For decades, the usual way to make an engine more powerful was to make it bigger. Today the usual answer is a smaller engine with boost, and the naturally aspirated engine is the one being phased out. It is not going because it is worse. It is going because a small turbo engine meets emissions and fuel-economy rules more easily, and the rules are what get built to.
What separates these engines is how air reaches the cylinders. Put another way, where does the push come from?
- Naturally aspirated: from nothing but the atmosphere. The engine simply draws air in.
- Turbocharged: from the exhaust. Waste gas spins a turbine that drives an air compressor.
- Supercharged: from the crankshaft. A belt or gears from the engine drive the compressor directly.
The two boosted designs squeeze more air into each cylinder, so the engine can burn more fuel and make more power from the same size.
Follow the air
Pick an engine to see the path air takes to the cylinders, and what drives the compressor.
Naturally aspirated
Air path
- Outside air
- Air filter
- Throttle
- Cylinders
What does the pushing
- The atmosphere alone
No compressor and no boost. The cylinder can never hold more than the atmosphere will push in.
Read the naturally aspirated sectionTurbocharged
Air path
- Outside air
- Air filter
- Compressor
- Intercooler
- Throttle
- Cylinders
What drives the compressor
- Exhaust gas
- Turbine
- Shared shaft
- Compressor
Energy the engine was about to throw away. The catch is the wait: no exhaust flow, no boost.
Read the turbocharger sectionSupercharged
Air path
- Outside air
- Air filter
- Throttle
- Compressor
- Intercooler
- Cylinders
What drives the compressor
- Crankshaft
- Belt or gears
- Compressor
Boost whenever the engine turns, paid for with some of the engine's own power. Shown for the common Roots and twin-screw layout, where the throttle sits ahead of the compressor.
Read the supercharger sectionWhy does an engine need help breathing?
An engine is an air pump that happens to burn fuel. Power depends on how much air-and-fuel mixture you can burn per second, and fuel is the easy half. Air is the hard half.
A naturally aspirated engine gets its air the passive way. The piston moves down, the pressure in the cylinder drops, and the atmosphere shoves air in to fill the gap. That is the whole mechanism. The cylinder can never hold more than the atmosphere will push in, and in practice it holds less, because the filter, the throttle and the intake plumbing all get in the way.
Forced induction breaks that ceiling by compressing the air before it reaches the cylinder. More air means more oxygen, which lets the engine burn more fuel on every cycle, so the same displacement makes more power. The pressure above atmospheric is what everyone calls boost.
So the real difference between the three engines in this article comes down to one question: where does the push come from? From nothing but the atmosphere, from the exhaust, or from the crankshaft. Lag, sound, efficiency and price all follow from the answer.
One catch applies to the two boosted designs. Compressing air heats it, and hot air is less dense and more prone to knock, which is why most boosted engines add an intercooler to chill the air on its way in.
Why would anyone still want an engine with no boost?
Because of the throttle. In a naturally aspirated engine, the pedal is connected to the cylinders by nothing more exotic than air. Open the throttle and the cylinders fill. There is no turbine to spin up and no belt-driven pump to wait on, so the engine does what you asked, when you asked, in proportion to how much you asked.
The power delivery follows from that. Output tends to climb steadily with revs, which is why the best of these engines are built to keep climbing, with high redlines and intake and exhaust tuned for the top of the range. It is a smooth, predictable curve, and a lot of drivers value that predictability more than a bigger number.
The price is a ceiling. Power is set by displacement, revs and how well the engine breathes, so extra power means a bigger engine or a faster-spinning one. The traditional answer was more cubic inches, which is honest and thirsty, and a fair summary of the American V8.
Two more limits are worth knowing:
- Low-rpm torque. With no boost, a smaller engine has little to offer below the middle of its rev range. The fix is a larger engine or a gearbox that keeps the revs up.
