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Originally Posted by Fizz
I've looked around online but still having difficulty understanding the difference between twin screw and roots. They both seem so similar....? Can someone educate me please?
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Well let's start with "centrifugal" vs "positive displacement":
"Positive displacement" superchargers: have moving parts that physically push the air into the engine. Spinning parts grab a volume of air and move it into an engine.
"Centrifugal" superchargers: have a turbine compressor with vanes (looks sort of like a pinwheel). The idea here is it pulls air into the center of the 'pinwheel', and as it spins faster it 'throws' the air toward the outer edges of the 'pinwheel', like a centrifuge. It compresses the air as it does this, and forces this air into the engine - but there are no physical moving parts that are physically pushing air into the engine, it's all from the pressure generated by the compressor.
Roots and twin screw are both positive displacement superchargers - roots has two spinning lobes which grabs air and just pushes it into the motor, there's no compression happening inside the blower itself (there is compression in the manifold). Twinscrew has two spiral 'screw' (for lack of a better word) parts inside that grab air at one end and push it along into the motor. The difference here is as the screws push the air along, it also compresses it inside the supercharger.
Since both roots and twin screw physically push fixed volumes of air per rotation, they are very linear vs rpm, which means they produce power across the RPM band (read power increases down low and up high). When twin screw supercharges first came out, they typically were much more efficient than the roots type superchargers. In other words, they heated up the air less - with combustion (and limited octane) heat is the enemy. Nowadays, roots superchargers have been refined greatly with advanced impeller designs and materials - the TVS units are really nice, and pretty much rival twin screws on efficiency.
That said, neither positive displacement unit is as efficient as a centrifugal compressor. A centrifuge increases it's effect exponentially in proportion to the compressor speed. So if you have a centrifugal supercharger driven proportionally to engine RPM, the amount of air forced into the engine will increase exponentially as RPMs increase. This essentially means that the compressor's effect will me most noticeable in the upper RPM band, and not as noticable in the lower RPM band (relative to the positive displacement units). A well matched centrifugal supercharger will likely produce greater 'peak' HP at redline than a positive displacement unit (greater efficiency, less heat, allows greater max manifold pressure, engine timing, etc.) - but the advantages over a well matched positive displacement unit will only be in the very upper RPM band (last couple thousand RPM). Good for a race car that stays in the upper band, but not as good for everyday street driving compared to a positive displacement unit (caveat: not to say centrifugal SCs are bad for street driving, and some people prefer the 'racy' nature of centrifugal SCs).