Mechanical type gauges are often more accurate, but I actually recommend an electric type oil temp sender. This is because:
- they are using to hook up, as there is no capillery that needs to pass through the firewall
- mechanical senders use a fairly large fitting and, unless you use the sump, it can be hard to find a good spot for them using off the shelf oil pedestal adapter
- electric sender have standard 1/8 NPT fittings which is usually what the oil pedestal adapters have.
Now, why not fit the gauge to the sump you may ask?
I had a mechanical oil temperature sensor in the sump which I fitted using a plate where the oil level sensor usually goes. I can't say for certain whether it was the position or the sensor itself, but the whole exercise was useless. It would take 30 minutes of driving or sometimes longer before there was an observable increase in oil temperatures in the sump, even though I could say for sure the oil had reached operating temperature (due to the e-shaft pellet and oil cooler thermostat bypassing).
I put this down to the time it would take for oil in the sump to be fully cycled through the engine and heat to be evenly distributed through the system. Based on my experience, I believe this takes a long time, as you effectively have a large reservoir of stationery oil, with only a relatively small proportion of that pumped through the engine and eventually returned to the sump.
Once the sump oil did finally heat up, the gauge would begin to register oil temp changes, but it took a fair bit of driving to get to that point.
It is like sticking a coolant temp sensor in the radiator core, which no person would sensibly recommend doing. Until the thermostat opens, the coolant temperature in the radiator could be sitting at 30-40'C while the coolant recirculating in the block is at 80'C. It is only once the thermostat begins to open and cold coolant from the radiator is metered into the engine that temperatures across the entire system equalise (UK/Aussie spelling, not going to use a z for you guys lol) and system temperature changes are properly measurable within the radiator.
In my view, it's best to measure temperature as the oil is entering the engine - the cooler it is, the more heat it will take out. Even though it is at the hottest point, you don't really care about the temperature after the oil leaves the engine, as that it is not what the bearings are seeing. In the water cooling system, you
do want to read the temperature at the hottest point, but this is because you want to prevent overboiling. That, and water coolant temps should be fairly uniform throughout the system once the thermostat is open.
To my mind, BLUE TII on rx7club.com has explained this all most succinctly:
Oil is a lubricant in the lubrication system of the engine. You measure oil temperature to make sure the oil can still do its job as a lubricant.
The oil in a rotary also cools the rotor. The cooler the oil before it cools the rotor the better, the hotter the oil after it cools the rotors the better (you want maximum heat exchange).
You measure the oil temperature before it enters the bearings because you want to make sure it can still do its job lubricating the bearings (if it is too hot entering the engine it cannot).
You don't measure the oil temperature after it has been through the bearings because you would also be measuring the heat added by cooling the rotors.
You don't worry about the oil boiling (boils at 570 F) and not being able to cool the rotors; the oil is constantly breaking down and will be changed between races or 3,000miles.
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Coolant, coolant/water or water is a heat exchange medium in the cooling system of the engine. You measure your coolant temperature to make sure the coolant can still do its job as a coolant.
What is important is that the coolant stays below its boiling temperature, so you measure it at the hottest spot (near the rear spark plugs).
It is not important to engine health what the coolant temperature is leaving the engine after it has circled around the cold side of the engine cooling down and is ready to enter the radiator.
It is important to the ECU to have a slowly increasing/decreasing input of coolant temperature while warming up instead of an exact value, so the ECU coolant input is at the outlet of the engine.
You don't want to monitor or log from this sensor except to see general trends instead of actual values.
We are used to looking at this sensor value though since it is the input to the ECU.
So, on balance, I think the best spot for oil temp sender is an adapter fitted to the oil filter pedestal. These usually have multiple ports, so you can stick an accurate oil pressure gauge sender there too.
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