Automobiles Forum banner

Flow Calculator


This is really not a "turbo" calculator, although it does have features that allow you to select a properly sized compressor if you plan on turbocharging a motor; it is really an intake tract simulator, performing thermodynamic analysis of the intake flow and approximating how much torque can be expected given some numbers that characterize the other efficiencies of the motor.


Environmental Parameters   Engine Parameters
Inlet air temperature°C   Boremm
Ambient IC temperature°C   Strokemm
Altitudefeet ASL   Cylinders 
Pressure% SL   Displacementcc
Pressurepsi   DisplacementCID
Inlet air densitylb/ft3   Displacementlb/cycle
Turbo/Supercharger Characteristics
Compressor efficiency%   Manifold boost psig
Temp gain TC°C   Temp out TC°C
Intercooler efficiency%   Pressure droppsi
Temp drop IC°C   Temp out IC°C
Water Injection
Water injector(2)   Methanol mix%
Water injection ratecc/min   Injected water temp°C
Temp drop WI°C   Temp out WI°C
Vaporized volumecc   Air:Liquid Ratio:1
Engine Variables
RPM    Air:Fuel Ratio:1
Volumetric Efficiency%   Specific Fuel Consumptionlb/HP/hr
Pressure ratio (3)    Density ratio 
CFM flowft3/min   HP (5)HP
LBM air flow (4)lb/min   Torqueft-lb
LBM fuel flow lb/min   BMEPpsi
Injector Sizing
Max Injector Duty Cycle%   Injector size (6)cc/min

Plot on turbo map:

Turbo Map


  1. This calculator is equally applicable to all motors, naturally aspirated (NA), turbocharged and supercharged. For an NA motor, just set the boost to zero and set the IC parameters to zero. Supercharged motors will have a higher SFC than turbo motors due to parasitic losses in driving the compressor, but are otherwise the same. Any boosted or non-boosted motor can have an intercooler or water injection, try it.

    You need to make educated guesses on three significant numbers for this calculator:

    • Volumetric Efficiency (VE)
    • Air Fuel Ratio (AFR)
    • Specific Fuel Consumption (SFC)

    If you have grossly bad values for any of these three quantities, then the estimated results will be correspondingly bad.

    Other values such as turbo and IC efficiency are not nearly so sensitive (try a test with 50% and 100%, then look at the difference in HP values.

  2. The water injection system modeled here allows you to select the location for the injector: before the compressor inlet, between the compressor and the IC, and after the IC. (With a little bit of thought you will see why when you have no intercooler, the injector should be first; when you do have an intercooler the injector should be last; in no cases does it make sense to inject between the turbo and the intercooler, try it. Here's a hint: if you inject before the intercooler and cool the charge below ambient, the intercooler will heat the charge back up).

    The model assumes dry air (0% relative humidity) and complete vaporization of the injected water/methanol stream, and hence grossly incorrect results will be produced if you attempt to inject large quantities of liquid (or if you even inject small quantities on a very humid day). If you see the post-injection temperature ("Temp out WI") dropping well below the ambient temperature, then you are pretty surely in the realm of incomplete vaporization and the results are wrong.

    Note that only the thermodynamic effects of the injected liquid on charge temperature are considered here; all combustion-related effects are ignored (it is assumed that you will tune the SFC value correctly to compensate).

  3. Use the value in the "Pressure Ratio" output field as the Y-axis (vertical) coordinate on a pressure map. Pressure ratio is measured at the turbo outlet, and is the sum of the ambient pressure, manifold boost and the pressure drop across the intercooler. Most texts compute this value without regard for this last term, which for some installations can be quite large.

  4. Most pressure maps use the LBM flow value on the X-axis (horizontal) coordinate, but some use the CFM flow value. LBM is much more accurate because it takes into account density ratio, whereas CFM varies grossly with temperature.

  5. Hey, these torque and HP numbers are ESTIMATES based on your own faulty data! Do not quote these values to your local law enforcement officer or on some internet newsgroup, they are not real numbers. Remember that you are merely guessing at the VE, SFC, AFR and cetera, so unless you really know what you are doing, they are probably off by ±20% or more.

  6. Injector size is computed directly from the SFC, AFR and maximum injector duty cycle values that you enter. If you have accurately estimated these values, then the injector size should be very close to correct. Note that injector size is estimated for the displayed configuration, so you should record the setting only when you have a maximum HP estimate shown. Note also that this is the actual injector size you should use. Some tables show injector sizes in terms of 100% duty cycle, beware that you will need larger injectors than these tables show!

  7. When you click the "Save" button, all of the parameters that are shown in fields are placed in the location bar of your browser. You can then copy the link into an e-mail, or save it and come back to that same configuration later. You can often shorten the link line up by deleting parameters that are not useful, or have default values.