Why did Nissan use such a low temp TStat: 76C/170F? ('90 KA24E)

[pgtrr]

When replacing my TStat on my 90 KA24E I was shocked to find the original and the replacement were only 76C or 170F.

Cars in the late 60s switched from 160 to 180 for Ethylene Glycol and improved oil performance and I started seeing 190 and 195s as standard by the mid-late 80s for among other reasons, improved emissions.

Nissan's 170F opening temp left me scratching my head. Anybody know the story behind this engineering decision?

[steve s14]

probably because the engine runs better that way there are probably a few reasons that it's that way but only the guy that designed the engine probably truly knows.my guess would be to ensure that there is adequate cooling of the engine without having it run too cold. back then, they didn't have tools they do now to test designs before going into production so some things had to be done by best guess and experience.most of the reasons engine temps have gone up is because of emissions reasons and not because it is optimal for power output.

[pgtrr]

probably because the engine runs better that way there are probably a few reasons that it's that way but only the guy that designed the engine probably truly knows.my guess would be to ensure that there is adequate cooling of the engine without having it run too cold. back then, they didn't have tools they do now to test designs before going into production so some things had to be done by best guess and experience.most of the reasons engine temps have gone up is because of emissions reasons and not because it is optimal for power output.

In my experience most engines (My Nissan expected) don't run better at below 180. In fact that's generally considered too cold and long since out of practice w/ the advent of ethylene glycol based coolants from the mid 1960s. It doesn't allow the oil to reach optimal temperatures nor does it allow for the most optimal combustion. Engine R&D capabilities have been surprisingly sophisticated for decades. White papers dating back to the mid 1950s (20 years before it became practice) point out the advantages running higher combustion temps.

Keep in mind the 170F is a minimal temp (tstat's do not regulate maximum) - it's a surprisingly low minimum by modern practices. Eg. it does seem too cold. I'm curious if anybody has any insights from an R&D perspective as to why they default to such a low opening temp on their tstats.

[steve s14]

Engine R&D capabilities have been surprisingly sophisticated for decades.

depends on what you would consider sophisticated. they sure didn't have even remotely close to the computer aided design capabilities they do now.

notice how much stiffer the chassis on an s14 is compared to the s13. most of the design revisions were done with the help of a computer which they didn't have during the design of the s13.notice how engines get more powerful and burn cleaner every year. most of these leaps coincide with advancements in computer technology.so as far as being "sophisticated for decades". i'd have to disagree.

no where did i say that 170 is where the engine temps should be for proper operation. i was speculating that the person or persons that designed the engine 17 plus years ago, knew what they were doing.for all i or you know, maybe the coolant circulates slowly in one region of the engine. maybe the thermostat didn't respond as well as predicted.my point is that most people build engines by knowing what is optimal and building it as close to that ideal with the resources available, so there is no way of truly knowing without talking to the people who designed it. everything else is opinion and speculation.

[rs13-1]

Look where nissan's thermostat is located. In the LOWER rad hose.

All of the cars you mentioned with 180 or 195 degree thermostats have it in the Upper rad hose.

Measueing temperatues at different locations require a different thermostat.

Think. (for example a 20 degree warm up)If all thermostats were in the lower hose the 170 would maybe have the coolant at 190 exiting the head.The 195 would have the coolant exiting the head at 215.

Can you see what i am getting at.

[pgtrr]

Look where nissan's thermostat is located. In the LOWER rad hose. ...

Eureka! Thank you very much - makes PERFECT sense, thanks for pointing that out.

[pgtrr]

depends on what you would consider sophisticated. they sure didn't have even remotely close to the computer aided design capabilities they do now.

notice how much stiffer the chassis on an ... (SNIP)

Uh, re-read my second post and note the presence of the word "engine" not chassis. Let's focus shall we? (Though you should be informed that the practice of using computer based modelling goes back to the, gasp!, 'unsophisticated' late 1970s. Indeed, at work we stopped physically modelling frames in the R&D stage altogether as our computer models are cheaper, faster and if configured correctly more accurate. But our physical models are VERY sophisticated by any standard)

so as far as [engine R&D] being "sophisticated for decades". i'd have to disagree.

Your response suggests that you have neither a background in nor an appreciation of the historical evolution of the IC engine. This ignorance of engineering history makes your response, in my opinion, particularly arrogant in it's implied bias that modernity equals sophistication.

no where did i say that 170 is where the engine temps should be for proper operation.

Oh? Actually you did pretty much say that. And I quote "probably because the engine runs better that way". :nono :nono :nono Obviously the context of "that way" in the thread is referring to 170F topic. As was noted by another poster - the engine's internal water temperature is indeed similar to other engines and the lower tstat is merely a function of the difference in location. We were both in error in overlooking that fact. You merely compounded upon the erroneous presumption by assuming it 'probably runs better that way'.

i was speculating that the person or persons that designed the engine 17 plus years ago, knew what they were doing.

Just as they knew what they were doing 25 or 50 years ago.

[Magnes]

Sorry, I don't like to see someone get beat up like that just because he's moderately sarcastic...

SteveS14's reference to improved chassis designs was to illustrate in a tangible way the point about computing power and it's impact on vehicle design. He wasn't avoiding the subject of engine design, he was addressing it indirectly.

Secondly, if he thinks engine R&D hasn't been sophisticated for decades then all we can do is disagree. There is no Right or Wrong way to interpret 'sophistication'. For some people cars will never be sophisticated until they learn to read our thoughts, so be it.

Thirdly, he in fact did not say that 170 is the right temp for this engine, he just said 170 was the right thermostat. As you yourself pointed out, thermostat nominal ratings and engine temps are not the same.

Finally, his point about the very capable engineer 17 years ago who decided, based on the best information available to him (or her) at the time, that the thermostat should be rated 170 was not to suggest that engineers 25 or 50 years ago were not just as capable. Things change, and there is no law of nature that says that thermostat temps can not change too.

As for myself, I hardly think that amateurs theorizing about the optimum thermostat rating on an online chat forum are going to be able to come to a better conclusion that that engineer from 17 years ago, even with the terrific advantage of 'web chat' and 'online forums' tools that we have now.
Modified by Magnes at 8:52 PM 8/15/2006