Wot Mobil !

[Rory McLeod]

smile.gifsmile.gifThat conjured up an interesting picture at this time of the morning - good play on words - not good for my blood pressuresmile.gifsmile.gif

 

Edited by - Rory McLeod on 12 Sep 2000 06:37:07

[Peter Carmichael]

Arnie,

 

Are you talking about running oil pressure or idling when you come back into the pits?

 

I can imagine that a 40 rating is a nominal thing in a range from, say, 35-44.9. Two 40 grade oils could be at different extremes of such a range. The viscosity is only one part of the oil suitability equation, with film strength being the other important aspect.

 

SAE values give you information that the viscosities are roughly equivalent. There is no such metric for film strength and Mobil have been busy dumbing down their marketing information now that Mobil One is established as a premium brand.

 

Apparently Redline used to make the base-stock for the original Mobil One and the recent stuff has been a simplified formulation direct from Mobil mode>. There is nothing to stop you buying the Redline stuff.

[Simos]

Arnie,

the W in the grade stands for Winter and is designated as the viscosity number at 0oF (-320C) the second figure is the hot grade and is the viscosity number at 212oF (100oC).

 

The following is all IMHO [:-)]

 

Viscosity number is an arbitary grade based on a range of actual laboratory measurable viscosities (as Peter pointed out) and is not a fixed measure of how runny it is.

 

The spread of the grades (15 - 50) gives an indication of the change of (change of) viscosity with temperature.

 

So what does that mean and how does it affect me ?

 

What we want from our *ideal* engine oil is an oil that doesn't get thinner when it gets hotter, we spec the ideal viscosity and its associated film strengths etc for our engine and whether we're 100 yds down the road on a frosty morning or pulling 9 grand down the straight at Monza, it's always correct.

 

Well it aint gonna happen! oil gets thinner as it gets hotter and for a "single grade oil" it gets thinner by a set amount depending on the starting viscosity (and other properties but lets keep it simple).

 

If we choose a thin enough single grade oil that is good for our frosty morning then it will be too thin and the film will break as we increase forces when it's very hot... we're not going to reach that chicane, our engine will have seized.

 

If we put in a thicker oil to cope with Monza, it'll be too thick to get pumped to the top of the cams on that frosty morning, we're not going to make it to work after all...

 

So what we need is a "multigrade oil", an oil that has a viscosity like one kind of oil (say SAE15) at one temperature (say -32oC), but has a different viscosity (say SAE 50) at a different temperature (say 100oC).

 

Make a note of the temperatures 'cos that's the really important bit. It's NOT saying that mobil1 is thicker when it's hot than when it's cold. That's bollox we can see it's thinner when hot, our oil pressure drops !

What it says is that 15/50 is as thin as 15 grade oil at -32oC and as thin as 50 grade at 1000C. *BUT* 50 grade at 100oC is *THINNER THAN* 15 grade at -32oC even though it is a thicker oil (AT THE SAME TEMPERATURE).

 

To give you some idea of how big a spread those grades cover and why your oil pressure might differ with different oils purporting to have the same hot (or cold) grades here's a grade conversion chart showing multi grades at the bottom and single grades (like the EP80 you put in your gearbox) at the top vs absolute viscosity on the axis.

 

SAE single grade Gear Viscosity Numbers

 

/__/75W_/80W__/85W/____90________/________140_____________/

 

SAE multi grade Crank Case Viscosity Numbers

 

/__/10/_20__/_30_/_40__/__50__/

 

2__4__6__8__10_12_14_16_18_20_22_24_26_28_30_32_34_36_38_40_42

viscosity cSt @ 100 degrees C

 

 

 

The different grades and spreads, synthetics and minerals all have their own suitabilities but I've typed too much already I doubt anyone's read this far and I have to go catch a train so I'll stop here.

 

Cheers, Simon.

