posted on February 8th, 2018

What’s under the hood?

Terms like I4, H6, or V10 might sound like secret gearhead code, but it’s not so complicated. All they do is indicate the layout and cylinder count of different engine types. The letter relates to the arrangement of the cylinders, while the number shows how many cylinders it has. Still, understanding these alphanumeric pairings takes explaining, so let’s explore different engine layouts and what they mean.


Inline engines (abbreviated as “I”) have their cylinders arranged in a row. Since there’s only one row (or bank) of cylinders, they’re easier to design and manufacture, and are usually smaller and lighter than other layouts. With this emphasis on simplicity, inline engines are common, particularly as four cylinders, and are usually under 2.0 liters. However, inline engines become longer as cylinder count increases, so how they fit inside a car becomes a concern. Therefore I6 engines are uncommon, and I8 engines are obsolete. But by adding a turbocharger or supercharger, an I4 can be as powerful as a six cylinder counterpart while maintaining efficiency. Today, I4 engines — once regarded as cheap — are found in luxury and economy cars alike.

Harold’s 2018 Jeep Renegade with 1.4L I4
Daniel’s 2016 BMW M4 with 3.0L twin-turbo I6


V engines address the problem of increased length on inline engines with higher cylinder counts. Instead of one cylinder bank, V engines use two banks angled away from each other. They get their name because the banks connected at the bottom form a V shape when viewed lengthwise. Picture this: split an I6 into two banks of three cylinders, then angle them next to each other instead of in a row. Presto! This V6 engine is only about half as long as an I6. Consequently, a V layout makes an engine with eight, 10, or 12 cylinders compact, without forcing the car to have an extra-long hood. Since, in a nutshell, the more cylinders an engine has, the more powerful it can be, it’s understandable why muscle and sports cars frequently use V engines.

Luie’s 2018 Ford Mustang with 5.0L V8
Ryan’s Dodge Viper with 8.0L V10

Horizontally opposed

Horizontally opposed engines, commonly referred to as “flat” or “boxer” and abbreviated as “H,” also use two cylinder banks like a V layout. However, instead of angling the banks next to each other, flat engines spread the banks out so the tops of the cylinders point in opposite directions. While this results in an engine that’s wider than an I or V layout, it reduces the height, which lowers a car’s center of gravity for better handling. Perhaps, then, it’s no wonder why the venerable Porsche 911 has only ever used a flat engine.

Parker’s 2015 Subaru Outback with 2.5L flat-four
Ali’s Porsche 911 with turbocharged 3.0L flat-six


Also called Wankel engines in tribute to their inventor, rotary engines do away with the circular pistons that move up and down inside cylinders like are found in I, V, and H layouts. Instead, they use a rounded triangle which spins inside an oblong housing to create combustion. Getting these two disparate shapes to work together requires a bit of mechanical wizardry, which explains why rotaries are rare — and why they’ve garnered a cult following. On the plus side, rotary engines are compact, have high horsepower, and deliver power smoothly. On the negative side, they aren’t as torquey, efficient, or durable as conventional engines. The last rotary was found in the Mazda RX-8, which ended production in 2012, and it’s unknown when this layout might make a return.

Paul’s Mazda RX-8 with 1.3L Wankel
Mark’s Mazda RX-7 with 1.3L Wankel

So there you have it: you can decipher the gearhead code. While vast volumes have been written further detailing what makes these layouts unique, now you know how to answer when someone asks what’s under the hood.

Alex has been a car fanatic for as long as he can remember. At 6'10," he might be the tallest auto writer in the world, and whether it's engine timing, exotic car design, or race strategy, there is no automotive topic beyond Alex's interest.