I think the amount of boost an engine will "take" is going to be dictated by things like compression, chamber design, type of fuel, cam, tune, timing, etc...all after the fact of whatever the source of forced induction is. I think those types you mentioned are more efficient and will make more power for the same amount of boost (I think mainly because of less heat generated under boost) but I think the amount of boost (cylinder pressure) will be the same...but I could be wrong...haven't really built and tested enough different blower deals to know. I'm building a (non marine) Vortec blown 540 BB Chev right now that will be on alky, MFI/blow through and should make around 2500. I'm also building a twin turbo 511 BB Chev EFI pump gas deal so when we get this on the dyno I'll remember to take notes on all this.

My point was, lets say you have a roots combo making 1200hp at the flywheel, and a turbo making 1200hp at the flywheel. The roots will probably have quite a bit more parasitic loss , and making more cylinder pressure to net the same power at the flywheel. Not sure if im making sense the way im explaining it lol

I dont know much about BMEP except its a calculated number? I will say this about BSFC/vs load/vs boost/vs hp/vs what makes the boost. I am making these numbers for sake of discussion but they are supported by some past testing I have done/read about, they just will be exaggerated a little. Heres the example: we build (2) 1200 hp blower motors, motor A has the worst example of a roots supercharger on it, motor B has whatever centrifugal that is most efficient. The roots motor has a BSFC of .625 (typical in real life), the centrifugal motor has a bsfc of .520 (have observed this personally a few months ago), REMEMBER, this is for sake of discussion, not to start a fight about. So both motors are giving us 1200 hp at crankshaft, BUT motor A is using 750 lbs per hr of fuel, motor B is using 624 lbs per hr of fuel. For sake of discussion I will over simplify this too, boost equals extra hp, this motor B in our example makes 650 hp on its own, we give it 14.7 lbs of boost the hp doubles minus parasitic losses so roughly motor now makes 1300 but takes 100 hp to turn supercharger so we see 1200 at the crank. So the cylinders are making 162.5 hp and using that amount of fuel, shedding that many btus, cylinder pressure goes with this accordingly, i'm sure at some point the BMEP could be calculated from this. Motor A also makes 650 hp on its own, and 1200 crank hp boosted but is using 20% more fuel to do it, which in turn means each cylinder to do it has to make 195 hp, equals extra btus, more load per cylinder, more detonation etc. Now we could start arguing about latent cooling from the fuel going thru the blower rotors and a whole lot of other things, this is just a example, not complete fact. In reality though, for motor A to make that same hp as motor B it would need to run MORE boost to make that extra hp per cylinder to get us our same crank hp so at same boost motor A would make LESS crankshaft HP, same boost, probably same cylinder pressure prob same amount of detonation, at same hp level however, more prone to detonation, FWIW, Smitty

Makes sense to me. ^^^

In that regard, yes absolutely. Artic explained it pretty well but I think the discussion would have to go in the direction of WHY the centrifugal makes more power with less boost. It's not 100% parasitic drag, i's also efficiency in the compression of the air/fuel. Compare a Whipple or PSI to a traditional roots and you'll see what I mean. The physical demand to turn the rotors is equal for all discussion purposes and doesn't take much HP at all but if you look at the charge efficiency of both, you can see where they drastically differ.

In my example though a old school roots supercharger uses signifigantly more hp to turn, I recently "dynoed" a procharger at less than 75 hp parasitic loss to make 14 psi of static boost at 5800 engine rpms, hence the reason why you can drive a centrifugal with a 8 or 12 rib serpentine and old school roots blowers need a much larger drive belt, they would shred/slip a 8 rib unmercifully. There are DEFINATELY other issues with pumping losses between rotors, heat generated if sized small and turned up high too on roots but overall the main reason bsfcs are so much higher on it takes more hp to physically turn the roots, fwiw, Smitty

I've heard this argument for years and it just isn't true. Think about it...how much mass is there really in a pair of aluminum rotors? None to speak of...certainly not what we see HP wise in the difference between types of blowers, especially when you start comparing the rpm those other blowers have to turn and the mechanical disadvantage they have. You take away the fact that a roots is technically a compressor...remove that load form the rotors and it doesn't take squat to turn them. You're not measuring parasitic loss due to the mechanics of the blowers, you're measuring parasitic loss in efficiency of how they treat the air. Do the same calculation on a turbo, where there is ZERO mechanical loss and you'll see what I mean.

Ive been running a 10 rib serpentine belt on my b&m 420s with no slipping issues, and the belt lasts quite a while. Mercs 600/ 800sc used the same setup, many guys put intercoolers on them and ran 8-10lbs with 10 rib, although some did the 16 rib swap.

I dont think the roots are as bad as people think as far as power to turn at low boost. Now, 35lbs of boost, they definitely do take some power.

The majority if marine and street roots blown setups, dont need a cog belt.

The other thing is, look at the pulley sizes. A roots might run a 5 inch rib pulley on top, and 5 or 6 inch rib on the bottom. Where a procharger runs what, a 8 or 10 inch crank pulley, with a tiny top pulley? I think that plays a part in pulley slippage issues with the centrifiguls due to their massive overdrive

I turn my centrifugal 58,000 rpms, its internally stepped up 4.4 to 1 and overdriven by the crank over 2to1 also, fwiw, smitty

Do you get decent belt life ? My buddys procharged setup was hard on belts . Like you, he was turning it pretty hard . That dam belt had to be like a guitar string or he lost a significant amount of boost.