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Old 11-01-2011, 01:40 PM   #1
Power6
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Default Stock oil cooler theory...

08+ WRX do not come with a stock oil cooler. STI does. Actually outside of the US, the WRX still comes with an oil cooler. If Subie feels it is needed for everyone except cheapskate US customers, that is good enough for me to want it ;-) I am looking at what creative way one could be added.

I've looked at how it works, the cooler and the hoses would be easy. The outlet(or what I think is the outlet flow) of the cooler goes right to a nipple on the block by the filter. The WRX simply has a plug there, so the nipple should be able to be swapped right in.

The big issue is the WRX has the "non-turbo" water pump, which is missing an outlet that feeds the inlet of the stock oil cooler. I'd like to avoid having to buy a new water pump to do this. If I have to do that, I'll just wait until timing belt time or something.

I have looked at some old instructions for adding an OEM oil cooler to the GC 2.5RS, using parts from a Forester. In that case a Forester thermostat housing is used. It has an extra nipple on the housing for the hose going to the oil cooler.

That housing won't fit, but it gives me can idea...perhaps the thermostat housing could be drilled tapped with a fitting for an oil cooler inlet hose to be attached...

Here is where you can poke some holes in my idea: The stock oil cooler, if I am correct here, circulates the coolant constantly as the engine warms up. It seems the housing tap idea, being on the other side of the thermostat, would delay coolant flow through the cooler until the thermostat starts to open. The heat up function would still be there, just not sure how much it matters. It seems Subaru didn't think it was a big deal. Anyone have a thought on that?

** I am interested in the stock oil/heater cooler for its oil heat up ability and minimal cooling. This is not a track car, if it were I would just get an external radiator setup. If you do know of another option that heats as well as cools the oil lets talk about that too.
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Old 11-01-2011, 07:02 PM   #2
ki4syr
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Default

If you tap into the thermostat housing, won't the coolant be the hottest since it's already made its cycle through the engine? Seems you would want the coolant from the water pump like the STI.

I just done the first service on my STI this weekend and noticed how low the oil filter was. At first I was "wow, much easier to change", then I realized it had a cooler. My first STI btw. Always hated having to change the filter on our '10 Forester, it was way up in a hole with exhaust all around.
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Old 11-01-2011, 07:06 PM   #3
eg33GC
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do you understand how the cooling system works?^^
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Old 11-01-2011, 08:11 PM   #4
Power6
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Default

I did some googling and found a few other OEM oil cooler setups that take the coolant feed off the thermostat housing. Seems like it is a viable option.

I will pick up a spare thermostat housing to see how a fitting can be tapped into it where it will fit with clearance to get a hose on it when it is installed.

It is my understanding that yes the coolant will be the hottest going into the oil cooler. I guess there are pros and cons to that. It seems to offer the greatest heating ability which is a big function of the OEM cooler, and also prevents the coolant from slowing the oil from coming up to temp becuase it always gets the warmest coolant pre-radiator. Of course that means on the other side, it doesn't cool the oil as well.

It is more of an oil heater, or oil temp regulator than a cooler, which is exactly the function that I am looking for.

Last edited by Power6; 11-01-2011 at 08:18 PM.
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Old 11-01-2011, 11:00 PM   #5
ki4syr
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Quote:
Originally Posted by eg33GC View Post
do you understand how the cooling system works?^^
Hot water on top hose, cool water on lower hose. duh
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Old 01-16-2012, 11:16 PM   #6
Power6
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I finally did this:










Note on effectiveness. I have an oil temp gauge. To take out ambient temp variable, I noted times from the point that oil temp hit the first numbered mark on my oil temp gauge, at 40C(104F). The load and how you drive is still a big variable, that's why my notes are ranges of time, noted over a number of days of cold starts.

before oil cooler:
Oil temp time 40-80C(104-176F)
6-10 min
Coolant temp time, from 40(104F)(oil)-half gauge (fully warm from experience)
2-5 min

Lag time:
Coolant half gauge to oil temp 80C(176F)
2-5
Shortest lag is idling or stop and go traffic, longest is getting right on to freeway

Cruising oil temp is 95C(203F) consistently
1-2-3 WOT run will raise the oil temp to 100, 2-4 min to cool back to 95C

After oil cooler:
oil temp time 40-80C(104-176F)
4-7 min

Coolant temp time, 40(oil)-half gauge
4-7 min

Lag time
Coolant half gauge to oil temp 80C(176F)
no lag

Cruising oil temp is 90C(194F) consistently
1-2-3 WOT run will raise the oil temp to 95C, 1-2 min to cool back to 90C.

Cliffs notes: w/o stock oil cooler, coolant heats up quickly and oil lags. At hot temps oil easily gets well above the coolant temp. w/ stock oil cooler, coolant heats up slower and oil heats up quicker. At hot temps oil runs a bit cooler, closer to coolant temp. There is slightly more headroom for breif hard running oil temps. Probably not a solution for sustained hard running like a track day, where an oil to air type would be the preferred solution.

You may also notice the stock STI pump will draw coolant through the oil cooler behind the thermostat, meaning it has full flow all the time, before the thermostat opens. My "hack" relies on thermostat opening to get flow through the cooler. It is possible my solution has reduced performance. Service manual states that the thermostat starts to open at 80C/176F. It is hard to say at what point the temp gets to 80C in the thermostat area. I do know that the oil temp comes up very quickly as soon as coolant gauge hits the second hash mark from cold. Perhaps also the "bypass route" around the radiator from the block through the oil cooler warms the radiator return coolant heading into the thermostat with this setup, which serves to keep the thermostat cracked and flowing coolant through the oil cooler, compared to stock w/o oil cooler.

Last edited by Power6; 01-19-2012 at 10:37 AM.
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