The key ingredient to maximizing engine horsepower (horsepower at the crankshaft) is to maximize the mass airflow used by the engine. Mass airflow is defined as the mass of air (measured in pounds, grams, etc.) used by the engine per unit time - a common unit of measurement is pounds per minute (lbs/min.) Many dataloggers are able to display the mass airflow calculated by the ECU directly, which makes this a convenient value in estimating crank horsepower. The general rule of thumb is:
crank horsepower = mass airflow (lbs/min) * 10
For example, a mass airflow value of 31 lbs/min corresponds to a crank horsepower figure of approximately 310 crank hp. So the important question is how can I maximize mass airflow? There are four primary parameters that can be adjusted to maximize mass airflow:
Increasing any of the above or a combination of the above parameters will result in an increase in mass airflow. Each parameter will now be discussed in further detail.
Increase Intake Manifold Pressure
On normally-aspirated engines (or turbocharged engines, for that matter), one way of increasing intake manifold pressure is by minimizing any restrictions to airflow into the cylinders. In the case of normally-aspirated engines, this pressure will always be less than atmospheric pressure (a partial vacuum) - unless you consider setups which use a "ram-air" effect. Outside of this exception, normally-aspirated engines will be limited to a maximum manifold pressure of atmospheric.
On the other hand, turbocharged engines can take great advantage of this parameter. In this case, manifold pressure can be increased significantly above atmospheric. This is done by increasing boost pressure. For example, setting your boost pressure to 15 psi (measured at the manifold) results in a 2X increase in manifold pressure compared to atmospheric. This would result in a 2X increase in mass airflow when compared with a normally-aspirated engine, all other things equal. In reality, the increase in mass airflow would be somewhat less due to heating of the air as it is compressed by the turbo. Even so, this demonstrates how a small-displacement engine can really benefit from a turbocharger.
Increase Engine Displacement
Generally, this is one of the primary advantages of the "muscle" cars - there is still a place for good old-fashioned displacement. Displacement, along with manifold pressure, determine how large of a "gulp" of air the engine can consume for each rotation of the crankshaft. For a given manifold pressure, a larger displacement engine will ingest a larger gulp of air, all other things equal. The result is higher mass airflow for a larger displacement engine at the same manifold pressure.
Increase Volumetric Efficiency (VE)
Volumetric efficiency quantifies how much of your engine's total available displacement is actually used. For example, a volumetric efficiency of 95% in the case of a 2.0L displacement engine implies that you are really only utilizing 2.0L*95% = 1.9L. Therefore, your "effective" displacement is only 1.9 liters. Your "effective" displacement is all that matters in terms of actual airflow. Hence, you want VE to be as close as possible to 100% (the theoretical limit.) One issue that complicates matters is that VE is not a constant for a given engine - it varies as a function of RPM. One of the most common methods of maximizing VE is to install a set of performance cams, which affect the timing of the opening and closing of the valves to give the air more time to enter/exit the cylinders. Performance cams can allow the engine to breathe better, especially at higher RPM's.
Increase Maximum Engine RPM
The previous three parameters determine how much air the engine ingests for each gulp (each rotation of the crankshaft.) Now, to maximize airflow, we want the engine to take-in as many gulps per second as possible - this is where engine RPM comes into play. If the engine takes-in twice as many gulps per second, this will result in a doubling of airflow, all other things equal. By increasing engine redline, an increase in airflow can be achieved at the same manifold pressure (assuming that a high VE can be maintained at the higher RPM's.) For example, at a constant boost pressure, a given engine will produce twice the airflow at 6000 RPM than at 3000 RPM, assuming a constant VE.
I like to use the following "equation" to remember how the above parameters determine mass airflow:
mass airflow (lbs/min) = (lbs of air per gulp)*(number of gulps per minute)
where (lbs of air per gulp) is determined primarily by the first three parameters and (number of gulps per minute) is determined by engine RPM.
Now that there is a high mass airflow into the engine, there must also be sufficient fuel flow capacity to "keep-up" with the airflow. After all, the engine produces power by combusting fuel - more mass airflow means that more fuel can be combusted in less time, resulting in more power. The flow capacity of the fuel system is determined primarily by the flow rating of the fuel injectors as well as the flow capacity of the fuel pump at the required fuel pressure. The fuel injectors only have a limited amount of time available to activate at each engine cycle and inject the required quantity of fuel - therefore high-flow rated injectors are needed. The fuel pump must also provide sufficient fuel flow to supply all of the injectors at their maximum duty-cycle.
