Oil issues: Dry Sump, Accusump for road racing?

[kb58]

I tend to write epic...

Though an outsider to both Subaru drivetrains and Factory Five products, I've used baffled oil pans, Accusumps, and a dry sump, so would like to contribute to this discussion. My one-off (Midlana.com) uses a Honda K24 mounted mid-engine and is turbocharged. Honda K-series engine suffer similar oil starvation in turns but in a different way from Subarus. The Honda I4 engine is mounted with the timing chain on the right side of the car and in long left turns, the oil sloshes to the right and covers the front sprocket and timing chain. The timing chain unfortunately acts like a conveyor belt, lifting oil up out of the pan and flinging it into the cylinder head. A very different engine but with similar results due to hard cornering.

So there's the Accusump. For those who don't know what it is, think of plumbing a rubber balloon into the oil system. When the car starts, oil pressure inflates the balloon until equilibrium is reached. If at any time oil pressure drops, the balloon pushes oil back into the oil system to maintain pressure - that's pretty much it in a nutshell. I had one but got rid of it, and to explain why, well...

The mechanical shutoff valve should be used for mechanical reasons, better flow, reliability, and lower cost. The problem is that you *will* forget to open it before starting the engine, losing the "pre-oiling" feature. That in itself isn't a fault of the product, it's just something you have to remember before heading out on-track. Likewise, when you come in off-track and shut everything off, if you forget - and you will - to close the valve first, 2-3 qts of oil get pushed back into the system when you kill ignition. That's fairly harmless, but it means when you go to start the car, you again don't get the "pre-oiling" feature, and have 2-3 extra qts of oil flaying around until you open the valve and the Accusump charges. Again, it's not really the product's fault but is, again, something you have to remember to do. There's that, and then there's something someone told me that ate at me until I decided it was a real problem. Disclaimer: This is only a theory of mine which may not be completely true, but I've fully bought into it.

The theory is that there are problems with using an Accusump and assuming it's fixed the oil supply problem. The problems are all related to how oil in a traditional oil pan takes on the consistency of chocolate milk when the engine's running hard due to air getting whipped into it, sometimes a lot of air. So you head out on-track and really start leaning on the engine. As you probably know, the crank doesn't actually touch the bearings, it's floating on a layer of oil, and that's great, until it isn't. The problem is that when on-track and at high rpm, oil viscosity drops way off and the high rpm "stirs" a tremendous amount of air into the oil, sometimes as much as 50%. This means that the "wedge" of oil keeping the crank away from the bearings is no longer 100% oil, but is maybe as low as 50% oil. This effectively cuts the viscosity in half again because the air compresses easily in comparison with the oil, and means the crank is riding dangerously close to the bearings. Unfortunately, the oil pressure gauge is no help because 60 psi oil pressure reads the same as 60 psi air pressure. Additionally, the Accusump can't help because the air/oil emulsion is at the same pressure as the oil in the reserve tank, so the Accusump doesn't push any oil back into the system.

Another situation: You're cornering hard when all the above is going on and oil sloshes to one side of the pan, allowing the oil pump to start sucking in larger bubbles. Because they're bubbles, they're followed closely by more oil, so the air gets compressed along with it. In this situation you're right back to the above situation, where you have 60-psi air bubbles in your 60-psi oil, so again, the Accusump doesn't get involved, the oil pressure gauge continues to read normally, but your steady supply of oil to the bearings is now "hiccupping", further worsening the oil situation.

So during the hard corner, the pump finally sucks in enough air that pressure drops off. If this is the first time it's happened, the Accusump works as designed and pushes fresh oil into the system to prop up pressure. But suppose this is the second, fifth, or tenth time it's happened on-track. When pressure is regained after the corner, the oil pump dutifully recharges the Accusump - with an air/oil froth. Have that happen a few times and now the Accusump now contains a 60-psi oil - and air mixture - ready to be pushed back into the system. Is it better than nothing, yes, but just that.

Related to the above, air takes a long time to separate out from the oil. In a dry sump tank, the returning oil is fed into the tank tangentially such that it swirls around the wall of the tank, imposing high G-forces on the mixture. The force de-aerates the oil far faster than it does sitting still inside an oil pan or Accusump. In the case of the Accusump there's another potential problem, that if and when that air separates out from the oil, where does it go? It stays in the Accusump, pressurized to 60-psi just like the oil, ready to be fed back into the engine the next time there's a need. If the Accusump is oriented with the outlet upward, that big air bubble is going to be the first thing into the oil system. If the tank is oriented with the outlet downward, the air remains trapped within the tank. This is a problem that gets worse over time because as the air continues to "off-gas", it forms a bigger and bigger bubble at the top of the tank. After a while, the "2-3" quart Accusump actually contains maybe 1.5 quart of oil and 1.5 qts of air.

In addition to all the above, there's one more thing which may or may not be a problem, which happens coming out of a turn that required the Accusump to "dump its load" into the engine. In that situation, the OEM oil pump is having to supply full oil pressure to not only the engine, but also the partially-empty Accusump reservoir. Does the OEM pump have the capacity to send oil to both at the same time? Who knows.

I'm afraid that in the case of the oil system, you really do get what you pay for. The harder you plan to run the engine and the more G forces it's going to see, the more a dry sump becomes less optional. As said above, this is a working theory of mine that I've "decided" is true without doing any testing with an Accusump. If someone using an Accusump to fix a known oiling problem finds no bearing issues during a rebuild, well, maybe I've decided too much.