- Altitude. Thinner air means less oxygen per cylinder-fill. As a rule of thumb, a naturally aspirated engine loses roughly 3% of its power per 1,000 feet (about 300 metres) of climb.
What does altitude take from a naturally aspirated engine?
At 5,000 ft, a naturally aspirated engine making 300 hp at sea level makes roughly 255 hp.
- Sea level100% of sea-level power
- 3,000 ft (about 900 m)roughly 91%
- 6,000 ft (about 1,800 m)roughly 82%
- 9,000 ft (about 2,700 m)roughly 73%
- 12,000 ft (about 3,700 m)roughly 64%
Uses the 3% per 1,000 ft rule of thumb, so treat it as an estimate. A turbo engine recovers much of this loss. A supercharger helps, but cannot spin faster to make up the difference.
The usual claim is that this design is simpler and so more reliable, and there is something in it: fewer parts, lower cylinder pressures, nothing running red-hot in the exhaust. But the claim gets oversold. A modern turbo engine is designed for boost from the first drawing, and the reliable ones are not held together by hope.
Why did turbochargers win, and what happened to lag?
Because a turbocharger recycles energy the engine was about to throw away. Exhaust gas spins a turbine, a shaft carries that rotation to a compressor wheel on the intake side, and the compressor packs extra air into the cylinders. Nothing is taken from the crankshaft to drive it.
Calling it free is a stretch, since the turbine is an obstruction in the exhaust and the engine has to push gas through it. But the trade is a good one, and the industry took a while to make it work. General Motors tried in 1962, with the Oldsmobile Jetfire and the Chevrolet Corvair Monza Spyder arriving within weeks of each other. Neither started a trend. Jetfire owners had enough trouble that Oldsmobile offered to swap the turbo system for a conventional four-barrel carburettor, which is a blunt way of reading customer feedback.
The turbo became credible in the 1970s, with the BMW 2002 Turbo in 1973, the Porsche 911 Turbo in 1974 and the Saab 99 Turbo in 1978. The real takeover came in the early 2010s, when many European manufacturers started replacing big engines with small turbocharged ones.
And the lag?
It is built into the principle. A turbo needs exhaust flow to spin up, and at low revs on a light throttle there is not much of it. Press the pedal and there is a pause while the turbine catches up and boost arrives. The early cars were known for it.
Engineers have attacked it from every side: smaller, lighter turbines that spin up quickly, twin-scroll housings that make better use of exhaust pulses, variable-geometry vanes, two turbos working in sequence, and electric motors that spin the compressor before the exhaust can. Lag has not disappeared. In a modern small turbo engine it is mostly a matter of degree.
The electric fix is the newest, and it arrived in two forms. Audi's SQ7 TDI went on sale in 2016 with a separate electrically driven compressor, run from a 48-volt system, to fill the gap before its two conventional turbos woke up. In 2022 the Mercedes-AMG SL 43 went a step further and put an electric motor on the turbo shaft itself, an idea taken from Mercedes' Formula 1 engines. The motor spins the compressor up before the exhaust has caught up, which is about as direct an answer to lag as exists.
Is it really more efficient?
Sometimes. Cruising gently, a small turbo engine behaves like a small engine and uses little fuel. Under boost it needs fuel to match all that extra air, and it drinks like a bigger engine, because it is acting as one. Official test cycles are gentle, which flatters it. Hard driving does not.
The altitude advantage is more solid. Thin air gives the turbine more room to spin faster, so a turbo recovers much of the power a naturally aspirated engine loses on the way up a mountain.
The costs are heat and complexity. The turbo sits in the exhaust stream, where it runs very hot, and it needs an oil supply and often coolant to survive. Add the wastegate that controls boost and the intercooler, and there is simply more to go wrong.
What does a turbo ask of its owner?