One last item to keep in mind is that an increase in mass airflow into the engine means that there is also a need for the engine to expel exhaust gases at a higher rate as well. This highlights the importance of a free-flowing exhaust system from the exhaust valves to the tailpipe.
crank horsepower = mass airflow (lbs/min) * 10
For example, a mass airflow value of 31 lbs/min corresponds to a crank horsepower figure of approximately 310 crank hp. So the important question is how can I maximize mass airflow? There are four primary parameters that can be adjusted to maximize mass airflow:
- Increase intake manifold pressure
- Increase engine displacement
- Increase Volumetric Efficiency (VE)
- Increase maximum engine RPM
Increasing any of the above or a combination of the above parameters will result in an increase in mass airflow. Each parameter will now be discussed in further detail.
Increase Intake Manifold Pressure
On normally-aspirated engines (or turbocharged engines, for that matter), one way of increasing intake manifold pressure is by minimizing any restrictions to airflow into the cylinders. In the case of normally-aspirated engines, this pressure will always be less than atmospheric pressure (a partial vacuum) - unless you consider setups which use a "ram-air" effect. Outside of this exception, normally-aspirated engines will be limited to a maximum manifold pressure of atmospheric.
On the other hand, turbocharged engines can take great advantage of this parameter. In this case, manifold pressure can be increased significantly above atmospheric. This is done by increasing boost pressure. For example, setting your boost pressure to 15 psi (measured at the manifold) results in a 2X increase in manifold pressure compared to atmospheric. This would result in a 2X increase in mass airflow when compared with a normally-aspirated engine, all other things equal. In reality, the increase in mass airflow would be somewhat less due to heating of the air as it is compressed by the turbo. Even so, this demonstrates how a small-displacement engine can really benefit from a turbocharger.
Increase Engine Displacement
Generally, this is one of the primary advantages of the "muscle" cars - there is still a place for good old-fashioned displacement. Displacement, along with manifold pressure, determine how large of a "gulp" of air the engine can consume for each rotation of the crankshaft. For a given manifold pressure, a larger displacement engine will ingest a larger gulp of air, all other things equal. The result is higher mass airflow for a larger displacement engine at the same manifold pressure.
Increase Volumetric Efficiency (VE)
Volumetric efficiency quantifies how much of your engine's total available displacement is actually used. For example, a volumetric efficiency of 95% in the case of a 2.0L displacement engine implies that you are really only utilizing 2.0L*95% = 1.9L. Therefore, your "effective" displacement is only 1.9 liters. Your "effective" displacement is all that matters in terms of actual airflow. Hence, you want VE to be as close as possible to 100% (the theoretical limit.) One issue that complicates matters is that VE is not a constant for a given engine - it varies as a function of RPM. One of the most common methods of maximizing VE is to install a set of performance cams, which affect the timing of the opening and closing of the valves to give the air more time to enter/exit the cylinders. Performance cams can allow the engine to breathe better, especially at higher RPM's.
Increase Maximum Engine RPM
The previous three parameters determine how much air the engine ingests for each gulp (each rotation of the crankshaft.) Now, to maximize airflow, we want the engine to take-in as many gulps per second as possible - this is where engine RPM comes into play. If the engine takes-in twice as many gulps per second, this will result in a doubling of airflow, all other things equal. By increasing engine redline, an increase in airflow can be achieved at the same manifold pressure (assuming that a high VE can be maintained at the higher RPM's.) For example, at a constant boost pressure, a given engine will produce twice the airflow at 6000 RPM than at 3000 RPM, assuming a constant VE.
I like to use the following "equation" to remember how the above parameters determine mass airflow:
mass airflow (lbs/min) = (lbs of air per gulp)*(number of gulps per minute)
where (lbs of air per gulp) is determined primarily by the first three parameters and (number of gulps per minute) is determined by engine RPM.
Now that there is a high mass airflow into the engine, there must also be sufficient fuel flow capacity to "keep-up" with the airflow. After all, the engine produces power by combusting fuel - more mass airflow means that more fuel can be combusted in less time, resulting in more power. The flow capacity of the fuel system is determined primarily by the flow rating of the fuel injectors as well as the flow capacity of the fuel pump at the required fuel pressure. The fuel injectors only have a limited amount of time available to activate at each engine cycle and inject the required quantity of fuel - therefore high-flow rated injectors are needed. The fuel pump must also provide sufficient fuel flow to supply all of the injectors at their maximum duty-cycle.
One last item to keep in mind is that an increase in mass airflow into the engine means that there is also a need for the engine to expel exhaust gases at a higher rate as well. This highlights the importance of a free-flowing exhaust system from the exhaust valves to the tailpipe.
