Melling M55 or M55a?

[135 Honolulu]

I have a project truck that has a basically stock engine. It is a 1994 GMC Sierra SLE 5.7L. I need to pull the engine to change out the freeze plugs. I figured that while the engine is out, I'd install headers and the oil pump and water pump. I have a question.
Which oil pump should I use? Melling M55 or Melling M55A?

 

[89GMCJOHN]

I run the m55a standard volume high pressure pump ,,,,,z28 pump as its also called...I think they have a 78 psi spring in them. From Melling
Melling Engine Parts: Technical Bulletins

Quote:
That is what a high volume pump will do. Now let Is consider what it will not do.


It will not replace a rebuild in a worn-out engine. It may increase pressure but the engine is still worn-out.

It will not pump the oil pan dry. Both solid and hydraulic lifters have metering valves to limit flow of the oil to the top of the engine. If a pan is pumped dry, it is because the holes that drain oil back to the pan are plugged. If the high volume pump is also higher pressure, there will be a slight increase in flow to the top.

It will not wear out distributor gears. The load on the gear is directly related to the resistance to flow. Oil pressure is the measure of resistance to flow. The Ford 427 FE "side oiler" used a pump with relief valve set at 125 psi and it used a standard distributor gear. Distributor gear failures are usually caused by a worn gear on a new cam gear and/or worn bearings allowing misalignment.

It will not cause foaming of the oil. With any oil pump, the excess oil not needed by the engine is recirculated within the pump. Any additional foaming is usually created by revving the engine higher. The oil thrown from the rod bearings is going faster and causes the foaming. This is why high performance engines use a windage tray.

It will not cause spark scatter. Because of the pump pressure there is a load on the distributor gear. The number of teeth on the oil pump gears determine the number of impulses per revolution of the pump. In a SB Chevy there are seven teeth on each gear giving 14 impulses per revolution. At 6000 RPM the oil pump is turning 3000 RPM or 50 revolutions per second. To have an effect on the distributor, these impulses would have to vibrate the distributor gear through an intermediate shaft that has loose connections at both ends. Spark scatter is usually caused by weak springs in the points or dust inside the distributor cap.

 

[L31MaxExpress]

^^^^All that being said I use a standard volume high pressure big block pump myself. It has more teeth and turns more smoothly under load as well as smoothes the distributor drive from excess impulse shock. It also has an internal bypass which helps keep the oil from aerating by returning the oil directly into the inlet side of the pump rather than the pan. It was the prefered setup of Smokey Yunick in his endurance race built 302s. Harmonics can definately be generated in something spinning 50 times per second. There is a reason an engine has a harmonic dampner and valve speings have an inner dampner as well as driveshafts having a critical speed.

 

[MarkZ28]

Im using a high volume bog block pump in my 350 build. M77H I believe. You will need a special driveshaft and pickup tube, but are easy and cheap to get.

 

[135 Honolulu]

Thank you, Guys!
And so, the adventure continues...
Aloha, and have a great week!