The SC boost controller would make use of the recirculating valve that is already part of the system.
The recirculating (or bypass) valve is a butterfly valve that blocks a tube from the intake manifold (on the pressure side of the SC) to the intake snout (suction side of the SC).
This valve is there on all the whipple / Autorotor / SOS setups, and it's purpose is to be opened under VACUUM, to reduce the parasitic drag of the SC when the car is under part throttle.
There is a vacuum dashpot connected to an arm that actuates the valve when the intake snout sees vacuum, which happens when the throttle plate is mostly closed during part throttle / cruising.
When this valve opens, it allows air to be sucked directly from the intake snout to the manifold, effectively bypassing the SC. By doing this, the SC just spins (and does move air) but does not generate any pressure and consequently does not require nearly the power to spin it. Think of pumping a bicycle pump when it's not actually inflating a tire - it still takes some effort to move air, but not nearly as much as when you are compressing it.
When the throttle is opened, the vacuum disappears in the intake snout, and the bypass valve closes, so all of the air entering the engine goes through the SC, which, due to it's swept volume / driven speed delivers more air than the motor can use, resulting in compression of the air in the intake manifold (boost) as more air is physically forced into the manifold than goes through the intake valves.
Spinning the SC faster (smaller pulley) will result in more air supplied to the motor at any given rpm, resulting in more pressure at any given rpm (and consequently more heat).
The boost controller concept makes use of the same bypass valve that ordinarily allows air to flow from the intake snout to the manifold.
By opening this valve under pressure, it creates in effect a massive boost leak when compressed air in the manifold is allowed to go back into the intake snout for another trip through the SC.
The resultant bleeding of pressurized air lowers the intake manifold pressure (boost), so essentially by controlling the bleedoff of this air, you can control the maximum boost pressure, but it can only be lower than the amount of boost that the pulley / SC combo will make with no air bypassed (eg, if your SC produces 9psi of boost, you can't use a boost controller to get more than 9 psi)
To open the valve under boost (as the supplied bypass valve is designed to open under pressure), you either need to have a vacuum supply handy to admit to the bypass valve (think brake booster for instance), or use a different bypass valve actuator that has a port on both sides of the diaphragm, one of which will open the valve under vacuum (which you would use just like the original one) while the other port would open the valve under boost. (Whipple sells a dual port actuator that bolts right in place of their single port one supplied with most of these kits fwiw).
- C
