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Drazy began his quest for a better supercharger based on the premise
that the Rootes blower was never designed to handle the task of
supercharging an engine with 35 to 40 pounds of boost pressure.
Historically, he was correct in that assumption. The Rootes used in
drag racing originally came from GM diesels.

They were designed by GM in the late '30s to blow fresh air -- at zero
boost -- into the combustion chambers of their two-stroke diesels to aid
in the exhausting. Early drag racers adapted stock GM, and later,
aftermarket GM Rootes blowers, for the purpose of supercharging, or
boosting cylinder volume above atmospheric pressure.

However, the Rootes was never designed to supercharge a drag
racing engine to 30-plus psi levels. It can supercharge an engine, but
not to the degree required in drag racing without the high cost paid in
overall operating efficiency. A Rootes maintains fairly good efficiency
up to 10 psi, but beyond that, its efficiency drastically drops off.

One of the problems with a Rootes is that it compresses air by back
flow. Air is taken in at the inlet port at the top of the Rootes at 14.5 psi
-- atmospheric pressure at sea level. It carries this 14.5-psi air in the
spaces between its lobes and along the outer cases until it reaches the
outlet port at the bottom, 180 degrees later. The air is deposited in the
intake manifold where it piles up under pressure. That is where the
problems begin.

First, the high-pressure plenum air continually wants to flow back into
the low-air-pressure rotor spaces. This force tries to turn the rotors,
and crankshaft, backwards. It takes a tremendous amount of power
from the engine to turn the rotors against this pressure.

When Drazy began working on his screw supercharger several years
ago, he built a supercharger dyno to measure, among other things,
how much horespower a Rootes consumed at a certain rpm and boost
pressure. He found that a 14-71 Rootes required 818 horsepower at
11,000 rpm and 35 pounds of boost -- that horsepower figure
represents between 30 and 40 percent of an alcohol engine's
horsepower output.

Second, back flow causes a tremendous amount of air heating. Moving
pressure waves of air between the plenum and the rotor spaces at the
rate of six cycles per revolution, according to Drazy, produces
typhoon-like conditions inside the intake manifold. When the air is
moved around to that degree, high temperatures are encountered due
to friction.

As a result, air density is decreased, which makes the supercharger's
job all the more difficult. (It was fitted to the engine to increase air
density in the first place.) To make up for the rapdily thinning air, the
Rootes has to be turned faster, which really takes a toll on efficiency.

Third, there is a significant pumping loss through the rotors as the
high-pressure air pushes its way up past the rotors and out the inlet
port.
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written by John Brasseaux
from Drag Racing Magazine
page 46-49; 52-53 - September, 1989

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urmas: Ma väitsin, et ülelaadimise meetodist olenemata tekib soojus, millest on vaja vabaneda.

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urmas: Pärtel, ma kahtlen su jutus. Rõhu tekitamisel vabanev soojus ei ole sõltuv rõhu tekitamise meetodist.

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Seegi väide on mööndusega, et konkreetsele mudelil pole kohta, kuhu vahejahutit lisada. Erandliku näitena vaata fordi 3.8L roots-kompressorit, kus on õhk-õhk vahejahuti olemas.

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