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Which engine oil

Chris_R
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Re: Which engine oil

Post by Chris_R »

There are several inaccuracies in this thread. I have written several times on the topic of oils in this forum.
First about ZDDP. The ZDDP value is simply the average of the zinc and the phosphorus values, then rounded down to the nearest 100 ppm (parts per million).

ZDDP was originally introduced as a bearing corrosion inhibitor, not as an anti-wear additive but it was discovered to have anti-wear properties. The introduction of TEL (Tetraethyl Lead) and increasing compressions in engines lead to a build up of lead oxides in the engine. Lead scavengers were introduced to counter this but these then resulted in acidic by-products which reduced the effectivemess of ZDDP and levels were increased to a peak of around 1,400 - 1,600 ppm. These high levels were there to counteract the effect of other additives, not to increase anti-wear protection. Following the clean air acts in the USA starting in 1975 and the availability of unleaded petrol in the UK and elsewhere from the 1980s the concentration of ZDDP declined as it is toxic both to humans and to the aquatic environment with long term adverse effects and since 1994 was limited to 1,200 ppm.

A high level of ZDDP is no guarantee of providing sufficient wear protection. And to make matters worse, excessively high levels of ZDDP can actually cause damage to your engine, rather than prevent it. Motor Oil Industry testing has found that oils with more than 1,400 ppm ZDDP, increased long-term wear. And it was also found that oils with more than 2,000 ppm ZDDP started attacking the grain boundaries in the iron, resulting in camshaft spalling (pitting and flaking).

Although you may think that a cam follower or cam lobe is smooth, when you get to the molecular level the surface is like a mountain range. ZDDP works by laying down a very fine layer of molecules that sit in the troughs between the peaks of the iron surface and smoothing them out allowing the two surfaces to more easily move across each other supported by a film of oil. The molecules of ZDDP bond to the steel under heat and pressure, the heat coming from the engine and the pressure coming from the cam lobe running across the cam follower surface. Once the troughs are filled no amount of additional ZDDP will have any benefit, no more of a layer can be laid down.

The question therefore asked is what is the correct or ideal amount of ZDDP. In truth there is little available research into this question. In the 1950s and again in the 1970s General Motors did extensive testing of low levels of ZDDP at 800ppm and at 600ppm on their flat-tappet engines and found no detrimental effects.

High levels of ZDDP is one of those myths that is perpetuated by marketers to sell their products, likewise is the myth that "modern" oils do not contain ZDDP which is also not true. All oils, synthetic, semi-synthetic and mineral contain an amount of ZDDP.
Current minimum levels are 600ppm with a maximum of 800ppm in xxW-20 and xxW-30 oil grades. Other grades do not have a maximum but is typically between 800 and 1100ppm (not including racing oils). If an oil has the ACEA A2/A3 with B2/B3 or B4 performance levels, phosphorous will be at 0.10% to 0.12% which will exceed the anti-wear requirement for older engines by a good margin. Ironically older blends of oil e.g. SF or SG formulations will usually have phosphorous content of around 0.08%, less than the later formulations. Mobil 1 0W-40 fully synthetic oil for example has 1,000ppm of ZDDP.

What about additives such as STP? The STP oil additive contains ZDDP. The amount is proprietary and so not known. In some documentation it claims to increase the ZDDP content to the levels of older oils but what "older oils" it is referring to is not made clear. Tyey are just playing on this myth that high ZDDP levels are needed to protect your engine. Adding STP to an oil which may already contain 1,000ppm of ZDDP may take final ZDDP levels into the toxic range which far from protecting your engine may cause damage in the long term.

Second about oil viscosities.

