VS57 Limitations

The following graph shows the boost characteristics for a McCulloch VS57 supercharger.

The graph is actually very useful at illustrating the benefits that the variable ratio drive offers over conventional centrifugal and roots type superchargers. As can be seen the boost output from the VS57 ramps up very quickly to 5 psi when the supercharger is in high ratio where it is held at 5 psi by the governing actions of the solenoid, air piston and idler arm control system which is detailed in the VS57 Operation section of the site.

The supercharger only enters the high ratio when under high load conditions, i.e. under full throttle conditions, consequently for the majority of itís life the supercharger runs in low ratio where itís boost output is comparable with that of conventional centrifugal superchargers. I.e. when cruising at 3000 rpm a boost of 1.75 psi will be generated, thus allowing the engine to rum more efficiently under cruise. In reality the supercharger will often vary itís boost output between the values given by the high and low range graphs, as the road load varies under driving conditions.

The supercharger is designed to generate itís maximum boost at itís maximum safe impellor speed, 5 psi at 29,000 rpm. When the supercharger goes into high drive ratio (10 to 1) at 2,900 engine rpm the supercharger produces 5 psi. When 2,900 engine rpm is exceeded the solenoid, which is preset to 5 psi, bleeds the excess pressure and uses that to drive the air piston on the variable ratio drive, consequently reducing the drive ratio as the engine rpm increases. Obviously engine rpm can increase to a point where the supercharger is incapable of reducing itís drive ratio any further via use of the pressure bleed, which is when it reaches itís low ratio of  5.7 to 1. Given the 29,000 rpm impellor limit this is when the engine rpm reaches 5100 rpm. Any engine rpm increase above the 5100 rpm limit will result in increased boost above 5 psi as a result of increased impellor speeds above 29,000 rpm.

Boost versus Impellor Speed

Although itís a gross oversimplification, itís generally accepted that the pressure output from a supercharger varies as a square of the impellor tip speed. Using the graph of McCulloch boost pressure against engine rpm shown previously we can generate an equation of the relationship for the VS57 supercharger. Iíve used the high ratio curve and come up with the equation

Boost = 5.945E-9 * ( Impellor RPM) * (Impellor RPM)

We can confirm this works by feeding in RPM values and checking that the boost output agrees with the graph. Iíve included the confirmation values, along with values calculated for higher impellor rpm in the following table of impellor speed versus boost output.

Impellor Speed (rpm)                                    Boost Output (psi)

5000                                                                           0.15

10000                                                                         0.59

15000                                                                         1.34

20000                                                                         2.38

25000                                                                         3.72

29000                                                                         5.00

30000                                                                         5.35

32000                                                                         6.09

35000                                                                         7.28

40000                                                                         9.51

As can be seen the boost output ramps up considerably once we exceed 25,000 rpm impellor speed. It can also be seen that the VS57 could conceivably generate significant amounts of boost if we could increase the impellor speed safely. Iíll cover more about this later.

What dictates a safe Impellor Speed?

The 29,000 rpm impellor speed safe limit is actually a misnomer. The maximum safe impellor speed is actually a function of the power transmitted by the supercharger, which is based on the air flow requirements of the engine that the supercharger is hooked up to.

The airflow requirements of an engine is actually based upon the horsepower output of that engine. Typically a 100 hp engine requires 720 lbs per hour, a 200 hp engine requires 1440 lbs per hour and a 300 hp engine requires 2220 lbs per hour. The following graph, although designed to demonstrate the adiabatic efficiency of the McCulloch VS57, also shows us the power consumed by the supercharger, at 29,000 rpm impellor speeds, for the given engine air flows.

As can be seen for a 100 Hp engine, with a 720 lbs air flow requirement, the supercharger consumes almost 8 hp, and produces 4.8 psi boost. For a 200 Hp engine, with a 1440 lbs air flow requirement, the supercharger consumes almost 13 hp, and produces 5.2 psi boost. For a 300 Hp engine, with a 2220 lbs air flow requirement, the supercharger consumes almost 17 hp, and produces 4.2 psi boost. Why does the boost value vary for the 29,000 rpm impellor speed? Basically because the impellor shape and ducting can only be designed to reach maximum efficiency at only one air flow point, and any variation from that point will result in a drop off of the boost.

