GMC thingee

[Robert Harris]

First, the reason for its love, survival and continued abuse is Daves Law
"Cheap is a virtue unto itself" - it really pisses the technogeek wannabe off
to have some dumb ass redneck with a blown big block embarrass his whiney
Farting Rice Rocket - specially when he paid 5 times as much for it.

Yes - properly done, the Whipple, the turbo etc can do a better job - but hey
not bad for a device that is well into its third century of use - first use
was in the mines pre 1800 pumping air.

Next, its a back pressure device - no internal compression.   The pressure you
read is the pressure above what the downstream system can consume - in other
words - a measure of the RESTRICTION.  The rotors merely trap a volume of air
and move it into the manifold - there is no compression in the Roots unit
itself.  And as such responds as well or better to air flow tuning and
enhancement as a turbo.

The low ve you see is due to wide rotor clearances.  GMC ran the rotors dry
with large clearances to swallow dirt without destruction - these were on
industrial units.  Raising the back pressure raises the resistance to the
movement of the air, thereby reducing the amount of air to be pumped. 

And GMC never used a Roots in the exhaust side.  Makes sense only to the
purveyors of fuel magnets and pogue carburetors.

Lowering the blower rpm increases the time for the air to bleed backwards past
the rotor tips.   Raising the rpm reduces this bleed back as does clearance
reduction and ultimately teflon tips to seal the tips.  

Adding liquid to the air prior to the rotors also greatly increases sealing.

Turbo and other devices still impose loss's on the system - you would be
amazed how much of a restrictive loss that turd up your tail pipe is.  Unless
you sell Pogue carbs - TANSTAAFL  ( There Ain't No Such Thing as a Free Lunch
- Heinlein ) applies and granted overall its less than the roots pumping loss.

But for fueling purposes, the roots tracks closely to a greatly increased
displacement NA engine.   Most OEM EFI stuff can be set right over the blower
and not even suspect its blown - if they can flow enough fuel to match the
air.  And the pulsing of a long duration cam is eliminated also - making it
easier.  And no sharply rising pressure curve to compensate for either.  The
ease of fueling was part of the reason that turbos did not make major inroads
until the 80's.