[Bicycle Bob]

If you have a bad muffler, you can hear wasted pressure bursting past the exhaust valves whenever you reduce manifold vacuum substantially. You have to start the compression stroke well below atmospheric pressure to wind up with a heated charge not much above it. Any audible bang is an inefficiency alarm, however glorious they may sound in rapidly rising succession.

I think that the easiest way to test this is with an Eaton electric roots supercharger. It has about the right pressure differential range, but might want some high-vac oil. It could be rigged to go from idle vacuum right into boost if you wanted to, either producing or drawing power. However, doing that would make a variable compression ratio even more desirable. At high manifold vacuum, we should have a very high compression ratio to take full advantage of the charge not detonating. It might be best to never run even wide-open, and build the engine lighter. Such an engine would have a power density more suitable for air cooling, and I hear that 3-D printers make much better fins than casting.