Ha-ha Greybeard...
You make some very valid points with which I cannot disagree, but let me add a few extra ideas of my own to it.
Firstly, moving the throttle upstream of a supercharger has a several effects. Firstly, (hypothetically) without using an air bypass, the supercharger would be acting as a high powered vacuum pump working very hard against an almost closed throttle most of the time. The supercharger would certainly not be unloaded under those conditions.
But as you quite rightly point out, an air bypass will unload the supercharger when under vacuum, particularly a screw supercharger when configured that way with an up stream throttle.
But an up stream throttle is sometimes not a very desirable solution for another completely different reason.
The further the throttle is placed away from the engine, the more stored air volume there is in the pipework that has to fill and empty at varying pressure with every throttle movement. This can make throttle response noticeably worse. While it will probably work fine if the supercharger is bolted directly onto the inlet manifold, it will work less well on an inline engine with the necessarily long induction pipework between the combined throttle/supercharger, and the inlet manifold. Forget all about using any sort of intercooler with an up stream throttle, the car would be totally undrivable. Been there done that <grin>.
And any high boost supercharger project absolutely must run an intercooler, or you are just not serious.
A down stream throttle is far more desirable for both throttle response and most often for ease of installation. And you can run a massive intercooler with fairly long pipework without any significant degradation of throttle response. So, down stream throttle should always be first choice if it can be done that way, but it may not always be possible.
All of this depends so very much on the type of engine and type of supercharger, and is it going to be intercooled ? and generally, what you are trying to achieve and under what constraints. There are a lot of interrelated details that need to be very carefully thought through to suit each particular installation. There is no "best way" to go about this.
Now the question about volumetric efficiency arises. While I agree that a screw supercharger approaches very closely to the theoretical ideal of the perfect positive displacement supercharger, it is still not perfect, and even less so when mounted onto a real engine.
Heat soak, and the resulting inlet air density drops from hot duct work into the supercharger can reduce top end mass airflow more than many people realize. Heat is the greater enemy than actual measured pressure drop as far as supercharger inlet air density goes. One percent air density drop for each three degrees Celsius of additional temperature rise is the number.
Once the air density going into a positive displacement supercharger falls, nothing can get it back. If relative air density drops to 90%, (only 30C rise) then your 100% volumetric efficiency supercharger becomes 90% real volumetric efficiency.
When the whole installation is being cooked with 85 Celsius air from the radiator, and toasted by the exhaust system, feeding cold dense air into the supercharger is far from easy. A cold air intake going through scorching hot under bonnet pipework to the supercharger intake is only a partial solution to this problem.
The advantage that an intercooled turbo has over a supercharger is that boost pressure sensing right at the engine ensures that as the flow and density losses all mount up, the turbo just works a bit harder to keep the intercooled air density up to the engine. Flow or density losses into a turbo do not matter quite so much. The wastegate just closes a bit, and the turbo spins faster.
A supercharger can never compensate for density losses at it's inlet in quite the same way as a turbo can. The bottom line really is, that it is always a lot easier to get high top end airflow and horsepower with a turbo than it is with a supercharger on a practical road car that suffers from typical high under bonnet heat problems.
Not saying it cannot be done, but on an everyday road vehicle, a wastegated and intercooled suitably large turbo will beat a supercharger at the extreme top end every time. It may be worse in every other respect (lag, boost threshold etc..), but for sheer top end airflow and power the intercooled turbo is king.
You can build a supercharged engine without these thermal problems by sticking the supercharger up out through the bonnet, and using a bug catcher type air intake. So theoretically it is not a problem. But on a road legal car it is never quite so simple.
We can argue theory round and round in circles, but really an Opcon is a superb supercharger, and I would certainly never try to discourage anyone from fitting one.
But twincharging has it's place too, especially for the more budget conscious do it yourself adventurous types.
More and more people are becoming interested in twincharging, and I have yet to hear of anyone that has completed a twincharge project and not been absolutely delighted with the results. Many of these guys, like myself have been turbocharging and supercharging engines for decades, and wish to try something a bit different.
