Low temp thermostat - why?

I’ve been thinking about reliability of my 987.2 Cayman (supposedly one of the most reliable watercooled Porsches made), and cooling “upgrades” is one of the primary areas cited, at least for track use. This is my DD that will maybe get 1 or 2 track days a year. While some mods make sense to me (center radiator, larger PDK cooler), the one I can’t wrap my head around is the low temp thermostat. Why is this cited as a must have? Both thermostats open fully below over temp, and with all the hoohaa people get into about getting engines up to temperature before driving hard to reduce bore scoring chances, etc, making that harder to do just seems weird to me? LN claims that there are local hotspots in the cooling circuit that will cause damage that the lower temp tstat helps fix, but… really? Increasing heat rejection capacity with more and/or larger radiators, I’m onboard with 100%. Lower temp thermostat? I don’t get it. I have studied enough thermodynamics and heat transfer to be dangerous on this topic, but curious about what people with more hands on experience with these cars have to say about this.

I agree with you 100% @Caylman. The low temp tstat simply sends coolant thru earlier in the cooling cycle so it doesn’t help in any way with overheating. I had one for a short time and now have a normal OE tstat. There are three big issues in my view. First, it costs substantially more than the OE one. Second, mine lasted less than 3 years before it got stuck in the open position. Third, and most importantly, if you have the center radiator, your car will run at too low of a temp. I have an accurate CANbus connected coolant and oil temp gauge (thanks to a very creative and resourceful forum member who makes them). In normal everyday driving with ambient temps at 70F I noticed that my coolant temp would not exceed 165F. This is the temp that the low temp tstat opens. My center radiator was doing it job. But the engine isn’t efficient at such a low temp. So the fuel doesn’t get burned as completely. Furthermore, the oil doesn’t get up to temp. Once my tstat got stuck in the open position, I replaced it with the normal OE one. Now my coolant temp gets to 185 in normal driving. Much better.

Thanks for sharing your data. It is super interesting to see that in a very realistic scenario (3rd radiator, average temps), our cars can be overcooled. In LN’s defense, they do specify it as a track only modification, but I’m having a hard time seeing value in that scenario, either (not that I’m personally that interested as I have a low powered highway flyer that has no practical cooling challenges).

Yes, if the cooling system can't dissipate the heat generated a cooler thermostat will not help. However, it will delay the overheating by a bit, if that helps. The bigger difference is that for a car with sufficient cooling, lower temp water sent to the radiators will return cooler water to the engine. This can reduce any hot spots by a few degree and prevent damage.

I honestly don’t see the usefulness in track use. The issue one faces at the track is the coolant getting too hot. Opening the tstat earlier does nothing to improve this situation. The cars ability to dissipate heat is mostly related to the radiator capacity. Running coolant into the engine at a lower temp does nothing to improve how hot the coolant will get under hard driving. The earlier onset in full coolant flow with a low temp tstat is of negligible help as the coolant temp easily gets into the 200’s F very quickly. When at the track I can reach into the 230’s when ambient temps are in the 90s. The shop that services my car also recommended that I use the OE tstat even as they recommended I move to CSF radiators for additional cooling capacity. We will see how these CSF radiators work when I get back on track in March.

I am not suggesting a lower temp t-stat specifically, but if you start with lower temp incoming water and dump in tbe same amount of heat energy, the outflow (and all the fluid in between) will be cooler than if you you start hotter. Just like lower ambient temps. The real fix is making sure the radiators can dissipate the heat.

I can't see how it helps either. If the car is running too hot with a stock thermostat then the thermostat is already full open. The limit at that point isn't the thermostat, it's the heat removal at the radiators. A lower temperature thermostat can make a car run too cool (as Greg experienced) but it's not going to fix overheating problems at the limit. LN's suggestion (even assuming they're talking track cars) doesn't really make much sense unless I'm really missing something obvious.

You assume that the water temp is even through the engine. Ot is not. It enters cool(ish) and pocks up heat along the way. The hottest points can be significantly above the average (blended) temp at the t-stat/exit. Ot is those hot points that are cooled more with a lower t-stat, and lower return temp.

Sure, at low load conditions when there's not a lot of heat being generated. That's why Greg found his car running too cold around town. But that's not really what we care about, right? We want to keep the car cool at high load. Let's say you're on the track going all out. The stock thermostat is full open at (I'm making up some numbers here) 220F but the engine's running at 225F because the engine is generating a lot of heat. At that point the thermostat is out of the picture: it's fully open, and the engine temperature is being controlled by the radiators' ability to remove heat. A different thermostat won't make a difference in that situation because the stock thermostat isn't limiting cooling at all: it's already fully open. If you take the same car and drop in a thermostat that's full open at, say, 210F and then drive it in the same conditions it's still going run at 225F. A thermostat is basically used to keep the car from running too cold, not from running too hot. If you want to make a car more resistant to overheating putting in a colder thermostat doesn't help (again, because the stock thermostat is already fully open at overheating conditions...the colder thermostat can't be "more open" than fully open). You'd need to add cooling capacity, like larger radiators.

