Turbo vs supercharger

How a turbocharger works and How superchargers work looked at each one on its own. Both squeeze extra air into the engine; the difference is what drives them. A turbo is spun by the exhaust, and a supercharger by a belt from the crankshaft. This short lesson puts them side by side.

The demo runs the same 2.0-litre engine twice through the same drive: once with the turbo from the Wastegates lesson, and once with the twin-screw supercharger from the superchargers lesson. For a fair fight, both make the same 0.7 bar (10 psi) at full boost. The graph shows each one’s boost over the drive, with the full-throttle stretches shaded, and the readouts show their power.

1. Cruise. Neither needs boost, so both make the same power. The supercharger still turns with the engine, but its bypass valve is open, so it costs very little.
2. Pull away. You floor it from 1,500 rpm. The supercharger squeezes the air at once, while the turbo waits for the exhaust to spin it up. For the first two seconds the supercharged engine makes noticeably more power. Then the turbo catches up.
3. Cruise again. Back to light throttle.
4. Overtake. Kickdown at 2,500 rpm. Over the first second the supercharged engine averages about 71 kW (95 bhp), the turbo one about 62 kW (83 bhp). Once the turbo has spooled, it pulls ahead by about 5 kW (6.7 bhp), because it isn’t paying a belt to drive it.

Over the whole drive, the turbo engine uses about 1.4% less fuel while making a little more power on average.

The model is simplified, and real cars blur the differences: modern small turbos spool much faster than this one, and a supercharger’s bypass valve doesn’t make it quite free.

• Response. Supercharger: instant, in step with your foot. Turbo: a short wait for boost, though modern ones keep it small.
• Power delivery. A Roots or twin-screw supercharger gives strong, even pull from low revs, like a bigger engine. A turbo gives a surge once it’s on boost, though modern ECUs smooth it into a flat torque curve.
• Efficiency. The turbo wins: it runs on energy from the exhaust, while a supercharger takes power from the crankshaft. The higher the boost, the bigger the gap.
• Boost. Turbos make high boost easily and cheaply; a supercharger big enough for high boost costs a lot of power to drive.
• Altitude. A turbo can spin faster to make up for thin air. A belt-driven supercharger can’t, so it loses power like a naturally aspirated engine.
• Heat and plumbing. A turbo sits in the hot exhaust and needs oil and coolant lines and an intercooler with long pipes. A supercharger is cooler, simpler to fit and easier to add to an engine that never had one.
• Sound. A supercharger whines; a turbo whistles and hisses, and muffles the exhaust a little.

Nearly every new car with forced induction uses a turbo. The reason is efficiency. Rules on fuel use and CO2 pushed makers towards small engines, and a turbo lets a small engine use little fuel most of the time, then make the power of a bigger one when you floor it, as Why forced induction makes more power explained. A supercharger would keep costing power whenever it’s boosting.

Turbos also got much better at what superchargers did best. Smaller, lighter wheels, twin-scroll housings, variable geometry and electric assistance have made lag hard to notice in everyday driving.

• Big performance engines where instant response and the sound matter more than fuel, such as supercharged V8s.
• Adding boost to an engine that never had it. A centrifugal supercharger kit bolts to the front of the engine without changing the exhaust, so it’s often the easier route.
• Filling the gap before a turbo wakes up. Twincharged engines, and electric superchargers on mild hybrids, use a supercharger for instant boost and a turbo for efficiency. Electric superchargers are how the supercharger idea is most likely to live on.