Oil has to work in 2 conflicting ways. In liquid form (defined as a quantity of oil capable of being poured) in the bearings and in film form (defined as a quantity of oil too small to flow or be poured) in locations such as camshaft/followers and in the rockers. In liquid form in the bearings oil is incompressible and a wedge of oil forms as the crankshaft rotates and that wedge is what supports the bearing and keeps the surfaces apart. It is important to remember here that the oil pressure does not keep the parts separated, the oil pressure serves only to supply sufficient oil to be pulled in between the parts by the rotations of the crankshaft.
In film form it is very different. The oil is the last barrier between two metal surfaces rubbing together. In locations such as the cams and cam followers there is not enough oil to form an incompressible wedge. Any liquid oil is simply pushed aside by the passing surfaces leaving only a thin film between the parts. Here the critical factor is the film strength of the oil. Unfortunately there is no official comparitive test of engine oils and engine oil manufacturers data sheets might not tell you this information either.

Oil viscosity is measured in centi-Stokes which measures the resistance to flow, or how easily the oil flows to the different parts of the engine and also in milliPascal second which is a measure of the internal resistance and may be thought of as the fluids internal fluid friction of the strength to support a load. The first is measured at 100c and the second at 150c.
SAE 30 oil has a viscosity rating between 9.3 and 12.5
SAE 40 oil has a viscosity rating between 12.5 and 16.3
SAE 50 oil has a viscosity rating between 16.3 and 21.9
SAE 60 oil has a viscosity rating between 21.9 and 26.1
All these measurements are taken at 100c. For a multigrade oil, e.g. a 0W-40, a 10W-40 or a 20W-50 to get it's -40 or -50 rating it must meet that specification. But notice that each rating has a range of values. One multigrade oil could achieve a hot viscosity rating of 16.2 and be graded as a -40 oil and another could achieve a rating of 16.4 and be graded as a -50 oil but for all practical purposes the oils are as near identical to each other but will be labelled completely differently.

The W rating in an oils specification is the cold rating. The other figure is the hot rating. The higher the number, the thicker the oil. At operating temperature a 50 grade oil, and here it make no difference if it is a monograde oil or if it is a multigrade, will be thicker than a 40 grade oil. This is fact. Conversely the W rating gives an indication of the performance of the oil when cold. A 0W oil will flow more quickly when cold than a 20W oil. Anyone who has an oil pressure gauge will notice that oil pressure rises more slowly when the engine and the oil is cold compared to how the pressure rises when the engine and oil is hot. What you are seeing is the resistance that the oil has to flowing. A 0W rated oil would reach normal pressure more quickly on cold startup than a 20W rated oil. It is often said that 0W, 5W or 10W oils are too thin. This is another myth. All oils, even 0W oils, are too thick when cold. Yes it is true that thicker oils thin out more than lighter oils when hot but that is because they started out much much thicker to begin with. This picture shows how viscosity changes with temperature.
Viscosity vs Temperature.jpg
Why is this viscosity important? If the oil selected is too low in viscosity, heat will be generated in the bearings due to an insufficient film thickness and some metal-to-metal contact will occur. If the oil is too high in viscosity, heat will again be generated, but due to the internal fluid friction created within the oil. Selecting an oil which is too high in viscosity can also increase the likelihood of cavitation. The high- and low-pressure zones, which are created within the oil on each side of the area of minimum film thickness in the bearing journal, can cause oil cavitation in the bearings. Cavitation is a result of expansion of dissolved air or a vapour (water or fuel) in the low-pressure zone of the bearing. The resulting bubble implodes, causing damage, as it passes through the high-pressure portion of the bearing. If the implosion or collapse of the vapour bubble occurs next to the metal surface, this can cause cavitation pitting damage to the metal. This can sometime be seen when bearing shells are removed and there is marks on the shells such as these.
Damage Cavitation1.jpg
What is the correct viscosity? This depends on the bearing clearance. Not tolerance. This is another myth. It is often said that manufacturing tolerances were not so tight when classic car engines were made and therefore need a mineral oil. Do not confuse tolerances with clearances. The manufacturing tolerance represents the range of clearances that would be acceptable. Tolerances have reduced which produces a more consistent product instead of one where the size varied slightly. Reducing the manufacturing tolerance does not reduce the clearance, it just reduces the variation of clearances. Any item outside the tolerance and therefore too large or too small a clearance would have been rejected. Modern techniques and precision mean that far fewer components are rejected because they are outside the specified range of clearances. Crankshaft main bearing design clearances have been consistent at least from the 1950s right through to the end of the century. Engine designers used engineering principles in the calculation specifically of bearing sizes and bearing clearances and required an oil to meet those principles. The design clearance of a typical engine from the 1950s or 1960s e.g. the 2.5l V8, the 4.5l V8 or many other engines will be almost the same as the design clearance of an engine from the 1990s.

