Fuel Pressure and Volume
The relationship of pressure to volume is directly proportional. That is, as pressure increases the volume will increase,
everything else being equal. A certain amount of fuel pressure is always required to maintain engine performance by assuring
that fuel is available on demand. Also, other factors and conditions must be taken into account such as acceleration
G-forces and friction within the fuel system itself. At the same time, however, an adequate fuel volume is needed to
ensure that the proper amount of fuel can always flow to the engine, especially during peak demand situations. A basic
understanding of this critical pressure/volume relationship is needed when designing the proper fuel supply system for
your vehicle.
Fuel Line System
The fuel line system should be routed to avoid running near hot spots, such as various exhaust system components, and
designed to promote maximum fuel flow. Most factory stock fuel systems utilize 5/16" fuel lines. This size works well
on street applications with stock engines. When the horsepower requirements go up, however, the inadequacy of this line
size soon becomes apparent. A #6 (3/8") line size is sufficient for all street performance applications and some racing
applications. #8 (1/2") fuel lines are used on everything else, including alcohol applications. Avoid using rubber fuel
lines, or use them sparingly, for two reasons. First, rubber is more resistant to the flow of fuel than any hard line.
An actual pressure loss can be measured over distance. Second, for safety's sake, it's not a good idea to use rubber
fuel line, especially when using a high pressure performance fuel pump.
Fuel Line Fittings
Like the fuel line, fuel line fittings are also a very important element in the total fuel line system and should not
be overlooked. Obviously, the fittings should be the same size as the fuel line. Also, if at all possible, you want to
minimize the use of 90° fittings. Avoid sharp turns or bends in the fuel line routing; these cause undue restrictions
to the flow of fuel.
Fuel Filters
What's the use of designing and building a good fuel line system and then choke it down with a restrictive fuel filter?
It just doesn't make sense. Therefore, the fuel filter is another important consideration when building and designing
your fuel system. A filter that's too small for a particular system is a potential high restriction area that will hinder
performance by not allowing the fuel pump to perform to its maximum. Holley offers standard in-line filters that can
be used on the street. For the Pro-Series VOLUMAX fuel pumps, Holley recommends using either Holley p/n 162-514 or 162-515
VoluMAX filters.
Fuel Pressure Regulators
The needle and seat assemblies that are installed in Holley performance carburetors can satisfactorily control fuel
pressure up to about 8 PSI. If the fuel pump is putting out more than 8 PSI, a regulator should be used to keep the fuel
pressure within safe limits and avoid the possibility of flooding. Holley manufactures a number of regulators for most
any need. A street regulator is available in either a satin finish (p/n 12-804) or a chrome finish (p/n 12-500). A performance
regulator is available in either a satin finish (p/n 12-803) or a chrome finish (p/n 12-501). Both regulators feature
a 3/8" NPT inlet port and two 3/8" NPT outlet ports with a .220" restriction (7/32"). The street version regulates pressure
from 1 to 4 PSI while the performance version regulates pressure from 4-1/2 to 9 PSI. NOTE: These regulators are designed
to work with a single carburetor installation. If two carburetors are used then two regulators will be required, one
for each carburetor. Also, two of the P/N 12-803 regulators MUST be used when running a VOLUMAX fuel pump - one for each
fuel bowl.
Holley also offers two VOLUMAX regulators. The first, P/N 12-704, is basically a larger version of the two previously
mentioned regulators but with more flow capacity. This regulator features a single 1/2" NPT inlet port and two 1/2" outlet
ports with a .437" restriction (7/16"). Fuel pressure is regulated from 4-1/2" to 9 PSI. The other regulator, P/N 12-707,
is designed for dual carburetor installations. It features four "-6" AN (approximately 3/8") outlet ports and one "-8"
AN (approximately 1/2") inlet port with a .437" (7/16") restriction. Three 1/8" NPT fuel pressure gauge ports are also
included. Without a doubt these two Pro-Series regulators are the least restrictive, highest flowing, production regulators
currently available.
Installation Tip
For vehicles without fuel return line to the tank: (SEE FIGURE #1) Install the regulator close to the carburetor. Fuel
lines from the regulator to the carburetor should be #6 (3/8").
Mechanical Pumps
Various Holley mechanical fuel pumps are available in the AVENGER line. 80 GPH pumps are chrome plated and are designed
to work with stock or mildly modified engines. A regulator is not required since the fuel shutoff pressure is pre-set
at 7-1/2 PSI. (Fuel shutoff pressure is the maximum outlet pressure that a fuel pump is capable of generating.) A selection
of inlet/outlet fittings is included to facilitate installation. 110 GPH pumps are designed for street/strip applications
where substantially higher than stock fuel delivery requirements are necessary. 3/8" inlet and outlet ports are utilized
and, with fuel shut-off pressure in the area of 6-1/2 - 8 PSI, a regulator is not required. 130+ GPH pumps are available
when maximum fuel delivery is desirable. 1/2" inlet and outlet ports are included and fuel shut-off pressure is between
7-1/2 - 9 PSI. A pressure regulator is definitely required. Both the 110 GPH and 130+ GPH pumps utilize a high capacity
six-valve design that will ensure an adequate fuel supply is always available.
