Dry Sump Secrets
Most people who want a dry sump know its most important function -- it provides oil to the pump even in high-G turns and sweepers. But there are many inobvious facts about its functions.
1. Fact. A dry sump system removes foam from the oil so it lubricates better. Subfact: It doesn't do it perfectly. NASCAR uses centrifugal oil/air separators added to the dry sump system. The rest of us rely on an anti-foam agent in our oil to reduce foaming, which is the same as in stomach pills: Simeticone!
2. Fact. Dry sump tanks must be round in order to maximize oil swirl on entry, which greatly helps "deaeration" (getting the bubbles out) of the incoming oil. Subfact: Round is better but there are many square or tall rectangular tanks that race every day. Swirl counts, but apparently depth matters. Depth gives the oil more of a chance to deaerate as it gets sucked down to the outgoing hose.
3. Fact. A dry sump system still needs an oil cooler. Subfact: A dry sump, like a deep sump, contains more oil than a standard system and therefore it takes longer to get too hot, but it will get there. And a dry sump tank and lines contribute to cooling, but not as much as you'd suspect.
4. Fact. Dry sump systems still need catch cans. Subfact: A dry sump tank should be filled to about 7/8th capacity, to provide expansion room (the "headspace" for the hot oil. Most guys never think about oil expansion, but it does expand. When an over-filled dry sump "burps" out of its vent, you are likely to get a litre of oil out of it all at once.
5. Fact. Most dry sump systems need an oil tank heater for 40-degree (F.) or less weather. Subfact: Heaters commonly are AC current and consist of a "electric blanket" wrapped around the tank. But most hobby or daily driver VW's do not have heaters, so a respectful warm-up interval is advised to avoid blowing oil filters or coolers just getting out of the driveway. Special plumbing configurations with thermostatically controlled bypass valves can be installed to dump oil pump overpressure back to the oil tank until you reach about 180 degrees F.
6. Fact: A small oil tank, depending on application, can serve as well as a large tank. Porsche ran 6-litre oil tanks on their Type 547 engine 4-cam Spyders. One reason was oil is heavy and the factory only wanted to carry just enough to do the job. Today, the average round oil tank for the VW holds a heavy three gallons. Subfact: There is always more oil in a 6-litre system than 6 litres. Don't forget the oil coolers (2 each on a 550 or 550A Spyder), the oil lines, the engine case residual, and the oil filter.
7. Fact. The CB Performance dual-stack dry sump pump is only a single pickup pump, as opposed to pumps that have scavenging pickups in the heads, valve covers, etc. Subfact: Even being a single pickup, the CB withstands endurance racing, 7 to 24 hours at a time very well. With the engine running, the CB system will show about 1/4 inch, or 6mm of oil at the bottom of the stick. Kind of scary the first time you see it. The truth is, the VW dipstick exits its guide tube and fully depressed, stops about 11mm short of the case floor.
8. Fact. The CB pump system is simple but so efficient it sucks all the running oil out right down to the oil pickup level. Subfact: This fluctuates, and at times, the pump is also evacuating case pressure, which really helps internal case windage and adds a little power on its own.
9. Fact. A dry sump system is really two oil systems as opposed to one. Subfact. This takes a little getting used to -- one oil system is delivering oil to the engine bearings. The other oil system is pulling oil out of the case and sending it to the filter, cooler, and oil tank. As a real-world example of the two systems, we've had the case pickup get clogged (long story how), but the oil delivery side of the system was working. The engine case filled up because no oil was being returned to the oil tank. And the opposite can happen as well.
This is by no means all there is to know about a dry sump system, just a highlight overview.
FJC
