E. Woods,
I have a 3 qt. Accusump and I'll echo what Tom Wells said. Here's the way I see it working. You start with an initial small charge of air inside the Accusump at relatively low pressure. When the engine starts and the
oil system pressurizes,
oil will force its way into the Accusump, compressing the air into a smaller space. When the pressure of the compressed air matches the
oil pressure, the Accusump will be fully charged. The amount of oil in the Accusump and the amount of pressure it is under is determined by the size of the initial air charge and your engine's oil pressure.
When your engine loses oil pressure (e.g., a long sweeping turn pulls oil away from the pickup in the pan), the compressed air will begin to push oil back into the system. As oil is pushed out, the space for the air expands and its pressure will begin to drop. The rate at which it drops depends on how much air you had in the Accusump. The dilemma is this: If you start with a larger air charge, the pump will hold less oil, but it will maintain a higher pressure longer when it has to push it back into your engine. Conversely, you can use a smaller air charge to get more oil into the Accusump, but the pressure will drop more quickly when the Accusump is pushing oil back to the engine. Because neither situation is ideal, it is a good idea to have a larger Accusump so you can have both a larger oil reservoir and a larger air charge.