As for performance engines... from a "performance" perspective we are more interested in how the oil behaves when:
- temps are high (heat soak from track use with high proportion wide open throttle)
- revs are high (engine has been tuned)
- you have/don't have an oil cooler
- you have/don't have a dry sump
- the engine is an older design
- the engine has done more miles
Here is engineering 101 about hydrodynamic lubrication (main and big end bearings):
- the design of the engine determines the optimum viscosity. By design I mean:
-- diameter of the bearing journals
-- width of the bearing journals
-- surface speed of the bearings (combination of diameter and revs)
-- surface roughness of bearing surfaces
-- filtration
-- load
Forget the much vaunted "tighter clearances of modern engines" bunch of generalised arse. Here is what is actually going on.
Modern engines tend to have narrower bearings (more compact design, less crank flex, etc.). Modern engines tend to have larger diameter bearings. This means that the surface speed of the bearing is higher. Modern engines tend to have higher capacity oil pumps because oil spills out of the sides of this type of bearing quicker. Overall you get greater throughput of oil, keeping the buildup of temperature in the bearing to a minimum (which keeps the bearing material from breaking down). With the higher bearing speed you need a lower viscosity oil in order for the bearing to establish the film thickness (hydrodynamic wedge carrying the load) you require to cope with your surface roughness and the contaminant particles left behind by your filtration system.
Please note that of all of the above design parameters I haven't yet mentioned "load", but so far our modern performance engine finds itself happily using a lower viscocity oil.
This is why modern engines come from the factory with lower hot viscosity oil recommendations than older designs.
If you put a higher viscosity oil in there, the film thickness has no problems, but you get greater build up of temperature in the bearings, which could lead to premature bearing damage. If you drive your engine hard before the oil gets up to temperature you could get some problems from more thermal gradients across the bearing materials. There will also be mechanisms to do with shear stability of the oil molecules and oil film.
When we bring load into the equation it becomes interesting because the critical loads depend on reciprocating mass and reciprocating accelerations. A modern engine tends to have lower reciprocating mass. Many modern engines also have rev limits in factory spec that were unheard of a few years back.
Now we take our engine (archaic or modern) and as a result of tuning we increase its rev limit, resulting in increased bearing loads. If nothing else had changed, a greater load would demand a more viscous oil to sustain a film thickness to cope with the bearing surface roughness and the contaminant particles. BUT, the bearing surface speed has also increased, which reduces the need for a more viscous oil. The two effects play against each other. Without RESEARCH or a really good MODEL you cannot really tell whether the engine needs a more viscous oil (this is where the manufacturers spend millions on testing). What would be CERTAIN is that if we used a more viscous oil we would still get the temperature build up in the bearings that we are so keen to avoid.
TRACK USE
Track use results in heat soak conditions, but the revs are no more than the design standard. Load is no more, viscosity will be affected by the temperatures. Shallow sumps tend to make the windage problems worse, but will only be as bad as shows up in the oil temperature. The manufacturers testing programme for a car will include hot/cold testing, so you are unlikely to exceed those temperatures, but the characteristic of using full revs at those high temperatures is extreme. If extreme oil temperatures can be avoided, avoid them. If not, you'll be wanting an oil that retains stable viscosity behaviour to high temperatures to support the max revs loads with an adequate film thickness.
HIGH MILEAGE ENGINES (as distinct to modern/archaic designs)
High mileage engines may have picked up some bearing damage along the way; the surface roughness will certainly be worse than when it came from the factory. This alone is the reason why you should use a higher viscosity oil. You should also not demand the full performance of the engine (max revs, track use), especially once that higher viscosity oil is in there. The higher viscosity oil and reduced loads (revs) will keep the film thickness sufficient to cope with the increased surface roughness and will prevent the engine from accelerating into decrepitude. If you can't keep away from the revs, sell the car or plan to witness it lasting less long than it would given more gentle treatment. A rebuild can sort all this out, with bearing journals refinished and new bearing shells fitted the surfaces will be as good as new and you can return to the original viscosity specification oil..
TO RECAP
From my original list of concerns, how the oil behaves when...
tick - temps are high (heat soak from track use with high proportion wide open throttle)
tick - revs are high (engine has been tuned)
- you have/don't have an oil cooler
- you have/don't have a dry sump
tick - the engine is an older design
tick - the engine has done more miles
What about what happens to the oil recommendation for those remaining two?
If you can't manage your oil temperatures, your oil will be running in your bearings with a lower viscosity. This will reduce film thickness and the potential exists for the film to be inadequate to cope with the bearing surface roughness and contaminant particles. You can pay close attention to the high temp, high shear viscosity of the oil you use. The performance at 150degrees is more relevant to you than the broad categorisation of SAE viscosity at 100degrees.
Or, you could manage your temperatures.
As long as a dry sump system keeps up with the oil demands of the engine (modern engines circulate oil at a greater rate for the reasons stated above), it is always a good thing.
A FINAL RECAP
In all of the above I have limited myself largely to talking about running oil viscosity. I have written it many times, but the selection of appropriate viscosity is where you start with oil selection. The super-duper characteristics of some of these mega-oils come into play around the fringes of the main event.
Shear stability - if your oil doesn't stay the right viscosity then sooner or later it becomes no good (but ACEA A3/B3, B4 and A5/B5 specify stay in grade characteristics that are good enough for all manufacturer warranty periods).
Low temp performance - if your oil is woefully the wrong *running* viscosity at startup, then... your bearings stay lubricated, but the oil suffers shear damage if you start gunning it before proper warmup. Also acidic blowby doesn't evaporate out through the crankcase breather system, but condenses in your oil, so both your oil and your bearings suffer. Also, your oil system has a pressure relief valve, so the pressure doesn't guarantee to move enough treacly oil to keep up with engine demands at high revs. The thermal gradients aren't brilliant either. The synthetics stand up to this sort of abuse better because a good basestock VI means that there are fewer viscosity improvers to break down in shear and the greater chemical stability against acid attack is a good thing. The preference of 0W over 15W keeps this sort of nonsense to a minimum, but if you are trying to start an engine at -35 degrees C you *must* have 0W.
If you read all of the above, it contains general information about engine design which is where your oil characteristics matter a damn.
I would narrow my oil choice selection down to:
1. What does the book say (original engine manufacturer rather than Caterham/tuner etc.) about viscosity and ACEA rating?
2. Do I manage my oil temperatures? If I don't, then a super-duper HT/HS oil might be a good thing.
3. How many miles has the engine done since new/last bottom end rebuild? If low mileage, then run with manufacturers new spec oil and thrash the nuts off it. If high mileage, run one grade thicker (max SAE 50) and keep away from the last few hundred revs OR run one grade thicker and run shorter sessions on track so the oil temp is always managed OR run one grade thicker and be super-vigilant on oil temps/take extraordinary measures of oil cooling.
4. How much have I tuned my engine? Extra oil temp = super, duper HT/HS oil, same as general point on managing oil temperatures. Extra revs means anybody's guess on viscosity, but higher viscosity oil will exacerbate the temperature situation.
So there are some bits in there where it is useful to know that a higher spec oil might be worth having, but not everybody needs it by any means.
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