Good oil explanation from another (oil discussion) board

[PalmerWMD]

Quote an Australian Chemist:"OK fair enough, i just hope i dont loose people with the chemistry because i have done that before. Thin oils have thinner characteristics at low temps compared to thick oils - obvisouly. 0w and 5w are both fine. This is good because it means it will circualte with ease at cold temps and start up. The reason why it good for a turbo is because it allows for better and more efficient spool up. It also gets to oppertional temp quicker. A fluid with a high density has a higher specific heat capacity (with al things being equal) which means it takes longer to store heat. Oils dont like being cold. When the car has warmed up the turbo has not, or not that well i should say. It takes time when the car is on boost for the turbo to get up to temp. Thin oils are quick to respond (matter of seconds) thick oil are not (upto minutes). That time is a killer for turbine wear. But no one hear has issues with the lower viscosity (except the 25w dont know what on earth is going on there) So 5w30 is fine for that.

Where the problem comes in is the 30 bit. Its to low. That means the oil molecules will begin to dissociate upon thermal energy past its flow rate. This is oxidation or break down or in my terms the lose of electrons from outer shells in atoms. Therefore when the engine gets hot, the break down of the oil begins to occur and happens quickly. This means its life in the engine is limited. Thicker oils have larger aromatic hydrocarbons and stonger carbon to carbon bonding between the alkanes. Basically its a bigger molecule. Hydrocarbons are just that - hydrogen and carbon molcules and they start off simple gases like methane CH4, to liquids like octane C8H18 up to waxes as high as C80 ect and then into tars. The higher the carbon content the higher the viscosity and the harder it is to break down into simpler forms. We know that butane (C4H10) from a cigi lighter burns easily cause its small but a larger one like petrol (which is a mixture of octane, pentane and hexene) is less volitile and then diesel is less flamable again cause its bigger - feels greezy between the fingers. Gas is better than petrol, which is better than diesel which is better than oil ect...with respect to breakdown or flamability and thus resistance to oxidation.

So....the thicker, the heavier, the more resistant to heat, the longer it lasts. The sr and ej and 4g engines have pretty small sumps and small oils ports. They need something that circulates easily. Presure is related to density and corss sectional area of the passage in which it flows and density is related to viscosity and x-sectional area is related to port size. Sucking water through a straw is easier than sucking honey.

When the temps get to high the 30 oils is out of its range and it starts to become ineffective. I can hunt down some mass spectrocopy plots and titration curves comparing the multigrades we use for different tracks and requirements for the f1 engines. It shows how as temp rises oxdation occurs at a rate of 0.056m/mol per Kelvin. This is for the 'special' oil in the race engines, the trends would be the same in road oils as well.

Also, the stoke on a 2L motor is quiet small. This is why our engines rev to 7000rpm and most pushrods v8 go to 5500. The longer the stroke of the engine the more stress occurs on the bottom end bearing from internal inertia. Like dropping an object from two fifferent heights and each will hit harder depending on height and load (revs). Thin oils find it hard to maintain the thin film lubrcation on bearing when the stress becomes high. So if the engine is working hard for long times or has massive power and or long stroke then the oil needs to become thicker - what resists spreading easier out of grease and wax - both HC's but wax is thicker. Thicker oils resist destortion upon movement. " End Quote

Fred...

[w1ngzer0]

tired.... too... tired... no ready.... uhhhh

[9095240SXDUCK]

So is he saying

We should use 5W since it is thin but use something like 5W-40 or 5W-50 so the oil can stand the heat of a Turbo?

[Jesda]

Awesome read!