How Much Horsepower Does a Turbo Boost Provide?

For years, turbochargers have played a crucial role in boosting horsepower, a trait that has made them a favorite in the world of race cars and high-performance sports cars. The query of "how much hp does a turbo add?" now goes beyond sheer power; contemporary turbos also improve fuel efficiency, enabling smaller engines to perform better while still reaching highway speeds effortlessly.

This evolution not only showcases how turbochargers amplify an engine's power but also underscores their contribution to enhancing overall engine efficiency.

What Are Turbochargers?

Turbochargers function as a forced induction mechanism, pressurizing the air entering the vehicle's engine. By compressing the air, the engine can intake a greater volume of air into each cylinder, allowing for more fuel to be injected. As a result, each cylinder's combustion generates increased power.

An engine equipped with a turbocharger delivers greater overall power compared to a non-turbocharged engine of the same size. This enhancement significantly improves the engine's power-to-weight ratio. Additionally, a smaller engine can achieve higher horsepower more efficiently, leading to reduced fuel consumption and fewer trips to the gas station.

To achieve this performance boost, the turbocharger harnesses the engine's exhaust flow to drive a turbine, which powers an air pump. The turbine within the turbocharger typically operates at speeds ranging from 80,000 to 200,000 revolutions per minute (rpm) — up to 30 times faster than most car engines. Given its connection to the exhaust system, the turbine also endures extremely high temperatures.

Turbochargers and Engines

Enhancing engine performance and extracting more power often involves increasing the engine's capacity to burn air and fuel. This can be achieved by adding more cylinders or enlarging the existing ones. However, when such modifications aren't practical, a turbocharger offers a more straightforward and space-efficient solution, particularly as an aftermarket upgrade.

How Much Horsepower Does a Turbo Add?

Turbochargers enable engines to burn more fuel and air by compressing additional amounts into the cylinders. A typical turbocharger delivers a boost of 6 to 8 pounds per square inch (psi). Given that atmospheric pressure at sea level is 14.7 psi, this means the engine receives approximately 50 percent more air. While this suggests a 50 percent power increase, real-world efficiency varies, often resulting in a 30- to 40-percent improvement.

The horsepower gain from a turbocharger varies based on factors like turbo size, engine type, and tuning. For example, a 200-horsepower engine could see its output rise to between 240 and 280 horsepower with a turbocharger.

A key inefficiency arises because spinning the turbine requires energy. Placing a turbine in the exhaust flow creates additional exhaust resistance. During the exhaust stroke, the engine must work against this higher back pressure, slightly reducing the power output of the active cylinders.

Turbocharger Design

The turbocharger is attached to the engine's exhaust manifold. Cylinder exhaust drives the turbine, which operates similarly to a gas turbine engine. A shaft links the turbine to the compressor, positioned between the air filter and intake manifold, where it pressurizes incoming air for the pistons.

Exhaust gases flow through the turbine blades, causing them to rotate. The volume of exhaust directly influences the speed at which the blades spin.

At the opposite end of the shaft connected to the turbine, the compressor forces air into the cylinders. Functioning as a centrifugal pump, it pulls air into the center of its blades and propels it outward as it rotates.

To withstand rotational speeds of up to 200,000 rpm, the turbine shaft requires precise support. Conventional bearings would fail at such speeds, so turbochargers typically employ a fluid or hydrodynamic bearing. This bearing type uses a thin oil layer constantly circulating around the shaft, serving dual purposes: cooling the shaft and other components while minimizing friction during rotation.

However, excessive boost can be problematic. As the turbocharger pressurizes air into the cylinders and the piston further compresses it, the risk of knock increases. Knocking occurs because compressing air raises its temperature, potentially igniting the fuel prematurely before the spark plug fires. Turbocharged vehicles often require higher octane fuel to prevent knocking. If boost pressure is extremely high, reducing the engine's compression ratio may be necessary to avoid this issue.

The turbo system might also incorporate an intercooler positioned between the turbocharger and the cylinder. This component cools the air before it enters the combustion chamber, minimizing the risk of knocking.

Designing a turbocharger for an engine involves numerous trade-offs. In the following section, we'll explore these compromises and their impact on overall performance.

Turbocharger Parts

A significant drawback of turbochargers is their delayed power response when accelerating. There's a brief moment before the turbine reaches optimal speed and generates boost, creating a lag sensation. Once the turbo engages, the car surges forward.

Many automotive turbochargers incorporate a wastegate, enabling the use of a smaller turbo to minimize lag while preventing excessive spinning at high speeds. The wastegate acts as a valve, diverting exhaust away from the turbine blades. It monitors boost pressure; if it becomes too high, indicating excessive turbine speed, the wastegate redirects some exhaust, slowing the blades.

Certain turbochargers utilize ball bearings instead of fluid bearings to support the turbine shaft. These are not ordinary ball bearings but high-precision bearings crafted from advanced materials to withstand turbocharger speeds and temperatures. They reduce friction compared to fluid bearings, enabling the use of a smaller, lighter shaft. This design helps the turbocharger accelerate faster, further diminishing turbo lag.

Using Two Turbochargers & More Turbo Parts

Some engines employ two turbochargers of varying sizes. The smaller turbo spools up quickly to reduce lag, while the larger one delivers increased boost at higher engine speeds.

Compressing air causes it to heat up, and heated air expands. Thus, part of the pressure increase from a turbocharger comes from heating the air before it enters the engine. The objective is to increase the number of air molecules in the cylinder, not just the air pressure, to enhance engine power.

An intercooler, or charge air cooler, resembles a radiator but allows air to flow both inside and outside. Intake air moves through sealed channels within the cooler, while external air is blown across cooling fins by the engine's fan.

The intercooler boosts engine power by cooling the compressed air from the turbocharger before it enters the engine. For instance, if the turbocharger delivers 7 psi of boost, the intercooler ensures that 7 psi of cooler, denser air—packed with more air molecules—enters the engine.

Turbochargers are particularly beneficial at high altitudes, where air density is lower. Conventional engines lose power at high altitudes due to reduced air intake per piston stroke. Turbocharged engines also experience power loss, but it's less severe because the turbo's air compression compensates for the thinner air.

Modern fuel-injection systems use oxygen sensors in the exhaust to monitor the air-to-fuel ratio, automatically adjusting fuel flow when a turbo is installed. However, adding a turbocharger with excessive boost may overwhelm the system, as the controller's software or fuel pump and injectors might not support the required fuel supply. In such cases, additional modifications are necessary to fully leverage the turbocharger's potential.