Paxton-McCulloch Supercharger History - Part III
Paxton / McCulloch Supercharger History – Part III
The McCulloch-Paxton VR57
Another product that Paxton Products designed during 1955, and developed during 1956 was the variable ratio supercharger. This was developed to provide a general-purpose supercharger that was more compact, and easier to install than the VS57, and also had greater boost and airflow handling capabilities. The resultant supercharger, which is commonly known as the McCulloch-Paxton VR57 differed from the VS57 superchargers in that instead of the planetary ball drive being a fixed ratio, and the drive pulley being a variable ratio, the planetary ball drive had a variable ratio, and the drive pulley had a fixed ratio. Another more significant difference, which no doubt benefited from Paxton’s involvement with the Novi, was the use of engine oil for lubrication and cooling.
The variable ratio planetary drive was achieved using split inner and outer ball races that varied in separation according to the input shaft speed. At low engine and input shaft speeds the inner ball races are pressed together with springs, forcing the balls between the races to ride higher and forcing the outer ball races apart, giving an impeller ratio of 3.5:1. At higher engine speeds, engine oil pressure forces the outer races together via a piston, forcing the balls into a tighter circle and giving a ratio of 5.5 to 1. This ratio varies between 3.5:1 and 5.5:1 against engine speed due to engine oil pressure gradually overcoming the spring pressure against the inner ball races. A supercharger drive pulley ratio of between 1.7:1 and 2.0:1 allowed the supercharger impeller to be driven at ratios of up to 11.0:1, which, as for the VS57 units would allow impeller speeds in excess of their rated 32,000 RPM maximum. To limit the impeller to it’s maximum, boost output from the supercharger was used to reduce the oil supply to the split races, allowing the planetary drive to drop back into low ratio as engine speed increased. Interestingly the impeller is the only component that is common to both VR and VS superchargers.
As with the VS57 the VR57 could be forced into high ratio, and in this case this was achieved using a throttle activated valve, which diverts the oil supply direct to the piston acting on the outer ball races, thus increasing oil pressure on the outer race. This was also regulated by the increasing boost pressure as the boost output of the supercharger increased.
The use of the variable planetary drive arrangement in the VR57 eliminated the need for the variable ratio pulley and tensioned idler arm set up used in the VS57. In the case of the VR57 a conventional supercharger drive pulley could be used in conjunction with an adjustable idler pulley for applying tension to the drive belt. This elimination of the idler arm and variable ratio drive pulley allowed the VR57 to be slimmer in profile than the VS57 unit, although it’s diameter was slightly larger, in order to accommodate the larger drive balls and races required to give the variable ratio planetary drive. This slimmer profile reduced the installation requirements of the supercharger, and allowed for more compact installations, minimizing the need for fan blades and radiator re-positioning. The use of a standard drive pulley on the supercharger input shaft also provided the added bonus that standard ½” drive belts could be used in conjunction with standard crankshaft and idler pulleys, allowing for an easier installation by the home mechanic when bought as an aftermarket kit, and again offering minimized clearance problems.
Ford, at that time was looking for a means of increasing the performance of their racing engines as a part of their long running NASCAR battle with Chevrolet. With the Rochester Fuel Injection looming on the horizon Fords prospects looked weaker for 1957 and they were looking at all options. The decision to use superchargers was made late in 1956, and the decision to use McCulloch superchargers was probably due to the fact that Ford did not yet have a system of their own, and the fact that McCulloch superchargers were well proven in the field. Former Indy driver Peter DePaulo is often credited as being the person who suggested McCulloch superchargers to Ford in late 1956 however Ford was certainly already aware of McCulloch at that time, indeed one of the rumors that came from Detroit in 1955, and was reported in the Sept 1955 Motor Life magazine, was that the 1956 Ford Thunderbird would appear with either fuel injection or a McCulloch supercharger. McCulloch themselves reported in 1955 that Benson Ford had driven a McCulloch powered Thunderbird and was very impressed, and also reported in early 1956 that more than 50% of the VS57 supercharger installations were to Ford Thunderbirds. Regardless, at some time in 1956 Ford were shown the direct oiled VR57, which would have been at, or near, the end of it’s development life, and ready for production, and were sufficiently impressed that they wisely engaged McCulloch’s Paxton Products for an exclusive one year contract for the VR57.
The 1957 homologation rules for NASCAR stipulated that only factory “stock” engines could be raced during 1957. Factory “stock” was defined as being advertised as a regular production option, and also required a minimum of 100 powerplants being manufactured prior to the start of the race season, January 1, 1957. To meet these homologation requirements 100-125 VR57 units were delivered to Ford in December 1956, as a result of intense effort at Paxton. These were Phase I units, i.e. pre-production units, which differed from the later Phase II units. The Phase I blowers were sensitive, and suffered with unreliable control valves, although their performance was reputed to be a lot higher than the later Phase II blowers. They were also apparently tedious to work with, however they were never meant for use by the public, so this was tolerated. The most obvious difference with the Phase I is in the design of the case, which lacked the ribs of the later unit and used a large clamp instead of screws to hold the two parts together. Ford produced a total of 12 “D” coded 1957 Thunderbirds in January 1957, and an unknown number of 1957 passenger and convertible cars using these first phase I VR57 units, which incidentally had serial numbers beginning with the designation VR57A, or Ford experimental serial numbers.
