Oil Temps, Relationship of temp to rpm & speed Options

[ThinAir]

Today I took the teener from Flagstaff to Sedona. A change in elevation from 7,000 ft to 4,500 ft and back again. Morning temperatures at 7,000 ft were probably about 40 and temps at noon when I made the climb back to Flagstaff were probably about 70 in Sedona.

My car is a 2.0L 1973 with stock fuel injection, running 87 octane unleaded fuel. I've got the stock center console fuel temp gauge.

On the way to Sedona this morning I took the interstate at 75 mph (about 3400 rpm) and the oil temp gauge had the needle right over the T where the vertical & horizontal strokes cross.

On the way back to Flagstaff I drove through Oak Creek Canyon, which is 40 mph max, but is still about 3400 rpm in 3rd gear. After about 1/2 hour at that speed the oil temp gauge had the needle straight up between the E & M. As I climbed the switchbacks in 2nd gear (20 mph, but still about 3400 rpm) the temp climbed to where the needle split the M down the middle.

Once I was back on top of the canyon, at 7,000 feet, it was back to 55 mph in 4th gear (again about 3400 rpm) and the temp started coming back down until it got down to about the middle of the E.

So... do these readings sound right for the conditions? The thing that struck me about it is that the oil temperature seems to be more related to speed than rpms. Is this what you would expect, or does this indicate that there is a problem with my cooling system?

[Bleyseng]

That sounds about right and what a pretty drive that is. I did it last year in a Daewoo which is no teener! I loved Flagstaff and Sedona.

Geoff

[ArtechnikA]

... oil temperature seems to be more related to speed than rpms. Is this what you would expect...

yes - oil temp is more an indication of load. the increasing fan speed helps mask the effects of rpm, but ultimately, it's the load (power) that makes heat.

[ThinAir]

yes - oil temp is more an indication of load. the increasing fan speed helps mask the effects of rpm, but ultimately, it's the load (power) that makes heat.

It's good to hear that this appears normal. I don't want to be destroying my engine by running too hot.

I was thinking that the engine fan should have kept the temperature more even since the rpms were about the same regardless of the speed the car was going. I've heard that the fan starts to "cavitate" at about 4000 rpms and thus looses effectiveness, but I expected more even temps in the 3400 rpm range. I suppose that a cylinder head temp gauge would be a better indication of what is really going on?

[ArtechnikA]

...I expected more even temps in the 3400 rpm range. I suppose that a cylinder head temp gauge would be a better indication of what is really going on?

the two gauges measure different things, and CHT is related to a bunch of things in nonlinear fashions i'm not prepared to quote -- start trying to correlate that to EGT and you realise why designing the optimal digital engine control unit is much more difficult in the real world than it appears on paper ...

but think about it - you are not producing nearly as much power going 20 mph (for example) at 3400 rpm in 2nd as you are going 70 mph at 3400 rpm in 5th. so - since you are producing way more power but moving the same amount of cooling air (discounting for the moment the effects of density altitude...) you'd expect a temperature increase with more load...