Holley 12-125 User Manual

Installation Instructions 

 

 
 

APPLICATIONS: 

 

Billet Aluminum Pump.  One 
3/8” NPT Inlet, One 3/8” 
NPT Outlet 

125 GPH 
Free Flow 

Built In 

Yes 

7 PSI max 

Recommended  

(P/N 12-810) 

Billet Aluminum Pump.  One 
3/8” NPT Inlet, One 3/8” 
NPT Outlet 

150 GPH 
Free Flow 

12-803 included 

Yes 

17 PSI max 

Recommended 

(P/N 12-810) 

 
 

INTRODUCTION: 

 
Congratulations on your purchase of the Holley HP fuel pump.  This instruction sheet contains all the information needed to install this fuel 
pump.  Please read all the WARNINGS and NOTES.  They contain valuable information that can save you time and money.  Holley Performance 
Products cannot and will not be responsible for any alleged or actual engine or other damage, or other conditions resulting from misapplication 
of the HP fuel pumps and fuel pressure regulators described herein.  However, it is our intent to provide the best possible products for our 
customer; products that perform properly and satisfy your expectations.  Should you need information or parts assistance, please contact our 
Technical Service Dept. at 1-270-781-9741, Monday through Friday, 7 a.m. to 5 p.m. Central Time.  Please have the part number of the product 
you purchased when you call. 
 
NOTE:   A top quality fuel filter at 70 micron or less, fuel hose and clamps, 12 gauge wire, fuel fittings, assorted terminals, a relay, and an 

optional safety shut-off switch is required to complete the installation of the HP fuel pump.  These parts are not included with the HP 
fuel pump. 

 
NOTE:   The 12-150 HP fuel pump requires the use of a fuel pressure regulator.  Holley P/N 12-803 is included for this reason.  
 
NOTE:   For optimum performance and precise fuel control, Holley ALWAYS RECOMMENDS using a Fuel Pressure Regulator. 
 
 

 
In applications where the fuel pump is subjected to high temperatures due to climate, towing, close proximity of exhaust components, high 
under-car temperatures, etc., Holley highly recommends the use of a bypass style regulator (such as Holley PN 12-803BP) to eliminate potential 
vapor lock issues that may occur due to the sensitivity of new fuels. If you are experiencing fluctuations in pressure readings or no-flow 
conditions when temperatures rise, you will need to install a bypass regulator. 
 
 

PUMP MOUNTING AND INSTALLATION: 

 
The best location for mounting any electric fuel pump is the rear of the vehicle, near the fuel tank and in a position that will allow the fuel to be 
gravity fed to the pump.  The pump should be mounted on a solid member, such as the chassis, in a vertical position with the pump motor on 
top.  Avoid exposure of the pump and fuel lines to moving parts and to any hot areas, such as the exhaust manifold.  The pump should not be 
mounted in an enclosed area, such as the vehicle’s trunk.  Follow the steps below for mounting the pump. 
 
WARNING!  THE PUMP MUST BE PLACED WHERE INTERFERENCE BETWEEN THE VEHICLE’S BODY AND ITS CHASSIS MOVEMENT IS 

 
1. 

Select a mounting site as close as possible to the fuel supply and away from possible sources of heat as detailed previously. 

 
2. 

Use the mounting bracket attached to the pump as a template and drill two clearance holes for 5/16” bolts. 

 
3. 

Mount the pump in the vertical position (motor on top) using two 5/16 bolts. 

 
NOTE:   To ensure pump life and flow efficiency, a quality fuel filter, such as Holley P/N 162-515 must be installed between the tank and each 

pump inlet.  The filter should be supported in such a manner that it does not hang from the fuel lines.