NOTE: The following is original research and may contain errors. If you spot any errors in any of the equations, spreadsheets or graphs, please contact me and I will correct them as soon as possible.
Introduction:
This thread is meant to explain some of the science behind turbocharger selection. To make the information contained herein more applicable to the 8thCivic community, the K20Z3 engine is used for all examples. It is my that this thread, with contributions from the community, will become a reference for Si owners who are considering turbocharging.
Calculation:
Several processes are detailed online for analyzing compressor maps and selecting the proper turbo for your application. The system used here takes a slightly different approach to maximize the range of applications addressed and to allow for changes in elevation. Living in Colorado, the effects of altitude have a significant impact on the data. All of the spreadsheets and charts included will have two versions: one for an altitude of 5280 ft and one for sea level.
You will also notice that all calculations are done in SI units and then converted into imperial units as needed.
Step 1: Volumetric Displacement
The first step is to calculate the approximate airflow through the engine as a function of RPM. In theory, a four stroke engine (also known as an "Otto Cycle" engine) will move a volume of air equal to its total displacement every two complete revolutions. In reality, airflow restrictions and other sources of inefficiency prevent an engine from flowing all of the air it is theoretically capable of. The amount of air that an engine will actually flow is dictated by its Volumetric Efficiency. To get an exact value for Volumetric Efficiency requires testing an engine on a flow bench. For our purposes, we are going to assume that the K20Z3 has a high Volumetric Efficiency of 90%.
Step 2: Air Density
Calculating the density of air the plenum is where we can account for changes in elevation. It is also where we could go way overboard on the math, accounting for water vapor, compressor efficiency, intercooler efficiency, etc. To keep things simple, but maintain accuracy, we assume a constant intake temperature for an intercooled system of 338 K (~ 130 F) and that the incoming air has no water vapor. (Anyone who has been to Colorado knows this isn't far from the truth
). The intake pressure is equal to the atmospheric pressure adjusted for altitude minus 6.8 kPa (~ 1 psi) to account for restrictions in the intake and air filter. Our variable in this equation is the Compressor Pressure Ratio, the value commonly seen on the y-axis of compressor flow maps.
Step 3: Mass Flow
Step three is a fairly basic application of volume and density to find mass. We take our Volumetric Flow from step 1 and multiply it by the Air Density from step 3. The result is our mass flow rate for our engine at a certain Compressor Pressure Ratio and RPM.
Step 4: Power
While not necessary for analysis of compressor maps, having an estimate of your engines power output under boost conditions is always handy. To calculate power output we are going to use a value known as Brake Specific Fuel Consumption (BSFC). BSFC is a measure of how much fuel an engine consumes to create a certain amount of power. Lower values of BSFC indicate a more efficient engine because less fuel is needed to create the same amount of hp. To get an accurate estimate of the BSFC for the K20Z3 I used the known hp number published on the Honda website (197 hp @ 7800 rpm). Amazingly, the result was 218 g/kW*hr. The average four stroke gasoline engine has a BSFC of 322 g/kW*hr! Having a BSFC as low as 218 is usually only found in diesel engines!
As a side note, using the data from the above calculations matched perfectly with AJP's dyno results when adjusted for sea level.
