what oil in 944S2?

[Copperman05]

Ha Oil again.... Dont we love this subject... There seems to be a lot of myths and old wives tales surrounding oil and its use in our cars. I always seem to hear people saying fully synth oil wasnt around when are cars were made, well they def were and are even recommended in my 1989 944 manual (Simon will dispute this as that is what he likes to do) but the fact remains there in black and white, if your car doesnt have an issue with leaks, then there is no reason you cant use 5W oil, 5W is just the cold temperature rating after all, 5W-40 10W-40 amd 15W-40 all have the same viscosity at working temp so wheres the issue, they are the same thinkness at working temp...

This debate will continue and so shall the differing views, which one you agree with is up to you, personnaly I use 5W-40 oil in my 100,000 mile engine and it runs at 3 -4 bar pressure at warm idle and does not leak or use any oil. If I were you I would invest in the future, as oli says, a cheap oil changed regularly will be better than an expensive oil changed rarely, but I would take it a step further, a good quality fully synth ester based oil changed regularly will offer you the very best wear protection available, you then only need to decide what viscosity oil you need and that will depend on whether intend to use the car mainly on the road, track or a mixture of both...

Edd

 

[sawood12]

ORIGINAL: Hilux

Bearing clearances have not changed over 20yrs or engines will be seizing. Piston rings are an interference fit in the cylinder so there are no clearances.

Modern engines are almost blueprinted, old Porker engines arent/werent to the same degree.

Clearances grow with wear and age so a lower viscosity oil will lose pressure as the required volume is decreased. As you say the secondary function (almost parallel in importance) of oil is to remove heat and slowing down its flow adds heat (and wear)

The clearances at the bottom end are tiny (consider the mimimum oil warning light pressure) and the pressure therefore required to flow oil to the bottom end is surprisingly low. Much higher at the top end. What is key is volume/quantity passing through so as to remove heat so with a given head pressure if an opening increases you lose pressure at that point and reduce flow.

If you reduce flow by using a thicker oil you reduce both of these properties.

Equally in an older engine with increased clearances/reduced tolerances, if you use too thin an oil you reduce flow hence I would err on the side of at least 10w

Blueprinting doesn't mean tightening up clearances. Blueprinting means selectively fitting parts with specific toleraces so you achieve the nominal clearances to maximise the engines performance. On a normal production line you will pick parts at random and so the clearances will vary such that on some engines by luck you might hit upon an effectively blueprinted engine and on other engines where the tolerances might be diametrically opposite and you have the max clearance and so lose a small amount of performance.

And in anycase the wear properties of machines mean that blueprinted engines don't remain blueprinted for long - you have an exponential rate of wear initially that settle down and flattens out - that is why race engines are rebuilt so frequently. Bearing clearances are dictated by achieving the right running clearances whilst allowing for location of the rotating parts and the thermal expansion of the materials to allow continued running of the bearing without losing location or seizing. Oil plays no part in the design considerations here.

As engines get older the clearances will settle in after running in and will remain pretty much constant for the remainder of the life of the engine i.e. the wear rate is so small. Therefore there is no need to change oil viscosity as engines get older and in anycase there is enough flow rate capacity in the oil pump to maintain oil pressure by the small rates of change as engines get older. You don't regulate oil pressure by varying oil viscosity. You are chasing your tail. If clearances do increase then pressure will reduce, but flow rate will be maintained.

There is only rule here that applies to everyone (assuming your engine is in fine fettle) - use the Porsche recommended grades.