Oil temperature - where to measure?

Competition engines and ancillaries - general discussion
cantfindausername
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Oil temperature - where to measure?

Post by cantfindausername »

Another topic has thrown a question about where oil temp should be measured?

And as I'm in the process of specifying an oil cooler system for my car this is something of particular interest to me.

My initial plan was to run 2 oil temp gauges. 1 pre cooler, and 1 post cooler. So I could ensure that the oil temp was being reduced to a suitable level. From here I changed my design so that I will measure from the return line that goes from the oil stat to the block as this will be both pre-cooler when its not being used, and also post-cooler when the stat is opened.

However, is it better to measure oil temp once its been through the engine? Should it be measured from the sump? Pre turbo? Post turbo?

Maybe an ideal world would measure as many points as possible (from a data logging point of view?) but in real world, and realistically, what is the best "single" place to read temps from?

Thanks,
Ant
Snakeman
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Re: Oil temps - where to measure?

Post by Snakeman »

I have an oil cooler and an in-line capilary oil temp gauge. I have positioned it on an oil pipe just as the oil exits the engine before the remote filter and cooler. That way I know the oil temp at it's highest.

Too hot = worry
Not too hot = get on with driving

G
Guy Croft
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Re: Oil temps - where to measure?

Post by Guy Croft »

The critical oil temperature measurement is the temp of the oil going_in_to_the_engine -ie: at the block gallery.

Of course you can measure it coming out too and thus assess the effectiveness of the oil cooler,

GC
cantfindausername
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Re: Oil temps - where to measure?

Post by cantfindausername »

Guy Croft wrote: Of course you can measure it coming out too and thus assess the effectiveness of the oil cooler,
THis was what I was initially going to do, but figured as long as the oil going into the engine is at the right temp then really thats the most important thing covered.

Flip side to this though is how hot its getting while its doing its job. There are plenty of things to cool on its journey round the engine > What if its already above the working level by the time it reacahes the turbo? Can I expect the turbo to not last as long as it should if it wasn't so hot?

Ant
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Re: Oil temps - where to measure?

Post by Snakeman »

I would argue that it depends on your reasons for wanting to know the oil temperature.

If you want to know that the temperature of the oil going into the galley is good, and the cooler is functioning, then you measure it on the inward pipe just as it enters the engine. But if you want to know if the oil coming out of the engine in ok then measure it just as it exits the engine.

My personal view is that the purpose of oil temperature monitoring is to determine if there are early signs of overheat in the engine. Ergo it should be measured on exit from the engine.

If the cooler is the correct size and not blocked then it must be ok, so knowing the ingoing temperature becomes ‘nice to know’ and, to my mind, not critical. It may be of use on a very hot summer race day where there is high risk of the cooler having a reduced effect, but otherwise not.

I remain open to persuasion.

Whats a 'Turbo'? ;o)

G
cantfindausername
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Re: Oil temps - where to measure?

Post by cantfindausername »

I would imagine that the turbo would be heating the oil considerably more than the rest of the engine. People have commented that measuring from the sump would give a good indication too.

Maybe I'll look at fitting multilpe sensors and a switch panel to flick between them. Not as a long term thing, but I think some data logging around this could be a benefit.

Ant
Guy Croft
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Re: Oil temps - where to measure?

Post by Guy Croft »

Geoff, hi

you're open to persuasion so perhaps I can persuade you. I will say as an engine builder that I want to know the temp of the oil going in - because I know that if it's too cold it won't function properly as a cleaning agent and can give power loss and if it's too hot I'm going to get excessive leak down.

I'm not overly interested in the temperature coming out of the engine, because I know that an engine running properly, well, whilst it's going to transfer heat to the oil, it's not going to 'cook it'. And when it comes out hot, well, that's where the cooler comes in. If the oil inlet temp suddenly skyrockets - then I know there is a serious engine problem.

As for the temperature of the oil in the feed line to the turbo, well here we don't have to worry so much about bleed-down (leakage from bearings) because the turbo shaft doesn't exert much load on the bearings normal to the shaft axis - unlike a crankshaft assy and the truth is that whilst excessive oil temp is a very bad thing inside an engine because of the bleed-down issue, modern oils can easily cope with temperatures over 120deg C and thus still lubricate and cool the turbocharger, and where bleed-down would potentially be an issue, in the tubo thrust bearings, the end thrust load in a turbocharger is miniscule compared with the bearing load in a reciprocating engine. The principle problem with overheating oil in a turbo is oil going past the sealing rings at part throttle, this makes them smoke.

The only time I'd read oil exit temp is if I was proving out the temperature drop over the cooler.

GC
Snakeman
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Re: Oil temps - where to measure?

Post by Snakeman »

Hi Guy,

Your points are valid, but what if the oil temperature on its exit from the engine is, say 130 deg C, and the oil temp after passing the cooler is 110 deg C. In that circumstance the driver (who is reading a gauge with 110 deg C on it) believes the oil temp is 'on the borders' but accepts that it is OK to continue driving/racing. But in reality the oil temp is past the capabilities of the engine oil where engine damage is almost certain?

