How to Install a High Press Spring in a Small Block Oil Pump
I just spoke with Renee at Melling, since they're 20 minutes from my house, I thought I'd give them a jingle.
Melling DOES NOT sell the Melling M-72HV with a high pressure spring. They sell the standard volume pump with a relief of 72 and the HV with a relief of 75.
Does a 3psi varience constitute the difference between a std spring and a high pressure one?
Sticking to the premise of providing information...
The stock spring will relieve pressure at an indicated pressure of about 55psi. That's regardless of the size of the pump's vanes... A higher volume pump will reach that point much earlier than a std volume unit because it's displacement in one revolution is 25% larger. So it's basic physics. What we interpret as pressure is resistance to flow within a system. The relief valve, being right within the pump itself, will be instantly exposed to any rise in output, and subsequent rise in pressure. Because of that,
all HV pumps come with a spring that is a bit stiffer than a standard pump relief spring. Just as Bad notes above. If they didn't, the pump would open the bypass at a much lower rpm, but at the same pressure as a std pump. That's why when you take a worn engine and stick in an HV pump you get 60psi of presssure when the old pump had 35.
From another perspective - look at the relief springs themselves. The resistance in any spring is a function of the material, wire diameter, and number of coils (length). The one in the stock HV pump has 13.5 coils. There is no picture of the two springs side-by-side, or a standard stock output relief spring, but I'd wager the overall length of the MP one is a little longer, because the MP one has 14, and the stock one will be shorter, made of smaller diameter wire, and have a different number of coils. The two Karl shows are very similar which also jives with Bad's Meliing representative's comments.