One of the novel features of the McCulloch VS57 supercharger (and indeed itís successors the Paxton SN superchargers), apart from itís variable ratio drive, is the planetary ball drive used to give a quiet ratio multiplication (4.4 to 1) of the superchargers input shaft speed. Like all ball drives there is a certain amount of slippage or skidding between the balls and the races that contain them. This slip increases with the amount of horsepower being transmitted by the planetary ball drive, and when that slip reaches 2% (when rubbing speed is two percent of the rolling speed) friction generated temperatures shoot up, the lubricating oil film breaks down, and the drive self destructs. For a typical VS57 supercharger this 2% slip occurs at about 17 hp power transmission, which as we have seen correlates to 4.2 psi boost on a 300 hp engine at 29,000 rpm. Hence 29,000 rpm is the maximum safe impellor speed for a 300 hp engine.

It should be noted that when considering 300 hp we are talking about 300 hp as a result of the Superchargers boost Ė and not prior to installing the supercharger. In reality if we tried to run a McCulloch set to 5 psi on a 300 hp engine the impellor speed would exceed the 29,000 rpm safe limit, due to the fact that the supercharger control system would not limit the impellor speed until 5 psi was reached. To prevent this from happening the solenoid regulator on the VS57 would have to be adjusted so that it limits the boost to 4.2 psi.

How do we determine the safe boost limit for a given application?

Roger Huntingdon came up with this rule back in 1956, which I believe is still applicable.

ďFirst, figure that your true horsepower as installed is roughly 10 percent above the advertised rating of the engine in question. If this horsepower figure falls below 150 you can safely adjust the screw regulator to hold as high as 6 Ĺ lbs boost. If the calculated power falls in the 150-210 hp range, Iím sure you could stretch a point and adjust for 5  Ĺ - 6 lbs, with a decent safety margin. Above the 210 hp point you should either regulate down to the 4 Ė 4 Ĺ lbs range or use the Heavy Duty drive. With the Heavy Duty drive you can safely pull five lbs boost up to a calculated hp figure of at least 300, and 5 Ĺ lbs up to maybe 275. In the higher air  ranges the power requirement of the impellor rises at such a rapid rate that the McCulloch design is all but useless for anything over 350 hp.Ē

Iíll address each of the points Roger made in turn starting with the method of calculating the true horsepower. McCulloch advertised that the addition of the VS57 supercharger resulted in a 40% increase in power. In the case of my 1956 Mercury the advertised horsepower was 225 hp, although Hot Rod magazine (Dec 56) dynoíd it at 181 hp. Applying 40% increase to the 181 hp I end up with a figure of 253 hp. Using Rogerís method of multiplying the advertised horsepower (225 hp) by 10% I end up with 248 hp Ė which is pretty consistent.

Assuming 250 hp I can estimate the air flow requirement to be 1800 lbs per hour (based on 720 lbs per 100 hp), and using the adiabatic efficiency graph this equates to a boost of 4.8 psi with an impellor speed of 29,000 rpm. The horsepower transmitted by the supercharger would be slightly less than 16 hp which is uncomfortably close to the 17 hp limit of the standard McCulloch VS57.  Dropping the boost to 4 Ĺ psi as recommended by Roger would drop the impellor speed to about 28,000 rpm and would give a safer horsepower transmitted figure of about 14 hp. So again I tend to agree with Rogerís statement on safe boost levels.

As for the Heavy Duty drive, Iíve yet to ascertain how to identify which superchargers have the Heavy Duty drive installed, and which donít. I suspect that the superchargers which are prefixed with O (as in OVS57) have the Heavy Duty drive, although Iím not prepared to commit on that yet. What I do know is that the Heavy Duty drive, which basically involved the use of  heavier duty spring pack, was rated to handle power transmissions of up to 21 hp before the 2% slippage problem occurs. Obviously the increased power transmission will allow for increased boost levels, although the increases are not significant, and tend to agree with Rogerís stated figures of 5 lbs for calculated horsepowerís of 300 hp to 350 hp, and 5 Ĺ lbs for less than 275 hp.

Summary

The VS57 has the following limitations:-

1)     They should not be used on an engine with a calculated true horsepower of 300 hp or more. The Heavy Duty drive VS57 units have a limitation of 350 hp.

2)     Boost should be limited to between 4 and 4 Ĺ psi for engines with a calculated true horsepower between 210 and 300 hp.

3)     Maximum boost should be 6 lbs for a calculated true horsepower of  less than 210 hp and may be as much as 6 Ĺ lbs for a calculated true horsepower of less than 150 hp.

4)     Engine rpm should be limited to below 5100 rpm for a standard installation. Iíll cover this aspect in more detail later.