I've been putting off writing this for a couple of reasons: Supercharging can be complex and I intend to write extensively about it on the information site. We will only touch on centrifugal supercharging here and leave Roots-type, turbocharging, and other schemes out of it. Also, I think that turbocharging an engine of less than three cylinders is a waste of effort that could be better applied elsewhere. Lastly, real superchargers are expensive and don't give the payback that they should unless you modify a lot more. The units that ATI (my source for superchargers) suggests for engines of the Virago's stock output only yield about a hundred or so horsepower. To merit the use of their more robust units involves building the engine to some extent. I wouldn't consider running less than a B1 unit as cost-effective.
Centrifugal superchargers will be understood from here out unless I need to reference a different type. Centrifugal because the only heat they gain is self-generated and their lubriction is self-contained. Sadly (?), they are precision pieces of equipment and the prices reflect that. Also, the margin is razor-thin. ATI expects its dealers to make their money through engineering, installation, and dyno-tuning.
As one can see, the top-of-the-line for NA (naurally aspirated) 920s just barely merits a B1. At maximum output a B1 takes about twenty five horsepower to turn. Before I get too far afield: I've been told several times that superchargers like around 112° LBA (lobe center separation), which repesents a lot of over-lap. This suggests that a lot of unburned mixture simply gets pushed out the exhaust when on boost. And that the unboosted bike is a handful to ride already. Boost is regulated by a "sneeze" valve which keeps the engine unboosted most of the time. It must be triggered by some event: The throttle suddenly whacked full open; the revs passing a magic number; something. The rest of the time the valve vents anything over 1 bar (one atmosphere) out into the open air and the bike runs like there is no supercharger present. It is a breathe-through system - easy for EFI; tricky for carbs.
It would take a 1200 plus motor with a trick ignition to merit our favorite: the C1-R. At maximum output it requires forty horsepower to spin it. All boosted engines must run some kind of an inter-cooler and all of ATI's are air-to-air. Most of these are side mounted and that is considered good enough. Personally, I would prefer an air-to-water setup with at least one fan. Compression heats the air above ambient and an inter-cooler brings it back down. There are even dry ice-acetone systems that bring the air temp well below ambient. Lots of mixture gets fed into the cylinder(s).
One must tap power off the engine some where for the lower pulley: carefully seal off the right crank end and run an extension, or machine a longer nut to run off of the rotor on the already wide left side. The blower itself can be mounted in front low enough to avoid obstructing the air flow to the top-end. Thence ductwork to the inter-cooler and out to the cavity between the heads. On Gen I bikes this could go straigh into the frame. Don't forget to filter it.
This represents one blower, at least two pulleys, a belt, shrouding for it, a "sneeze" valve, ductwork coming and going, an inter-cooler, and modifications to the air-box and carbs - or a switchover to EFI. Let's not forget the internal work to meet the preboost requirements.
As of this writing a bare B1 runs $1850; the C1-R $2900. Polishing $250; Black Powdercoat $350. Full kits including some castings run over six thousand for Harleys, Victories, and Indians.
