Its about air compression and making the most pressure with the least HP draw. If I could (on my car) run a 4-71 and fill the volume requirments I would because the amount of energy required to spin the rotors would be far less. Santos ran some big numbers years ago after switching to a screw blower, not because it made more pressure, it was because it took 300hp less to run it. Compressing air inside the manifold is fighting the rotors and trying to spin them backwords. the extra lenght of the "8" is leverge you are fighting if you are not depleting the air volume in the manifold. If your theory was true, why not run a 10,12 or 14-71 and underdrive it---because you want pressure and underdriving it will only take longer to make pressure.
When running Alc as a fuel the intake charge is around 64*. Gas being burned as a vapor, the droplets can heat up very quickly and become detonaton sensitive and thus intercoolers and chillers under the blower densify the charge, Alc burns as a liquid and at 2.1 times the volume as gas does not heat up and there for not likely prone to detonation.
