[quote=Trendz]Air speed goes down with boost. Air speed is highest with less density. Intake to exhaust pressure ratios are what you base overlap on. Duration is defined by operating range(rpm)
I agree obviously less dense air at the same pressure would have higher airspeed (fluid is "thinner").
But in the case of boost I don't really see it. I mean even a simple example of blowing out birthday candles. Purse your lips and blow softly doesn't get much airspeed, but push hard with your lungs/diaphragm and obviously the airspeed picks up. Bigger pressure differential = more air speed. Density and airspeed are not inversly proportional. In turbos it's multiplied together to get mass flow right? Density x speed = mass air flow rate.
I will use your car as my example...
Where is the pressure differential? Your exhaust pressure is higher than your intake pressure, so the combustion chamber never "sees" this blowing effect as your simplified example suggests. A turbo merely changes charge density. There is no "positive" change in air speed with increased boost pressure.
Density and air speed are not
linearly proportional, but, given a fixed area of flow, have direct effects on each other. The more contact surface/flow area, the greater the effect.
Your mass flow equation is leaving out quite a few more details as it relates to what I was describing. I believe you are equating compressor map math to port speed. It's not that simple. Compressor maps are basically telling you the efficiency of the wheel as a centrifuge(speed in rpm) as it relates to inlet mass.
Yes, a bit off topic, but not really.
And I whole heartedly agree with you in your advice to O.P. Much better off cash wise spending the budget on the turbo than on heads.
