Oil temp depends on where you measure it

#1

I wanted to collect some data on measuring oil temp in the flow vs. measuring it at the pan. I had a theory that if there’s no air flow to the pan, then the pan gets heated up by the block and if you measure OT at the pan you get an artificially high reading. An experiment last month indicated that OT at the pan changed by 35deg with an oil pan shield blocking air flow. What did not get done last month was to see how OT at the pan compared to OT in the flow.

Raced this weekend at Roebling with 3 OT sensors. One in the pan and another in the flow. Both went to a single gauge via a switch. The 3rd sensor was a digitil gauge in the flow. The “in the flow” sensors were in an oil filter sandwich adapter.

No oil cooler.

What I found is that the sensors in the sandwich adapter read crazy high at 260deg while the sensor in the pan read 170deg. The digital sensor read 10-12deg higher then the analog sensor, so “crazy high” was not only confirmed but was kinda “crazy high + 10%”.

There’s no way my oil temp was 260deg, not when my engine was at 180deg with my fabulous new double-pass radiator. I just can’t buy that. My theory is that the sandwich adapter, when fastened directly to the block in the absence of an oil cooler) got heated up by the block, and maybe the exhaust manifold.

The digital sensor was pointed down and the analog sensor was pointed forward on the sandwich adapter. So it’s hard to buy that radient heat from the exhaust manifold was affecting the sensors too much, especially the digital sensor and especially at speed. And recall that it was the digital sensor that read higher.

So what does this mean…

  1. You can’t compare OT #'s unless you’re measuring it exactly the same way. The difference can be a lot.

  2. The most accurate way to get OT’s are to put the sensor in plumbing, away from the motor.

  3. Getting OT from a sandwich adapter fastened to the motor leads to artificially high measurements. Getting OT from a sandwich adapter with the OEM oil filter adapter would be better, but still not ideal.

  4. Getting OT in the pan is distorted by airflow to the pan. The pan gets cooled by airflow and that makes the sensor read artificially cool. This is significantly impacted by OT probe length. Likewise, if the oil pan isn’t getting any airflow the block heats up the pan and the pan makes the sensor read hot. The longer the probe the better.

#2

how about if you drill/tapped the oil filter adapter bolt and used that location? i was planning to do that but didn’t get around to it. i’m happy with the location in the pan for now. not THAT worried about the difference since its more the range i’m interested in rather than a precise number.

#3

kishg wrote:

I’d probably use the sandwich filter before I’d put a hole in that bolt. Both solutions would have the same disad…getting hot from the block.

Consider this… If the sensor says your 190deg oil is 260deg, what will the sensor say when you have a genuine problem and the oil temp climbs up to 240deg? Your guess would be as good as mine.

#4

well it all depends on the baseline for what normal is doesn’t it? is 260 deg bad?

#5

kishg wrote:

Our oil shouldn’t get that hot. An aircooled motor might. Certainly turbo charged engines run hot oil because some of the oil goes thru the turbo. I can’t speak for other performance applications, I just don’t know enough about them.

It’s not just about the baseline. If the most significant variable affecting the temp displayed on the gauge is the hot engine block, or the air flow or lack thereof, then things get confusing. Lets say that you have your oil sensor in a sandwich adapter and you are used to seeing hot oil temps of 260deg. One day you see 270deg. But now you have to interpret that. Does it mean the oil is ten degrees hotter? If the heat flow is such that 3X as much heat is coming from the block as from the oil, then if the oil temp did go up 10deg, the sensor would only go up 3deg.

If the gauge did read 10deg higher, then you’d be stuck trying to puzzle out why?

IMO opinion it’d be better to sense all variables in isolation. That way if something goes up/down, there’s no ambiguity. Pressure you can do almost everywhere. Temps however, not so much.

#6

I’ve got my OT sensor (SPA Dig) tapped and bolted in the oil filter/oil cooler adpt bolt. Been running 178-cool and 210(Tops)-hot. Isn’t the oil pressure highest there? Pulling from the pan…

#7

NASA144 wrote:

The oil that comes down from the head thru the channels in the block (not down the pipe) is probably the hottest. That mixes with oil coming out of the crank bearings and the oil coming out of the intermediate shaft bearings. The oil from the latter being not so hot.

Oil splashes around inside the motor like crazy. Visualize a crazy whirling 6000rpm hurricane. Although water is cooling the middle of the block, there’s nothing cooling the bottom third of the block other then air flow and oil spray. I’d guess the the bottom third of the block gets pretty hot.

Oil pressure is highest right after the pump. The oil goes immed to the oil filter, so it’d be plenty high there too. Where exactly the oil’s temp could be measured at it’s peak tho…I dunno.

Clearly you are getting better temp data then I am at largely the same location. I wonder if some subtlty of my sandwich adapter causes oil to be stagnent right where my two OT sensors are. The stagnent oil could then get heated by the block and I’d see really high temps. The OT sensor that is reading highest, on the sandwich adapter is the one that is farthest away from the water jacket (it’s lower). That could explain why it’s reads 10-12deg higher.