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Re: [T3] synthetic oil and temps and pressures


On 6 Nov 2005 at 22:31, Dan Hoopes wrote:

> well, don't know if anyone remembers my resurrecting the synthetic oil
> debate a couple of months ago, but i made the switch to 10W-30 Mobil 1
> full synthetic and have some observations and questions.

 From below, I gather that you have a full flow filter. With that in mind, this 
sounds to me like it should be fine.

> when i switched i noticed two things.  first, my VDO oil temp gauge
> showed that temps were going up about 20 to 30 degrees higher than
> they did with regular oil on a 25 mile freeway commute i used to do. 
> the sensor is sensor in the pressure relief hole and while not
> completely accurate (who knows), is pretty repeatable.  the gauge used
> to always show about 215 degrees during the drive.  the temp crept up
> to almost 245 after putting in synthetic, so i reduced rpms to not
> risk it.  this is in august in socal, and winter temps are easier on
> the setup.
> 
> also, the new synthetic was giving lower presures when i got to stop
> lights during the drive, like down to about 20 psi where i used to
> keep 30 or 40 psi at stoplights when hot.

I had to think about your question for awhile before responding, because you've 
got real data here that doesn't seem like what we would have expected. I've 
thought this over and over and I just don't seem to be able to come up with a 
nice neat explanation for your experience.

Given that, I guess I'd recommend pulling the pressure relief valve(s) out of 
your case, just to verify that they are working and that the pistons in them 
are not stuck.

> should i switch to 10W-40 synthetic to maintain pressure?  is there
> anything else that would explain the higher oil temps and lower
> pressures?  should i worry about the numbers?

I don't think so. Pressures over 30 psi just mean that your relief valve is 
being overwhelmed. Perhaps it was stuck when you were getting these numbers, 
and now the Mobil 1 has freed it up. I used to see 30-40 with a cold engine at 
speed, but at idle, even with a 25mm pump, it would fall to 20, or, on a hot 
engine, much less. The oil light is supposed to come on at around 6 psi, and 
the pressure relief valve is supposed to limit the pressure to 28 psi.

It's easy to get into the feeling that more oil pressure can only be a good 
thing, but this really isn't true. The pressure is only there to deliver oil to 
the bearings, and the bearings only need a certain VOLUME of oil, the pressure 
they use to lubricate themselves is developed internally and is NOT the 
pressure we see on our gauges.

The only thing I see here that bothers me is the fact that your oil temp has 
gone up. Berg says that synthetics don't carry off heat as well as dino oils. I 
don't know how that can be, so I've always been skeptical about this statement, 
but maybe he was right. If so, that is certainly a problem.

IF that is the problem, going to a higher viscosity oil would only make it 
worse, because it would reduce the volume of oil thru the cooler and increase 
the volume that gets to bypass the cooler.

One more consideration: The temp you measure at the relief valve is dependent 
on a number of things. One of them is the speed at which the oil gets there 
from the sump. If the synthetic is lower viscosity, it might be possible that 
it gets pumped around more quickly, making the temps around the different parts 
of the engine more uniform. This MIGHT make your gauge reading higher, even 
though your overall engine temp had not significantly changed.

It's even possible that the synthetic has cleaned a lot of crud out of the oil 
passages and changed (improved) the temp distribution. To test this, you could 
try switching back to dino oil.  

Interesting problem.  ;-)

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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