The viscosity determines how easily the oil is pumped through the engine, pumped to the working components, how easily it will pass through the filter and how quickly it will drain back to the engine. The lower the viscosity, the easier all this will happen. This is important to why cold starts are so critical to an engine because the oil is cold and so it is relatively thick. But the story is quite different once the engine has warmed up, the oil flows freely to the bearing and to all the other parts of the engine where it is needed. The viscosity is also important to the amount of load that the oil can support, as you lower the viscosity, the less the load the oil can support in the bearings. Conversely, the higher the viscosity, the better the load it can support. However, the friction in a crankshaft bearing is directly related to the viscosity of the fluid lubricating it and a higher viscosity has a trade off in higher drag, potential power loss, higher fuel consumption and increased oil temperature in the bearing potentially leading to increased bearing wear so there is a balance between the load supported and the drag experienced.

The ideal viscosity in a bearing at normal operating temperature is around 10 – 15cST depending on various factors such as load, rpm and the design or dimensions of the bearing shells. Historically engine designers would call for an oil of approximately that thickness at operating temperatures. Much higher than this and drag results, much lower than this and boundary lubrication occurs.

Oil that is too thin will not generate a sufficiently strong enough wedge in the bearings to maintain the fluid gap and under load metal to metal contact will occur. Oil that is too thick will not generate a thicker wedge as there is no more space available for that wedge, it will create an extra drag on the rotation of the bearing with consequential loss of power and increased heat in the oil which perversely may result in a lower instantaneous viscosity. It also increases the shear forces in the oil which over time will degrade the multigrade capability of the oil and it will slowly revert to its base stock. Note that in a mineral oil a 20W-50 oil is basically a thin SAE20 oil that through the use of viscosity improver (VI) additives behaves like an SAE50 oil when hot. As those VI additives are destroyed in use the oil gradually reverts towards an SAE20 rating. Frequency of oil changes is important to maintain protection.

The thickness of the oil required to form this wedge depends on the journal clearance in the bearing, a “50” grade oil (monograde or multigrade) will support a larger clearance than a “30” grade oil. Bearing clearance and the thickness of the oil required to support the journal in the bearing are of course related. Larger clearances in a worn bearing can sometimes be supported with a thicker oil until the wear level reaches a point when the oil leaks out of the sides of the bearing faster than the incoming flow can replenish it at which time there will not be enough oil to form the wedge.

Christopher Storey
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Re: Which engine oil

Post by Christopher Storey »

I am not an expert on oils, but I do know that there is , as mentioned earlier, a lot of hocus pocus talked about the subject. Too many people think that the only important factor is pressure . In fact , because a major function of the oil is to carry heat away from the bearings and underside of the pistons, flow rate is also of great importance, and thus thicker oil( and commensurate higher pressure ) is not always superior in performance . ZDDP is important for preservation of the cam lobes, and this has been reduced greatly in modern synthetic oils, hence their perceived undesirability . As far as the question of low gearing leading to premature wear is concerned, the rule of thumb even when lubricants were rather primitive was that the mean piston speed should not continuously exceed 2,500 feet per minute, and on the 2548 cc V8 this equates to 5443 rpm, so that even at say a cruise of 4,500 rpm = c.75mph on the auto saloon and c.95 mph on the Overdrive manual saloon, this was well within accepted limits. The true cause of premature main bearing wear in my view is that the centre main is far too narrow for comfort, and there is not much one can do about that

PS my post has crossed with Chris R's above ( which I think says , in much more erudite and interesting form, rather the same as mine )

