Matching a turbocharger to an engine can be difficult given all the metrics, calculations, and considerations involved. One must know the specific engine as well as its applicable use. However, with a realistic mindset, anyone can find the right turbo for their vehicle. While some might want to increase their horsepower by 300 percent, such a boost is unlikely. Realistically, turbos provide a 50 percent increase to a vehicle’s engine, and finding the right turbo to match can lead to more satisfactory results. This guide on how to match a turbocharger to your engine will help. See how knowing your vehicle’s operation standards, calculating the metrics, and understanding these metrics (like volumetric efficiency, compressor matching, and trim) all have a role in selecting the ideal turbocharger.
Figure Out What You’re Looking For
Before anything, you must know what you’re looking for to find the right-size engine turbocharger. Realistically, turbos add about 50 percent more horsepower to an engine. Prioritize and understand the amount of horsepower your engine needs to produce the power needed for its application. A performance vehicle does not need the same turbo as a work pickup truck or boat, so it makes sense that turbos will be available in different shapes and sizes built from various manufacturers. If possible, find a turbo that matches your vehicle’s brand, such as Mitsubishi, Ford, John Deere, etc.
Calculate the Metrics
There are a few calculations and metrics involved with finding the ideal turbocharger. The main calculation is finding the amount of air your engine uses at certain boost pressures. Use the engine’s RPM, size, volumetric efficiency, and boost pressure to find this. You’ll see the resulting pressure ratio and the amount of air in three figures: CFM, lbs./min, and m3/min. Each calculation pertains to the revolutions per minute of the crankshaft connected to the piston rods. It also indicates the engine’s displacement in liters or cubic centimeters. Additionally, the result shows the ratio of mass density of the air and fuel mixture that travels into the cylinder and mass density of air traveling into the intake manifold. Any airflow restrictions in the intake system cause a drop in pressure, which reduces the air density in the cylinder. Larger turbos produce more airflow at a lower boost level with longer lag time, whereas smaller turbos have less lag time but run at a high boost level to produce more airflow.
Volumetric Efficiency and Air Density
Volumetric efficiency is an important metric in turbo performance. Maximizing the volumetric efficiency, or VE, raises the potential horsepower and RPM. An engine’s VE refers to the calculated volumetric airflow compared to the actual capability. Engines have fixed displacements that allow for a certain flow of cubic inches per revolution. While there should be a linear relation between airflow and engine RPM that doubles the revolutions per double air displacement, this is not the case. Most engines fail to meet 100 percent VE because of the air cleaner filter, small valves, or fewer valves per cylinder. However, one can increase the VE with larger valves, improved intake manifold design, enhanced valve timing, different camshafts, and/or a free-flowing exhaust system.
Compressor Matching
Compressor matching is also important. It commonly requires a series of assumptions to match a compressor to the right turbo. The varied RPM, manifold system, and corresponding fuel flow all affect the match. Since the turbo helps increase the total airflow through the engine, one must consider the airflow and fuel passing through the engine as mass flow. More fuel and air traveling through the engine means the engine will produce higher horsepower. Airflow is measured in pounds of mass per minute or cubic feet per minute (CFM) in a naturally aspirated engine. Once compressed, it is measured in pounds of mass since it has a different amount of oxygen content than at ambient pressure. Temperature and atmospheric pressures also affect the CFM, so mass-flow sensors are essential for correcting these variations.
Trim
Trim is the ratio for the turbine and compressor wheels. It is a reference to the flow potential of a wheel’s dimensions. Wheels have several configurations within their castings that determine the flow range and pressure characteristics. The inducer and exducer diameters make up the wheel trim. The inducer is the small inlet diameter where fresh air enters the compressor wheel. The exducer is the maximum diameter that accounts for the overall diameter and the tip width or height. As trim increases, the wheel supports more mass airflow. Typically, there are more compressor trims than turbine trims since turbines are not as sensitive to flow changes as compressors. Large inducers and small exducers will flow large volumes of air at low pressure, whereas small inducers and large exducers flow less mass at higher pressures. Note: a compressor wheel’s trim characteristics greatly influence the compressor map and turbine flow potential.
Turbine Matching
It can be more of a challenge to match a turbine than a compressor end because there are more variables involved. Turbines, on the other hand, are complex, given the compressor’s characteristics and constant airflow feeding into the engine. A turbine’s variables pertain to the air-to-fuel ratio, VE, charge-air coolers, manifold, and more. One way to match your turbine to your engine is to place a pressure tap in the turbine inlet. This will provide turbine inlet pressure without exceeding boost pressure or back pressure. When matching the best turbine to your engine, be realistic about its application. The compressor’s sizing should indicate a realistic turbine selection to provide the best flow range and power balance between both ends of the turbocharger.
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