Ask Away! with Jeff Smith: Can I Use a Big Block Oil Pump on a Small Block Chevy?

A friend of mine suggested using a big-block oil pump on my small-block Chevy in order to get smoother flow and more volume with a stock volume pump? Will that work?

D.B.

Jeff Smith: It seems there’s quite a bit of confusion regarding oil pumps and oil pressure so let’s focus on what’s happening. We’ve covered a part of this question before, so we won’t go into all the details again, but it is possible to use a big block Chevy oil pump on a small block. Several years ago, Steve Brule’ and I performed an oil pump pressure and oil viscosity test on a small block Chevy using Westech Performance’s dyno. The last pump we tried was a standard-volume big block Chevy oil pump compared to a high volume/high pressure small block oil pump. The reason you would want to use the big block pump is for additional volume as the Rat pump’s body is deeper than a stock small block pump and the pickup tube is larger in diameter.

What we found was the big block pump appeared to enjoy a slight advantage in terms of power over a small block high-volume/high pressure pump. Comparing the big block pump to the small block, the biggest change besides larger gears was the big block uses a greater number of teeth on the spur gears used to create the pressure. We think that the small advantage might be attributable to the greater number of teeth on the spur gears creating a more even flow (reduced pressure fluctuations) compared to the small block’s courser tooth pattern. We have no way of supporting this theory.

There are other issues that may affect this decision. First of all, because the pump is deeper, the pickup you choose may or may not fit the oil pan properly. It’s critical that the oil pump pickup be located roughly ¼ to 3/8-inch above the floor of the oil pan in order to not suffer from cavitation issues with low oil levels during hard acceleration. The oil pump will also have to fit between windage trays, pan baffles, and other obstructions that may require custom work in order to facilitate the use of this big block pump.

So the answer is yes, you can use a big block pump, but only if you can mount it with the proper position of the oil pickup. If the pickup tube cannot be properly located, then you have just lost any advantage you might have over a small block pump.

It’s also worth mentioning that the ability to create more oil pressure is not a good reason to use a big block pump. High oil pressure – above 60 psi for example – is not something to strive to achieve. There are plenty of examples of engines using high-pressure/high-volume oil pumps that produce much more pressure than is necessary. The old rule of thumb was 10 psi for every 1,000 rpm of engine speed. That isn’t a bad plan but frankly it is on the high side.

For street engines, oil pressure at idle are near ideal at 20-25 psi and maximum pressure at peak rpm can be 50 to 60 psi, assuming a maximum engine speed of less than 7,000 rpm. Hopefully, this information will help you with a decision as to whether you really need a big-block pump for a mild street engine. Keep in mind that oil viscosity also plays a part in how much pressure is created, especially when the engine is cold. It’s important to not put too much load on the oil pump drive shaft and gear with thick oil when the engine is cold as this can create excessive wear on the entire system.