I am about to rebuild a 350 small block Chevy that originally came out of a truck. We’re going to add new forged pistons, better rods, and if I can wrangle up the cash perhaps a nice set of aluminum heads. Here’s my question. With all these good parts I was going to add a stock oil pump since I don’t plan to race this engine or anything but a friend said I should use a high volume, high pressure pump to protect the bearings. I never really considered that before. Does my engine need that?J.D.
Let’s start with a fact or two and then you can decide.
First, there have been literally millions of small block Chevy engines built and driven hundreds of millions of miles using a stock volume and pressure oil pump. I think that sometimes car guys just look for a reason to add a high performance part to their engine. In your case, it sounds as if a high volume, high pressure pump is really unnecessary.
However having said that, there is certainly nothing wrong with adding a high volume pump to your engine if it makes you feel better.
You mentioned that you’re trying to save money for a set of really good heads. Good cylinder heads are an excellent way to help make more power. A high volume pump will not help make power but may in fact hurt the power. Remember that if you are moving more oil at higher pressure, that energy has to come from somewhere and that amount of energy required (as small as it may be) will be subtracted from flywheel horsepower. It’s that simple.
Another way to look at this is that a stock small block Chevy oil pump is designed to be able to push sufficient oil against all the restrictions in the engine in order to supply oil throughout the engine with more than sufficient pressure. So now, if you add a pump with greater volume potential this means the pump has to work harder to push oil into the engine because it’s moving a greater volume of oil. So common sense would dictate that this is expending additional work toward an objective that isn’t necessary.
Now, if you build the engine with much larger clearances, then that’s a different story. A normal 350 small block Chevy’s main journal bearing clearance is right around 0.0025 inch. If the clearance is closer to 0.0030 inch, then that additional space will allow more oil to flow and should probably be accompanied by the use of both thicker viscosity oil and a high volume oil pump.
There are no real advantages to increasing clearances. In fact, NASCAR small blocks run at very tight clearances to reduce the amount of windage (oil leaking out of the bearings) that will hurt horsepower at higher engine speeds. So it’s actually to your advantage to keep the clearances in the middle (around 0.0020 to 0.0025 inch) to reduce the amount of oil escaping the bearings.
A long time ago, Steve Brule’ and I did a oil pump test at Westech where we ran a stock volume pump, a high volume, then a high volume/high pressure pump, followed by a big block Chevy pump. Not surprisingly, as the volume and pressure demands went up with bigger pumps, the power suffered and the stock pump made the most overall power. What was surprising was how well the big block Chevy pump performed. We theorized that the greater number of teeth on the big block pump improved its performance compared to the fewer number of teeth on the high volume small block pump.
Speaking of gear designs, Melling offers a complete redesigned oil pump gear set they call the Shark Tooth.
The original small block Chevy oil pump design uses straight spur cut gears that tend to exhibit excessive leakage past the gears that contribute to pressure fluctuations that load the distributor gear drive unevenly. The Melling Shark Tooth design reshapes the gears into an asymmetrical helical shape that evens out the pressure fluctuations and reduces leakage past the gears. The Shark Tooth pump is quite a bit more expensive than the standard small block pump but for a high-tech, high rpm small block it might be a great addition.
A closely related point to your question is the viscosity of the oil that you will be running. The move in modern cars is to thinner viscosity oils now like a 5w30 or even a 5w20. When running a standard bearing clearance like the above-mentioned 0.0025 inch for the small block Chevy, it’s best to run a 10w30 or 10w40 oil in order to create the proper lubrication barrier between the bearings and the crank journal. Thinner oils can be used if you go with tighter clearances. Generally, thinner oils will allow the engine to make a slight amount of power increase due to reduced pumping losses but these are one to two horsepower gains at most. Conversely, you can see a one or two horsepower drop by running a 20w50 instead of a 10w30.
So the bottom line for a mild 350 small block or any mild street engine is that you don’t need a high volume or high pressure pump to do an adequate job. Think more about perhaps a windage tray and better oil as a way to help your engine manage the lubrication task.