The DB2-4911 (5088?) was the last mechanical injection pump upgrade for the 6.2-6.5 series. One late revision, indicated by Blue Label, corrected the hot-soak, warm-start problems in some aging pumps, where excessive clearance between head and rotor resulted in injection pressures lower than required down around 100 pump rpm.
Early pumps were Black Label.
Most warm-start problems proved electrical in nature - low battery(s), bad cable(s), worn starter bushing(s), bad fuel, etc.
So, how does the 4911 compare with the 6.2 series pumps, and how can we tell the difference? Let's take the nomenclature on the tag -
J2-DB2-831-4911-J and J5 DB2-829-4126-J
The first two digits represent build date month and year. In this case, J is September, 2 is 1992, and 5 on the 4126 indicates it was built in 1995, as validated by the '95' casting mark.
DB is for rotary distributor-type head, where each cylinder gets it's own injection port and pipe, supplied by single barrel.
2 indicates two opposed pumping plungers, driven by steel rollers in ceramic pillow blocks
8 indicates number of cylinders
31 indicates 0.31" pump plunger diameter, used in the 6.5 pumps. 29 is 0.29" used in 6.2 pumps.
4911 indicates specifications to which pump was built, J is assembly plant
Also on label is inj pump serial niumber, and customer part number - GMC, Chev, Ford, etc
Refer to the two drawings in the members area - DB2 injection pump cutaway - starting at input flange and working rearward to the distributor head.
The input shaft rides in single needle bearing, with one seal toward engine, two seals toward pump. Driven counterclockwise by pump gear, this shaft drives the two-roller pump rotor inside the camring, where internal ramps force the rollers\blocks against the two pump plungers, creating the high pressure required for injection. An 8 cylinder engine requires 8 ramps.
An arched leaf-type spring, adjustable by the fuel metering screw, limits roller\block travel and pump stroke.
The rotor has two opposed pump discharge ports for transferring high pressure fuel into the radial charging passage
A rotary valve in the head, actuated by the throttle-governor slide via the throttle shaft, meters fuel in the charging passage,
When not powered, the fuel shut-off solenoid forces the rotary valve fully clockwise, blocking fuel flow.
Fuel to each cylinder is not de-pressurized after injection, such that no time is required to re-pressure each pipe.
A metered orifice setscrew controls housing fuel flow.
The camring ramps, in conjunction with injector design, are designed to initiate injection slowly as cylinder pressure reaches combustion temperature, increase flow during combustion, then taper off fuel as oxygen is depleted.
The 6.2\6.5 gained the reputation as the "smokeless Diesel" due to the success of the Stanadyne Roosa Master injection pump.
The camring is rotated counter-clockwise for retard and clockwise for advance injection timing by the advance piston, operated by hydralic pressure from the pump.
Manual control is via a cam on the throttle shaft forcing a rocker lever against a rod driving a servo valve spring. The servo valve unbalances end pressures on the advance piston, which then rotates the cam ring.
Electrical control is via the HPCA (housing pressure cold advance) solenoid, which vents housing pressure to the fuel return line. This also offsets pressures on the advance piston, allowing it to force the camring clockwise, for cold advance.
Rear of rotor shaft drives the four vane transfer pump. A transfer pressure regulator, adjustable, is located in the rotor shaft. A nylon filter screen is in the transfer pump housing.
6.5 pump diferences
Note: all internal parts are interchangeable between housings
The 4911 aluminum casting is greatly strengthened in the flange to housing area, preventing housing movement caused by increased injection pressure forces.
Camring ramps and timing changed to compensate for higher injection pressure
Pump plunger diameter increased from 0.29" to 0.31" to supply increased volume at higher pressures
Roller travel limit (fuel metering) spring increased in length allowing greater pump plunger stroke
Head recieved two transfer channels
Advance piston end caps reduced in length dimensions to permit longer piston stroke
Note: as explained to me, while the 6.2 rotor and head may be used in clockwise or counterclockwise rotation service by flipping the camring, which is marked CW on one end and CCW on the opposite end, the 6.5 rotor and head contain timing differences such that they are specific to rotational direction.
GM pumps are counterclockwise driven, IH (Ford) is clockwise; both used Stanadyne DB series pumps thru 1993.
Governor weight retaining plate is notched, rotor recieved groove for ball, firmly locking plate to rotor, preventing wobble.
Throttle guide stud is externally adjustable for governor tip-in.
Throttle\governor slide assembly strengthened, recalibrated with internal spring
Throttle return spring added to shaft inside housing.
#1 injector line gets pressure pulse timing sensor screwed onto pump fitting, requiring shorter piping
6.5 injector piping inside diameter increased
In street terms, what's it all mean?
The 4911 pump got the rotor\pump and head assembly from the 6.9\7.3 IH Navistar (Ford) engine injection pump, which required strengthening the aluminum pump housing for timing accuracy.
To what end?
More fuel with on time delivery, as required by the 6.5 turbo engines , with less wear on the camring and rollers.
This info resulted from my tear-down and part-by-part comparison of 1982 -4126 (new), 1989 -4723 (rebuilt), and 1993 -4911 injection pumps, where I subsequently rebuilt the 4911 for my truck.
I took my findings to an old hand shopowner with Bacharach flow bench, working on inj pumps and magnetos since 1947, for validation\confirmation\repudiation, and was offered a job, on the spot. Feel good time,, oh yeah!
Was my investigation fun? Definitely.
Educational? Well.....no use trying to tell me you can turn up my 6.2 pump and make it perform like a -4911. That Bacharach cannot test everything Stanadyne spec'ed.
For me, the proof is in the putting, as it were.
Confirmation, validation, and\or repudiation welcomed.
jd
[ 08-07-2004, 07:10 AM: Message edited by: gmctd ]
!
