Why must both of these 4- and 8-cylinder engines run on 89 octane gas?

Dear Car Talk

Dear Car Talk | Jun 01, 1997

Dear Tom and Ray:

I've been enjoying your amusingly informative column now for
many years thinking I might never need your help. This despite
the fact that I gave my stepdaughter my '82 Chevy Caprice
station wagon with over 200,000 miles on it! But it was a good
swap. I now have her mom's '89 Volvo 240 (as well as her mom!).
Both my car and I have much more mileage on us than she or her
car! Now for the question. The Chevy had a much more powerful
eight-cylinder engine and ran great on regular gas. The Volvo is
only a four-cylinder engine, yet both the manual and the Volvo
employees say it must run only on 89 octane gas. Are they nuts?
Or do they know something I don't know? -- Floyd

TOM: They know about compression ratios, Floyd.

RAY: You've probably noticed that the Volvo is more sluggish
than the Chevy was, right? But it's not HALF as sluggish, even
though it has only half as many cylinders.
TOM: And that's (partly) because the Volvo engine is squeezing
more power out of each cylinder in part due to a higher
compression ratio.

RAY: The compression ratio is the difference between the volume
of the cylinder before and after the compression stroke. What
does that mean in plain English? It's a measure of how much the
engine compresses the fuel-and-air mixture just before the spark
plug ignites it. And the more compressed the mixture, the more
power it produces when it "explodes."

TOM: And our Mitchells database tells us that the Chevy's
compression ratio is 8.6 to 1, whereas the Volvo's is a much
higher 9.8 to 1

RAY: So why the higher-octane gas? The more you compress the gas
and air, the more likely they are to pre-ignite; that is, ignite
just from the pressure and heat, before the spark plug even
makes a spark. That's called "knocking," "pinging" or
"detonation," and all those extra, early explosions are bad for
the engine.

TOM: Higher-octane gasoline is simply gasoline with a higher
ignition point. That means it takes a higher temperature to
ignite the 89 octane than it takes to ignite the 87. So using 89
eliminates the pre-ignition in this engine.

RAY: Some high-compression engines require even higher-octane
gas, 91 octane, to prevent detonation. And that's why not all
cars have them. High-compression engines are usually found on
more-expensive cars, where the buyer cares more about a few
extra horsepower than about a extra few bucks at the gas pump
every week.

TOM: So swallow hard and fill it up with 89, Floyd. Don't worry,
you still made out OK. Even without factoring in the kid's
mother, you still got the better end of this deal.

* * *

TOM: Hey, do you think you're taking good care of your car? Are
you sure?

RAY: If you're like many of our customers, you may be ruining
your car without even knowing it. Yes, even you! Find out how.
Send for your copy of our informative pamphlet, "Ten Ways You
May Be Ruining Your Car Without Even Knowing It!"
TOM: Send $3 and a stamped (55 cents), self-addressed, No.10
envelope to Ruin No.1, PO Box 6420, Riverton, NJ 08077-6420.

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