Pony Up

Normally the word explosion has sort of a negative connotation. If, for example, it were to appear in either of the following sentences: (1) "So I was working out at the gym last night when...," or (2) "I decided to take a little stroll along the beach and...," that would be bad. And yet, properly managed, explosions are responsible for getting many of us safely to work in the morning, in the form of the internal combustion engine.

If all goes according to plan,1 these explosions happen in the combustion chambers inside a vehicle's engine. The chambers sit on top of cylinders, which contain the pistons and valves that manage all of the important manipulation of  fuel and gases flowing into and out of the engine. As a basic introduction/refresher, here's what's going on inside a typical car engine when it's running:

And here's a simplified interactive diagram that lets you scrub through what's happening on each of the cylinder's four strokes:

Even though most engines2 follow the same basic pattern, not all engines are created equal. For one thing, size matters. Some terminology might be helpful here. As the piston moves through the cylinder, it displaces a certain amount of air. The diameter of this imaginary cylinder is known as the bore, and its height is called the stroke. The total volume of all the cylinders in an engine (typically between four and eight, but sometimes more) is called its displacement.

Since the volume of a cylinder determines how much fuel-air mix it can hold, larger cylinders can produce more powerful explosions, which leads to a more powerful engine. In other words, more displacement means more horsepower. So people who care about horsepower care about displacement and, thus, the engine's bore and stroke measurements.

If you're interested in trying to get more power out of an engine, one of the things you can do is increase its bore and stroke.  By drilling out the cylinder in order to create a larger bore diameter, or adjusting how far the piston travels in order to create a longer stroke, you can increase displacement, and therefore horsepower.  This all seems reasonable and sensible, until you run across a situation like the following. Here's the Dodge Challenger R/T, which is already a pretty beefy machine at something like 372 horsepower from an engine with 5700 cc of displacement.

You might be rightfully impressed by that kind of power, that is until you run across this steroidal monster: the Dodge Challenger SRT Hellcat.

Even though those two cars are at their most basic level the same, their performance seems to indicate two entirely different species. Can more engine displacement really account for the huge discrepancy? If so, with roughly double the horsepower, this would suggest that the Hellcat would need to be almost double the engine size! An engine block just can't support that much modification. It's not like there's a lot of extraneous metal you can just drill away.

In reality, the Hellcat only has a 6200-cc engine, so clearly there must be other factors at work. For one thing, you might have noticed the "Supercharged" badge on the car. A supercharger compresses air before it's drawn into the cylinders, which means more air can fit in the same volume, which means more fuel can fit in the same volume, which means more giddyup without more displacement. That's just one example.

For really souped-up cars, engine displacement is actually only part of a much larger engineering puzzle to squeeze every bit of power from these machines. After all, presumably one of the things you're paying a lot of money for in a really souped-up car is the expertise necessary to solve the tricky puzzle of cramming more horsepower into a given engine size.

But for the do-it-yourselfer, you can always head out to the garage and grind away a little metal. You might not get yourself a Hellcat, but you can get a few extra ponies.

Teachers, want to have this conversation in class and do some work with expressions and cylinder volume? Check out our new lesson, Pony Up.


1.  For instance, the Fundamental Theorem of Internal Combustion Engines states that, ideally, all combustion should remain internal.

2.  Well, most car engines, anyway, which is what we're interested in here.  Diesel engines have the same basic cycle of operations, but without spark plugs.  How?  The air is compressed first, which increases its temperature to above the fuel's autoignition point, so when fuel is sprayed into the already-hot air, it spontaneously combusts.  If you've ever had trouble starting a diesel vehicle when it's cold outside, now you know why. There are also two-stroke engines, which fire on every revolution instead of every other revolution, through some nifty tricks with pressure that allow a bunch of stuff to happen simultaneously.  Because they're simpler and cheaper to produce, two-stroke motors are usually found in less driveable appliances like your weed-whacker or chainsaw.


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