Non A*E* Roots Numbers

[Robert Harris]

Reference " A Do it Yourself Guide to Street Supercharging"
Pat Ganahl   SA-Design part 17-2  ISBN 51694 00017 3

Chapter 4 - page 33  - reprint of Detroit Deisel charts

Horsepower required to drive a 6-71 blower at various pressure levels and
amounts of boost - non anally extracted.  20 " HG ( approx 10 PSI )
GMC 6-71  Swept Volume 420 cubic inches per revolution

Lower effective limit - 1600 blower rpm - 15 bhp
Upper charted rpm   - 5200 blower rpm - 65 bhp

Volumetric Efficiency -  1600 blower rpm - aprox 20 %
Volumetric Efficiency -  5200 blower rpm - aprox 73%

GMC-6V-71 Swept Volume -334 cubic inches per revolution

Lower effective limit - 1600 blower rpm - 12 bhp
Upper charted rpm   - 5200 blower rpm - 52 bhp

Volumetric Efficiency -  1600 blower rpm - aprox 22 %
Volumetric Efficiency -  5200 blower rpm - aprox 72%

Temperature Increase with back pressure increase.

Ambient Temperature 75 F  Test of GMC 6-71
Partial restriction

2000 RPM    1PSI (manifold)  92f (manifold)
4000 RPM    5PSI                  98f
6000 RPM   12PSI                 200f
7200 RPM   15PSI                 230f

Repeated with no restriction ( all valves open )
2000 RPM    .5PSI                  80f
4000 RPM    2PSI                   82f
6000 RPM   3.5PSI                100f
8000 RPM   6.5PSI                 210f

All above rpm  *** BLOWER RPM ***

Since the HP and VE figures come from a Detroit Diesel chart published by
someone who is not selling any form of turbo wazoo sheeeet hot ting of the
week, I tend to trust them more than say  Corko Bell - we be turbos - figures.

Besides all the combined high performance sales of GMC superchargers probably
would not total up to a line item in Detroit Diesels petty cash account.

Another little tidbit - a roots type compressor is an external compression
device.  No compression takes place within the roots unit - its all done in
the backpressure of the manifold.  

Why use an ancient idea - invented and in use a hundred years before saint
otto and even before saint carnot for pumping air into mines ??  

First, its a positive displacement device.  Within its normal range of
operations, air gets pounded into the engine, unlike whirly things that sorta
maybe move air into the system - if it really wants to.  And the whirly things
are extremely rpm dependent and take a finite time to speed up.

The butt simple roots is more of an engine magnifier.  Take a largish amount
of cubic inches, multiply by roots drive ratio and you have monster stump
puller big block.   

And yes, after many thousands of dollars from the people who bring you 4" Fart
Pipes for Rice Rockets - you can get the same response as simply putting a
6-71 on it plus you can sneer at antique technology that you do not understand
and brag about how with massive defecation complication you achieved a power
band similar to a half a**ed roots.

Then for the truly insane who covet Lancia technology, you can take a couple
of whirly thingees and use them to pump into a roots.   Its a little known
factiod that roots blowers efficiency in all matters goes up with increasing
air density and with twin turbos densifying the air prior to the roots, the
Lancia got insane power and torque at any point of power curve.

And its not fair to compare an un-aftercooled roots to an extensively after
cooled whirly thing.   Roots systems respond as well or better to after
cooling.   Ps  you can't "inter" anything unless you have one before and one
after the "inter" - but I forget that technical stuff has been dumbed down to
comic book level to sell overpriced parts to the idiots and inter has more
marketing sauve than the truth.