Throttle body location (sc vs tc)

Why is it I see the throttle body before the compressor on an eaton supercharger but most commonly after the compressor on a turbocharged engine?

Quick Google says that throttle response comes into play but i cant help but think if that was all there was to it then they would ALL be post compressor (the idea being the further from the engine the tb is, the slower the response)

What would the plusses and minuses be if I moved my throttle body from the supercharger to the plenum of my intake (post compressor and intercooler)?

Thanks!

Tb before supercharger allows modulation of boost. I think. Blow off valve does it for turbo.

A roots blower is always spinning, so it will always create boost if there is air to act on. With the throttle before the supercharger, its basically spinning in a vacuum when the throttle is not open, so almost no wasted effort. A turbo is not creating boost until the exhaust stream is increased by opening the throttle. They also generally include something to prevent the column of air from hammering the throttle plate when it closes.

and with the Blower mounted on the intake no place to mount one. I was going to use a blower in front of the engine like a Vortec type blowing into a Carb but was told by a blower EXPERT Don Hampton that for my deal I should just use Nitrous, Burst my Bubble!

I think the two biggest reasons are packaging (many Rootes blowers are mounted right to the manifold) and that positive displacement blowers react even worse to having a downstream throttle snapped closed - you end up with a lot more air to vent. I have seen Rootes blowers with an upstream and a downstream throttle to partially get around this.

Positive displacement superchargers (they aren't compressors, except for a few screw-type units Not Relevant Here) move air when they are spinning. If you have a closed throttle downstream of it, you'll bend the throttle plates. So you have the throttle upstream of the blower, or you have a goofy mechanically linked bypass valve like VW used on the G60.

Centrifugal compressors aren't positive displacement. You can completely shut off the airflow downstream of them and the world doesn't end. You can try that with a hairdryer, the fan speeds UP because it isn't moving air anymore.

MadScientistMatt wrote: I think the two biggest reasons are packaging (many Rootes blowers are mounted right to the manifold)

This part is sort of backwards -- one of the main reasons people go to the effort of building roots blowers into an intake manifold is because they're forced to put the throttle plate before the blower, and in order to minimize the throttled volume they need a custom intake manifold.

But yeah, this is all yet another reason why turbos are inherently superior to belt-driven superchargers. :-)

Knurled wrote: Centrifugal compressors aren't positive displacement. You can completely shut off the airflow downstream of them and the world doesn't end. You can try that with a hairdryer, the fan speeds UP because it isn't moving air anymore.

Excellent example

Thanks everyone, I appreciate the info