Kevin Johnson
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The slant six has a well deserved reputation for durability under normal operating conditions. However, there are some very real issues with the oil system that should be considered if you really "push" your engine or race it.
In most automotive applications the slant six pan design (and there are numerous pans) has a very shallow sump that allows the sump oil to migrate backwards and/or collect there if the car is under steady acceleration forward -- at even fairly modest levels. This can happen under braking as well in the front of the pan. The oil is then struck by the rotating assembly and whipped full of air. This "entrainment of air" is different from simple oil foaming. It takes much longer for this air to be released than for foam to be dispersed and broken.
This air entrainment can also happen in competition when the car enters high speed left turns and the oil collects in the rear corner of the pan. The clockwise moving rotating assembly tries to push the oil up the side of the block and in the process churns it full of air. This is simply an inherent issue when using an engine tilted to its side. BMW racers are very familiar with this issue and have dealt with it for at least 50 years now.
If enough air is entrained into the oil this can lead to rod bearing failure and other issues. When highly aerated and pressurized oil passes through the lubrication circuit any abrupt changes in the flow path can cause localized pressure drops. The localized pressure drops allow air that is dissolved in the oil to come out of solution and collect into bubbles. Yes, air really does dissolve into your oil - it is a normal thing. The higher the oil pressure, the more air that can be dissolved, typically at 9% per Bar pressure.
It is extremely important to realize that this high aeration or entrainment can occur at relatively low to medium engine rpms. Try to imagine the big end of a rod whipping through a puddle of oil at ten times a second -- that is merely idling speed (600 rpms). People testing engines alone on stands or in cars on roller dynos can sometimes forget what happens when significant movement of the engine/car is introduced.
These entrained bubbles can cause damage in the bearing shells. In fact, the slant six has been known over its production history for losing the number 5 rod bearing and now you know why. There are fixes to the oil circuit that have been developed by very talented tuners and engineers, notably Doug Dutra.
When the slant six had its crankshaft redesigned for lighter weight and higher efficiency one of the major changes was to dramatically reduce the swept path of the counterweights. At the same time, this helped to reduce the amount of air churned into the oil. Why?
Hydraulic lifter circuits were being developed for the engine. Hydraulic lifters will not tolerate extremely aerated oil: by the time oil reaches the lifters in a dynamic hydraulic circuit the pressure has lowered and air that was dissolved under higher pressure at the pump would be coming out of solution. A fluid with air bubbles in it is compressible and will cause erratic lifter operation.
Another more esoteric problem that emerges at high engine speeds (approximately 6000 rpms or that neighborhood) is vibration from cavitation at the oil pump -- from the aerated oil. This chaotic vibration and shock loading was not contemplated when the small "point contact" oil pump pinion drive gear was designed. Racers have reported loosing pumps in this rpm range. Then too, six cylinder inline crankshafts have a third order harmonic that emerges around this rpm. Overall, a difficult situation.
All in all, the best solution to the entrainment problem is to try to prevent the excess air from being churned into the oil in the first place.
On dot org there is a technical article I suggest reading: http://www.slantsix.org/articles/oil-pump-gear/failure-fix-report.htm
The performance oil pump mod suggested below should help:
One thing this article does not address is the aeration of the oil. The gerotor designs of the pumps (both the cloverleaf and star gear pumps are gerotor designs) have some elements that help to reduce aeration and entrainment of actual bubbles in the oil as well as possible cavitation damage.
The passages or cavities in the pumps allow the changing volumes in the pumps to communicate with each other. In doing so they allow free air in the oil to transfer backwards and be worked more gradually into the oil whether by dissolution or entrainment. The pressure relief bypass back into the low pressure side of the pump makes it more efficient with respect to the volume of oil needed to feed it, directly from the sump.
The modifications in the picture below introduce a dedicated deaerator circuit into the pressure cavity. The small amount of oil and large amount of gas allowed to be bypassed back into the sump area performs a useful secondary function by oiling the pump drive gear. This additional oil will help with the known camshaft and pump drive problems. By allowing much more entrained gas to escape more quickly from the internals of the pump, the shock loads to the gears and their loading areas will also be reduced. People running hydraulic cams may notice an immediate power boost at high rpms due to more stable lifter operations.
