Why are horsepower numbers listed as different rpms?

Dear Car Talk

Dear Car Talk | Sep 01, 1996

Dear Tom and Ray:

I've noticed that my '95 Taurus has 140 horsepower at 4,800 rpm. The '96
Taurus has 145 horsepower at 5,250 rpm. Would my Taurus have 145 horsepower
at 5,250 rpm, too? Why are the horsepower numbers listed at different rpm?
Is this just a numbers game, adjustable by the manufacturer? It's the same
3-liter Ford engine. Thanks. -- Phil

RAY: Great question, Phil. Now, if we could only use our hands to explain
the answer!

TOM: Here's the story, Phil. If you take a piece of graph paper and plot
how much horsepower an engine delivers over how fast the engine is spinning
-- in revolutions per minute -- you get a curve that looks something like
the Liberty Bell.

RAY: Well, it's not quite as neat as that. It actually looks more like a
sideways question mark. But it does go up and then down.

TOM: What that curve tells you is that as engine speed goes up, horsepower
also increases, up to a point. In your case, that point is 4,800 rpm, at
which point the horsepower starts to decrease again, even though the engine
keeps going faster.

RAY: So it should come as no surprise that the folks trying to sell the
cars pick the highest point on the curve when they publish a horsepower

TOM: When they plotted the horsepower for your '95 Taurus, the highest
point on the curve was 140, and it happened to fall at 4,800 rpm. And after
they did some work on that 3-liter engine -- they might have lightened the
pistons, for example, or changed the valve springs -- they plotted the
horsepower curve again. And this time it got up as high as 145, at 5,250
rpm. So that's what they advertise.
RAY: So it's actually not the same engine, Phil. It's a similar engine, but
it's not the same anymore.
TOM: And while there is some degree of salesmanship and gamesmanship going
on -- I mean, how often does the average Taurus owner rev the car to 5,250
rpm? Never! -- the numbers are based on fact. The manufacturer does not
simply make them up.
RAY: Right. The numbers are not infinitely changeable, like my brother's
age, which seems to vary widely, depending on the attractiveness of the
female customer currently in the garage and on the size of the senior
citizens discount being offered that afternoon at Louey's Sub Shop.

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