Engine size is one of the most important vehicle characteristics that experienced buyers look at when comparing cars. Car buyers have been conditioned that the size of the engine directly correlates to what they want from the vehicle they are considering. Bigger engines, such as those in performance cars, for example, indicate more power, while smaller engines achieve better fuel economy, and we can quickly look at the size of an engine and know basically what to expect from that vehicle even without knowing other specifications.

This number that indicates engine size is called displacement, and it’s actually a variable that can be manipulated to achieve different objectives. More cylinders in an engine usually means more power, but not necessarily. Instead, it's the displacement that plays a larger role in horsepower and torque output. In the era of muscle cars, for example, it was quite common to simply play with the engine’s cylinders, such as by increasing the bore, to incrementally increase power. Read on for a deep dive into vehicle displacement – how it works and what it means to your vehicle.

What Is Engine Displacement

The literal definition of displacement is the movement of something from one place to another. By application, displacement is the amount of a substance (usually liquid, but it certainly applies to a gas) forced out by the occupation of its space by something else. Further, it is also the volume pumped by a reciprocating system, such as an engine.

In an internal combustion engine (ICE), displacement is the amount of air “displaced” by the internal workings of the engine. Sometimes, that isn't as simple as assuming that a 5.0-liter engine displaces five liters, or 5,000 cubic centimeters (cc). Engine displacement is not affected by the mass of the engine (i.e., a 500-pound engine that has thicker walls, versus a slimmer 400-lb one).

How To Calculate Displacement

1. Pi x bore radius x bore radius
2. Multiply by stroke length
3. Multiply again by the number of cylinders

You can use the above formula to calculate engine displacement yourself.

The cylinder is the tube in which the piston moves. The volume of the cylinder is calculated by taking the area of the engine bore (the width of the hole) and multiplying it by the stroke (the distance the piston moves from the top of the cylinder to the bottom).

Bore is the diameter of the circular cylinder, but area is calculated using the radius of the circle (half the diameter), so you can’t just multiply bore times π (pi) times stroke, as some on the web suggest.

Multiply the bore radius by itself, then multiply by π (about 3.14) to get the area of the circle, then multiply that by the stroke to get the volume of each cylinder (basically circles stacked on top of each other). Multiply the total by the number of cylinders in the engine to get the engine displacement, either in cubic centimeters (cc) or cubic inches (cid).

Is Bigger Better For An Engine?

There was a time when the most reliable way to get more power from an engine was to increase the displacement, by increasing the bore or the stroke. Though that is still a solution for making more power, there are also more practical and cost-effective ways, rather than physically altering an engine block.

The old adage about “no replacement for displacement,” still holds true according to the laws of physics, but it is not the only solution in these days of rampant forced induction and, soon to be, widespread electrification.

That said, adding components instead of rebuilding the engine block adds load and stress to engine components and may affect their, and the engine’s, durability.

How Engine Displacement Affects Power

We’ve established that an engine is basically a self-propelled pump, sucking in air and blowing it back out, and it generates power because the air it sucks in is rich in combustible fuel molecules that are ignited to create a precisely-choreographed series of explosions to generate power.

The more cylinders an engine uses, the more volume it has and the more air it can displace. You can use more cylinders to displace that air, or fewer, larger cylinders, but it all comes down to that ability to displace air.

The Correlation Between Displacement And Power

By displacing more air, an engine can generate more power. There are other forces and variables at work in drawing the proper amount and right type of air into an engine cylinder, but simply put, drawing more air in directly translates into the fuel system supplying more fuel.

The more fuel that mixes with the fresh air, the denser or “richer” the air/fuel mixture that is ignited in each cylinder to drive a piston down. That forcible movement of the piston is called the “power” stroke, and it makes the engine work. In a nutshell, more air means more fuel to create the potential for more power.

How Can Engine Displacement Be Modified

It should be noted that displacement is strictly the cylindrical volume of an engine. It does not take into account the combustion chambers. Changing those will certainly modify the power an engine makes (and we’ll cover that further down), but it will not change the engine displacement.

There are only two ways to change displacement – change the bore or change the stroke. Either of them changes the volume of the cylinder and the amount of air to displace, hence modifying power.

Modifying Bore And/Or Stroke Changes Power Output

• Widen the cylinder to make the bore larger
• Shorten the stroke to make a cylinder shorter
• Lengthen the stroke to make a cylinder deeper

Some of the most powerful engines have large bores and short strokes (think Ferrari), so if you’re looking to change the displacement of an engine, you can make a wider cylinder and/or you can shorten the depth of said cylinder by limiting the travel of the piston after combustion.

A bigger bore will make a bigger cylinder meaning more air, which means more fuel and therefore more power. A longer stroke will also make the cylinder bigger (virtually, since the cylinder tube remains the same size, but the travel of the piston uses more of the cylinder’s volume).

Conversely, shortening the stroke decreases the virtual depth of the cylinder and the amount of air/fuel mixture drawn into the cylinder, which would appear to be counterproductive to increasing power.

How Modifying Bore Or Stroke Affects Power Output

(Source: Over-Drive Magazine)

A shorter stroke means the piston doesn’t travel as far during the power stroke, which means the combustion cycle would be shorter, and the crankshaft will then turn more often each minute. More revolutions per minute (rpm) means more combustions per minute, more power strokes per minute, and more power output.

A longer stroke allows more air/fuel mixture and the combustion forces the piston over a longer distance and takes more time, having an effect on engine torque at lower rpm, since the crank isn’t turning as many times per minute.

The chart above shows the evolution of a Pontiac V8 engine, showing the change in displacement as a result of increasing the bore or the stroke, and the corresponding gains in power and torque.

External Variables Influence Power, But Not Displacement

There are variables outside of displacement that can make an engine produce more power. The obvious one is forced induction, such as supercharging or turbocharging. Force-feeding air into the cylinder forces the fuel system to add more fuel, making a denser gas for the combustion explosion. Superchargers have been forcing air into engines for about 150 years; turbochargers for 100.

You can also play with the designs of piston and cylinder heads, altering the volume of the air/fuel mixture that is drawn into the combustion chamber, and the level to which it is compressed. Higher compression means more power.

Cylinder heads are domes; changing the dome shape changes compression and can improve power output. Changing the head gasket or milling the head will shorten the height of the dome and increase compression. Some tolerances must be adhered to, to make sure intake and exhaust valves can function properly.

Combustion Chamber Design Impact On Power And Torque In The Same Engine

(Source: Over-Drive Magazine)

What's Compression Ratio?

The compression ratio is another engine performance aspect that can be modified to enhance power. Piston heads are scooped out; changing to a flatter head will increase compression. And again, there are tolerances that must be adhered to, to make sure all the combustion components continue to work in harmony.

How Different Compression Ratio Affects Power In The Same Engine

(Source: Over-Drive Magazine)

Vehicles Moved Away From Displaying Engine Size

Engine displacement is probably the prime indicator of a car’s power, and it used to be quite easy to see at a glance, as engine manufacturers proudly displayed the engine size on their engines, or via the vehicle’s badges.

However, in this day of cosmetic engine covers and alphanumeric nomenclature, the badges can be deceiving. A Mercedes G 400, for example, used a 4.0-liter biturbo V8 as implied in the name. However, the current G 550 uses a 4.0 twin turbo V8, too.

The old adage about no replacement for displacement may not be completely true in these days of forced induction and electrification, but there’s no denying a turbocharged large engine will still out-power a supercharged small one.