Iíll go through the major parts first, starting with the obvious. The descriptions may seem rather simplistic, and/or insulting and patronizing in places, please donít take offence as what is obvious to some people may not be obvious to everyone. Hopefully there should be sufficient information here for you to be able to build up the supercharger system, using modern replacement parts if necessary.
Well covered elsewhere on this web site, this forms the heart of the system and is basically an engine driven air pump designed to boost the air supply into the carburetion system at all engine speeds in excess of 1500 rpm in response to driver demand.
Bonnets force the boost from the supercharger directly into the air horn of the carburetor. This presents two basic problems, which have to be addressed before a bonnet can be used successfully. Firstly, the boost pressure from the supercharger tends to be higher than the fuel pressure of the fuel pumped into the carburetor air horn, i.e. at high boost the fuel is discouraged from entering the carburetor air horn thus causing starvation. Modifications can be made to the carburetor and fuel pump to overcome this, and these will be discussed separately. Secondly, many carburetors have external venting (Holleyís are a good example) which can seriously reduce the boost pressure. In fact the boost pressure can also be reduced through leakage from bad sealing at throttle bearings. To enable a bonnet to be used this also has to be addressed, and again this will be covered later.
The bonnet replaces the air cleaner on the carburetor inlet and is generally connected directly to the McCulloch supercharger output port via use of a large bore pipe. The bonnet is required to have an air tight seal against the carburetor, to prevent the loss of boost pressure, which is generally achieved via use of a rubber gasket in the interface. The pipe connection to the supercharger is also required to be air tight, with normal pipe clamps being used to achieve this.
A brace of bonnets
The pictures show some variations of the many different types of carburetor bonnets produced by McCulloch. The first two are designed for the Holley ďTeapotĒ four barrels, the third for Carter and Rochester four barrels. Two barrel versions of these bonnets were not generally available, as it was possible to connect the pipe from the supercharger output directly to the carburetor air horn, using a pipe clamp to achieve an air tight seal.
The Holley bonnet is probably the commonest bonnet due to the number of Ford installations of VS57 systems. The earlier Holley bonnets have different designs to the ones shown and can be easily identified by the distinctive series of concentric circles stamped on their tops. The Holley bonnets all have valves on them which allow fuel vapor to vent, to alleviate hot start problems, when the engine is stopped. In itís most basic form the valve consists of a rubber flap covering holes in the top of the bonnet. When the engine is running, and some boost is being produced by the supercharger, the rubber flap is pressed against the holes by the air pressure, thus blocking the holes. When the engine is stopped, gravity clears the rubber flap from the holes, allowing vapor to escape. The Holley bonnets also have a nipple on their side, and this is to allow a pressurized air supply to be fed into the secondaries, balancing the air pressure there.
The Carter bonnet is quite basic in that it has no vent valve and no nipple for an air feed to the secondaries. The Rochester bonnets are, as previously stated, a modified form of the Carter bonnet. These have a vent valve, and they also have an extension tube (normally aluminum) to allow the bonnet dome to be bolted onto the carburetor.
Some variants of the above bonnets came with additional nipples to allow pressure gauges to be connected to them, and these can still be found. Itís not too difficult to add a nipple, extension pipe, or a vent valve to a bonnet, and worth considering if you cannot get the exact bonnet you need. Although Iíve never tried it, it probably isnít too difficult to make your own bonnet.
Multiple bonnets can be used for multi-carburetion set ups, providing there is enough clearance, and these tend to be connected to the supercharger outlet via Y pipes or similar. Itís probably worth looking at the photo pages, if you are interested in multi-carburetion set ups, as they show a few variations in the way that the bonnets have been hooked up to the supercharger.
I personally donít like air boxes as I think they look ugly, but they do however eliminate one of the problems caused by the use of carburetor bonnets. Air boxes surround the whole carburetor, and as a result, donít cause any problems with outside venting of carburetors due to the pressure being the same around the carburetor, as it is inside the carburetor. The fact that the carburetor has the same pressure internally as it does externally means that it does not require any modification. Air boxes also have the significant advantage of making multiple-carburetion installations easier, as only one pipe connection is required to the supercharger, all carburetors will receive equal boost, and no carburetor modifications have to be made. The use of an air box still requires modification to the fuel pressure supplied to the carburetor however, as the same problems with the fuel pressure being less than the boost pressure apply.
