Fuel Pump

This section describes fuel pump modifications required in order to ensure that adequate fuel delivery occurs when the carburetor is operating under boost. Essentially this involves boost referencing the fuel pump so that its output fuel pressure rises with the boost pressure maintaining a constant difference between the fuel pressure of 2 to 5 psi above the boost pressure dependant upon the carburetion demands of the engine.

The fuel pump modifications described are essentially the modifications made by McCulloch during their kit installations. However before covering these it is probably worth considering the fuel system modifications made by the factory installations of McCulloch superchargers by Kaiser, Ford and Studebaker. All three incidentally did not use a supplementary electric fuel pump in their installations, a practice that seems to have been restricted only to the McCulloch aftermarket installations.

Kaiser on their supercharged Manhattans used the Carter  M2145S model fuel pump which was a standard mechanical diaphragm pump sealed and modified for boost referencing.. The pump had a static output pressure of  4.5 PSI and boost referencing increased the pump output pressure in line with the boost pressure increase. This was achieved by sealing the pump (no external venting) and then feeding the boost behind the fuel pump diaphragm in order to help the spring displace the diaphragm. The three valve design of the pump allowed for a greater fuel volume to be pumped.

Kaiser Fuel Pump

Studebaker and Packard used a carter mechanical pump as well, the M2623S. This had an uprated diaphragm spring compared to the standard M2573S used on unsupercharged Packards, and had a static pressure of 6 – 7 psi compared to the standard 3 ˝ to 5 psi. Boost referencing was also used in order to increase the pump output pressure in line with the boost pressure.

Ford had two different fuel pump arrangements. The pumps used with the 100 Phase I installations had static pressures of 9 psi (achieved using dual springs and modified diaphragm)  and were boost referenced. This high pressure often caused roughness or flooding of the engine at low speeds and resulted in the service replacement of the diaphragm spring so that the pump produced 5 to 6.5 psi static pressure, which in itself could cause problems with starvation or leaning out under high boost. The Phase II installation resolved these problems by using a boost pressure differential circuit on the same pumps which maintained a 3 psi differential (fuel pressure to boost pressure) under all boost conditions.

Fuel pressure differential circuit

To maintain the pressure differential a differential pressure circuit was set up which used manifold vacuum (taken from the base of the carburetor)  to work against the fuel pump diaphragm spring, and reduce the fuel pump pressure output to  2.5 – 3 .5 psi. As boost pressure increases the manifold vacuum decreases and the fuel pressure rises, maintaining a fuel pressure which is 2.5 – 3.5 psi above the boost pressure.

 

AC Fuel Pump and Filter used by Ford

As a precaution against excess fuel pressure at the carburetor the Ford Phase II installation also utilised a special inverted fuel filter which had a central drain tube. If the fuel pressure pushed the fuel level above the drain tube level the excess fuel was drained back to the fuel tank.

Standard Mechanical Fuel Pump Modification

Standard Mechanical Fuel Pump Modification

The above diagram shows the modifications required to a standard mechanical fuel pump to allow it to be used in conjunction with the McCulloch supercharger. Part numbers referred to in the diagram are McCulloch part numbers and modern equivalents will have to be found in order for the modification to be conducted. Mechanical fuel pump modification is required because it is necessary to increase the fuel pressure when the engine is being supercharged in order to force fuel into the carburetor when the carburetor is pressurized. To accomplish this, air pressure from the supercharger is directed beneath the fuel pump diaphragm to help the spring pump fuel at a high enough pressure.

To modify the fuel pump the fuel pump needs to be disassembled sufficiently enough so that the fuel diaphragm can be removed. A 5/16” hole should then be drilled into the side of the pump, where the diaphragm chamber vent holes are located, to allow the supercharger pressure fitting (which is basically a pipe nipple fitting) to be inserted. The location of the hole should be such that it provides direct pressure feed into the air chamber behind the diaphragm. The pressure fitting can be fixed either via tapping the drilled hole (assuming there is sufficient depth of metal to accept a thread), or via a nut inside the diaphragm air chamber which screws onto the pressure fitting. In both cases a rubber sealing gaskets should be used to ensure an air tight connection of the pressure fitting to the fuel pump. Any vent holes remaining after the nipple has been installed will need to be sealed, and this can be accomplished either by using an epoxy resin, or via use of lead shot.