Mostly clean oil, changed on time. The turbo shaft spins at enormous speed on a thin film of engine oil, so old, dirty or wrong-grade oil is the quickest way to kill one. The other habit worth keeping is not to switch off straight after hard driving. Many modern turbos are water-cooled, and some cars keep a coolant pump running after shutdown, but taking the last minute or two of a drive gently still lets the turbo cool before the oil stops moving. Look after those two things and a modern turbo has no reason to fail early.
What does a supercharger do that a turbo cannot?
It answers instantly. A supercharger is a compressor driven by the engine itself, through a belt, chain or gears from the crankshaft. Because it turns whenever the engine turns, it does not wait for exhaust flow. Open the throttle and the boost is already there.
It is also the older idea in cars. Supercharged road cars such as the Graham eight-cylinder (1934), the Cord 812 (1937) and the Studebaker Golden Hawk (1957) all appeared before the first production turbo did.
Not all superchargers feel alike, though. Roots and twin-screw designs push air from near idle, so the shove arrives low in the rev range and stays. A centrifugal supercharger builds boost as revs rise, which gives it a character closer to a turbo, only without the delay. Anyone who claims a supercharger gives the same power at every rev has been describing one type.
What does it cost?
Power, to start with. The engine spends part of its own output driving the thing that makes more of its output. The net result is still a gain, but the supercharger is the least efficient of the three designs when it is working hard.
It also heats the air it compresses. Hot intake air costs power and invites knock, and the fix is the same intercooler a turbo needs. The Dodge Challenger SRT Hellcat, with its twin-screw supercharged 6.2-litre V8 rated at 707 horsepower, carries two air-to-water intercoolers for exactly this reason.
The Hellcat also shows the attraction. It shipped with two key fobs, and the black one limits output to about 500 horsepower, which is the automotive equivalent of a parental control setting.
Installation is simpler than a turbo, since there is no exhaust plumbing or oil feed to route. That makes superchargers popular with aftermarket kits. But more air still needs more fuel and a proper tune, and the engine has to survive the extra load.
Can you have both?
Yes, and Volvo did. Its 2.0-litre T6 four-cylinder uses a supercharger below 3,500 rpm for instant low-rev torque and lets the turbo carry on above it. Volkswagen built a similar twincharger engine and walked away, reportedly over cost and reliability. The idea works, it just costs a great deal to do.
Which one should you actually want?
It depends on the job, but the choice is less open than the debate makes it sound.
- The car you will live with: turbocharged. It gets the most from the least, it holds its power up a mountain, and its worst habit, lag, has been engineered down to something most drivers never notice.
- The engine you would pick for a good road: naturally aspirated. Nothing else gives the same direct line from pedal to cylinders, or the same reason to chase the redline. Buy one while you still can.
- The big V8 with something to prove: supercharged. Boost from idle, the whine, and enough torque to make the gearbox nervous. It is the least efficient answer and often the most fun one.
What matters most to you?
Pick one to highlight it and see which engine comes out ahead.
| Engine | Throttle response | Low-rpm torque | Fuel efficiency | High altitude | Complexity |
|---|---|---|---|---|---|
| Naturally aspirated | Immediate | Weakest; needs size or revs | Good at light load, poor for the power made | Loses roughly 3% per 1,000 ft | Lowest |
| Turbocharged | Small delay, much reduced in modern designs | Strong once boost builds | Best at light load; closer to a bigger engine under boost | Holds power best | Highest: hot-side plumbing, oil and coolant feeds, wastegate, intercooler |
| Supercharged | Immediate | Strong from near idle (Roots and twin-screw types) | Weakest under boost, since the crankshaft drives it | Helps, but the compressor cannot spin faster to compensate | Moderate: belt drive and intercooler, no exhaust plumbing |
If the money were mine and I had to pick one, I would take the naturally aspirated engine and accept the lower ceiling. If it were not available in the car I needed, I would take the turbo without feeling cheated. The supercharger is for when the V8 is the point.
Choose the turbo if you are being sensible. Choose the naturally aspirated engine if you are being honest about why you like cars.