you turn the switch,
we're still working on that one ourselves, Jeffrey. Write to us again in
six months. Maybe we'll have it figured out by then.
Tags (Browse All)

You must be logged in to leave a comment. Login / Signup
Support for Car Talk is provided by:

Donate Your Car,
Support Your NPR Station

...and get a tax break!

Get Started

Find a Mechanic


Go



Submit


Rocket Fuel