The major difference between an inline-six and a V6 engine is easy to spot. I6 engines have six cylinders in a straight line, while V6 engines have two rows of three cylinders aligned in a V formation.
In general, the compact shape of a V6 has made it the standard for manufacturers. It allows them to use one engine in many different models. However, many enthusiasts love the I6 due to its unique sound, smooth performance, and simplicity. Let’s take a look at some of the less obvious differences between I6 and V6 engines.
What is an Inline-Six Engine?
An inline-six, also called a straight-six, is composed of six cylinders arranged in a line. The term “inline-six” is often shortened to the abbreviation “I6.” Occasionally, the abbreviation “L6” is used, which is short for “Line-Six.” There’s also a motor called the slant-six, which was used almost exclusively by Chrysler. It was essentially a straight-six that was angled 30 degrees to one side.
Due to the packaging within the engine bay, most front-wheel-drive cars cannot use an I6. They tend to use a V6 or an I4. This means that I6 engines are fairly rare in today’s cars. Many coveted sports cars from the ‘90s used a straight-six engine. A prominent example is the original Toyota Supra’s legendary 2JZ motor. There are a few modern exceptions. BMW is the best-known manufacturer that still uses inline-sixes.
Inline-Six Balance and Smoothness
A straight-six engine is sort of like two I-3 engines put together in a row, mirroring each other. This means that the pistons in cylinders 1+6, 2+5, and 3+4 all move in tandem. Plus, each pair is at a different part of the up-down cycle at different times, canceling out vibrations. This gives the I6 a naturally balanced layout. Many enthusiasts feel that even a well-counterbalanced V6 simply doesn’t feel as smooth as an I6.
Inline-Six Firing Order
To attain perfect balance, most inline-six engines use a 1-5-3-6-2-4 firing order. This allows the load of twisting movement to be evenly distributed. Cylinders are not firing simultaneously and adjacent cylinders don’t fire one after the other. The perfect balance is not possible with V-style engines and is a major factor in why I6 engines are so desirable for drivers.
Pros and Cons of an Inline-Six Engine
There are quite a few characteristics of the straight-six that make it favorable. However, its length makes it hard to fit into the engine bays of most modern cars.
Inline-Six Engine Pros and Cons
|Inline-Six Pros||Inline-Six Cons
|Simple to build and work on
||Large, awkward size
|Naturally less vibration
||Higher center of gravity
|Unique engine sound
Pros of an Inline-Six Engine
The inline-six layout is considered the de facto way of making an engine, as its design is rather simple. For that reason, straight-six engines are more straightforward to repair than a V6. Since the cylinders are more spread out, each can be clearly seen and worked on. With one engine block, cylinder head, and valve cover, they’re less expensive to build as well.
The long, longitudinal layout also means that there’s more room on the sides of the engine bays, for things like turbochargers.
Additionally, straight-six engines are composed of an even amount of cylinders. They can be fired off in a way where the vibrations of one piston counteract the other, making them run smoothly. Combined with the long configuration, this makes them very balanced.
Unique Engine Sound
Some automotive audiophiles claim the straight-six sounds better than the V6. But this claim is as controversial as it is subjective. While the general consensus is that the straight-six sounds good, real-world acoustics will vary. The exhaust, muffler, and the specific make of the engine all impact the exhaust note. At the very least, the sound of an inline-six is unique.
Cons of an Inline-Six Engine
Large, Awkward Size
The biggest problem with straight-six engines is that they’re simply too long for many vehicles. The large size means that straight-sixes typically can’t be mounted transversely (perpendicular to the vehicle and driveshaft). Therefore, vehicles require a long nose to fit an I-6. That’s why they’re very rarely used in cars today.
I6 layouts usually only work for sports cars. Designing a car to accommodate a bulky inline-six engine, plus all the modern safety tech and other components, simply isn’t profitable for most manufacturers.
The I6’s length can also cause the camshafts and crankshaft to flex while they rotate. This isn’t a huge issue with modern engine design, but is something to note for older models.
Higher Center of Gravity
The awkward packaging of inline-sixes and the necessary engine components mean that they can’t be mounted as low as a V6. This means a higher center of gravity, which leads to worse handling and control.
What is a V6 Engine?
A V6 engine has two rows of three cylinders, called banks. The cylinders are angled away from each other and slightly offset. In other words, they don’t form a perfectly symmetrical V. If you’re familiar with the design of a V8 engine, it’s not a stretch to think of a V6 as a V8 with two fewer cylinders. Since it’s only three cylinders long, it’s shorter than an inline-four cylinder engine. This has made the V6 the most popular six-cylinder engine.
In recent years, there’s been a trend away from V6 engines in favor of turbocharged four-cylinders. You’ll still find V6s in large crossovers, pickups, and luxury vehicles though.
V6 Balance and Smoothness
A V6 engine won’t have the natural vibration cancellation that an inline-six has. This is because each cylinder bank has an odd number of cylinders. Unlike an inline-six, they don’t operate in tandem, as this would cause imbalance.
The vibrations can’t be fully mitigated without counterweights, but the V-shape helps a little. By angling the cylinders away from each other, engineers are able to smooth out some of the up-down motion of the opposite cylinder.