If the cooling system can dissipate the heat, the system should be operating as a "closed-loop" feedback system, with the t-stat fully open infrequently. Track use may be an exception, but it shouldn't be if the system is designed properly. P.S. yes, you need to not overcool am engine, and a significantly low t-stat can do that.

Caylman is specifically asking about racing. Also per the LN page and they say the stock thermostat is full open at like 210F anyway. So once you hit 210F the two thermostats perform the same, and that's basically a temperature you could see driving on a hot day (and you'd see a lot more a the track). So what's the cooler thermostat doing for you? Basically a lower temp thermostat can make the car run cooler in conditions when you don't care, and it won't help the car run cooler in conditions where you do care (when it's really hot). So I'm with the guys above: it doesn't seem to make any sense. Especially when they're promoting it for racing?

Basically the only condition it provides a “help” seems to be increased coolant flow between 180 and 210 (ish). My big hang up on the value is that any benefit presumes the engine can not run at extended WOT. I’m not personally stressed about this as my DD isn’t rallied on, and my only track days have been in cool weather, but it has been interesting thinking about this little problem. I don’t see myself ever doing the low temp thermostat, but central radiator? That seems like an overall good idea.

Not really though: the thermostat only blocks flow to the radiators, right? Any coolant "blocked" by the thermostat is looped back through the water pump and engine block. So the overall flow through the block should be the same, unless I'm missing something on the 987.2 cooling system? This is a 991 coolant flow diagram but what I'm reading it looks like it's basically the same as the 987.1 & 987.2. The flow through the block looks like it's the same in all conditions, it's just how much of it is coming from the radiators and how much is recirculated back from the block that gets varied. The other hiccup here is that the 987.2 thermostat has both electronic and mechanical control (I think) so the ECU can make adjustments. That's apparently how it runs cooler in sport mode (and also why the low temp thermostat can apparently throw check engine lights as noted on the LN page). So I dunno what to tell you on that: if you want it to run cooler then a third radiator + the 987.2 sport mode button upgrade from Porsche might be your best bet?

987.2 does not have an electronic connection to the T-stat. That diagram shows the T-Stat in the return path. (It could be) All the "It doesn't matter for racing folk, depend on the assumption that the system is out of the control band of the T-Stat most of the time. If you notice the Temp sensor is well away from eth T-State after the cylinders have heater the coolant. When you are indicating 210, you are seeing the incoming temp plus heat load. When you are adding a lot of extra heat it is beneficial to start with "cooler" coolant. Seeing that diagram further convinces me that eth T-Stat is in its operating range most of the time.

Yep, you're correct: it looks like the heater was added on the 981 , the 987.2 is purely mechanical. The temperature sensor is out the outlet of the block, yes. That's typical and is the temperature you care about controlling. Regardless of what it's doing at moderate temperatures, the low temp thermostat can make your engine run too cold on the street (see Greg's result), can throw check engine lights (see LN's page), and won't increase the maximum cooling capability of the system, which given that they're saying it's for track-only seems to limit its usefulness. So again, why would you use it? Flow through the block is the same regardless of thermostat opening percentage, right? So if hot spots are an issue why would you expect significantly different temperature differentials in the engine with either thermostat? Like, if the thermostat were restricting flow through the block it could cause hot spots, but it's not doing that. If the coolant coming in is a bit warmer then the engine will obviously run a bit warmer at the output, but at the same flow rate I wouldn't expect any (significant) difference in hot spots, if they exist. It just...seems like a really bad idea for the street? And it won't help on the track at the cooling system limits, which is where you'd care. So I'm still struggling to see the point.

First paragraph is right. Second paragraph assumes that you are beyond the design/control limits of the cooling system. (I don't agree for most peopel most of the time, even on the track) Third paragraph is 1/2 right. "Temperature differentials would be similar but the PEAK temps in the hot sops goes UP as the base or average temp goes up. It is those PEAKS you shave with a lower t-Stat a lot of the time. It is a fairly bad (depending on how cool a T-stat) for the road, and especially in cooler ambients (winter). It may or may not be a good idea on the track. Porsche obviously thinks so since they added an electronic system to open the T-state earlier. (Best of both worlds)

Thanks for the robust discussion! That diagram was definitely helpful. I am now seeing the case for the low temp tstat in certain circumstances. As mentioned (a few times!) on having cooler coolant enter the engine, so I wonder how much cooler the third radiator makes the coolant (lower flow rate addition of more effectively cooled coolant) vs 2 radiators and the lower temp thermostat, and what the engine inlet temp would be at that point? It is kind of a pointless query, though, as I can’t see someone jumping to only the low temp thermostat as their cooling solution. Anyway, thanks for expanding my understanding!