With the homologation requirements met, further supercharged Ford production was not scheduled until later in 1957 when the Phase II VR57, a which produced nearly the same boost levels as the slightly larger Phase I units, were used. These were nearly as potent as the earlier Phase I units, and proved to be a lot easier to maintain and repair. The actual number of supercharged Fords produced in the Phase II production runs, are unknown, although between 208 and 211 of this production is believed to have been in the form of Thunderbirds, and a similar number (maybe as many as 300) were produced as Fords. All of the Phase II VR57 equipped Ford engines were designated as F code engines, and power outputs were 325 hp @ 4,800 rpm or 340 hp @ 5,300 rpm dependant upon the installed cam (256 or 290 degree duration), although Ford conservatively claimed only 300 hp. Interestingly Paxton Products claimed that the output was actually 360 hp with the hotter cam.
The VR57 equipped Fords basically dominated motorsport during early 1957, and if it weren’t for the NASCAR ban on the use of superchargers (and fuel injection) from racing in April 1957, would probably have continued dominating for the rest of the season. A supercharged Ford even held the NHRA National title for three years. However, as happens with all good things, the use of superchargers was banned from NASCAR racing in April 1957. Ford was the only company out of the big three who honored the AMA ban on high performance in 1957. This was a gentleman’s agreement that member companies would disband all involvement in racing and eliminate all references to horsepower from advertising.
The one year agreement with Paxton Products for the VR57 ended in late September 1957, and on 1st November 1957 Paxton publicly announced that the VR57 would be available for sale, with the serial number prefixed VR-57B. These would retail without an installation kit for $295, or with an installation kit for $495, intended for 352 CID 1958 Ford and 361 CID 1958 Edsel installations only. In actual fact the installation was the same for both engines, with only the scroll rotation being different between them. Paxton also announced that they did not intend to produce the VR57 to back-fit 1957 or earlier automobiles, with the possible exception of a few isolated models such as the Chrysler 300C and the 1957 Corvette. Instead they would continue offering the VS57 unit for general use. The VR57 actually had a greater air flow capacity than the VS57 units so speculation is that Paxton reserved the VR units for the newer, larger V8 units being produced, which would have otherwise exceeded the capabilities of the VS57 units.
Paxton actually had a policy of using existing components during installation in order to minimize cost. So many VR installations by Paxton used VS57 idlers, pulleys and brackets, and the supercharger drive belts could either be the standard McCulloch 7/8 inch belts, or in some cases 1/2 inch belts. As for the VS57A units intended for Thunderbirds, Paxton stamped a letter at the end of the serial number indicating the intended application if it wasn’t the standard Ford installation (i.e. the letters ‘CH’ for a Chevrolet installation). Additionally the VR57B serial numbers did not continue on from the VR57A serial numbers, instead starting all over at 1.
Paxton Products produced the VR57 superchargers in limited numbers through the rest of 1957 and into early 1958. Why production was limited is not known, although the supercharger requirements of Studebaker and Packard, coupled with strong aftermarket sales of the VS57 units may well have heavily utilized Paxton’s production capacity. Servicing problems also existed with the VR57 units. Because Paxton did not generate sufficient service information for these superchargers, from mid-1958 and on all VR superchargers had to be returned to Paxton for any warranty claims or rework.
The Paxton VR58
Paxton introduced further improvements for the VR units during 1958. These improvements included higher quality German-made inner and outer races, Swiss-made steel balls, upgraded multiplier springs, and improved rear oil seal ring and the addition of an air deflector shield to deflect the boost output from the rear oil seal. The new VR units had serial numbers which started with the prefix VR58 and were supplied by Paxton with a 4,000 mile, or 90 day warranty, which was valid only if the unit was not operated in excess of 5500 rpm with a standard 7.7 inch diameter crank pulley, and if the unit was not modified to generate more than five psi of boost. As a reminder of this warranty the VR58 units were tagged with a warning that tampering with the boost pressure would invalidate the warranty. An optional oil filter, which greatly improved the superchargers life, was made available at the time of the VR58 introduction, basically as a safeguard against contaminated oil entering the supercharger. The VR58 was offered for the 348 CID 1958 Chevrolet, all 1955-58 Corvettes, the 1957-58 Chrysler 300, and the 1958 Fords, Edsels and Thunderbirds. Paxton fully intended that the VR58 supercharger would be available for all 1959 models however the continued popularity of the VS57, due to it’s higher boost levels at low rpm and the development of its replacement, the short lived DO-VS59, stopped all further development.
Click here for Paxton-McCulloch Supercharger History - Part IV
Last edited by Michael; 09-01-2009 at 08:50 PM.