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How can we actually measure actual (real) engine air flow cfm

EFI Tuning Fundamentals

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How can we actually measure actual (real) engine air flow cfm On a dyno as the lesson states

For our purposes we generally wouldn't measure actual airflow. At a professional or OE level it's possible to equip an engine dyno with an airflow turbine to measure airflow in CFM but this isn't something most of us in the aftermarket industry will ever do. Of course if you have a MAF sensor on your engine this will measure airflow and you can log this in your ECU however it's important to also understand that the MAF sensor calibration relies on the entire intake track to be stock for the reading to be accurate.

I live in a rotary world a 13B in a FD has 40ci per rotor 2x = 80ci at 8500 RPM (there is one intake stroke per eshaft rotation) 3 face per rotor so if I got this right I have 6 face x40ci=240ci per engine period. 240ci x 8500rpm / 1728 = 1180.55 cfm theoretical CFM

So for VE effeciency I cannot calculate the effective % ? I can calculate the theoretical CFM but not my actual consumtion so this is a no go to find ou my actual % VE am I correct or is there a rule of thumb?

Maybe that can help you:

As for how to measure real air flow (actually it's mass flow), that's how it's done:


The rotary engine is a little unique but the same techniques are still applicable. I think the thing you have perhaps missed is that the rotors turn at 1/3 of the eccentric shaft speed so you aren't actually seeing 6 combustion events occur during 360 degrees of engine rotation - This actually takes 1080 degrees or 3 full rotations. If we compare a rotary engine's displacement to a piston engine then it's easiest to compare the two over 720 degrees of crank rotation as this is what a piston engine needs to complete a full engine cycle. The piston engine will displace its engine capacity over this time however in the rotary we would have 4 combustion events occur in 720 degrees of rotation and hence the engine would displace 160 ci or twice it's nominal capacity.

Rotary engines are often discussed as 2 stroke engines as each rotor will have one combustion event per 360 degrees where as each cylinder in a piston engine has a combustion event per 720 degrees rotation.

Does that help clear things up for you?

Sorry to bring this thread back from the dead, yet I have a question.

Say I'm tuning a VTwin NA Harley-Davidson engine.

Engine has the MAP sensor (and temperature sensor).

Bike is on a DynoJet bike dyno.

How can I measure the actual "Mass of Air"? From one of the lessons, I believe we have two options to measure mass of air:

- MAF (mass air flow) sensor

- Speed Density Principle (some math to help us get there)

In this Harley-Davidson with a MAP sensor, we would use the speed density principle... yet there my question: wouldn't the calculations give us 100% perfect VE?

Or is the process:

#1 calculate theoretical VE (displacement, rpm, temperature, pressure, etc)

#2 calculate actual VE with the MAP's output at the same rpm?

I'm trying to make the connection from theoretical VE to measured VE (without the engine room MAF sensor).

Thank you!

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