Understanding Engine Boost and Power Output in Turbos
When considering engine modifications, the integration of turbos can significantly boost power output. However, it's important to understand the mechanics behind this process to achieve optimal performance. This article delves into how the addition of turbos affects an engine's power output, with a focus on the crucial variable of pressure ratio (PR).
The Basics of Boost and Power Output
Boost, in the context of an engine, refers to the additional air pressure that is forced into the engine's intake manifold. This is achieved through the use of turbochargers, which compress air to increase its density and allow more fuel to be injected. However, the increase in power is not purely linear but follows a specific mathematical relationship defined by the pressure ratio (PR).
Pressure Ratio (PR) and Its Impact on Power
The pressure ratio (PR) is defined as the pressure in the intake manifold divided by the atmospheric pressure. Atmospheric pressure is 14.7 pounds per square inch (psi). For instance, if your engine generates 5 psi of boost, the pressure ratio would be calculated as follows:
PR 5 psi / 14.7 psi 0.34
However, the increase in power is proportional to the square root of the pressure ratio. This is expressed by the formula:
Power Increase √PR
For example, if the pressure ratio is 1.34 (due to 5 psi boost), the power increase would be:
Power Increase √1.34 1.16
Given that the base engine was making 330 horsepower, the boosted output would be:
330 bhp * 1.16 380 bhp
Limitations and Required Adjustments
A normally aspirated engine is designed to operate with naturally occurring air, making it less capable of handling significant boost pressure. This is because the pistons, cams, and valve overlap are specifically optimized for higher compression without additional air pressure. Typically, such engines can safely handle no more than 5 psi boost, which corresponds to a pressure ratio of 1.34. At this pressure ratio, the power increase would be approximately 15.7%.
To push the engine beyond this, a complete rebuild with forged pistons, thicker crowns, stronger connecting rods, and optimized camshafts is necessary. These components are designed to handle higher pressures and boost levels. For instance, a pressure ratio of 1.68 (corresponding to 10 psi boost) can lead to a power increase of 29.6%, bringing the total output to around 428 bhp.
Modern Turbocharged Engines and Oil Coolers
Most current production turbocharged engines include oil squirters on the main oil gallery, which are designed to cool the pistons under higher pressure and boost conditions. These oil squirters require a higher flow oil pump to ensure proper lubrication and cooling. Without these modifications, the engine might overheat and fail.
Boost Pressure and Custom Dyno Tuning
Manufacturers typically set the base boost pressure to around 10 psi. Tuning (customizing the engine's performance) may increase this to 14.7 psi, equivalent to 1 bar. This adjustment leads to a pressure ratio of 2, resulting in a significant power increase of 41 bhp over a naturally aspirated setup.
It's crucial to note that while any engine can handle some boost for short periods, sustained high boost pressure requires robust engine components. The weight and intended use of the vehicle also play a role in determining the boost tolerance. For example, a light sports car can usually withstand higher boost pressures compared to a heavier vehicle.
In summary, the integration of turbos into an engine can dramatically increase its power output. However, this increase is contingent on the pressure ratio, which is derived from the boost pressure and atmospheric pressure. To achieve maximum performance, it's essential to rebuild the engine with the appropriate components and ensure proper cooling systems are in place.