Question? Oil Temp vs OAT

I just re-cut my plenum to match up to the cowl and added baffeling to the cowl for proper air flow from the cowl to plenum.. Also re-sealed all leaks of air. I do not have high oil or cly temps. Just wondering the relationship between the OUTSIDE AIR TEMP and AILTUDE.
Question
Day 1 at 8500 da OAT is 30 degrees oil temp is 170
It is not summer yet so what could I expect when temps go to
Day 2 at 8500 da OAT is 80 degrees. Assuming every thing is EQUAL what would you expect the OIL TEMP to be on day 2 ???

Larry Parham said:

I just re-cut my plenum to match up to the cowl and added baffeling to the cowl for proper air flow from the cowl to plenum.. Also re-sealed all leaks of air. I do not have high oil or cly temps. Just wondering the relationship between the OUTSIDE AIR TEMP and AILTUDE.
Question
Day 1 at 8500 da OAT is 30 degrees oil temp is 170
It is not summer yet so what could I expect when temps go to
Day 2 at 8500 da OAT is 80 degrees. Assuming every thing is EQUAL what would you expect the OIL TEMP to be on day 2 ???

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I don't know - what size oil cooler do you have and does your engine have a vernatherm installed? You may not see any difference, or you may see a significant difference.

My experience is, once temperatures are high enough to tell the vernatherm to get oil going through the cooler, that there is a direct relationship b/w OAT and OT in cruise flight. For example, my oil temp on a 90F day is going to be about 30F higher than it would be on a 60F day.

All that said, I'm exploring alternate coolers in order to try to get OTs down in the hot summer months. That direct relationship might not exist with a more efficient cooler or installation.

So many questions.

Where is your cooler located?
Does your engine have oil squirters for the pistons?
Do you fly LOP?
Which oil cooler are you running?
Etc.

You mentioned a "plenum", how is it installed? Is it mounted close to the cylinders or high up, away from the cylinders? The further away from the cylinders, the more the air can slow down before it goes through your cooler.

If you have squirters, you may not see much change.

If your cooler is close to capacity, you may not see much change.

LOP operations usually lowers CHT's, which lowers oil temps.

One data point:

I flew yesterday, OAT at 8000' was ~0* F, OT was 153*.
A flight last Oct, OAT at 8000' was ~31*F, OT was 174*.

My OAT relationship to OAT runs at about 135-140 deg higher once the Vernatherm routes the oil through the oil cooler. That is to say if the OAT is 60 deg, I am expecting to see 200 deg oil temp (straight and level at best power only). If I run lean of peak I have another curve but I don't remember off the top of my head.

You can easily figure out your relationship by graphing oil temp and OAT as functions of time.

You can determine when the Vernatherm opens up because the slope of the line changes to a lesser slope. You can see it happen around 175-180 deg if you use the standard Vernatherm.

Question oat and oil temp

Steve, if I am hearing you correctly if it were a 30 degree day your oil temp would drop another 30 degrees. So the temps are directly proportional.

Larry Parham said:

Steve, if I am hearing you correctly if it were a 30 degree day your oil temp would drop another 30 degrees. So the temps are directly proportional.

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Probably not exactly, but pretty close, at least during the summer months. Like Axel, I generally ballpark OAT + ~130-140 (though mine are for cruise.)

Again, though, on cold days the vernatherm might not send oil to the oil cooler at all, so the curve would look a bit different--I don't have a ballpark for that.