Why Roots Blowers Suck (or blow... your pick)

Perhaps I'm biased against the in-bred V8 crowd (in my town), opinioned, or just well educated... but "Roots" type superchargers really bug me.  Allow me to explain...

First a little history.  The original 6-71 & 8-71 blowers that started the rage came off the V6 & V8 cylinder Detriot Diesel engines.  Believe it or not, these blowers were never used to supercharge the DD engines.  Most Detroit's are 2 strokes which means that in order to be efficient, they need to do two things: 1. Fill the cylinder with as much cold fresh air as possible and # 2.  Sweep out all the combustion gases after to maximize mass of air from #1.  For a variety of reasons, the engineers at GM decided it would be better to fit the engines with an external pump to help accomplish these goals.  This type of pump is typically known as a scavenge pump because it aids cylinder filling by pushing out the combustion by products instead of relying on residual pressure to blow them out.  Since the air just needs to be moved along adn not pressurized, the displacement of the pump was matched to the engine displacement & they were operated at a pressure ratio of 1.  This means that the pressure difference across the compressor was virtually zero.  Thermodynamics tells us that if you don't change the pressure or the volume, the temperature won't rise.  As such, the blowers designed for Detroit engines were never designed to actually work as a supercharger (pressure ratio > 1).  In fact, most of the Detroit engines were available with a turbocharger on top of the supercharger.  In this application, the turbocharger supplier all the "boost" and the blower spun along w/ a pressure ratio of 1.

Quick disclaimer before I rant on... I have now doubt that the first x-71 blower made it on a V8 because they were plentiful & cheap.  So, I really don't think that people that run roots blowers are dumb or otherwise comprimised upstairs.  It's a simple $$/hp ratio problem and I'm pretty sure that a roots blower beats them all hands down.

To be continued...