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Lubrication
| Viscosity | Other oil properties | Oil changes | |
| Oil filters | Oil level | Oil flush |
Oil plays an extremely important role in an engine, even more so if it's turbocharged.
- It keeps the metal parts from touching each other and becoming one large metal blob
- It cools many hard-to reach places, as the coolant passages are not everywhere
- It keeps the engine clean by storing all solid contaminants in the oil filter and keeping the rest in suspension until they are drained out along with the oil.
Lubrication is not equally critical in all parts of the engine. Some parts may go on running with no damage even if they are starved from oil flow for a few seconds, but others can't. The turbo bearing is one such example.
It's called a bearing, but it's not like the camshaft or the crank bearings at all. It's a floating bearing, meaning that it's a ring within a ring and these two are a bit 'loose'. The bearing surface really is pure oil under pressure between them.
As long as there's oil pressure this works well and leads to low frictional losses and negligible wear. But we have to remember that during full boost the turbo shaft revs at 100Krpm, maybe more. To complicate matters, there are strong forces trying to 'rock' the shaft about, and shift it along it's axis as well. It gets worse: air pressure differences may develop between the intake and the exhaust side of the shaft, and these will lead to the bearing oil being sucked in the exhaust or the intake. That doesn't help the bearing's effectiveness.
The oil is meant to be delivered to the turbo under pressure, (the banjo bolt at the top) then follow the path of gravity and go through a maze of tiny oil galleys, through the bearings and finally drain into the sump. Hence the sump oil level should always be lower than the turbo drain pipe. Sometimes this doesn't happen, either because there is too much oil (and it's kicked around by the crank) or because the sump is abnormally pressurised (faulty ventilation, piston blowby). In this case the turbo oil return cannot flow - it's stuck. New oil is shoved in the turbo bearings under pressure, and the old one has nowhere to go (or drains at a lower rate). This excess of oil will inevitably be pushed either through the intake or the exhaust side of the shaft, depending on the engine revs and load (the air pressure ratio at the time). Won't do the turbo any good, that's for sure...
During the short stay at the turbo bearing, the oil has a very hard time. It gets a beating that could break it apart. It looks like whipped cream, so much is the froth and the thermal stress. There is no place in the engine where the oil is tested so hard: this is important when deciding on oil quality.
This is the most well-known, and probably most important of oil properties. Oil of SAE (Society of Motor Engineers) 10 is thinner than SAE50. But that stands for single-viscosity oils. Manufacturers tend to recommend multi-viscosity oils, like 10W40. This means that as you start the engine in the winter (when oil wants to be 'thicker') this product will perform like a straight SAE 10 and at operating temperatures (when oil wants to be thin as Kate Moss) it will behave as if it were a straight SAE 40.
How do they do that? In the case above, they would typically use a base of SAE 10 and then add Viscosity Improvers. Simple (for them) and it works most of the time.
...But there's a snag: under extreme stress this 'cocktail' may momentarily break apart and revert back to the base (in this case SAE 10). Cams and turbo bearings may not be happy at that point (that's why many cam manufacturers recommend 'breaking in' their products with straight SAE 30 or thereabouts). Turbo bearings can get extremely hot, and the whipping action of the shaft brings out the worst in this sort of oils. If not changed very regularly they completely break apart and the VIs evaporate - you're left with a thin crispy base: this may be fine for a pizza, but the turbo (and the rest of the engine) will disagree.
VIs are a real pain for turbos, because they are the same substances that tend to carbonise on the ultrahot surfaces inside a turbo. Getting rid of them would be nice, but then we'd have to change oil every season (manufacturers' marketing departments would never agree to this). So we're stuck with them, and try to minimise their effect by using only fully synthetic oils (they are far more stable) and regular oil changes.
