Thanks for the response.

A second way of playing with efficiency would be to play with the compression ratio if I'm following this idea right. I suspect that would be done by using a slightly larger hole in the mounting plate.

I notice that your model showed a square hole where Adrian used a round hole in front of the driver. I would expect a 4 1/2 inch square to have a lot more surface than a 4 1/2 inch circle which would affect compression ratios.

Still playing.

Grant.