|... that's of course meant to say - 14.7|
Supercharger displacement: 45 c.i. ~ 737cc
Engine displacement of my TR1.1 engine: 1063cc = 531.5cc
Ratio: 1:1 = 1
There's quite a few reasons, why in real life you (most likely) won't see exactly this figure:
1) engine efficiency and blower-efficiency are blatantly ignored in this formula
2) blower efficiency varies with blower RPM
3) overall pressure has simply be assumed to be 14.7PSI and varies greatly with altitude
Assuming that we roughly hit the numbers calculated above, why can't we spin the blower faster and faster or why can't we simply use a much bigger blower. As with every mechanical machine, there's friction and all sorts of losses involved. Friction being one of the main contributors (among the work of compressing air. Luckily supercharger-manufacturers usually provide spec sheets, outlining where the "efficiency island" is in the pressure map and even more importantly where the redline for a supercharger is. This redline basically marks the point, where your blower turns into a mechanical hairdryer and only pumps hot air into the engine. (Incidentially on an Eaton M45 this is 16,000RPM, just to give you an idea!) On the other hand, a blower needs a certain minimal RPM to actually pump air efficiently, which is why you can't simply fit an Eaton M90 and then only run it on half engine speed and expect it to work somewhat efficiently. And as if this weren't enough, add the fact, that you have to apply some power to actually drive the supercharger, which can actually go up into the two-digit horsepower figures with said Eaton M90.
I hope you find this post (mildly) useful and I'd love to see/hear/read about at least one supercharger build one day that was sparked by this post outlining that it is a lot less of a dark art as some professional companies try to make you believe...