An additional seal (102305) is indicated at the base of the diaphragm air chamber. This was originally a disc shaped composite of metal and rubber and is designed to prevent pressurized air leakage into the shaft where the diaphragm stem connects to the pump drive arm. This can be reproduced using a rubber disc with a central hole which has a smaller diameter than the diaphragm shaft, thus allowing a tight seal against the shaft when the diaphragm shaft is pushed through it. A metal disc which has a larger central hole should then be placed over the rubber disc. This metal disc is held against the rubber disc by the diaphragm spring, and prevents the rubber disc from lifting when the pump drive lever pushes the diaphragm shaft upwards. The central hole should be sufficiently large enough to allow the rubber disc to flex with the diaphragm shaft movement, and small enough to provide a contact point for the diaphragm spring.

Dependant upon the pumps capability it is sometimes necessary to add an additional spring within the existing diaphragm spring, to increase the output fuel pressure from the pump. It is also possible that high pressure replacement pumps are available, or can be built up due to the interchangeability of fuel pump components. The fuel pressure check section described later provides an example of the required fuel pressures for specific boost pressures. Determining the required spring rate in order for the mechanical fuel pump to meet these required fuel pressures is pretty much a trial and error exercise unfortunately.

Supplementary Electric Fuel Pump Installation

A supplementary electric fuel pump can be used to increase the fuel pressure under high boost conditions, effectively boosting the pressure output from the mechanical fuel pump. McCulloch used Bendix fuel pumps for their installations (I believe some of the Land Rover pumps are similar) which were flow through pumps (do not need to be operating for fuel to flow through them) – and I believe that some Land Rover pumps are similar. These are quite low pressure pumps (3 – 4 psi) and when activated their fuel pressure is added to that generated by the mechanical pump, therefore a typical 4 psi mechanical pump used in conjunction with one of these pumps would produce in the region of 8 psi.

McCulloch supplied these supplementary electric fuel pumps presumably because high pressure mechanical pumps were not available for many applications, or were a rarer option, and they are quite effective in that a substantial fuel pressure increase will occur when it is most needed – when the blower is forced into high ratio. Nowadays high pressure pumps are available for most applications or can be built up due to the interchangeability of pump parts – so a supplementary electric fuel pump is not necessarily required if sufficient fuel pressure can be achieved using an uprated or performance mechanical pump.

If a supplementary electric pump is installed it has to be installed inline between the modified mechanical fuel pump and the carburetor, and has to be mounted such that it is not close to hot engine components, in order for potential fuel vaporization problems to avoided. Many of the McCulloch mounting brackets actually have mounting points for the electric fuel pump on their rear, and if this is the case for your bracket then simply mount the fuel pump to the bracket. Tank mounting, or mounting between the existing mechanical fuel pump and the fuel tank, is also possible, however I do not recommend this. Two potential problems exist with this sort of setup, the first being that the mechanical fuel pump can actually restrict the fuel flow, and effectively inhibit the additional fuel pressure generated by the electric pump. The seconds reason is that it has been shown that a gradual drop in fuel pressure, followed by fuel starvation, can occur on a  very hot day, or on an exceptionally long,   fast, high gear hill at not quite full throttle, using this set up.

If your bracket does not have provision for the electric fuel pump, or if you feel that potential fuel vaporization problems could occur using the bracket mounting point, then it is recommended that the fuel pump is mounted on the front bulkhead near the radiator, or alternatively on the inner wing. If either of these mounting positions are adopted then flexible fuel lines must be used to connect the electric fuel pump to the carburetor and the mechanical fuel pump. If flexible lines are not used then it is practically guaranteed that rigid pipework will fracture and leak as the engine vibrates, with the potential fire hazards associated with fuel leakage.

Supplementary Pump Plumbing & Wiring

The fuel supply from the mechanical fuel pump is fed to the electrical fuel pump (via flexible fuel lines if the electric pump is not mounted directly on the engine or supercharger mounting bracket), and the supply from the electric pump is fed to directly to the carburettor. It may be necessary to increase the diameter of the piping used for the fuel connections, as in some cases fuel starvation can occur under high boost and high load conditions, due to insufficient fuel capacity in the stock fuel lines. If this is the case the whole fuel system should use larger bore piping, i.e. the connection from the mechanical fuel pump to the fuel tank should use this piping as well.

The electrical connections for the supercharger control system are shown in more detail in the diagram below.