McCulloch air boxes
Airboxes were initially produced in 1956 and were used extensively in the í57 and í58 Studebaker/Packard factory installations. The air box is composed of two parts, the mounting plate, and the box. The mounting plate is designed to be bolted directly onto the carburetor mounting flange on the intake manifold using the existing carburetor mounting studs. The carburetor then bolts onto the mounting plate using the existing studs. Gasketing may be required, and the standard carburetor gaskets can be used.
The fuel supply to the carburetor is normally routed through the air box mounting plate. This can be either through a fuel block screwed onto the plate, or via a grommet. In both cases an air tight seal must be achieved otherwise a drop in boost pressure will be achieved. The throttle linkage to the carburetor is also routed through the mounting plate, this again must go through a grommet and achieve an air tight seal. The same also applies to any vacuum pipe connections to the carburetor, and/or choke wiring. Any pipe work within the air box must be rigid enough to withstand the boost pressure, otherwise the fuel supply of vacuum supply might be cut off. How air tight sealing of the throttle linkage, in conjunction with the re-routing of the throttle linkage through the mounting plate, was achieved, I havenít got a clue, as Iíve never seen an air box in the flesh. Although I imagine that graphite impregnated neoprene seals were used, along with a bit of ingenuity. The box of the air box bolts directly onto the mounting plate via studs set into the mounting plate, thus encasing the carburetor. The box may contain vent valves and/or nipples depending upon the application.
The mounting bracket, unfortunately, is very application specific, and as a result is different for each vehicle that the McCulloch Supercharger is mounted on. It primarily serves as a support for the supercharger, as well as placing the supercharger in the correct position, geometrically, with respect to the crank pulley and idler pulley. As with all belt driven rotating assemblies it is critical that all the pulleys align exactly with each other, otherwise the lateral loading can cause damage to bearings as well as damage to the drive belt.
Chrysler and Ford Thunderbird Brackets and Idlers
The idler pulley, is mounted on a spring loaded shaft to provide tension on the drive belt, and the idler pulley shaft is mounted on the supercharger mounting bracket, with suitable mounting points being cast into the mounting bracket to ensure correct alignment and belt tension. A second fixed (i.e. non sprung) pulley is required for some installations, notably Cadillac and Chrysler Group vehicles, presumably to allow the drive belt to clear some part of the host engine. Idler arms vary between applications, and the correct idler arm must be used for a specific application, otherwise the belt tension will be incorrect, causing incorrect operation of the supercharger.
The belt tension is critical to the correct operation of the supercharger. If the belt tension is too high, then the supercharger may be driven at the high ratio continuously. This is because the boost driven air piston in the supercharger may not have sufficient force to counteract the belt tension, and force the input shaft flanges together. This sounds great, but in practice means that the supercharger will eventually overheat, because at high engine rpm (e.g. 4500 rpm), when the supercharger is supposed to be in the low ratio, producing an impellor speed of 25700 rpm (1 x 1.3 x 4.4), the supercharger will be in high ratio producing an impellor speed of 45,500 rpm (1 x 2.3 x 4.4). This is in excess of the design speed of 30,000 rpm for sustained operation, and will cause excessive heat build up, over a period of time, due to the contact friction at the planetary drive ball interface. The ATF lubrication system, which also acts as a coolant, is not capable of sinking the heat generated by sustained high rpm operation and will eventually break down, resulting in failure of the supercharger.
If the belt tension is too low, the idler arm will never overcome the force exerted by the air piston on the sliding flange, and consequently the flanges will always be together, driving the supercharger at a low ratio. This causes no problems for supercharger, as it will behave like a typical centrifugal supercharger producing low boost at low engine rpm, and high boost at high rpm, however the advantages of the variable ratio drive will be lost, and high boost will not be available on demand.
To complicate matters McCulloch produced air piston springs with two different spring rates, 45 pounds and 65 pounds. These were designed to be used in conjunction with matched idler arm springs. As a rule the early VS57 units had 45 pound springs, and later units (1957 upwards) had 65 pound springs, with the springs being interchangable between all units. If you find that the idler arm has insufficent tension to be able to counteract the air piston spring, when the supercharger is at idle, although the idler arm can be moved manually to counteract the air piston, then it may mean that you have a 45 pound spring with a later idler arm.
