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Scargo
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- TL;DR Summary
- If I experience low oil pressure in a high G situation, how can I best intervene to stabilize the oil pressure? Where in the path of oil flow would I want to provide supplemental oil pressure and flow?
Subaru flat fours (a boxer engine design) are notorious for losing oil pressure under high lateral G's. The usual solution is using a dry sump. Dry sumps are expensive and complicated. They are, based on currently available remedies, unnecessary until you exceed 1.5 G's. I am at that threshold. So far I have not had an oil pressure related failure. One such remedy to losing oil pressure is questionable as to efficacy. The oil accumulator, which is an Accusump in my case, provides oil to the engine when the oil pressure drops below a certain threshold. Two or three quarts of oil can be added to the capacity.
There is debate about whether it works. One engineer I know says it is a placebo and that the engine's oil pump is working to recover a reasonable oil pressure and having to refill the Accusump at the same time. This issue can and should be an integral part of the discussion as far as I am concerned. This issue seems equally relevant and a part of the equation (or possible solutions).
Here is a link to a post and illustration of the oiling system.
During racing I have seen a 10 to 15 PSI pressure drop from the pump's output to the point that the remaining oil is feeding only the cams and turbo. The consumption of the main and rod bearings have lowered the pressure by that much. The oil accumulator is designed such that oil can flow out of it when the pressure is below the desired setting. 10 to 15 pounds is recommended.
Here is where I get into the weeds. A lot of torque, low RPM's and/or low oil pressure is a recipe for disaster in any engine. Oil protection can vary by viscosity, oil properties, heat, bearing clearances and resistance to friction of the mechanical components. Perhaps, for the sake of no arguing about brands, we leave the lubricants out of this? Let's just assume a "normal" operating situation where we are not overheated and we are using a good racing oil.
So, there is a magic number, at below which the pressure will not support a protective film of lubricant. There is also the possibility that the pump has started sucking air and you have aerated oil.
Regardless, the pressure has dropped to the 10-15 PSI threshold of the oil accumulator and it starts to release the oil. Where would you like for it to go? If it's before the pump wouldn't it need a check valve? It could be after the pump. Might it then fight the pump by providing a pressure resistance? Do you inject the oil before or after the main bearings? If after, (I'm assuming) some will go to the main and rod bearings and some will go to the heads.
In this engine, like most pressure systems, the farther from the pump you go the smaller the passages become to maintain the pressure. Whether it's my choice of bearing clearances (standard for a racing Subaru engine) and/or exceeding the design parameters of the pump and oil galleries, I am seeing a significant drop in oil pressure as it transitions from the crankshaft bearings to the heads, with the current design. The design is such that it's the dynamics of the fluid and pressures (and possible additional variables) that I'm have trouble with. Where and how can I best supplement the oil supply? Other remedies?
There is debate about whether it works. One engineer I know says it is a placebo and that the engine's oil pump is working to recover a reasonable oil pressure and having to refill the Accusump at the same time. This issue can and should be an integral part of the discussion as far as I am concerned. This issue seems equally relevant and a part of the equation (or possible solutions).
Here is a link to a post and illustration of the oiling system.
During racing I have seen a 10 to 15 PSI pressure drop from the pump's output to the point that the remaining oil is feeding only the cams and turbo. The consumption of the main and rod bearings have lowered the pressure by that much. The oil accumulator is designed such that oil can flow out of it when the pressure is below the desired setting. 10 to 15 pounds is recommended.
Here is where I get into the weeds. A lot of torque, low RPM's and/or low oil pressure is a recipe for disaster in any engine. Oil protection can vary by viscosity, oil properties, heat, bearing clearances and resistance to friction of the mechanical components. Perhaps, for the sake of no arguing about brands, we leave the lubricants out of this? Let's just assume a "normal" operating situation where we are not overheated and we are using a good racing oil.
So, there is a magic number, at below which the pressure will not support a protective film of lubricant. There is also the possibility that the pump has started sucking air and you have aerated oil.
Regardless, the pressure has dropped to the 10-15 PSI threshold of the oil accumulator and it starts to release the oil. Where would you like for it to go? If it's before the pump wouldn't it need a check valve? It could be after the pump. Might it then fight the pump by providing a pressure resistance? Do you inject the oil before or after the main bearings? If after, (I'm assuming) some will go to the main and rod bearings and some will go to the heads.
In this engine, like most pressure systems, the farther from the pump you go the smaller the passages become to maintain the pressure. Whether it's my choice of bearing clearances (standard for a racing Subaru engine) and/or exceeding the design parameters of the pump and oil galleries, I am seeing a significant drop in oil pressure as it transitions from the crankshaft bearings to the heads, with the current design. The design is such that it's the dynamics of the fluid and pressures (and possible additional variables) that I'm have trouble with. Where and how can I best supplement the oil supply? Other remedies?