Chris_R
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Re: Which engine oil

Post by Chris_R »

Thanks Chris! I would only take you up on one point in that ZDDP has been reduced in synthetic oils. This is in general a myth. The reduction only applies to the xxW-20 and xxW-30 grades. The other synthetic oil grades i.e. 5W-40, 10W-40 do not have such restrictions and as I posted earlier Mobil 1 0W-40 synthetic has 1,000ppm of ZDDP, probably more than many so called "Classic" oils.
The topic of mineral vs synthetic oils is a wholly different and interesting topic in itself but does cross over into much of what I already wrote in my previous post.

tjt77
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Re: Which engine oil

Post by tjt77 »

45 lbs is sufficient for short stroke engines that run with light loads and constant running speed kept below 4.500rpms...(the turner V8 is a short stoke engine..its 'over square' like many US made V8s) .and its true that many US made V8s run at 45lbs or less..the post 90s small block GM V8 for instance..but tuners always build them to produce higher oil pressure to avoid possible lower end bearing failure. Post 90-s GM road cars are fitted with overdrive auto boxes and are geared to run at around 1800-2300 rpm at 'normal' cruising speeds.. 45lbs was considered 'optimal' to keep the lower end lubricated, whilst providing minimal frictional and hydraulic losses, when legislation for greater fuel efficiency was put into play ..
The 2 1/2 V8 daimler in SALOON application is low geared and runs at comparably high speeds.. 3,500-4,000 typical cruising speed.. and was never supplied with oil cooler, so the oil has to work hard to do its job. The common rule for engines who's full performance is used is as stated above is 10 lbs per 1,000 rpms.. given that the Turner V8 is well known for premature main bearing wear in the saloon application, it is wise when rebuilding to set the relief valve for an additional 50-10lbs of pressure.. a 1/4"- 5/16" length of round 1/2" bar stock with the outer edge that fits in the plunger chamfered, allied to a new oil pressure relief spring will achieve this result..(Im talking on a fully rebuilt engine.. not an 'iffy' dodge to try and make up for main bearing wear giving low readings when hot. ) 50-55lbs is where it needs to be for optimum lubrication in normal use.. its not productive to go above 60lbs as to do so will add extra load the the oil pump drive and will cause premature wear on the bronze gear that drive oil pump and distributor.. and its also essential to run on recommend viscosity oil so as not to load up the oil pump drive.. no thicker than 20/50 or straight 30w.. On longer stoke engines that produce higher bearing loads (such as the BMC A and B series engines from same time period) most engine builders that modify for increased power outputs set the oil pressure relief valve for 70-75lbs so as to be sure the lower end will be adequately lubed at high speed..
I've found from observations over the past 40 years.that the Jaguar inline 6 really benefits from an additional 5-10lbs of pressure..hence I always set the pressure relief valve to achieve 60-65 lbs when rebuilding those engines.. rather than the standard 45lbs they came from factory with.. although good number of 3.8s did run @ 55-60lbs out of the factory.. especially common on the 3.8 E type engines..

tjt77
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Re: Which engine oil

Post by tjt77 »

Chris-R in his comprehensive and highly informative post brings up the issue of 'volume' of oil flowing.. the oil pump in the Turner V8 is barely adequate as regards volume pumped.. that is the main reason why, when wear starts to show in the main bearings,( down to the copper layer on the original vandervell bearings) the oil pressure drops off dramatically when hot.. a higher volume pump running @45lbs would likely be adequate.. but due to the fact the pump does not have much in the way of 'excess' capacity, I recommend a higher setting for the relief valve to achieve 55-60 psi .. it may well be possible to machine out the pump body to accept larger gears, and likely this has been done by various tuners over the years..
Incedently:- the old jaguar inline six had a higher capacity oil pump introduced around '66 with the 4.2 litre engine and this larger capacity pump became the 'standard' replacement .. it was originally upgraded for more capacity for the '55 model year (XK140 onwards) and the two types are NOT interchangeable without replacing the front main bearing cap on those engines..

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