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Remembered seeing in different places, that Throttle position (as 0-100%) and Air flow aren't linearly dependent on each other, which is true. Because of this is isn't recommended to create TPS based LOAD axis with equally spaced intervals, as in such way you would loose resolution in regions, where small changes in TPS give large change in airflow, and gain unnecessary resolution in regions where change is less meaningful. But it would probably make sense to base LOAD axis on actual airflow. So I decided to calculate formula, which lets you convert desired relative airflow (%) into TPS value. Of course this still isn't perfect, as it isn't really airflow, but just throttle opening area. Actual airflow will be affected by opening area, as smaller area means faster velocity and so on. But still this could be useful.
The end results are:
Relative "Airflow" = 1 - cos( T * 0.5 * Pi ) , T = throttle position, from 0 to 1.
Throttle relative opening = ( 2/Pi ) * arccos( 1 - Xr ) , Xr = Relative "Airflow", from 0 to 1.
Lovely work, nice to see someone thinking about a problem laterally. You should consider that a boundary layer would reduce the effective diameter of the intake (rule of thumb is around 20% but this is highly dependant on gas speed and Reynolds number and you could look in to applying Darcy-Weisbach principle, Bernoulli's theorem, the Moody chart and Poiseuilles equation to solve for relative airflow etc.
With regards to TPS mapping ypou are correct, we tend to group the breakpoints around the low to part throttle openings and space them out from 50-60% TPS.