Electric Pumps
Holley offers a complete line of electric in-tank and externally-mounted pumps.
Our electric in-tank line offers coverage for the most popular Ford, Chrysler, GM and import applications. Utilizing
a proven gerotor design, these pumps are available in either a 190 or 255 liter per hour (lph) flow rate.
Holley externally-mounted electric fuel pumps are also available in various flow ratings. The "red" pump, P/N 12-801,
is rated at 97 GPH and it is designed to work with stock or mildly modified engines. Pressure is pre-set to 7 PSI and
a regulator is not required. The "blue" pump, P/N 12-802, is rated at 110 GPH and it is designed for street/strip applications.
Pressure is pre-set at 14 PSI and a regulator is included as part of the package. Neither pump is compatible with methanol
or alcohol fuels nor should they be used with fuel injection systems. The "black" pump, P/N 12-815, is rated at 140 GPH
and is designed to work with either gas or alcohol fuels. This one is similar to the "blue" pump but it kicks out more
fuel. These pumps all feature a simple, yet rugged, rotor and vane design which has proved itself over the years.
Two very powerful "VOLUMAX" pumps are available for gasoline or alcohol fuels. They are P/N 12-705, rated at 160
GPH and P/N 12-706, rated at 250 GPH. These feature a gerotor pump design which is extremely efficient and quiet. Fuel
pressure is pre-set at 15 PSI and both a pressure regulator and 3/8" return line to the tank are required. The high volume
output of these pumps make them the natural choice for racers who will settle for nothing but the best. NOTE: Because
of the design of these pumps, it is necessary to connect both fuel pump inlets to the fuel supply for proper operation.
Electric Fuel Pump Installation Tip:
Mount the electric pump (motor side up) close to the fuel tank but below the fuel level so as to realize a good gravity
feed to the pump inlet. Use of a good non-restrictive screen-type filter between the tank and pump will protect the pump
from dirt and contaminants from the tank and will also avoid a fuel restriction or pressure drop at this point. NOTE:
Pro-Series VOLUMAX pumps are equipped with a 45 micron fuel inlet screen.
Oil Pressure Safety Switch
It's always a good idea to place a safety switch in the circuit when installing an electric fuel pump. Holley has one
available under P/N 12-810. This switch will ensure that the electric pump will not work unless the engine has oil pressure.
It will prevent the pump from running in a situation where the motor may stall with the ignition ON. Wiring the switch
through the starter solenoid circuit energizes the pump on engine start-up. After the engine is running the switch continues
to provide power to the pump as long as there is oil pressure to keep the switch turned on.
Fuel Pressure Gauges
There are a number of places where mechanical fuel pressure gauges could be effectively mounted. One place is just before
the carburetor. Holley chrome dual feed fuel lines (except model 4500 Dominator) and fuel blocks all have a tapped 1/8
NPT provision for this purpose. Another would be at the outlet side of the pressure regulator. The Holley four-port Pro-Series
VOLUMAX regulator incorporates two pressure gauge taps expressly for this purpose. The electric fuel pressure gauge can
be mounted inside the vehicle so that fuel pressure can be monitored while driving. This is possible because, unlike
the mechanical gauges, fuel does not flow up to the gauge itself. The Holley electric gauge, P/N 26-503, utilizes a remote
sending unit which is the primary fuel pressure sensor. An electrical signal is then transmitted from the sending unit
to the gauge which indicates the pressure.
Holley offers a variety of fuel pressure gauges, depending on use. For carbureted vehicles there are two (2) mechanical
and one (1) electric gauge available in the 0-15 PSI range:
....... 26-500 - Mechanical, Non-Liquid Filled
....... 26-504 - Mechanical, Liquid Filled
....... 26-503 - Electric, w/ sending unit
The non-liquid filled gauge is budget-priced and is the least expensive gauge. The liquid glycerin filled gauge is
a premium unit. The liquid glycerin acts to dampen the needle movement to make it more linear in operation. Both gauges
feature a 1-1/2" diameter face and a chromed steel case. The electric gauge, as mentioned above, includes the sending
unit and features a large, easy-to-read, 2-1/4" face.
Vehicles equipped with low pressure (up to 30 PSI) fuel injection systems (like Pro-Jection) can choose from two
mechanical pressure gauges in the 0-30 PSI range:
....... 26-502 - Mechanical, Non-Liquid Filled
....... 26-505 - Mechanical, Liquid Filled
Fuel System Schematics
Following are various fuel system schematics for street and race applications. Although intended only as a guide, these
designs have been successfully used in many performance applications.
Figure 1 - One (1) #12-802 "blue" pump feeding single carburetor, without fuel return line.
Figure 2 - One (1) #12-706 VOLUMAX pump feeding single carburetor.
Figure 3 - Two (2) #12-802 "blue" pumps feeding single carburetor.
Figure 4 - One (1) #12-706 VOLUMAX pump feeding dual carburetors.