Vacuum Switch Wiring

As can be seen a wire is run from the primary terminal of the coil to one post of the vacuum switch, and wires are run from the second post of the vacuum switch to the electric fuel pump and the solenoid that is enclosed in the housing underneath the badge on the supercharger. This allows the supplementary electric fuel pump to be activated at the same as time as the supercharger is switched to “high boost”, and subsequently ensures that the fuel pressure is increased to the amount required under high boost to overcome the increased carburettor internal pressure. If a mechanical switch, such as one mounted on the throttle which is activated at ľ full throttle, then this is wired in place of the vacuum switch. If an additional switch is required to enable high boost to switched in directly by the driver (not recommended, but some people will want it), then this should be wired in separately to the vacuum (or throttle) switch, and is basically the duplicate of the wiring for the vacuum switch. Large capacity wires should be used as the load of both the solenoid and electric fuel pump is high, and can result in burn out if the wiring is adequate. I’ll try and quantify the actual load when I get the opportunity.

Six Valve Fuel Pump

Mechanical fuel pump modification and supplementary electric fuel pump installation, or high pressure pump installation,  will provide an adequate fuel flow on most installations however in the case of hard driving and racing applications the volume of the fuel supplied to the carburettor may be inadequate causing fuel starvation problems. The first modification that should be made in this eventuality should be increasing the size of the fuel lines from the tank to the fuel pumps with 3/8” inside diameter being the minimum requirement. This in itself will not resolve the problem if the mechanical fuel pump has insufficient fuel flow capacity to pass the extra available fuel, and to provide a solution to this problem Paxton marketed a six-valve fuel pump body from the late fifties.

  

Six valve body and AC pump with six valve body

This was an AC unit and as such fitted  all stock AC fuel pumps on V-8 Fords and Thunderbirds from 1955 through 1960 as well as V-8 Chevrolets and most other General Motors cars with AC pumps. Obviously this unit is not readily available nowadays however Carter and Holley manufacture fuel pumps with six valve units and it may be possible to use of modify one of these modern units, or the whole pump.

Fuel pressure Regulation

On the whole mechanical fuel pump modification in conjunction with a supplementary electric fuel pump will produce a satisfactory system that delivers more or less the required amount of fuel needed to maintain the correct air/fuel ratio under varying boost levels. However in some cases flooding may occur at certain or no boost levels, or mechanical fuel pump boost referencing may not be possible and/or unreliable, and in these cases boost referenced fuel pressure regulation can be used.

Boost referenced fuel pressure regulation essentially involves regulating the fuel prior to it’s entry into the carburettor, and to enable correct operation requires that the fuel pressure into the regulator is as a minimum at the pressure required by the engine (i.e. 3 to 5  psi above the boost pressure). Boost referenced pressure regulators regulate the fuel pressure into the carburetor at the set value, and as boost increases the regulation value increases with the boost level (obviously they need to be connected to the boost output of the supercharger, ideally at the bonnet side, in order to get the boost referencing signal).

In order to maintain the input fuel pressure to the regulator to the minimum required by the engine mechanical fuel pump modification is still required. Ideally boost referencing should still be applied to the mechanical fuel pump and a supplementary electrical fuel pump activated by the vacuum switch installed so that the input pressure to the regulator rises with boost. However if a boost regulator is installed due to the fact that the mechanical pump cannot be modified for boost referencing then it may be possible to get sufficient input pressure to the regulator by modifying the spring rate in the pump (thus increasing pressure output) and using more than one supplementary electric fuel pump.

If problems still exist, or a simpler more modern solution is required, then the fuel system can be modified to use an electric fuel pump in conjunction with a boost referenced fuel pressure regulator, in place of the mechanical fuel pump set up. This is a well proven solution as countless turbo and supercharger installations since the early sixties used electric fuel pumps in conjunction with fuel pressure regulators, however it has its pitfalls as well. In order for sufficient fuel to be available to the engine the electric fuel pump must be capable of generating fuel pressure at the level required by the engine under full boost conditions. This will be regulated against boost by the regulator, however as most electric pumps pump at fixed pressure levels it does mean that the pump is pumping fuel at a higher rate than what is being is used under low or no boost conditions, and consequently will run hot. Modern installations compensate for this by running a return line from the pump or the regulator back to the tank, so that the excess pressure is bled off, and ideally the same compensation should be installed by yourself when setting up an electric pump as a replacement for the original mechanical pump. 

