Engineering Explained: The Pros And Cons Of Different Engine Types

The most common engine types - the four-cylinder, the boxer-four, straight-six, V6 and V8 - have their own pros and cons. Here's everything you need to know in one handy guide...

Engineering Explained

18 May 2015

What makes more power, a 4.0-litre V6 engine or a 4.0-litre V8? The answer isn’t so simple. When discussing various engines, the layout isn’t the biggest contributing factor to how much power it makes. With a bit of ingenuity (and you know, cash), a four cylinder engine can make just as much power as a V12. So what makes manufacturers choose different engine layouts? Here are the advantages and disadvantages of each layout.

1. Four-cylinder inline four

Let’s start with one of the most common engines, the inline four cylinder. There’s a reason it’s common, largely because it’s so simple: one cylinder bank, one cylinder head and one valve train. Here’s all you need to know:

Advantages:

The four-cylinder, inline four is small and compact, meaning it easily fits in nearly any engine bay.
It’s also lightweight, and with only one exhaust manifold, weight is further reduced.
With only one cylinder head, there are fewer moving parts than engines with multiple cylinder banks. This means less energy is lost which reduces the probability of malfunctions.
Primary forces are balanced because the outside two pistons move in the opposite direction of the inside two pistons (see picture above).
Four-cylinder engines are easy to work on; the cylinder head is the highest point which makes spark plug jobs and valve train access very easy.
Four-cylinder engines require lower manufacturing costs.

Disadvantages:

Secondary forces are not balanced, which ultimately limits the size of the engine.
Inline fours will rarely exceed 2.5 litres to 3.0 litres.
Larger four cylinder engines will often require balancing shafts to cancel the vibration caused by the secondary imbalance.
High centre of gravity compared to some layouts (H4).
Not as rigid as some layouts (V6, V8).

Here’s a quick video explanation of the four-cylinder engine:

2. Horizontally-opposed

From a performance standpoint, there aren’t many options as attractive as an engine with horizontally-opposed cylinders. The boxer four isn’t nearly as common as the other engines on this list, but from an engineering standpoint it’s a logical choice for your race car.

Advantages:

Primary and secondary forces are well balanced. This is a smooth engine.
This allows for less weight on the crankshaft, resulting in less power lost to rotational inertia.
Low centre of gravity allows for better handling.

Disadvantages:

Packaging size: these are very wide engines.
Flat engines were once used in Formula 1 for their performance advantages, but due to their width they obstructed airflow and are no longer used.
Complexity - two cylinder heads/valve trains.
Rocking couple (plane imbalances) due to offset pistons to allow for the connecting rods to connect with the crankshaft.
Maintenance can be challenging if packaging is tight.

3. Straight-six

An engineer’s object of affection, the straight-six is the result of tacking on two more cylinders to an inline four engine. BMW loves them, and it’s the layout of one of the most well-known boost-ready engines, the 2JZ. So what’s so special about the straight-six?

Advantages:

The straight-six is Inherently balanced.
The layout combined with its firing order leads to essentially the smoothest engine out there.
V12s and Flat-12s are the next step in further reducing vibration, as they are two I6s matched together.
Lower manufacturing cost - single cylinder block with all the cylinders in one orientation.
Simple design, easy to work on much like the I4.

Disadvantages:

Packaging can be difficult due to the length.
Not ideal for FWD vehicles.
High center of gravity (vs flat engines).
Lower rigidity than V engines as it’s long and narrow.

Here’s a quick video explanation of the straight-six:

4. V6

Now cut that straight-six in half and match the two cylinder banks to a common crank. The V6 is a common layout when there are six spark plugs involved. It’s also the current layout for Formula 1 engines. Why use it?

Advantages:

They’re compact and can easily be used for both FWD and RWD vehicles.
Allows for greater displacement than four-cylinder engines, typically meaning more power.
Rigid design.
Formula 1 chose to use V6s rather than I4s for the 2014 season because they wanted to use the engine as a stressed member of the car.

Disadvantages:

Two cylinder heads means added cost, complexity, and weight.
Additional rotational inertia and friction (more moving parts).
High centre of gravity vs flat engines.
Cost is often greater than inline.
Secondary imbalance requires additional weight on the crankshaft.
Two exhaust manifolds means additional weight.

5. V8

When you add a cylinder to each bank of the V6, you get an icon in both American muscle and European exotics - the V8. It can produce a refined whine, or a shuddering burble. So what makes this layout such a popular choice?

Advantages:

Packaging size (short in length).
Good balance, depending on the crankshaft type and firing order (flatplane vs crossplane).
Rigid design.
Allows for high displacement.

Disadvantages: