About a year and a half ago I started this chase for the best oil pump and different options and willing to spend $500 range.
Melling is the only manufacture of the 6.5 oil pump-end of story. They make them and every company out there has a Melling relabeled to their own name.
There is the low volume pump. There is the taller gears,spacer and longer cap bolts to work with it to make it a high volume pump. There is one optional spring that is for the relief in the pump to maintain higher pressure in the pump body before the gear passage, but will of coarse lower the volume some. 3 options, no more.
The Melling engineer I spoke to in quite detail explained it like this: The volume difference between the 2 springs will have less effect than the ambient operating temperature you are in if your morning starts out at 50 degrees morning and gets to 85 degrees in the afternoon. (F not C - sorry Matuva). The clearances of your bearings determine your pressure far more than anything else. A worn out pump with proper cam, main and rod bearing clearances will have higher pressure than a new pump with the tighter spring if your bearing clearances are of by 3%. He did say not to panic much because the testing they showed with the 6.2 and non squirter 6.5 that 8 psi was ample and squirter 6.5 that 9 psi was ample.
As long as that pressure is maintained that means the flow is good enough to keep everything lubed and still float the rotating component at load.
Melling did a buy back program of engines after 150,000 miles and re-tests for real world analysis back in the day and the 6.5 turbo was one of the samples. They got 100 trucks,flushed the oil system recorded pressures. Then replaced the oil pump with new pumps and retested. Only a 4-5% gain, % not psi. They took 2 engines did cam bearings only,2 main only, 2 rod only, and 2 all bearings. The cam bearings are the issue. Doing the lowers barely helped, but the cam bearings were responsible for 75% of pressure loss.
The engineer I spoke to by the way that explained all this and did the testing is the one that when I read him the list of brands selling oil pumps sent me the snapshot of each of the boxes and labels. They box and label them there -shipped ready to sell. I found most the different companies selling the pumps, he laughed and said I missed one but wasn't allowed to tell me who it was, but can verify if you ask them -do you make the pump for "x"? The phone he texted the pics to went swimming in Lake Mead, so I can't share the mfr plant snap shots, sorry.
In the end get a new high volume pump at the best price you can, because it will be the same pump anyways. If you order the one with the spring kit option you can play with it like I did, but don't expect noticeable differences all my bearings were new and plastigauged mighty nice. If you cam bearings have 40,000 miles on them you will not see a difference in the springs.