In most automotive applications the slant six pan design (and there are numerous pans) has a very shallow sump that allows the sump oil to migrate backwards and/or collect there if the car is under steady acceleration forward -- at even fairly modest levels. This can happen under braking as well in the front of the pan. The oil is then struck by the rotating assembly and whipped full of air. This "entrainment of air" is different from simple oil foaming. It takes much longer for this air to be released than for foam to be dispersed and broken.
This air entrainment can also happen in competition when the car enters high speed left turns and the oil collects in the rear corner of the pan. The clockwise moving rotating assembly tries to push the oil up the side of the block and in the process churns it full of air. This is simply an inherent issue when using an engine tilted to its side. BMW racers are very familiar with this issue and have dealt with it for at least 50 years now.
If enough air is entrained into the oil this can lead to rod bearing failure and other issues. When highly aerated and pressurized oil passes through the lubrication circuit any abrupt changes in the flow path can cause localized pressure drops. The localized pressure drops allow air that is dissolved in the oil to come out of solution and collect into bubbles. Yes, air really does dissolve into your oil - it is a normal thing. The higher the oil pressure, the more air that can be dissolved, typically at 9% per Bar pressure.
It is extremely important to realize that this high aeration or entrainment can occur at relatively low to medium engine rpms. Try to imagine the big end of a rod whipping through a puddle of oil at ten times a second -- that is merely idling speed (600 rpms). People testing engines alone on stands or in cars on roller dynos can sometimes forget what happens when significant movement of the engine/car is introduced.
These entrained bubbles can cause damage in the bearing shells. In fact, the slant six has been known over its production history for losing the number 5 rod bearing and now you know why. There are fixes to the oil circuit that have been developed by very talented tuners and engineers, notably Doug Dutra.
When the slant six had its crankshaft redesigned for lighter weight and higher efficiency one of the major changes was to dramatically reduce the swept path of the counterweights. At the same time, this helped to reduce the amount of air churned into the oil. Why?
Hydraulic lifter circuits were being developed for the engine. Hydraulic lifters will not tolerate extremely aerated oil: by the time oil reaches the lifters in a dynamic hydraulic circuit the pressure has lowered and air that was dissolved under higher pressure at the pump would be coming out of solution. A fluid with air bubbles in it is compressible and will cause erratic lifter operation.
Another more esoteric problem that emerges at high engine speeds (approximately 6000 rpms or that neighborhood) is vibration from cavitation at the oil pump -- from the aerated oil. This chaotic vibration and shock loading was not contemplated when the small "point contact" oil pump pinion drive gear was designed. Racers have reported loosing pumps in this rpm range. Then too, six cylinder inline crankshafts have a third order harmonic that emerges around this rpm. Overall, a difficult situation.
All in all, the best solution to the entrainment problem is to try to prevent the excess air from being churned into the oil in the first place.
On dot org there is a technical article I suggest reading: http://www.slantsix.org/articles/oil-pump-gear/failure-fix-report.htm
The performance oil pump mod suggested below should help:
One thing this article does not address is the aeration of the oil. The gerotor designs of the pumps (both the cloverleaf and star gear pumps are gerotor designs) have some elements that help to reduce aeration and entrainment of actual bubbles in the oil as well as possible cavitation damage.
The passages or cavities in the pumps allow the changing volumes in the pumps to communicate with each other. In doing so they allow free air in the oil to transfer backwards and be worked more gradually into the oil whether by dissolution or entrainment. The pressure relief bypass back into the low pressure side of the pump makes it more efficient with respect to the volume of oil needed to feed it, directly from the sump.
The modifications in the picture below introduce a dedicated deaerator circuit into the pressure cavity. The small amount of oil and large amount of gas allowed to be bypassed back into the sump area performs a useful secondary function by oiling the pump drive gear. This additional oil will help with the known camshaft and pump drive problems. By allowing much more entrained gas to escape more quickly from the internals of the pump, the shock loads to the gears and their loading areas will also be reduced. People running hydraulic cams may notice an immediate power boost at high rpms due to more stable lifter operations.