Installation of an electric fuel pump and fuel pressure regulator is straightforward in itself. The original manual pump should preferably be removed and blocked off, although if it is a dual action type used for generating accessory vacuum it can be left in place with the fuel lines disconnected. If it is retained in the fuel system it can act as a bottleneck to the pressure generated by the electric pump. Most electric fuel pumps are push type, and consequently should be installed at the fuel tank, whereas the fuel pressure regulator should be installed as close to the carburetor as possible, to minimise pressure drop at after the regulation. When installing an electric fuel pump some form of safety switch (tilt type switch and/or oil pressure switch) should be installed as well, this is to turn off the pump in the advent of an accident.

Early Paxton Fuel Pressure Regulator

Sourcing a boost referenced fuel pressure regulator is relatively easy, Paxton and other supercharger/turbocharger manufacturers have produced several variants over the years, and Mallory currently produces a $70 regulator (the 4309). Some of these regulators required plumbing fuel return lines to the tank (Mallory 4309 for example) others are simpler to install but should be checked for possible pump overheating problems when used in conjunction with electric pumps (less of a problem for a supplementary pumps as they only operate for short periods). A cheap regulator setup (without fuel return line) that can be used is a $20 Holley HLY-12-803 fixed fuel pressure regulator. This has to be modified though - the modification basically involves filing down the air bleed notch on the top surface (or filling it with epoxy resin - or welding it up) and then fitting a pipe to connect the booost to to the top surface. Greg 'TurboStude' has some photos and a description of the procedure on his TurboStude site (http://cancermn.net/TS/chapter_7.htm).

Checking the Fuel System Pressure

McCulloch recommended the following procedure for checking the fuel pressures, which are governed by the air/fuel ration of the mix inside the carburetor.

An air-fuel meter, 0-10 lb vacuum, 0-10 lb boost and 0-15 lb fuel pressure gauges are required for the check. The vacuum gauge should be connected to the intake manifold vacuum, the fuel pressure gauge should be connected to the fuel line between the mechanical fuel pump and carburetor, and the boost gauge should be connected to the pressure line from the supercharger discharge throat to the mechanical fuel pump, with T pieces being used for the connections where required. The air fuel gauge connection should be as per the manufacturers instructions.

To conduct the check the car has to be run on the road or a dynamometer. Under sustained running (with full throttle), and over 3,000 RPM, the boost pressure from the superchargers should read 5 pounds maximum and should remain constant. Failure to read 4.5 to 5 pounds, event though the pressure rose rapidly when manifold vacuum reached zero, is an indication of a defective solenoid regulator.  Provided the electric pump has been correctly wired up, the pump will be energized at the same time as when the supercharger shifts into “high blower” and produces 5 pounds. At the instance of the shift the fuel pressure should start to increase until it reaches a pressure at least 2 to 3 lbs higher than that indicated by the boost pressure gauge.

Drive the car at road load conditions and watch the manifold vacuum, fuel pressure, supercharger boost pressure, and the air-fuel ratio at car speed increments of 10 mph. The air-fuel ratio should run from 12 to 13 off the idle circuit, and then back towards rich as the car progresses towards full throttle. The following tables will serve as a guide.

Cruise (Low Boost) Air-Fuel Check

MPH

Air-Fuel ratio

Boost pressure (lbs)

Fuel Pressure (lbs)

Manifold vacuum

20

30

40

50

60

70

80

90

12 to 13

12.5 to 13.5

13 to 13.5

13 to 13.5

13 to 13.5

13

13 to 12

12 to 11.5

0

0.75

1.0

1.3

1.7

1.9

2.0

2.7

3.5 to 7

3.5 to 7

3.5 to 7

3.5 to 7

3.5 to 7

3.5 to 7

3.5 to 7

3.5 to 7

12 to 15

12 to 13

10 to 12

10 to 10

9 to 10

9 to 8

8 to 6

5 to 3

 

Full Throttle (High Boost) Air-Fuel Check

MPH

Air-Fuel ratio

Boost pressure (lbs)

Fuel Pressure (lbs)

Manifold vacuum

20

40

60

80

100+

12 to 13

12 to 11.6

11.2 to 10.8

10.8 to 10.2

10 to 10.4

1 to 2

2 to 3

4 to 5

4.5 to 5

4.5 to 5

6 to 12

6 to 12

6 to 12

9 to 12

9 to 12

0 to 1

1 to 3

3 to 4

3.5 to 4

3.5 to 4

One thing to be aware of with regards to the vacuum values listed in the above tables is that they are only a general indication of the expected values. A “Golden Hawk” which has a powerful engine in a light chassis will cruise with a higher manifold vacuum than, for example, a heavy station wagon with a modest engine. Fuel pressure will also vary between engines, as it depends on the engine size and the number of pumps installed.