MOCAL Thermostat observations., Observations on a new installation. Options

[John]

While helping to troubleshoot a new project, we were discussing the operation of a MOCAL thermostat installation.

It was always my thoughts that the thermostat was installed to prevent cold oil (high pressure) from bursting the oil cooler.

After reviewing the two ways that you can plumb the thermostat I came to the realization that the cooler will always see full system pressure regardless of temperature.

On a MOCAL thermostat, all ports are open when the unit is cold. When the unit heats up, the ports on one half are separated from the ports on the other half.

When the thermostat is cold and the oil is thick, there is no flow FORCED through the cooler. The path of least resistance would be through the return line back to the tank.

As the thermostat heats up, the internal valve closes FORCING oil to travel through the cooler and finally back to the tank.

This means that the cooler ALWAYS sees maximum system pressure. The path through the cooler (when cold) requires more force than it would to return to the tank preventing cold oil from being cooled further.

I verified this to be the case in both my cars as well.

I always thought it odd that you could plumb them as either straight through or crosswise. It makes sense now.



So my question is: Why do they market thermostats as a protection for oil coolers?

[904svo]

Here's the page from Porsche 914 manual about the oil system

[maf914]

Interesting question.

I would guess that when the oil takes the shorter, less restrictive, that flow rate increases and the pressure from the pump decreases. If the oil took the longer path through the cooler, resistance would increase due to longer line length and the cooler core, the pump pressure would increase as the flow decreases. As the oil heats up and the viscosity drops, flow resitance drops, and pressure drops as the flow increases.

Pumps have a performance curve, pressure (head) versus flow. As flow drops pressure increases and as flow increases pressure drops.

I wonder if that's correct? (IMG:style_emoticons/default/idea.gif) (IMG:style_emoticons/default/laugh.gif)

[PRS914-6]

John, you hit the nail on the head. Some thermostats have a pressure relief built into them. The Mocal does not. However, the console in the oil tank has a pressure relief valve in it at the filter. If my memory is correct, the Mocal when open is a direct flow so if the cooler plugs up your screwed....something's going to break......

[John]

In the cases I was investigating, the systems were dry sump (911 engined cars with an oil tank).

The path of least resistance through the thermostats I was looking at was the line returning to the tank, not the path through the cooler, so the majority of oil would flow back toward the tank.

I remember reading that one of the reasons for having a thermostat was that it was meant to prevent the cooler from seeing the higher pressures of a cold oil system. I have now found that reasoning to be faulty. I did some on-line research since posting this and it would appear that most if not all the literature now simply states that thermostats prevent/reduce oil flow through the cooler until the engine is warmed up.

So MOCAL thermostats do NOT protect the oil cooler from overpressure conditions when cold.

I strongly recall 20 years ago, when we first installed a front oil cooler, that the recommendation of the thermostat was to reduce the chances of popping the cooler from cold (high pressure) oil. I now know that story is/was BUNK.

The MOCAL thermostats simply regulate oil FLOW according to temperature.

[flesburg]

John,
I think all of our thinking was/is wrong. The scavaging (spelling) pump on a 911 engine does not pump oil at very high pressure on its way from the engine to the external cooler, the filter and then the tank. I do not know where to find out what pressure it is forcing through the cooler, but probably not very high. So it probably would not blow a cooler even when cold. The thermostat is probably there to circulate the cold oil to the tank until it is up to 180 degrees, without the cooler having any effect on its temperature. Possible we could save some weight by removing the thermostat, since we do not start the engine when the oil is below 40 or so degrees anyway.

The high pressure pump in for internal pressure inside the engine and does run quite high pressure when the oil is cold (90 pounds or more).

[Steve Thacker]

I know someone here on the site had posted a picture of the MOCAL and what lines went where. Can anyone redirect me? I have a unit and I want to put it in, just too stoopid to figure out what goes where.