OK, the oil temperature going in is good to go (just) but, in that circumstance, there is likely to be a problem in the engine and the driver continues based on the gauge information provided. I would also accept the driver may also be reading a marked reduction in oil pressure due to leak down.

G
cantfindausername
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Re: Oil temps - where to measure?

Post by cantfindausername »

Would it be worth fitting a second temp sensor, and wire both sensors in to a switch or relay, so the driver/user can switch the display between pre and post engine temps? This way they could run with the prefered method on main few, and occasionally switch to check what the temp is on the other side?

I can't see a reason why this wouldn't work, or why it would't be a useful bit of info to have (both temps). If there is one, can someone let me know before I buy the parts to carry this out.

Thanks,
Ant
smckeown
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Re: Oil temps - where to measure?

Post by smckeown »

cantfindausername wrote:Would it be worth fitting a second temp sensor, and wire both sensors in to a switch or relay, so the driver/user can switch the display between pre and post engine temps? This way they could run with the prefered method on main few, and occasionally switch to check what the temp is on the other side?

I can't see a reason why this wouldn't work, or why it would't be a useful bit of info to have (both temps). If there is one, can someone let me know before I buy the parts to carry this out.

Thanks,
Ant
You would fit 2 gauges. You don't have time to switch/take arms of steering wheel to play about with the dash when competing.

Based on the advice above, i'll probably move my oil temp sensor from the OE sump location to coming from the oil cooler, or fit 2 (pre and post cooler).

I assume the sump location for a sender would give a reading somewhere between the pre and post oil cooler ?
205 8v Track car
cantfindausername
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Re: Oil temps - where to measure?

Post by cantfindausername »

Good point. I've drawn up what I'd need to do, so may still look down this route as an interim means of measuring - doubt I'll be on the track much for the time being.

Sump temps from what I would see would only give a ball park figure of engine oil temps, until the entire contents has heated to the temp of the oil temp incoming to the sump.

I was thinking about where to measure the post engine temps from. Obviously its different for a turbo to an N/A.

I would think your problem will be that the oil wont leave the engine block, so would make taking a measurement difficult. The only place it leaves the block on a turbo is for the feed/return lines. But then this oil it taken from the main gallery, and hasn't passed through the rest of the engine... so that raises the question "where is it best to measure post engine oil temp"?

This one is pickling my brain a bit.
Guy Croft
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Re: Oil temps - where to measure?

Post by Guy Croft »

Geoff,

110 in and 130 out is a difference of 20 deg which is perfectly reasonable, nay, modest really. But a driver seeing 110 at inlet should switch off mighty quick and get his system sorted out to give 85 deg C max entry temp under all conditions. A temperature rise of the oil of 20 deg would then give exit temp of 105 which, again is perfectly reasonable/safe. The oil is reaching the bearings at a low temp that assures good viscosity and sure, it's heating up, but a proportion of the rise will be after it's done its job, ie: on drain/bleed down.

This is my point - one expects the oil to get hotter inside the engine, but at 85 deg C entry temp it's not going to overheat (to the extent that it cannot lubricate effectively) unless the engine runs a bearing - or similarly malfunctions. If it does the entry temp is going to plummet, I mean shoot-up up well over 100 deg C., and quickly. The oil pressure will drop at a comparable rate and the low pressure oil light will start flicking on at idle (may stay on depending on the sender pressure setting) and under high G. Reading the exit temp will tell you exactly the same thing but will not tell you that the oil is at optimum temp/viscosity going in - which is all you need to know really.

And: Don't run the entry temp hotter than 85 deg C. I say again: over that no_power_gain and risk of metal to metal contact due to the high bleed-down rate. Under 75 deg C, power loss due to drag, poor circulation and lubrication, poor performance of oil at dissipating entrained water (white sludge) gasoline.

Anyone - at this juncture - who disagrees with me - I suggest you fit two gauges.

GC
Snakeman
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Re: Oil temps - where to measure?

Post by Snakeman »

Guy,

Thanks for the clarification. I was winging it slightly on the predicted oil temperatures (110 and 130 deg C, etc). Sorry, I didn't consult my GCT book where it is most likely fully documented.

My exit oil temperature, on my normally aspirated 2.0L TC is 80 deg C for road use, and goes up to 90 deg C on the track or 'spirited' road use.

From this discussion it follows that, if one wanted to know how capable the oil cooler configuration is at reducing the oil temperature, then it is a good idea to fit sensors for exit oil temp and entry oil temp. Once the figures are known then record the results in different driving situation then remove the exit temp gauge components to keep the race weight down.

Geoff
Guy Croft
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Re: Oil temps - where to measure?

Post by Guy Croft »

... you're 'right on the money' Geoff!

GC
L200
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Re: Oil temps - where to measure?

Post by L200 »

Quick question - if you are not running an oil cooler (this is just for an old work ute), where is the best place to locate the temp sensor?

thank you,

Darren.
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