Melling Sweats The Details On Wet Sump Oil Pump Technology Part 2
Wayne Scraba
Click Here to Begin SlideshowLast issue we began our look at oil pump technology by delving into the way an oiling system functions (using a typical small block Chevy as the example). With that we followed the path oil must take and we also looked at where it ends up and how it is distributed. We also looked at oil pressure and volume. But the as engine speed goes up (typically something that goes hand in hand with performance), the need for better hardware goes up.
Certainly there’s a need for an increase in pressure, but volume needs also change as RPM increases. Why? Consider this: As the rotating assembly spins faster, the rate of oil loss from the bearings increases. Should the oil pump not be able to keep up with the demand for volume then you’ll see oil pressure drop off quickly. That’s the big reason you need to have an oil pump capable of delivering 8 to 12 gallons of oil per minute.
The maximum pressure an oil pump can deliver is determined by the force applied to the pressure relief valve by the compression spring located behind it. When the pressure of the oil exiting the pump exceeds the pressure applied by the spring, the oil pressure relief valve lifts off its seat and excess oil is bypassed. The bypassed oil can be directed to the oil pan or the inlet side of the oil pump. We’ll look at adjusting the pressure relief valve spring in the accompanying photos.
Bearing clearances are critical too. The clearances in engine bearings play a major role when it comes to oil pump volume requirements. Should the clearances be set too loose then oil will spray from the bearings at a faster rate (sometimes much faster). The oil pump selected for the job might have sufficient capability to maintain flow demand. If it doesn’t, the result is a loss of oil pressure (sometimes significant).
So why don’t you just put in the biggest, baddest pump available (with the most pressure and the most volume) and call it a day? Should you go too far when it comes to oil pump volume and pressure, you’ll quickly discover it is possible to flood the cylinders with too much oil. The same thing can happen with excessive bearing clearances coupled with a slightly incorrect pump for the application. What happens here is oil that sprays from the crankshaft journals winds up on the cylinder walls. If the cylinders are flooded with excessive amounts of oil, the rings might not be capable of scraping it away fast enough. In addition, the extra oil pouring down on the crank becomes an added source of windage drag (but this doesn’t mean a pump can drain an oil pan – more on this down the road). Bottom line? Be absolutely positive you measure all engine bearing oil clearances and rod side clearances accurately. If it becomes necessary to juggle bearings and/or components or in a worst-case scenario, to replace or re-machine internal engine parts in order to maintain these critical clearances then do it. This is one of the most effective ways to ensure the durability of your race or high performance engine.
Years ago, the only option one had when it came to oil pumps were factory examples, modified factory examples, replacements for factory examples and replacement pumps that duplicated the modifications racers were making. You could (and still can) specify high volume, high pressure or a combination of both. Usually, the pumps made use of stock style pickup tubes. Some aftermarket companies offered longer-than-stock pickup tubes for deeper-than-stock pans. One innovation from the sixties (for common Chevy applications) was a bolt-on bottom pickup designed to replace the end plate on a pump. In order to gain the depth you needed for a specific pan, you’d have to purchase the appropriate pump spacer. This spacer effectively moved the pump deeper into the pan. With those spacers, you’d need a longer-than-stock oil pump driveshaft.
Although the spur gear wet sump oil pump has been around for a long (Long) time, it works pretty well when properly sized for the application. But it has its share of disadvantages: With an OEM-style pump, you're forced to live with all of the factory pump shortcomings. You’re faced with things like cavitation, pump chatter, scattered spark, broken pickup tubes and so on. Honestly, none of the fixes mentioned above from the sixties and seventies were perfect but there are some different ideas out there.
We’ll stop right here for this issue. Next time around, we’ll dig much deeper into the slick solutions the folks from Melling offer when it comes to conventional, in pan wet sump oil pumps. The level of technology might surprise you.
Click Here to Begin Slideshow
These simple springs are used to adjust the oil pressure produced by a given pump. Melling provides two additional oil pressure relief springs with these oil pumps. One spring ads 10 PSI to the base (which is in the pump). The other spring removes 10 pounds of pressure from the base.
In order to change pressure, simply remove this Allen plug. Replace the spring and re-install the Allen plug. It doesn’t get much easier.
Bearing clearances play a considerable role when it comes to oil pump volume requirements and pressure requirements. Should the clearances be set too loose then oil will spray from the bearings at a faster rate.
Don’t forget about rod bearing side clearances. Excessive clearance here can prove to be a major factor in oil pressure and volume issues.
Why not just install the most potent pump available? Doesn’t work. You’ll flood the bottom end of the engine with oil and the end result will be excess windage (amongst other problems).
