Coolant temp vs oil temp

Just an observation that may or may not be relevant but I�ll throw it out there. My new 997.2 4s (F6I 4.0 9A1) gauge cluster shows both temps while my 996.2 4s only displays coolant temp. While both engines tend to reach max coolant temp in relatively equal time, now that I can actually see how long it takes for the 997�s oil temp to reach its max, at least an additional 10+ minutes, going forward i will be �easier� on my 996 for an additional 10+ minutes upon a cold start. Im obviously making the assumption that 9A1 vs M96 oil temps warm similarly. I remember seeing threads discussing adding an oil temp gauge to a 996. Probably a better mod than people think for long term health of the car.

We bought a 2019 Audi SQ5 last year and it has a "dummy" engine water temperature gauge like all cars and it also has a digital oil temperature gauge (love that). I find it interesting how long it takes for the oil to get to 170 F or so (max is about 210 F or so). The temp doesn't illustrate until 122 F, the engine always is fully warmed up on the dummy gauge before it even hits that. And if you are just driving around town, it can take another 15 minutes of driving to get it to 170F. On the highway it takes about 10 minutes of driving.

I�ve just started driving the 997. At first i was concerned that the oil temp gauge wasn�t working because it didn�t move off its minimum reading while the coolant was already at 175.

Agree. Water has a pretty high specific heat number ( 4.186 joule/gram ) as well as a high convective heat transfer coefficient, so in theory, it takes way more energy to heat up the coolant than the oil. Why it heats up faster, though, is because there�s way more thermal conductivity and good surface area exposed to it via cylinder head, cylinder walls, block passages, etc. Many of the newer cars probably try to keep the volume of these passages as small as possible to get higher temps from a cold start to better emissions. They try to circulate the smallest amount of water through the block and heads by keeping the thermostat closed. And when the thermostat opens, hot water can quickly diffuse with cold water and not have much of a viscosity change. Motor oil has about 1/2 the specific heat, which means it requires 1/2 the energy to heat up than water, which looks promising to achieve faster heat up times. But it also suffers from being 1/3 the thermal conductivity of water, so it would take 1.5X longer to heat the oil than water with both factors averaged out. Oil gets to hit the top of the cylinder heads, cams, valves, springs, bottom of the pistons, cylinders, crankshaft, so it ends up with great surface area contact, except most of those areas are the least of the hot items in the engine, some of which take the longest to heat up. The cylinder walls and piston bottoms are the hottest, which represent a much smaller surface area to heat the oil. So this is why the oil cooler in our 996s is really just a heat exchanger, heating the oil from coolant when cold, and shedding heat into the coolant when hot. The placement of this exchanger and the capacity of it could affect how long all this takes. In the end, why not just use oil inside the engine instead of coolant? Because oil breaks down at higher temps and can leave deposits in areas that we don�t want. Water can heat up and turn to steam, but there shouldn�t be any impurities (hydrocarbons) when that steam reverts back to liquid a second or two due to having such pure water in the system. For those drivers who have been waiting until coolant is at temp to drive your cars hard, you are probably not risking your engines too much, on the basis of the idea that oil still might be at 100-120F, even as coolant is at temp, 170-200F. Oil viscosity at 110F is way more fluid than at 70F or 50F or 32F. Designing an engine to redline safely under full load should be achieved only when there is enough oil pressure and volume - both - to protect the engine, and that�s likely occurring well before oil temp reaches operating temperature. But only if the engine designers made it so. So the safe route is to assume that only full operating temp for both oil and water, as well as piston head temp, and cylinder head temp, and exhaust temp, and transmission fluid temp...are all up to operating temp. Which could take 45-90 mins! But I don�t want to wait that long, so I drive my car as if it has a BMW M-series tachometer - with a progressively moving redline.

Contact Porschetec3 for a parts list to DIY an oil temp kit

Air (oil) cooled Porsche owners know it takes a long time for oil to warm up. My Italian car also has gauges for both....I can drive 50 miles before the oil even starts getting reasonably warm whereas the coolant temp reaches normal levels within a few minutes I wait until my 996 has been at temp for at least 10-15 minutes before I start getting "frisky"

Quotes source post: 16515575

Ive been suffering from �premature friskyness� but i now have a better appreciation for my Porsches� need for more foreplay.

I've noticed that my oil pressure will drop to just below 2 when the oil is warmed up. Goes along with what was previously posted and takes about 10 mins of driving after it's reached coolant temp. Obviously the colder it is outside the longer it takes for the oil to warm up. My rule of thumb: In 30 degrees outside temp don't go above 2200 rpm while being very light on the throttle. Once coolant is about 30% warm, don't go above 3200 rpm, a little more on the throttle. Once coolant is 100 %, don't go above 4k rpm. When oil is up to temp, then it's "drive it like it's stolen!"

Quotes source post: 16515649

Driving at too low an rpm (especially lugging the engine) is bad. I believe the rule of thumb is keep the rpm at around 3K rpm as you are driving (even when cold). I seldom drive at anything less than 2800 rpm. If I drop below that I shift to the next lowest gear.

Quotes source post: 16515649

Good point. Dare I say that oil pressure is almost as good as an oil temp gauge? It works the same way cold as it does hot. When it�s really hot, my idle pressure is 1.25 bar instead of 1.5. I know my oil is hot when it drops from 2-3 down to 1.5 on idle (625 RPM with the AC off).