There are several different cylinder angles that a V-style engine can use. They include 60, 90, 120, and 180 degrees. 60 and 90 degree angles are the most common though.
A 60-degree layout is better than 90-degrees for a V6 because it offers more balance. It grants better vibration cancellation for engines with banks of three cylinders. In terms of packaging, it’s a bit taller but not as wide.
A 90-degree V6 can be advantageous to build, however. This is because V8 engines are usually in a 90-degree layout, as it’s a better angle for banks of four cylinders. Using a 90-degree V6 means that there can be shared costs, parts, and assembly lines with an existing V8. The Mustang, Camaro, and Challenger have all offered V6 engines in addition to their V8s. However, 90-degree V6 motors also need a special crankshaft for extra balance.
There are other angle options for this engine, such as 120 degrees. This angle provides better balance, but is too wide for most production cars. You can even arrange the cylinders at a 180 degree angle, which is called a flat engine. The flat layout results in a lower center of gravity.
A 180-degree V is very similar to another flat engine called the boxer engine. Though both are flat engines, the piston movement in a boxer is technically different than a 180-degree V. The cylinders in a boxer engine move in direct opposition to each other, cancelling out vibrations. In the much rarer flat-V engine, the cylinders will move back and forth together.
Porsche and Subaru are pretty much the only manufacturers that use boxer engines in their vehicles today.
V6 Firing Order
A V6 engine usually has a 1-2-3-4-5-6 firing order. For a longitudinal V6, this means the front left cylinder will fire first, followed by the front right cylinder across from it. Then the middle left cylinder will fire, and so on. Note that some manufacturers will number their cylinders differently.
Other V6 engines, usually those that were originally based on V8s, will use a 1-6-5-4-3-2 firing order. These will produce more vibrations that will have to be cancelled out with counterweights.
Pros and Cons of a V6 Engine
The V6 has a more complicated shape than an inline-six, but its compact, square shape gives it some important advantages. Because of its ability to be used in many vehicles, the V6 has become the default choice for modern six-cylinder engines.
V6 Engine Pros and Cons
|V6 Pros||V6 Cons
|Compact size and shape
||Complicated to build and work on
|Fits into many different vehicles
||Needs counterweights to combat vibration
|Lower center of gravity
Pros of a V6 Engine
Compact Size and Shape
In order to maximize the space both under the hood and in the rest of the vehicle, the more compact engine style of a V6 is excellent for car designers. Since there are two half-length rows of cylinders, the V6 engine is naturally about half the length of a straight-six.
Able to Be Used in Many Vehicles
The square shape means that it can fit in most cars with little fuss. V6 engines are more expensive and complicated to produce than straight-six engines. However, once that initial work is done, the versatility of putting one motor in many vehicles is appealing.
Since the V6 does have six cylinders, it’s a good option for making powerful yet small cars. In terms of power, the shape of a V6 engine opens up a convenient space for turbos as well.
Low Center of Gravity
Since V6 engines can be mounted low in the engine bay, the overall center of gravity is lower. This helps the car feel more planted, improving handling characteristics.
Cons of a V6 Engine
Natural Vibrations Require Counterweights
Unlike the straight-six engine, which has a single row with an even number of cylinders, the V6 is made of two 3-cylinder rows. Because of it being an odd number, it cannot compensate for the excess vibrations with its firing order. Each cylinder does not have a corresponding match to cancel out its vibrations. This is remedied by placing weights on the end of a balancing shaft.
Modern V6 engines are usually quite smooth, but there are added design and manufacturing costs to make them so.
More Complex Design
The V6 is harder to work on given its compact design. While that doesn’t mean that it’s impossible to repair on your own, it may be easier to find your way around the engine block if it was an inline-six. A V6 has balancing shafts and counterweights, plus extra camshafts and valves.
The Future of Six-Cylinder Engines
As fuel economy standards become stricter across the globe, engines are becoming smaller. Many performance vehicle's V8s have been replaced by turbo V6s. Where manufacturers used to use V6s, they’re now opting for turbocharged four-cylinder engines.
Six-cylinder engines have historically offered a good compromise of power and economy. They slot right between four-cylinders and V8s. However, the new technology means that small, efficient turbochargers can be fitted to non-performance vehicles.
Smaller crossovers, a popular and profitable segment, are increasingly shying away from V6 engines. Turbocharged four-cylinder engines provide similar power, but perform better in fuel efficiency tests. This is leaving V6 engines relegated to pickup trucks and SUVs.
Yet in the past few years, some new six-cylinder engines have indicated a shift back to the I6.
Mercedes’ M256 that came out in 2017 is a prime example. Using clever packaging, they added two cylinders to an inline-four motor and added a hybrid system that actually saves space. It uses a powerful alternator instead of a series of timing belts that would take up too much room.
Another electric-boosted I-6 motor was recently introduced in the Range Rover Sport. There’s also a Mazda unit rumored to be headed to production.
Advances in electric technology could end up saving the I6 from what seemed like extinction and actually bring it into a renaissance. As for the V6, expect it to become less and less common over time. Eventually, it may only be used in trucks.
Sources: Hagerty | Engineering Explained | Donut Media | Subaru | Car Throttle | Allpar | Car and Driver