Dear Car Talk:
I got into a discussion about engine life. I said, all things being equal, an eight-cylinder engine will last longer than a four-cylinder engine. My logic is that the more cylinders you have, the less often each cylinder will fire. Does this make sense? -- Mike
Uh, how best to put this? No.
No matter how many cylinders you have, every cylinder fires once for every two rotations of the engine's crankshaft. So if a four-cylinder engine and an eight-cylinder engine both are idling at 800 rpm, in one minute, every cylinder will fire 400 times.
Think about walking your dog: The dog's got twice as many legs as you have, but even if his legs were as long as yours, he'd still move four legs for every time you moved two ... assuming you both were going the same speed, and he didn't stop to lift one of his.
That said, there are some ways in which you could be right (I'm trying to bail you out here, Mike). Larger engines will generate more torque. That allows the use of a transmission that lets the engine run slower for the same given car speed. So while a four-cylinder engine might turn at 2,500 rpm at 65 mph, an eight-cylinder engine might turn at 1,800. That could contribute to longer life.
Also, if a four-cylinder engine is too small for a given car (if you had a 115-horsepower four-cylinder engine in a three-ton Chevy Suburban), the engine would have to work harder and run hotter to move the vehicle around. And that certainly could shorten its life, compared with a more appropriately sized larger engine. But assuming an engine's power is appropriate to its car's weight (which is true in almost all cases these days), there's little to no advantage to having an engine with more cylinders.
In fact, there are some disadvantages. First, you're adding the extra weight of the bigger engine itself, so some of the power of the engine now has to go to simply moving that bigger engine around with you. And you could argue that an eight-cylinder engine has more parts that can break: more spark plugs to change, more valves to burn out, more valve guide seals to fail, more rings to wear out.
And we're big fans of getting the right-size engine for the car. With the enormous improvements in power per cylinder in the past decade, eight-cylinder cars are increasingly going the way of the dodo, and the vast majority of gasoline-powered cars will be running on four or even three cylinders soon.