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Cummins 30 PSI Belt Drive P7100 Overflow Valve

Months of Testing Has Produced the Best Overflow Valve

for the DTT Assassin and Glacier Fuel Boss Belt Drive Pumps

NOW you can control your P7100 fuel presure with a turn of a wrench!

* Two pressure ranges available

* OFV030 - 30 PSI

* OFV035 - 45 PSI

* DTT Assassin mechanical pumps

* Glacier Diesel Fuel Boss belt drives

* Ford 6.0 to Cummins conversions

* Aeromotive A1000 pumps

I get a phone call from Steve Barnas in Chicago. He purchased a Glacier Diesel belt drive lift pump for his 97 Dodge Cummins 12V. Actually he was trying to install a VP44 kit on a P7100 injection pump because Glacier doesn't make a P pump kit. Steve had about 25 PSI fuel pressure at idle, close to 45 at cruise but the pressure spiked to 100 at full power. His pressure gauge was pegged so the pressure could be higher. He asked if I could help.

When doing flow calculations, it's critical to know the actual pump output in gallons per minute (GPM) so the entire fuel system can be evaluated based on this number. I've found over the years that pump manufacturers "fudge" the numbers on the high side to give them bragging rights.

 pumpcasedimension.jpgSo Steve sent me an exact duplicate of his Fuel Boss VP44 kit for measurement. Gear pumps are easy to calculate GPM flow rates. You measure the gear width, gear diameter, number of teeth and the case dimensions. The actual rates (calculated) is dead nuts accurate. They match the flow chart exactly (below). Now we have a base line to evaluate the Cummins P7100 fuel system using the gear-width.jpgGlacier Fuel Boss pump.


According to Van Haisley of Haisley Machine, you have to dump all of the fuel flow of the lift pump until you go to full rack on the Bosch P7100 pump. Otherwise the fuel pressure becomes excessive. This is exactly what was happening to Steve's truck. He couldn't dump the extra fuel back to tank and the fuel pressure was too high.

 fuel-boss-flow-chart.jpgThe first thing that had to be changed was eliminating the stock fuel filter. The OEM filter is rated at 52.8 GPH (gallons per hour) This rating is far less than the output of the belt drive pump which is 78 GPH at 2000 rpm, 115 GPH at 3000 RPM and 154 GPH at 4000 RPM. It's like stuffing 10 pounds of poo in a 5 pound bag. The fuel filter creates a restriction or an orifice in the fuel system and raises the fuel pressure.


So the first modification that Steve makes is a high flow fuel filter. Now the fuel pressure isn't pegging his gauge at full power. We're making progress but the fuel pressure is still too high at wide open. How do we know? Using this rule you want 10 pounds of fuel pressure per 1000 RPM of governor speed. 3000 gov = 30 PSI, 4000 gov = 40 PSI and so on.


I send Steve a prototype adjustable overflow valve with the inlet orifice opened up to match the flow rate of the Glacier mechanical pump. The prototype valve certainly helped to lower the WOT fuel pressure, but we are still close to 80 PSI at WOT. The inlet hole calculations are correct but many times the actual results will fool you. One good test is worth a thousand calculations!

 The next change that I suggested was to change the return line from the overflow valve back to tank to a 3/8" line. The stock steel line is not large enough to handle the flow of the pump. Once again, this a restriction that will increase fuel pressure if it is too small. The 3/8" line lowered the fuel pressure another 5 PSI.

 meters1.jpgNow it's time to get serious about solving this problem. The only way to measure the actual flow (GPM) thru the overflow valve is to build a flow test bench. The vertical meters are labratory grade and are very accurate. With four valves I can choose which flow meter to use.


At Tork, we use an industrial hydraulic power unit that has temperature control and a variable displacement piston pump. In order to get consistent results, the fluid remains at a constant temperature. BTW the fluid is a special oil that has the same viscosity as diesel fuel without the smell. In each test we slowly increase the GPM rate to a fixed pressure reading. What we are looking for is an increase in flow at the same pressure reading.

 And now the good stuff! We built 12 different valve bodies with different inlet diameters and enlarged exit holes with the number of them increased. With each prototype valve body we tested a different size stainless ball. So, we tested 15 ball diameters with 12 different valve bodies. A total of two days testing.

What were we looking for? Fluid dynamics. I'll admit that I know very little about this subject and that's why we test so thoroughly. We know that a combination of valve and ball diameter will produce "a sweet spot". The valve will flow at the rated pump GPM and maintain linear pressure readings. What we did find is a change of .001 in diameter totally changed how the valve worked!


The result resembles a stock overflow valve but the seat depth is different, the ball diameter is not stock, the inlet hole isofv030body.jpg enlarged and the exit holes are now six total.


We sent the three best performing valves to Steve for testing on his Dodge Cummins 12V. Based on the flow bench numbers we knew which valve would work but wanted to test in real world conditions. As expected, prototype valve 5B worked like a charm. His fuel pressure stayed close to 30 up to about 2200 RPM and climbed to the mid fifties at full power. Both of us were pleased with the results. If you want to use either the Assassin or Fuel Boss on your Cummins 12v, YOU NEED THIS VALVE.

The OFV030 is fully adjustable from 20 to 40 PSI while the OFV035 is factory set to 45 PSI. (adjustable from 30 to 65 PSI)

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