Supplementary components are often required to enable the mounting bracket and idler pulley set up to be bolted onto the engine. Cooling hoses and fuel lines may have to be rerouted, brackets and housing assemblies may have to be replaced using McCulloch components, or a spacer may be required on the fan and crank pulleys. In many cases the mounting bracket is bolted onto the engine using one or two bolts due to limited access, and a series of braces is used to attach the bracket to other parts of the engine, producing a rigid, but not very elegant, mount.
56 Lincoln, Chrysler & Chevy Pulleys
The crank pulley, which is nominally 71/2 inches in diameter for all applications to ensure the correct drive ratio for the supercharger input shaft pulley, is also very application specific. This has to be bolted onto the existing crankshaft pulley, or in some cases a supplied replacement crankshaft pulley, and must be set up to align exactly with the supercharger drive pulley and the idler pulleys. Some McCulloch crank pulleys were produced with auxiliary grooves for power accessories, such as power steering or air conditioning, and special mounting brackets were used in these applications when relocation of the relevant pumps was required. Others were designed to bolt on to the accessory pulleys that already were provided with the vehicles. Although different standards were used by the manufacturers for mounting drive pulleys to the crankshaft, it is sometimes possible to port a pulley designed for one vehicle to another vehicle. It may involve judicious grinding, re-drilling and re-balancing, but it is possible, although I do not recommend it unless it is very straight forward, as the end result might be a damaged pulley that could have been traded with some one else for the correct pulley.
A vacuum switch, controlled by manifold vacuum, was used on the McCulloch supercharger kit installations for switching the supercharger into high boost when demanded, and for energizing the secondary fuel pump to increase the fuel pressure when high boost was being produced. The switch is wired between the primary binding post of the coil and the control solenoid housed beneath the badge on the supercharger. A second wire is run to the electric fuel pump from the post of the switch that is wired to the control solenoid. The Kaiser and Studebaker VS57 installations used a mechanical switch attached to the carburetor enclosure base, and actuated by the throttle linkage under kickdown or high throttle demand conditions, to achieve the same results. This generally switches in the control solenoid and fuel pump when the throttle has achieved seven eighths of itís full travel range.
The vacuum switch is simple in operation, and works on the principle that the engine normally has a partial vacuum in the manifold at all times, except at full throttle.The switch has a diaphragm connected to a contact breaker, which when closed causes energisation of the supercharger control solenoid, energisation of the electric fuel pump,and a shift of the supercharger into high boost, The normal partial vacuum within the manifold is sufficient to pull back the diaphragm within the switch, where it stays in position until a positive pressure is applied, thus breaking the contact and electrical supply to the solenoid and fuel pump, and keeping the supercharger in low boost. Under full throttle conditions the manifold vacuum is replaced by a positive pressure (which is why your vacuum driven windscreen wipers stop!), which closes the switch contact, energises the solenoid and puts the supercharger into high boost.
Itís fairly easy to test a vacuum switch to see if it works, just wire it into a test circuit (bulb and battery) and suck/blow through the vacuum connection. I've measured a vacuum switch to see what the operating parameters are, and found that the contacts open (low boost setting) when 4 inches of vacuum is obtained, and the contacts close (high boost setting) when a positive pressure of 1 psi is obtained. It is possible to find modern vacuum switches that operate within these parameters, although I have yet to find one that is capable of carrying the current required to energise the solenoid. If one of these was used it could be used to switch a relay, which could then be used to switch the electrical supply to the solenoid and fuel pump.
A separate switch, in the passenger compartment, can be wired up to enable the supercharger to be switched in manually, allowing the supercharger to be switched to high boost earlier than normal, although over enthusiastic usage can cause the blower to ďburnĒ out due to sustained operation at high boost. Not to be recommended really.
The basic rule with regards to the fuel supply to the carburetor is that the fuel pressure should always be at least a couple of pounds higher than the pressure within the carburetor, otherwise the fuel is not allowed to get into the carburetor. Although this requirement seems fairly straight forward, it is complicated by the fact that the McCulloch superchargers were capable of producing a high boost at a relatively low engine RPM (3500 RPM), and consequently although a mechanical fuel pump can be modified to produce a higher fuel pressure, itís output pressure is always dictated by the engine RPM, and when the supercharger goes into high boost at low RPM, insufficient pressure may be available, resulting in engine shut down at best, or leaning out at worst. If the fuel pump was modified to produce sufficient pressure for high boost conditions, then when the supercharger is in low boost the higher pressure output would result in flooding.
