Non A*E* Roots Numbers

[Jim Davies]

On Sun, 20 Jun 1999, Robert Harris wrote:

> 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.

> 
> 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.   

For those who may be historically inclined, the turbo vs supercharger
debate has been going on for over 50 years. I dont think there is anything
new to bring to the debate and the reason the debate still exists is
because the bottom line is this: its six of one and half a dozen of the
other. 

> 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.

Historically speaking, Lancia can hardly claim authorship of this [not
that they are...] Detroit Diesel uses this idea on many -53, -71 and -92
series diesels and its use was so common in WW2 American aircraft engines 
that is too long a list to include here. Some basic aircraft types of WW2
that used a supercharger [single speed or two-speed] as a second stage
that was fed by a turbocharger first stage: B-29, B-17, B-24, P-38, P-47
etc.

Lest you wonder why old tech is relevent today; this stuff ran very high
boost pressures because the gasoline used allowed it.