Ideal engine oil operating temp range

[SirBrass]

So, there's been a lot of hullaballoo about engine oil temps and oil coolers.

In looking at oil coolers, my question is this: what should we WANT our engine oil temps to be at (range-wise) when running them hard?

I know that running well above 230F is bad, but what is good? Is it the low 200's-220? Is it 185-194F of the Perrin cooler?

I know that we can't have our oil running too cold, so what's the operating range we should want it at?

[Boss Paco]

So many numbers thrown around. I've read too much. It seems 210-230 is good/optimal. Over 300, or so I read on oil and racing forums, is when you're high. Under 180, or so I read, was too cold.

Watching the temps on my turbo car for two months, I ran about 210-230. No issues detected. Water temps stayed around mid-190's peaking just over low 200's. The other night I made several hard pulls (might've been racing) and my oil temp peaked just over 240. Car cooled down after cruising a mile and all was good.

Since then I have installed an oil cooler. I am looking for a 200* thermostat sandwich after talking with a good friend/Subaru guru/old drag racer.

Man, I wished someone who knew 0-20 and boxer motors would step in with some intelligent information and sound advice!

[Dezoris]

The motor is designed to settle at around 200-215 during normal operation. Most motor oils advertise their flow rates at 100C or 212F as targets for efficiency.

On street cars that's the ideal range. Those historically who want oil coolers on the street choose 200F thermostats for every car I have been around. In this car you see the common oil cooler running 185F thermostats and with 0w20 if you look at thickness at those temps you are still good.

The main reason why 185 was chosen was because its a good starting point for if you track the car or run FI and need the extra cooling. For track most 4 banger race cars and the guys who I know run spec cars want oil temps between 220 and 240 on track. Issue with this car is we don't know how sensitive the heads are to heat and the DI system. So its best to always keep it under 240, and if you run above that often swap out the 0w20 to an oil that maintains a thickness closer to 0w20 at 212F to 225F when it operates at at those higher temps to adjust for oil flow in tight places like bearings.

Running the oil cooler than 175 all the time on 0w20 is not a huge deal but its not in the optimal efficiency range of how the motor was designed. My feeling after running UOA is anywhere between 185-220 on the street is good.

Track 220-240 good.
240-255 OK. 255-265 red zone, namely for 0w20.
265+ back off, too much until we have more UOA and longevity testing.

[SirBrass]

I think then that I'll go with upgraded radiator (not sure if there's much of a difference between mishimoto and koyorad) and the forester XT oil cooler.

[BatStig]

I've had the Perrin oil cooler installed for just over a year now. Just a few weeks ago I swapped out the thermostat with Mocals 203F thermostat. The weather here has been roughly 50F at best and while the oil hasn't been hitting 200F, it has been better than the 180F-if-you're-lucky standard thermostat.

[Kostamojen]

Originally Posted by Dezoris

Track 220-240 good.
240-255 OK. 255-265 red zone, namely for 0w20.
265+ back off, too much until we have more UOA and longevity testing.

Stock I hit 260+ just driving on back roads, pushing the engine hard but not 100%. Temps outside were in the 70's, altitude was pretty high around 5000 feet.

After my OEM oil cooler, same conditions, it stays under 245.

This car needs an oil cooler from the factory...

[Boss Paco]

Originally Posted by BatStig

I've had the Perrin oil cooler installed for just over a year now. Just a few weeks ago I swapped out the thermostat with Mocals 203F thermostat. The weather here has been roughly 50F at best and while the oil hasn't been hitting 200F, it has been better than the 180F-if-you're-lucky standard thermostat.

Got a PN or a link to a vendor who sells it?

[Sportsguy83]

Like @Dezoris said you want to be between 200-215

[arghx7]

I can tell you from a lot of accumulated knowledge with aluminum block engines, anything under about 135C or roughly 270F is safe--give or take some degrees. Is it optimal for power? No, but it's safe. Notice that most modern engines will go into some kind of failsafe mode, begging to reduce power when you hit around 270F? You won't incur long term damage if you keep oil temps below that unless Subaru dropped the ball.