Great discussion guys! And it appears 100% of the people can't justify a low-temp thermostat for the street, and question it's value at the track. FWIW - I also have a marvelous gauge* where I can monitor engine/oil temps quite accurately, not filtered/buffered like the dash gauige does. I drove the car when NJ temps were staying in the 90'F range for days at a time (hotter than Phoenix AZ btw..) and saw the engine coolant temp get up to 209F, where it stopped, which makes sense if the cooling system was of adequate cooling capacity and had a thermostat what fully opened at 210F. The oil temperature generally followed the coolant temperature which makes perfect sense since the car has a water/oil heat exchanger (very good thing - keeps max oil temps reasonable, and helps oil to get to operating temperature quickly.) In more clement weather (70's, low 80's down to around 40F) the temperature seems to come up to around 197F and stay there, so the thermostat is modulating the temperature. Running the engine colder - as someone pointed out - is not optimal from an emissions, performance and longevity perspective. It's only benefit I can see is a possible addition of some time before the car overheats under track use in extreme temps. * = Link to marvelous gauge design, testing and purchase info.. Gotta love this place - civil discussion. A rarity on the web.

Just to add a bit of detail since I have both the marvelous temp gauge Don mentioned (which I highly recommend) and a center radiator. And I’ve had both the low temp and regular tstat. After the center radiator was added and I still had the low temp tstat, my coolant temp would stay at 165 when driving on the highway in 70F ambient temps. After I went back to the OE tstat, the coolant temp would go to 185 in the same conditions. So my guess is that my 3 radiators were able to keep up with the cooling demand and the coolant temp was fully controlled by the tstat. Thus, the low temp unit kicks in 20 degrees below the OE one. Agree about the great discussion all around. We are very fortunate on this forum and with Porsche folks in general.

Thanks for the added detail! What’s interesting to me about this is that based on the diagram above (admittedly not 987 specific), your coolant temps match thermostat opening temperatures, but based on sensor location, is measuring after the radiators. I would have expected lower numbers (but still roughly that delta between stock and low temp tstats). It would be interesting to see what the delta T is across the radiators. Another interesting thing to note about the center radiator is that it not only adds surface area due to the third radiator, but because they are all plumbed in parallel, the coolant flow through the original 2 will be slowed down as well, increasing their ability to heat transfer. Getting super into the weeds about flow, though, the coolant in this parallel configuration will take the path of least resistance, so depending on how balanced the flow characteristics of the 3 radiator paths are, one will be cooling more than the others, but 3 will obviously still cool more than 2.

This is what Barry Hart from Hartech says about LTTs (I think he is referring to 987.1 3.4L engines though):

Very interesting! Some of what is being said in that reported article is talking about a system with a thermostat before the engine block, which seems consistent with earlier gen boxters (and maybe 987.1 caymans ?). Does anyone know if this does apply to the 987.1 ? Also, is the 987.2 different and more similar to the 981 flow diagram posted earlier where the thermostat controls fluid temp coming out of the block?

Yeah, I'd read that before I added the third radiator on my 987.2 I have no idea on the M96 /97, but at least on the 9A1 those concerns don't seem to be warranted. On the 9A1 the thermostat is still at the input. But from the flow diagram it doesn't restrict flow, it adjusts the mix between hot coolant out of the block and cold coolant return from the radiators. So it's varying the temperature of the coolant being sent to the water pump but it's not varying the flow rate. I could be wrong, of course, but with that sort of a setup I can't see how adding additional cooling capacity at the radiators could cause issues. Seems like Porsche was aware of some of the shortcomings of the M96 /97 cooling system and made a bunch of improvements on the 9A1 (e.g., increasing coolant flow, moving the water pump out of the block, improving the block cooling channels). Below is what the 987.2 technical information guide says. No details on thermostat changes, though, and I don't know how the thermostat was set up in the earlier cars. For the record I don't think you have to add a third radiator on the 987.2 for the street; it was just a relatively easy and inexpensive mod at the time so I figured why not do it. Anecdotally these cars seem like they benefit from a bit more radiator area, and the fact that they felt the third radiator was needed on the PDK cars (PDK probably doesn't add that much heat load?) made it seem like a reasonable idea. But this is educated guesswork on my part: I have no hard data so don't take my word for it.