More boost doesn’t always mean more power

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 With turbocharged engines, the boost number–the pressure at which air is fed into the engine–is often equated with the amount of power on tap. But that isn’t necessarily the case.

“Boost itself is not a meaningless number, but it’s probably not as meaningful as many people think,” Jason Fenske of Engineering Explained said. Here’s what he means.

Gasoline engines need four things to make power–air, fuel, compression, and spark. But air is usually the limiting factor, Fenske noted. That’s where turbochargers come in. They feed more air into the engine than would be possible at atmospheric pressure, a process known as forced induction. Superchargers are another form of forced induction, but they work a bit differently and so this explainer only applies to turbos.

Turbos increase the pressure of air going into the engine, allowing more air to be crammed in. But pressure is only one of many factors that affect power output, Fenske said.

Two turbos of different sizes might make the same amount of boost, but one may do the job more efficiently than the other. Fenske noted that an undersized turbo will have to spin faster to make the same amount of boost, heating the air molecules to a higher temperature. That makes the molecules spread apart, decreasing air density and limiting how much air can go into the engine.

The same applies to intercoolers. These devices cool the incoming air, making it more dense, allowing the engine to produce more power. All other things being equal, an engine with an intercooler may produce more power than one without an intercooler.

A car is a system, and all of its components have to work together to maximize performance. Changing one thing–such as increasing boost–may not have the desired effect without looking at the bigger picture.

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