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reading the many threads about thermostats, but still have a few questions. I have read that 87-89 cars had a PET part number
ending in 16 that was 75 degrees, and after 1989, ending number was 17 and 83 degrees. the early 16 number has been superseded
to the 17 part, so, it appears, that the 75' thermostat is discontinued?? not sure why the early cars would use a lower temp and
confused as to why it would be superseded to the higher one?
hoping folks with either thermostat will chime in and report where their temp gauges reads with the different thermostats. I have the 83'
and my needle is either a needles width below the third white line, or right on the line after normal warm up
wondering if people with 75' see their gauges at a lower point?
some of the 75' T stats have a smaller base than the 83' and assuming that would not be compatible with the internal O ring?
is there a different temp switch required with each t stat for correct fan operation?
This is a good thread. When I did my refresh on my 78, I replaced my thermostat with the new one - my car always reads hotter now when I drive it. I does not overheat, but it does make me worry because the needle was always in the lower third, and now is always towards the top of the middle third or even the bottom of the top third. My preference would be to go back to the "cooler" t-stat.
The two different thermostats will have zero effect on the steady-state core engine temperature. The ‘cooler’ tstat just makes it take longer for the core engine temperature to get to the designed operating temperature.
If you perceive a change in steady-state core temperature under then same driving conditions then something else under the hood has changed.
I have never used anything but the 83oC thermostat, per Porsche's suggestion. If you don't have a sharktuner, you should come borrow mine so you can drive around for an hour, log the data, and you will see that the coolant temperature varies between 179 and 187 degrees regardless of the thermostat and whether you're cruising at 2800, doing WOT blasts, or doing stop-start through town. Then you can put a big piece of duct tape over your temp gauge and stop worrying about it..
I run the 75 and it's a clear difference between the 83 when the vehicle is in motion. Aside from that, temperature when sitting at a stop light is the same if not just a little lower.
The two different thermostats will have zero effect on the steady-state core engine temperature. The ‘cooler’ tstat just makes it take longer for the core engine temperature to get to the designed operating temperature.
That's not necessarily true. A lower t-stat will lower the operating temp IF the cooling system has sufficient heat rejection capacity - and the 928 absolutely does, when everything is working correctly.
I did multiple tests - changing nothing else - in my 79 with both temperature t-stats and one modified to be always open (since we all know you cannot run a 928 engine without a t-stat). My 79 ran so cool with the always open unit, the temp needle never got past the initial white mark unless I was on the track and only after a couple laps @ WOT.
Same results in my 81.
Temps verified with a heat gun & other measuring devices.
However....if the car cannot maintain 83c under optimal conditions, installing a colder t-stat is not the solution, something is wrong. You really need to verify with a heat gun or some other calibrated device to verify what temp it's running at before getting too concerned.
Also, with a bone stock car, there is little to no advantage to running a colder engine, especially if the colder temp is below the fully warmed up temperature compensation cell in the ECU mapping. I cannot remember what the levels are in the 928's computers (can someone post a screen shot?).
The fans operating temperatures are also an issue. This needs to match the t-stat for optimal performance. Both of my cars are using different temp switches, I don't think it's that easy on an 85+ car.
At some point the load could exceed the colder temp range and it won't matter much then either.
It's an odd balance since a colder engine is actually less efficient, but may be less detonation prone. In a perfect world the heads / block would have separate cooling systems since what you really want is a colder combustion chamber. Twin Turbo Todd has debated for years on ways to do just that, separate the cooling systems so you can run a warmer block and colder heads. With e85 his engine never gets much past 170F anyway.
Also need to remember, if the engine oil never gets much past 180 you risk moisture buildup.
If you don't think changing the t-stat will effect the steady state operating temp, install a 110C / 230F thermostat and tell me it doesn't run warmer.
Erik: my main issue is living in a very humid and nearly year round HOT climate, is there an advantage to running the lower
thermostat. they made the 75' for the earlier cars, and those used the same temp switch as later cars: same switch from 87-95.
I feel confident, that in Florida, my oil temps are never on the low side.
Mongo, can you tell us the difference you see on your temp gauge? thanks
That's my temps when warmed up when the weather is under 90 degrees here. Pic was taken after doing 80 on the freeway and just getting to the offramp stop light. Outside temperatures above 90 and it's about half way. I worry less with a 75 honestly. It's your car and you can put in whatever thermostat makes you feel warm and fluffy.
That's my temps when warmed up when the weather is under 90 degrees here. Pic was taken after doing 80 on the freeway and just getting to the offramp stop light. Outside temperatures above 90 and it's about half way
I believe the earlier cars were designed to run on the warm side as evidenced by the temp gauge. At one point years ago I tried the lower temp thermostat and found that the heater was at best blowing barely warm air in the winter, so I went back to original spec tstat.
05-30-2018, 10:02 AM
75 v. 83 thermostats for 87-94 cars