There is an exception, and that is Mobil1: This uses no VIs, but a patented sequence of distillations. As a result it doesn't coke and keeps it's viscosity for way longer than anything else. There was an independent test publicised on the internet a few years ago, where enthusiasts used their bikes to compare different brands of oil. A viscosity tester was used before and after each test. Synthetics came out ahead of mineral oils (in general) and Mobil1 was a long way ahead of the best synthetic: It managed to hold 90% of it's original viscosity after 2500miles, while the others had lost it after around 1000 miles. Even their competitors agree - if we read between the lines! (local copy here)
When I was into turbo bikes, most of them were air-cooled, so oil had a very hard time indeed. I had tried several oils on my 750turbo, and after stripping it down, Mobil1 was ahead by a long shot. It was the only oil that didn't leave a black sludge in the sump. Here is another 'independent test' that has done the rounds of the internet in several guises. Look at the bottom and in 'viscosity retention' Mobil1 is highest (it's based on motorbikes that also use engine oil for the transmission). Pity they don't have figures for 5K miles though. (local copy here)
Viscosity is not the only game in town. Oils do oxidise, even if they are not used at all. Antioxidants (of various shapes) have to be present. Also detergents and dispersants: contaminants and debris have to be moved away via the oil, and get trapped within the oil filter. If the engine is dirty inside, then a good oil will get dirty very quickly. That is a sign that it manages to keep all the rubbish in suspension, protecting the engine. It's also a sign that it's time to change it, or else the suspended garbage will end up everywhere!
There is a lot of debate on how often should engine oil be changed.
The truth is that this varies, depending on the:
- Engine design (very old engines might have different clearances and oil needs, but then oil spec is just one of the incompatibilities there)
- Car use - motorway miles are far easier on the oil than start/stop city traffic, or a track day
- Ambient temperature - cold winters degrade the oil very quickly
- Number of warm-up periods - each time the engine cools down, there is condensation everywhere inside, especially in the winter. For the first 10 minutes or so, all this condensation is picked up by the oil and you're effectively running on an oil/water mixture. When the engine reaches operating temperature (coolant and oil > 90C) then it starts getting rid of this water (evaporation) but some of it remains diluted in the oil, making it thinner and less effective. Repeated warm/cooldown cycles with the engine never really maintaining operating temp for a while, lead to oil degradation far sooner than one might think. These short runs in the winter can kill the oil within a month or two even if the mileage in that period is minimal (because it was minimal in reality!)
- Quality of oil itself - mineral oils tend to break up easier than semi-synthetics and these don't last as long as fully synthetics.
- State of the engine - a dirty engine will overwhelm the oil with rubbish. Even the best oil cannot cope with this and only an oil flush can ease the situation. Also poor-sealing piston rings will allow fuel to find it's way down to the sump. This is even worse than water dilution, because oil and fuel mix really well, and the result is dead oil. Apart from optimising crank ventilation (or fixing the rings!) frequent oil changes are the only remedy.
Car manufacturers like to quote long oil-change intervals, because it makes their products look like low-maintenance appliances (the marketing department really loves that!). They claim that this is down to improved designs, superior metals, availability of good oils etc. etc... All these are valid arguments, but after 10K (sometimes more) miles the state of the oil can be horrific. Even the best oil in the world will deteriorate if it's mixed with liberal quantities of water and fuel, then is heated up at 100C and stirred at 8000rpm.
Perversely, oil manufacturers don't try to dispel this 'low-maintenance' myth, maybe because it would make their own product look inferior to the competition (If Mobil were to recommend that their ultra-expensive top synthetic needs changing every 3K miles, and Shell recommended 10K miles for their base £10/barrel stuff, which one would YOU buy?) <local copy here>
My experience is that turbocharged petrol engines should only be fed fully-synthetic, changed every 5k miles (at least!). If a lot of start/stop occurs in short winter trips then this should be brought down to 2K miles.
A lot is expected from the oil during start-up, as it has all drained down in the sump and there's only a very thin film covering the parts that are about to start rubbing themselves. Turbos also create expectations for lubrication right after shutdown. There is acute need for lubrication around the turbo bearing immediately after the engine is stopped, but the oil pump is on lunch break and there is oil pressure no more! There is the myth that the turbo keeps spinning for a long time after the engine is shut - complete rubbish, if you look at a turbo idling and then shut, it stops almost instantaneously. If it were doing full boost and the ignition were suddenly killed (very stupid indeed!) then it might take a couple of seconds for the turbo to come to a complete stop. You get the idea.
It's not that the turbo stays spinning forever after, but that the temperatures around the turbo bearing suddenly shoot upwards (not that they were low before!). The flow of pressurised oil takes with it a lot of heat, and this stops in its tracks when the engine is turned off. Therefore the remaining tiny quantities of oil are expected to keep the shaft from melting within everything it touches (one side of the shaft has the turbine, which can be white-hot at times! How do oils cope with this? With difficulty!
