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Oil Viscosity Vs. Oil Pressure


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11 replies to this topic

#1
Stu

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Ive been reading through a lot of thread on oil today, and think i get the jist of the numbering now.

So, a thinner oil from cold, say a 5W vs 10w will circulate better as it is thinner, but give a lower oil pressure reading right?

My point of asking is that at a recent trackday my Mi oil pressure dropped off a little as it got hot, so by my understanding i need to go to higher viscosity oil to avoid getting the oil too thin and starving a bearing of oil.

Ive been using semi-synth 10w40 since i built the engine, which gives an excellent oil pressure from cold, but when hot it drops quite a bit, now im judging this off the factory gauge and im well aware of how crap they are.

So, my question is such, i do a lot of trackdays where the oil temp gets up, ive got a 13 row cooler which keeps it in check, and even half a lap of cool-down will see it back to normal, so would i be better off with say a 10w50 oil? When the oil is up to temp after a session at 2.5k-3k its somewhere around between the 2nd/3rd mark which i estimate to be between 20-30psi, which seems low to me.

Normal driving (say 3k-ish) is well over halfway, and approaching 3/4 if the car is revving (5k + ), cold start is a full gauge.

Any ideas?

#2
brianthemagical

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What oil temps are you running? If it's running hotter then a standard car during normal use then a thicker oil may be needed. The pressure isn't necessarily that inportant in this context as it's the presence of the oil that is needed, and the volumes will be very similar, regardless of viscosity and pressure. But, the oil still needs to be at the correct viscosity to work correctly.

#3
Stu

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Well after a full on 20minute session the oil temp gauge is up well over half way, dunno what the marks equate to though? and at around 2.5-3k the oil pressure's dropped off a bit.

Am i right in saying a 10-50 will be thicker when hot when compared to a 10-40?

#4
Ethos

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My mi is pretty much one line from max when cold but when it's properly warmed up it's pretty much at the bottom.

I didn't think this was anything to worry about and quite normal....

10w/40 semi here too. The haynes actually states 15w/40 as the correct oil last time I looked.

#5
brianthemagical

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10w-50 is more viscose when warm (at 100 degrees C).It will probably be better than a 40. Best bet would probs be to give an online oil company an e-mail and see what they reccomend.

#6
stuRALLYE

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ive always been told that as long as the oil pressure light isnt on then everything is ok, due to the standard guages being pretty crap. i have 5-30 in my 8v and at the last trackday in hot weather the light was just and just flickering after a heavy session! :lol:

#7
welshpug

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bear in mind that on the oil temp gauge half way is only 110c

Posted Image

as for the oil pressure it will be low at only 2.5 to 3k, should be 5 or 6 bar at 4k RPM hot or cold IIRC.

#8
SPGTi

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Viscosity drop in oil will also depend on the quality of the oil. I use fully synthetic 0w-40 in my 8v and never have a problem even when doing lots of 2nd gear 6000rpm running. For trackdays IMHo you need an oil that is shear stable at the higher running temperatures.

Steve

#9
C_W

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Garage View Garage

ive always been told that as long as the oil pressure light isnt on then everything is ok, due to the standard guages being pretty crap. i have 5-30 in my 8v and at the last trackday in hot weather the light was just and just flickering after a heavy session! <_<


That's too thin IMO and not the right oil, its designed for new/modern engines with tighter tolerances and probably explains the oil light on hot idle.

#10
James_R

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You don't want to swap the oil about to make up for a lack of proper cooling.

Depending how anal you are you don't want to see over 110deg although it's "safe" to get to 120-125 I killed my old nova engine from spiking the oil temp.

Would look at the oil cooler set up closely and consider re-siting the cooler or goign bigger again, then carry some cardboard round to cover it for road use, or ducktape then you can tune in the temp on the first session on track.

From speaking to some german VLN race teams they said they aimed at coolant fo 75 and oil of 85deg for max power if youw ant something to aim at.

#11
Stu

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Im would estimate that im getting to approx 110 James, based on what Mei said about the gauge, even after a relentless beating it doesnt seem to get higher than this, the oil cooler currently sits behind a cut out fog light, and ive got a seperate port let into the bumper to feed it also.

I think its just crap oil if im honest, so i intend to dump it out and thought it'd be worthwhile to try something else.

A lot of people have suggested Silkoline 10w50 fully synth, which sounds just the job, but at 44 for 5 litres its a bloody expensive oil change given the oil capacity of my car with the sump/cooler is now approaching 7litres... <_<

#12
jackherer

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Get a decent gauge first, the SPA dual oil pressure/temp digital gauges are ideal.