For decades, engine size was one of the easiest ways to talk about performance. Want more power? Build a bigger engine. It sounds logical: more displacement means more air and fuel can enter the cylinders, so the engine should be able to make more power. Except it doesn’t always work that way.
A modern small-displacement turbocharged engine can make substantially more horsepower than a much larger naturally aspirated engine. Even two naturally aspirated engines with the same displacement can produce very different amounts of power. The reason is that displacement tells you how much space the cylinders have, not how effectively the engine uses that space.
Bigger Displacement Gives You More Potential, Not A Guaranteed Power Figure
Engine displacement is essentially the total swept volume of all the cylinders. Think of it as the volume of water that would fill an engine if you were to pour it inside (don’t ever do that, by the way). A 5.0-liter engine has more displacement than a 2.0-liter engine, but that doesn’t automatically tell you how much power either one will produce.
Think of displacement as the size of the engine’s lungs. A larger engine has the potential to move more air, but the rest of the hardware determines how effectively it can breathe, compress the mixture and turn combustion into useful mechanical work.
SAE research comparing production engines found that maximum torque correlates strongly with displacement, but the relationship between maximum power and displacement is considerably less direct. The study specifically identified factors including engine breathing, valve configuration, variable valve timing, friction and intake and exhaust design as contributors to power differences.
That’s why simply looking at liters (or cubic inches) can be misleading. A big, low-revving engine designed for torque and durability can produce less peak horsepower than a smaller engine designed to breathe efficiently and operate at much higher engine speeds.
Engine Breathing Can Matter More Than Engine Size
Here’s where things get interesting. An engine only makes power when it can get air into the cylinders and get exhaust gases back out. Engineers call the effectiveness of that process volumetric efficiency.
A bigger engine that struggles to fill its cylinders at high rpm can lose out to a smaller engine with better-flowing cylinder heads, larger or more efficient valves, optimized intake and exhaust systems, and variable valve timing.
SAE research from Ford engineers found that volumetric efficiency is affected by numerous operating and design factors, including engine speed, intake temperature, manifold pressure, exhaust pressure and compression ratio. That helps explain why modern engines can make impressive power from relatively small amounts of displacement.
More valves can help the engine breathe. Variable valve timing can alter when and how long those valves open. Better intake and exhaust designs can reduce restrictions. And higher engine speeds allow an engine to complete more combustion cycles in a given amount of time.
In other words, a 2.0-liter engine that breathes extremely well can be a very different animal from another 2.0-liter engine with a completely different design.
This Is Why A Smaller Turbo Engine Can Beat A Bigger Naturally Aspirated One
Turbocharging changes the equation even more. A naturally aspirated engine relies on atmospheric pressure to fill its cylinders. A turbocharger uses exhaust energy to compress the incoming air, allowing the engine to force a greater mass of air into the cylinders. That means a small engine can effectively behave like a much larger engine when it is under boost.
SAE research has documented the power advantages of turbocharging compared with larger naturally aspirated engines, while also noting that turbocharger efficiency, response and operating characteristics affect the final result.
This is one reason modern automakers have been able to use engine downsizing without necessarily giving up performance. A smaller turbocharged engine can provide the power of a larger engine while offering potential benefits in weight, packaging and fuel economy.
Of course, boost is not free. The engine has to deal with higher cylinder pressures and temperatures, and the turbocharger itself introduces additional engineering challenges. That’s why simply bolting on a bigger turbo is not a magic recipe for unlimited horsepower.
Here’s What Actually Determines How Much Power An Engine Makes
There are many factors that influence an engine’s horsepower potential, and the most common of them are the following.
Displacement: More displacement generally provides more potential airflow and torque, but it is not a direct horsepower guarantee.
Engine speed: An engine that can safely operate at higher rpm gets more combustion events per minute.
Volumetric efficiency: Better cylinder filling means more air—and therefore more fuel—can participate in combustion.
Compression ratio: Compression affects the thermodynamics of the combustion process and can influence efficiency and power.
Valve design: More valves and sophisticated valve control can improve airflow through the engine.
Intake and exhaust design: Restrictions on either side of the engine can limit how effectively it breathes.
Turbocharging or supercharging: Forced induction can substantially increase the amount of air entering the cylinders.
Fuel and ignition control: Modern injection and ignition systems allow engineers to precisely control combustion.
Friction and mechanical losses: Not all of the energy produced by combustion reaches the crankshaft.
Engine design limits: Temperature, cylinder pressure, durability and emissions requirements can all restrict how aggressively an engine can be tuned.
SAE’s analysis of production engines found that improvements in breathing and valve technology could produce meaningful performance gains, while turbocharging and supercharging allowed even larger increases.
Drivers Have Been Asking Why Bigger Engines Don’t Always Make More Power
Reddit has had this debate plenty of times. In a 2022 r/AskMechanics thread, a user wondered why two naturally aspirated engines with similar displacement can produce dramatically different power figures. The discussion quickly turned to airflow, engine design and how manufacturers extract more power from the same amount of displacement.
Another r/cars discussion asked why a smaller engine can sometimes make as much or more power than a larger one. Commenters pointed to turbocharging, power delivery and engine design, while noting that two engines with the same peak horsepower can behave very differently because their power curves aren’t necessarily the same.
Turbocharging comes up repeatedly. In a 2020 r/cars thread about modern small turbo engines, commenters discussed direct injection, computer controls, variable valve timing, improved materials and better engine design as reasons smaller engines can produce impressive power.
Redditors have also debated the relationship between boost, RPM and displacement. A 2024 r/cars discussion centered on an Engineering Explained video examining those three variables, reinforcing the basic point: displacement is only one way to make horsepower. How efficiently an engine breathes, how fast it spins and whether it uses forced induction can be just as important.
Horsepower Is About More Than How Many Liters You Have
This is also why comparing engines solely by displacement can produce some strange results.
A large naturally aspirated V8 might make enormous torque at relatively low rpm, while a smaller turbocharged four-cylinder could produce a similar or greater peak horsepower number higher in the rev range. Neither engine is necessarily “better” simply because its horsepower-per-liter figure is higher. They’re optimized around different goals.
And that’s the key distinction: displacement is an important ingredient, but it is not the recipe. Engineers have spent decades finding ways to make engines breathe better, rev higher, compress their mixtures more effectively and force more air into their cylinders. SAE research has documented how these technologies have steadily improved the power available from a given amount of displacement.
The conclusion is that a bigger engine does not necessarily make more power. It can, but the number of liters under the hood is only part of the story.
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