What happens is that the oil is trapped between an extremely hot shaft and a superheated bearing. Lubrication is the last thing in the oil's mind - survival is more like it. It is baked and all the additives evaporate. The VIs turn into a carbon form, hard as a diamond. What's left is some black ash and not much more. Repeated cycles see this build-up altering the shape of the oil galleys. Not pretty, and it's practically impossible to clean this stuff without dismantling the turbo. No oil flush or 'magic in a bottle' will get rid of it. It just adds up.
Same as with oils, no heavy-weight independent tests can be found on this subject. The commercial interests are so large, that the cost of lawsuits afterwards will cripple whatever organisation tries to clear up the muddy waters.
Enthusiasts have cut down filters though, and put them to the test, posting the results and the photos on the internet. (local copy here)
The general picture is the following:
- OEM oil filters tend to be the best available
- Many expensive 'big name' performance filters are rubbish
- Some cheap generic ones are almost as good as the OEM ones
- Some aftermarket filters (irrespective of price) can be dangerous - either they let large particles through, or they are not robust enough and collapse under high oil pressure, or their bypass malfunctions.
Race oil filters have a lower 'micron' rating, meaning that they stop even the tiniest particles that would otherwise flow freely through OEM-spec filters. This is a dangerous thing for someone who might forget to change this filter after a few hundred miles. It will clog up quite fast (as it's meant to) and then the relief valve will override it, letting all sorts of garbage circulate. Very dangerous!
The oil filter should be always changed along with the oil. The new filter should also be filled with oil before installation, or else during the first few seconds that it takes the pump to fill it with oil, the turbo will be running on oil fumes! After a few such oil/filter changes people complain that the 'turbo burns oil', but it's their carelessness that killed it.
This is an important issue, as there are warring factions once again, the full mark, the low mark and the middle earth people.
First of all the oil level should be checked (ideally) with the engine warm, but after it's been left to cool down for 10 minutes. The difference between the LOW and the FULL setting on the indicator is usually 1lt. Running higher than FULL is stupid, since the crankshaft will splash on a lot more oil, eating up power and reducing the remaining oil into froth. At high revs a lot more oil will be stuck at the wrong places (due to centrifugal forces of the crank) and the sump pressure will be higher. Even worse, the turbo oil return will struggle to flow properly, and we know that's not a good thing.
Don't think that you're immune from something silly as that, because nobody is. I've lost count of the times garages have overfilled my sumps, I think it's the norm rather than the exception. I don't know the exact reasons behind this conspiracy, but probably it's a combination of 'can't be bothered' and the fresh oil being transparent and not easily visible on the stick (especially under bright light). Beware.
Maximum power will be available when the oil level is at MIN. However this is not safe, as during sharp turns the oil pickup may suck in air or bubbles, and they are not a good lubricant for the turbo. My favourite is slightly below the middle mark, but if you're going on a long trip better play it safe and keep it near MAX.
The following procedure should be credited to 'Mycroft', a colourful character from the Skyline forum The best flush is as follows:
- Drain the existing oil/filter while it's still warm. Let it drain completely.
- Pour in a good quality *thin* oil, like a straight SAE10 or SAE20
- Drive the car around for 20-30 miles without revving it too hard. Not motorway miles, just around the neighbourhood, it needs varying speeds and loads.
- Drain this oil/change the filter (again). Let it drain for as long as possible
- Pour in the new oil.
This is arguably much better than any flush can.
Importance of regular oil changes
This issue cannot be overstated. If there is a single maintenance operation that you intend to perform on the engine, that should be regular oil changes.
These are pictures of a supra engine, freshly imported from Japan, where the owners had missed an oil change or two. Maybe three...
They were posted in SupraForums some time ago and present a good example on the horrors that can be hidden inside a neglected engine
Link to the excellent no-nonsense oil forum
Myths about synthetic oils, as well as facts. Keep in mind this is a commercial site that also sells them, though (local copies here and here)
More than you ever wanted to know about oils from MKIV.com(local copy here)
Slick50: more sceptics than fans (local copy here)
Snake oil galore! (local copy here)
Even more interesting stuff, with comparisons (local copy here)
Finally, here is an interesting article about synthetic oils:
On to Fuelling...
