Why Aren't There Seven-Cylinder Engines In Cars? (2024)

3 Series Sedan

Make

BMW

Segment

Sedan

Have you ever wondered why we don't have seven-cylinder engines in cars? It doesn't seem like an overly complicated thing; in a world with two-, three-, four-, five-, six-, eight-, ten-, twelve, and even sixteen-cylinder engines, why did the seven-cylinder get left out? Well, it all comes down to pretty basic physics and engineering, and Drivetribe recently dove deep into it in an informative video.

Titled, "Why is there no such thing as a 7-cylinder engine?" host Mike Fernie takes us through how engines fire their cylinders and the complications that can arise from adding and subtracting cylinders. Vibration is the enemy of a smooth driving experience, and with a seven-cylinder engine, there's a heck of a lot of it.

The first thing you need to know is that seven-cylinder engines do exist. They're used in shipping, agriculture, and aviation, so they can work, but their limitations make them a non-starter for automotive applications.

When you're building an engine, opposing forces that cancel each other out are your friends. This is why even number configurations are so common. Whether it's the Subaru WRX and its four-cylinder Boxer engine or BMW's iconic straight-six engine in your M340i, even-numbered-cylinder engines are known for being smooth and manageable, making them the perfect option for most applications.

On the opposite end of the spectrum, you have engines with odd-cylinder numbers, and here it's complicated. Perhaps one of the most unstable setups is your three-cylinder engine, which has three pistons firing at different times and, therefore, doesn't have a chance to cancel out the opposing forces. What's more, there's actually a gap in any one piston firing which makes the vibrations even worse, and only by utilizing a balancing shaft do these engines normally work.

As you move up in odd-numbered engines, things get better. Every car fan knows of Audi's inline five-cylinder engines and just how good they can be. Part of their success is that once you add two cylinders, you now have some overlap during the firing order, which helps smooth out vibrations. Not to mention, adding two cylinders adds power, making the engine a solid option for those wanting to save on space but not wanting something gutless.

The seven-cylinder engine follows this trend. According to the video, in the same vein as the inline five-cylinder engine, adding more cylinders helps to even out the vibrations even further and again allows power to be pumped up. Additionally, with an inline seven-cylinder engine, you're left with a package that is slightly smaller than an eight-cylinder and will offer comparable power. So what gives? If it works then why have we never seen one in a passenger car?

There are a few reasons. One, the package only works in an inline configuration. This creates crankshaft issues. A crankshaft for an engine like that would be very unique and heavy. When an automaker makes an engine like a V8, they're able to use a crankshaft from a four-cylinder setup to save money. That doesn't work with an I7; this means more money and engineering need to be spent to make it work.

Even if you do make it work, unfortunately, the package is just never going to be as smooth as a six- or eight-cylinder engine, and thanks to forced induction, both of those options do most everything a seven-cylinder could do and more. A seven-cylinder just doesn't need to exist, and it seems every automaker has come to that conclusion since the automobile was invented.

Still, if someone out there wanted to make one and put it in their project car, we certainly wouldn't be mad.