How do engine manufacturers determine the redline for an engine?

Dear Car Talk

Dear Car Talk | Nov 01, 1999

Dear Tom and Ray:

How do engine manufacturers determine the "redline" for an engine? Is this a "self-destruct" limit? Would you recommend keeping the rpm to, say, half of the redline
during routine driving? -- Cliff

TOM: Well, I don't know about engines these days, but back in the glory days of the American Motors Corp., the redline used to be determined by trial and error. When
a guy came in with a blown engine, the service manager would ask him if he happened to notice the rpm just before the piston shot through the hood.

RAY: Actually, it's mostly determined in the laboratory and test track these days, using sophisticated engine monitoring equipment.

TOM: The redline, by definition, is the engine speed (in rpm) that you should not exceed. And if you do exceed it, you are in real danger of having your engine parts
start to fly off.

RAY: Virtually all cars with computer-controlled engine management systems now have switches that shut off the fuel injectors or spark when you hit the redline. This is
an attempt by manufacturers to make cars more and more idiotproof (although the idiots always seem to eventually regain the upper hand).

TOM: But in normal -- and even abnormal -- driving, you really shouldn't come anywhere near the redline. Not only are you putting significantly more wear and tear
on the engine, but you're not even gaining much. Most cars produce their peak torque well below the redline -- usually in the 3,000 to 4,000 rpm range. So you should
always be at that engine speed or well below.

RAY: I can only think of a few exceptions to that rule: when you're passing on a two-lane road, when you're accelerating to get on a highway ...

TOM: Or when you're driving a rental car!

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