Pavlo

Oil viscosity will affect turbo spoolup.

But a (genuine) fully synthetic 10W/40 will offer better protection for the bearings than a semi synthetic 10W/40 or even 15W/40.

Jolly Green Monster

iTrader: (4)

not effect it enough for someone to notice surely.

Pavlo

you can certainly notice it with cold oil that's for sure.

Jolly Green Monster

iTrader: (4)

I see.. never let my boost when cold so perhaps that is why I haven't noticed.

DaveW

The performance across the whole rev range drops off with a thicker oil. Not just the turbo spool, since both the engine and turbo use the same oil. Well thats what I noticed when a Subaru dealership decided to stick Magnatec in mine Some nice fully synth sorted that out though

Aztec Performance Ltd

iTrader: (234)

Paul: Surely the difference between 10/40 semi and 15/50 fully syn, are not noticeable in terms of lag, when oil is warm?

Surely you're not with 'THOMO' on this and recommend basing oil choice on terms of turbo spool-up?

Bob

oilman

iTrader: (9)

Here are the figures for viscosity in cst (centistokes) and temperature for different oils.

The first numbers are sae numbers for straight 30,40,50 and 60. The second set of numbers is for various multigrades as they are obviously thinner when cold than straight oils due to lighter viscosity basestocks being used.

Monogrades

DegC...........0.....20......40......60......100.. ....120

Sae 30....1600....315.....95.......39......11........7 ...
Sae 40....2579....473....135.......52......14.......9. ...
Sae 50....4592....771....205.......75......18.......11 ..
Sae 60....7865...1218...303.......105.....24.......14. .

Multigrades

DegC...............0...............10............4 0..............100

0W/20............329............181...........46..... ..........9..
5W/40............811............421...........92..... .........14..
10W/50..........1039...........539..........117....... ......18..
15W/50..........1376...........675..........130....... ......18..
20W/50..........2305..........1015..........148....... ......18..

These two graphs demonstrate the fact that a monograde is the same thickness at 100degC as a multigrade of the same sae number but the distinct benefits that a multigrade brings at lower temperatures. This is obviously benefits cold crank wear as the rate of flow of multigrades is much better at lower temps.

There is no temperature where oil suddenly starts to flow better. A 10W/40 for example will flow between -25C and 100C or more, but there is a big difference in the rate of flow (True viscosity at -25C is about 7000 Centistoke(cSt) units, dropping to 14 at 100C. ‘Viscosity’ is just another name for ‘rate of flow’.

The question is, at what (sump) temperature is the oil at a viscosity that suits a modern high-RPM engine.

Present day designs seem happy on an oil viscosity of 10 to 15 cSt. (But many are OK on less than 10.) 30cSt is too high at high RPM. It can lead to foaming, air entrainment and cavitation.

............Temp. for 30cSt (Deg. C).......Temp. for 15cSt.......Temp. for 10cSt

5W/40..................71...........................9 0........................117...............
10W/40................70...........................99. .......................118...............
10W/50................80...........................109 .......................130..............
10W/60................89...........................119 .......................142..............

This shows that a 5W/40 or a 10W/40 is perfectly adequate for all engines except those that run an unusually high temperatures.

Also, a thick oil can lead to trouble unless properly warmed up before high RPM is used.

I hope that these figures at least give some insight to what actually happens to the oil inside your engine at different temperatures.

Cheers
Simon

LIDDO

Straight from the horses mouth.......................Cheers Mr ED

Pavlo

I can't compare different types of oil when hot in terms of spoolup. But I have noticed that when the engine is fairly hot but the oil still cool, the turbo takes longer to spoolup to say about half a bar. High RPM on cool/warm is going to do more damage than a little boost on warm oil but at low RPM.

Paul