Fran,

No worries, you are correct, this is based on choked flow at the nozzle throat. That is why I said it is the maximum possible. I'm not suggesting that the engine will generate the pressure drop necessary (1/2 an atmosphere) to get there. Ron just wanted a back of the envelope type calculation, and this easily sets the upper limit of what is possible in terms of airflow. HP scales linearly with airflow, so I thought it would be good info to have.

As for flow "reversion", if the orifice is attached to the inlet of the plenum and there are more than 4 cylinders there is negligible pressure (and therefore velocity) fluctuations at the orifice. If there are 4 cylinders or less, then there is a significant pressure oscillation in the plenum. This would mean (based on the choked flow assumption) that velocity would oscillate between sonic and something less than sonic, so the mean flow rate would be something less than 1025 SCFM, but we already know that number is not realistic.

So Ron,

How much HP is possible? That is a more difficult question as pumping losses become a significant factor with restricted engines. We can estimate it, but it would take a few more pages of calculations. We would need to assume some maximum manifold vacuum, estimate the losses of the orifice to find the flow rate, estimate the gross indicated power, and then subtract the pumping losses to get a net indicated number. It can be done but the efforts will be lost on most people. How far do you want to go with it?