mcollinstn

Recap:

GM two-stroke diesel scavenging blowers were indeed rated from what # of cylinders and displacement of the cylinders from the motor the blower came from.

Current aftermarket blowers are rated at the cubic inches of air at atmospheric pressure that the blower displaces in one revolution.

Multi-speed blowers have been used in aircraft to normalize air density at altitudes (running them at high speed caused fuel mix and detonation issues, not to mention that the high-powered fighter and interceptor planes had enough prop torque on takeoff to make it really tough to keep from torquing the plane into a "donut" going down the runway and destroying it - more boost would make it even worse). Two speed blowers were used on some Mercedes road cars of the 1930's as well...
(kick in a boat? not really, since we all run our single speed blowers at the edge of detonation anyhow - another speed would turn pistons into objects of art)

Okay, now, who can tell us off the top of their head whether the rotors in a roots blower rotate "out" or "in" on a typical installation? And who can tell us WHY? (yes, I know the answer).

Cord

Rotates out. Because the air is trapped in the groove between the rotor and the case.

Iggy

On a "Roots" type the rotors rotate out.
The air is drawn in from the top and passes along the outer casing before discharging in the bottom center. The incoming air will also help keep the casing cool.
The rotors have low pitch spiral in them so there is constant contact (mesh) between the two rotors as they meet in the center and to help even out the discharge pulses.
A "screw" type (Whipple) is like having two large diameter screws with opposite threads in constant mesh. The long pitch of the rotors delivers a more constant flow of air with little pulsation.

Offshore Addiction

so now you guys know how hard it was getting my boy back to doing his homework!

Crazyhorse

It wasn't uncommon to see the pilots of The Merlin-equipped Mustangs pulling 63-67 inches of manifold pressure on take-off, but the power was applied in stages to help deal with the P-factor, the highest power settings being applied after the tail was off the ground andf positive rudder authority was gained. These engines could be run (and often were!) at power seetings as high as 71 inches mainfold pressure, what the pilot's operating handbook called "wartime emergency" power setting.
And yes, aircraft use superchargers and turbo-superchargers to allow them to make rated power at high altitudes, but not to make more than that.

MKast

Taken from engine builder at Reno Air Races

"Before we came to Reno we'd never had the engine over 60 inches. We ran it at 3050 rpm and 80 inches of boost there, still in low blower, the difference between a Merlin and a Griffon, a Merlin in high blower at low altitude, like Reno, all it does is make heat, it doesn't make any more boost at all and that's why nobody uses it, but a Griffon's boost is geared different in the blowers. At Reno altitude, in high blower, I think we could run at 90-95 inches of manifold pressure, which would have given us about 300 more horsepower.."

Cord

How does inches compare to psi?

MKast

At sea level, standard day, pressure is 29.92 in. hg. which is 14.7 psi.

Cord

So 65" of boost is equal to 32 pounds of boost. Pretty good for a gasoline engine.

Iggy

You have to remember that those Rolls Merlin engines were over 1700 ci in displacement.
It's not like you're trying to stuff 32 lbs of boost into a 454.
1700 ci divided by 12 cylinders = 141.67 ci/cyl.