Now will an oil cooler help keep your temperatures lower? Sure, as long as the system as a whole can reject that heat (radiator etc). Should you go get one if you're still relatively stock? if you can afford to, go for it. Do you need to get one for the health of your engine? IMO, not unless you are turbo and/or you are hitting 270F noticeably. On the subject of oil temperatures being too low, well you increase friction if it's too low, and friction is wasted energy. That's not an issue when running them hard.

As for the question of whether the car should have come with an oil cooler from the factory:

The way that determination is often made is that there is a steady-state "thermal design point" for the engine. That point will probably come from simulation when the engine exists on paper. It may be representative of towing a trailer up a grade in the desert, or running the vehicle on the autobahn. The thermal design point is some speed and torque point with some air and temperature setting, all run on the engine in a lab at a steady state.

If the engine can run at the thermal design point and the peak power point under the pre-determined conditions you're probably not going to have an oil cooler as standard equipment. The oil temperatures would have to be exceeding the limit, again roughly 135C-140C.

Now we've seen some relatively high profile recent cases where stock engines were easily going into limp mode due to oil temperatures. I suspect the root cause is a poor choice of thermal design point, or an inability to convince management that such conditions would be reached regularly by most customers. Some manager somewhere looked at a Powerpoint presentation and said "ok, based on what I'm seeing, you're not going to be getting an oil cooler for your engine"

[stugray]

I know that you want to allow the oil to get above boiling (212F) to boil off any water.

For DD that is not a problem as a good hard drive every once in a while will do that.

For those that want an oil cooler, but dont track the car much, you want a thermostat so the oil doesnt run below 212 for very long durations.

Another good reason for a thermostat if you live in a cold climate is that pumping very cold oil through a cooler can blow the cooler (I have seen it happen and the results were literally visible from orbit - if interested, I'll post the pics :-)

See the line of kitty-litter from the pits to the starting grid? That was at Hastings, NE

[SirBrass]

Originally Posted by stugray

I know that you want to allow the oil to get above boiling (212F) to boil off any water.

For DD that is not a problem as a good hard drive every once in a while will do that.

For those that want an oil cooler, but dont track the car much, you want a thermostat so the oil doesnt run below 212 for very long durations.

Another good reason for a thermostat if you live in a cold climate is that pumping very cold oil through a cooler can blow the cooler (I have seen it happen and the results were literally visible from orbit - if interested, I'll post the pics :-)

See the line of kitty-litter from the pits to the starting grid? That was at Hastings, NE

No thermostat for those of us who are going to get the forester XT oil cooler kit. Oil never flows to an external air/liquid heat exchanger. Rather the regulator core is fed by the engine's coolant lines (not the radiator cold return) & thus integrated into the engine's basic coolant circulation. It brings oil up to temp faster in the cold & keeps it down in the heat.

I just ordered a koyo radiator today. I'll source the forester XT oil cooler parts after my next paycheck.

On phone I am. Therefore, if mistakes I make, Frank Oz blame you should.

[Kostamojen]

Originally Posted by SirBrass

No thermostat for those of us who are going to get the forester XT oil cooler kit. Oil never flows to an external air/liquid heat exchanger. Rather the regulator core is fed by the engine's coolant lines (not the radiator cold return) & thus integrated into the engine's basic coolant circulation. It brings oil up to temp faster in the cold & keeps it down in the heat.

Yup. Simple efficient design, been used by Subaru for over two decades, proven to cool the temps properly, even used on some BRZ's we've seen Subaru showing at track days...



Why these weren't included from the factory, is beyond me... They even offered this style of oil cooler as an "accessory" for Foresters a decade ago!

[SirBrass]

Originally Posted by Kostamojen

Yup. Simple efficient design, been used by Subaru for over two decades, proven to cool the temps properly, even used on some BRZ's we've seen Subaru showing at track days...



Why these weren't included from the factory, is beyond me... They even offered this style of oil cooler as an "accessory" for Foresters a decade ago!

It also takes up so little room that can be used for a super charger, intercooler & the like.

On phone I am. Therefore, if mistakes I make, Frank Oz blame you should.

[BRZPDX]

How effective are these for a fi'd FA20? I like the fact that it doesn't mess with oil pressures, and doesn't add too much complications. mild PNW weather, street use, occasional autox, this should be good enough? I'm contemplating if upgraded radiator is a better route.