The factory installed McCulloch systems of Kaiser, Studebaker and Ford were provided with fuel pumps designed to produce the correct pressure at the various boost levels, with increased boost compensated for by feeding the boost output into the pump, so if you have one of these, no problem, however if you donít then the fuel pump system will have to be modified using the techniques adopted by McCulloch on their kit installations. McCulloch used a two stage solution to the fuel requirements of the supercharged engine. The first involved modifying the mechanical fuel pump to produce sufficient fuel pressure when the supercharger was producing low boost, and the second was to plumb an electric fuel pump (sometimes two depending upon the application requirements) in series with the mechanical fuel pump which was activated when the supercharger went into high boost. The electric fuel pump activation was controlled by the manifold vacuum switch (or mechanical switch on the throttle lever) which was also used to switch the supercharger into high boost.
Mechanical fuel pumps, on the whole, operate using a cam driven lever to pull back a spring loaded diaphragm which sucks in fuel as the cam drives the lever. When the cam releases the lever, spring pressure forces the diaphragm forward, thus pumping the fuel at the pressure dictated by the spring rate. Replacing this spring with a stronger one increases the fuel pressure, but this tends to be a fixed increase over the RPM range of the pump. To allow the fuel pressure to increase in response to the boost pressure (at low boost), McCulloch modified the fuel pump so that the cavity behind the diaphragm which contains the spring, is pressurized with boost from the supercharger, and this in conjunction with a stronger spring is sufficient for low boost requirements. This involves reinforcing the oil seal in the pump where the lever enters, tapping the atmospheric vent from the spring cavity, screwing in a nipple, and running an air line from the supercharger discharge throat to the nipple tapped into the spring cavity. Sometimes, dependent upon the application, the vent has to be sealed and a new hole drilled and tapped for the nipple. The boost pressure being fed behind the diaphragm via the nipple is added to that of the spring pressure, and causes the fuel pressure to increase proportionately with the boost pressure. Under minimal boost conditions the spring pressure alone generates the fuel pressure.
The electric pump is used to supplement this pressure under high boost conditions. The pump used has to be a flow through design ,allowing the fuel from the mechanical fuel pump to flow through without significant loss of pressure when it is not activated. When the electric fuel pump is activated it boosts the fuel pressure supplied by the mechanical pump to the level required when the supercharger is under full boost.
McCulloch originally supplied an electric fuel pump (or two for larger displacement engines), a variable resistor (to allow the electric pump output to be adjusted), as well as the seals, nipple, spring and hose required to modify the mechanical fuel pump, in their installation kits. It is extremely unlikely that you will be able to obtain any of these parts, as they were rarely removed from an engine when the supercharger was removed, however modern parts can be successfully used instead, and they are likely to be more reliable than the original parts due to technological improvements. Nowadays we have access to electric fuel pumps which can be easily set to variable pressures, and/or are capable of higher pressures, so theoretically we shouldnít have too many problems with that element of the fuel system. The only rules that apply when installing the electric fuel pump, are that it needs to be installed in-between the carburetor and the mechanical fuel pump, so that it assists the pressure supplied by the mechanical pump, rather than itís output being limited by the mechanical pump, and that when mounting the electric pump consideration must be given to potential vaporization problems if it is mounted too close the exhaust manifold (which invariably is the only place you can mount it with the supercharger in place). Shielding and/or insulation of the fuel lines may be required to prevent vaporization if you are forced to mount the electric pump near the manifold.
Typical fuel pressure requirements are 6 to 12 pounds when the supercharger is in high boost, and 3.5 to 7 pounds under low boost. Actually setting up the fuel pumps, both mechanical and electrical, so that the correct fuel pressure is achieved under the various engine load conditions is very much a trial and error exercise, involving swapping springs on the mechanical pump, and adjusting the pressure generated by the electric pump until the correct pressures are achieved. The procedure for this is covered in the installation pages of this site.