 

Edited by - simos on 15 Sep 2000 14:54:12

 

Edited by - simos on 15 Sep 2000 15:02:19

 

Edited by - simos on 15 Sep 2000 15:06:52

[Simos]

Ok, so the chart is hard to do, I've edited it and it should give the general idea, a 10w/50 oil is as thin as 75 single grade at 0F and as thin as 90 grade(ish) is at 100oC...

 

Edited by - simos on 15 Sep 2000 15:18:55

[Anna Rexia]

Simos,

Are you related to Peter "Perfect"?

[Alex Wong1697456877]

Simos,

 

I've wondered why the numbers seemed the wrong way round for ages. Thank you for putting me out of my misery!

 

Ah.... More trivia towards my anorak reputation!

 

Alex

[Peter Carmichael]

I've just remembered that it might be important to answer the original question.

 

>>>Which one is best the 0w- 40 or the 15w-50 which says on the bottle, for motorsport, or something like that. I wonder if the thicker one might be more suitable for the hydraulic tappets in Vauxhall engines.

 

er... No. The thin one will be better for the hydraulic tappet VX.

 

You thrash your engine with hot oil (~100deg C) and regardless of oil grade you use the same rev limit. The loading on the oil films in the bearing is predominantly determined by maximum revs. This is also true of the followers.

 

Hydraulic tappets are typically suitable up to ~9.5mm lift and 270 degrees duration on your cams. Once you get beyond this the follower accelerations would pump down the hydraulic followers leading to increased clearances and impact with the cam lobes because the followers would not make first contact on the take-up ramp of the cam profile. The longer the duration of the cam, the higher up the rev range it will be developing its power, demanding even higher accelerations.

 

When you choose cams that require a high rev limit you end up moving to a mechanical valvetrain. (even if a well-designed hydraulic tappet may cope with pretty high revs - I think Arnie mentioned this talking in favour of the VX).

 

What all of this means is that if you are running hydraulic tappets then by necessary consequence you will not be using extreme rpm and therefore you will not need a SAE 50 hot grade of oil. If you use the thin grade of Mobil 1 you will have better lubrication from startup.

 

The 'Motorsport' designation of the thicker Mobil 1 indicates its suitability for highly revving mechanical valvetrain engines and for engines which get warmed up at a fast idle in the pits, rather than being started and driven on-load immediately from cold.

[Alex Wong1697456877]

At Charlie Brown's in B'ham:

 

Comma SynerG - 20 quid for 5 litres

Mobil 1 (both grades) - 32 quid for 4 litres.

 

Is there any evidence that Mobil 1 is a superior product, or is it good marketing. I'm led to believe that all modern synthetics are extremely good. Is this true or are some more equal than others? In the end, I went for the Comma stuff after the engine was run in on mineral oil, purely on a cost basis (nothing to do with the Comma girls - I wasn't there!). Incidentaly, my Vx is on solid lifters.

 

Alex

[Peter Carmichael]

The answer is never go for a cheapo joke-brand oil but otherwise be ruled by grade and cost.

 

If the marketeers don't think it is important to tell you what their base stock and additive package is and we are not equipped to judge such matters even if we were told, then Mobil One's brand perception is the only thing which marks it out.

 

Go with the Comma SynerG if it is the right grade. A 5W40 is barely different to a 0W40 unless you live in very cold conditions or insist on driving the car hard immediately from cold. Change the oil often.

[Rory McLeod]

Oh God, I know I am entering a minefield here arguing with the professionals but;

Lubrication is about providing a low friction bearing surface between two or more high friction parts.

This requires the maintenance of a continuous 'film' betweenthe mechanical parts in question under all conditions.

This can be achieved by means of fluids that appear to be either/both fluid or viscous.

What actually counts is the continuity of the meniscus (ie the integrity of the film of lubrication) of the fluid being maintained both at extreme temperaures and high load pressures.

Fot this reason I am tending towards the Castrol R type oil simply because they are spanning such extreme temp variations that I feel this synthetic has to have addressed that continous meniscous detail for high impacts.

 

Edited by - Rory McLeod on 13 Sep 2000 23:11:20