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.
Back on topic:
What about this quote?
Some may question why you would want to spend the cash for porting when the supercharger can just use pressure to feed the same amount of air into the engine. The simple answer is that the amount of air moved by a supercharger is usually measured in pounds per square inch (psi) of boost. But this boost number is actually an indication of resistance to flow. By reducing the restriction to the inlet side by porting the heads, this immediately reduces the restriction to flow. This increases the volume of air the engine can ingest and power improves while at the same time reducing the actual boost in the manifold. Plus, reducing the boost demands on the supercharger also reduces the amount of work the supercharger has to produce, which reduces heat. The net effect is that moving 500 pounds per hour (lb/hr) of air at 10 psi requires less work than moving the same 500 lb/hr at 18 psi. The net result is more power.
https://www.google.com/amp/www.hotrod.com/articles/blown-gen-iii-hemi-centrifugal-wind/amp/ The quote above is absolutely true. Like I mentioned before... when I had home ported 308 iron heads on mine vs. the CNC ported EDDY's, it took LESS boost with the eddy's to make the same power. That totally supports what was written above.... and if you were trying to optimize a world record holder on a blower/turbo limited class it would make sense.
In the case of your undersized little blower... the head will matter. No doubt. If you're interested in getting every last hp out of it... the porting will count.
But, In the case of a properly sized blower/turbo... there is no reason to spend money on the head.
I'll share some real world results on how boost is so much more powerful of a tool then anything. My buddy ran my car the last time we had it out after I had made some passes on a test and tune night. I had run the car hard at 23psi of boost. He hadn't had any time in the car so we started him off slow. Turned the turbo down to ~7psi so he could get used to driving/shifting the push button.
The difference between 7psi and 23psi was 12.40@110 to 9.55@142mph Same night, same track, same head, same cam, just add boost... for free.
My point is... if you took the money you're spending on porting the heads, and put it toward the turbo... you're going to be WAY further ahead bang for the buck. Scrap that little blower project, and just put the proper turbo on it from the start (or even proper sized blower... either one would work)... You'll be WAY happier with the results for the money, rather then spending it on porting.
