Originally Posted by jorj7

When I first got my 90 S4 it ran hot. I changed the coolant (75% water,
the rest antifreeze with a bottle of water wetter), cleaned the radiator, replaced
one of the electric fans that wasn't working, disabled the front flaps, and installed
a 75 degree thermostat. One or more of these fixed the overheating problem.

Originally Posted by Doug Hillary

.... I never said or intended to infer such things - time perhaps for a re-read of the earlier posts?

Originally Posted by Doug Hillary

Hello

Thank goodness everybody can have an opinion and theories - we all have these and they make Rennlist a great place because of it

Some people here will judge these as such too!

The 928's integrated cooling system cannot be compared to an average car as it is substantially unique - a thorough understanding is VERY helpful and not always easy for new owners

FACTS are ALWAYS important to note (all temps shown +/-2C) ;

1 - the OEM coolant thermostat OPENS at 83C and is FULLY OPEN at 98C

2 - the oil thermostat OPENS at 87C and is likely to be FULLY OPEN at about 92C

3 - all aluminium engines SHED HEAT rapidly via air flow around it - not only via the coolant

4 - many engine management sensors are triggered and operate via the engine's "core" temperature and the cooling system's "management" as Porsche's Engineers intended

The following DATA from my ‘89 S4 which has been acquired over two years using an IR reader (backed up by similar data from others) may interest you (each reading was taken after at least 30 minutes and/or at least 30-50kms running);

5 - the core operating temperature is around 88-94C
(my Z3 2.8 operates at 88-92C)
(my heavy trucks average 87C with a warning at 100C and engine shutdown at 106C)

6 - the flaps remain at only 30% open up to an ambient of at least 34C (the fan speed will vary of course)

7 - the oil temp is usually around 88-92C
(my Z3 2.8 operates at about 98-100C)
(my heavy trucks average 103C with a warning at 115C and engine shut down at 121C)
See NOTE below

8 - the S4's "overheat" warning system is first activated at 118C and finally again at 120C. This is an accurate indicator that operating around 90-110C is quite normal and as Porsche intended!

Some OBSERVATIONS from oil and cooling system monitoring over two years in ambient temperatures ranging from -5C(air chill about -15C) to above 42C may be of interest:

9 - Most 928 owners do not understand how the integrated cooling system works. Many will never quite comprehend the coolant's flow characteristics. Some Service Providers don't know either. For normal use some Service Providers recommend lower crack point thermostats, lower triggered fan sensors, removing the S4's flaps ("...they don't do anything....") or even high viscosity oils! Many of these things MAY be totally counterproductive. The engine NEEDS to quickly get to 83C and then to the design operating temperature about the 88-94C range
(Racing use requirements MAY be somewhat different of course)

10 - If all of the 928's cooling system's integrated components are in "average" to "above average" condition the cooling system will function well to keep the engine operating as the Porsche engineers intended (88-94C)

11 - The "living" temperature gauge is NOT an accurate depiction of the engine's core temperature. At best it is an indicator that reads about 5C below the engine's "core" temperature or somewhere above that. I have seen the gauge on one S4 read nearly 100C when the core temperature was about 92C

12 - Some owners unfamiliar with a 928 will think it is overheating because of the gauge or what is a high under hood temperature. This should be checked to confirm what the engine's core temperature really is before spending a lot of money. Most "over heating" reports are usually wrong unless of course you have a true fault such as coolant loss, faulty thermostat, coolant pump and or sensors and etc

13 - Many 928 owners discount the very good operating characteristics of the (correctly working) viscous fan in earlier cars. These fans can and do the job very well indeed

14 - Raising about 16 litres of oil and coolant (about 8 litres of coolant is behind the thermostat) quickly to operating temperature is a real task. The other 8 litres of coolant may never reach the thermostat's opening point at all during many trips! And it may not even open the flaps or trigger the fans which of course require 79C measured at the coolest (almost) part of the radiator!
See NOTE below

15 - In very cold conditions the 928's engine WILL struggle to reach operating temperature (88-94C) and then maintain it! One reason of course is the engine's all alloy construction!

NOTE: ENGINE OILS - the Anti Wear (AW) components in the oil's additive package including the Viscosity Improver (VI) (especially in mineral oils) is activated by temperature and the sooner the oil temperature is above 80C the better. An oil's IDEAL operating temperature range is roughly about 85C-110C and the 928 engine is VERY EASY on its engine oil if it is of the correct quality rating A3/B3 and viscosity (typically 5w-40) for the temperature the vehicle is used in

If anybody would like a summary of the 120 temperature test results I have (high, low, average, median) please contact me privately

Again - everybody to their own appraisal, theory and opinion and no offence intended with the information provided here. I hope it helps some new owners understand their car a little better

Regards
Doug

Originally Posted by ErnestSw

Heinrich,
All other things being equal, a lower temperature thermostat will not cause ANY engine to run cooler once the engine reaches full operating temperature. In most engines the thermostat is either fully open or fully closed. In the 928 it is seldom in either state. When it is fully open in the 928 it is no longer a variable in the cooling equation and other mechanisms come into play.

Originally Posted by ErnestSw

..... In most engines the thermostat is either fully open or fully closed. In the 928 it is seldom in either state. ...

Originally Posted by Doug Hillary

Hi,
heinrich - I never intend to be seen to be an "expert" on 928s - I'm not, nor do I want to demean the input from others. All input is valuable

I am also not "brain dead" on Porsches' either having owned more Benzes (I once worked for DB) and VWs and the same number of BMWs in an ownership trail covering more than 60 cars

In answer to your question:

"If you add a lower-temp thermostat to a system .... that one fact does not, in your opinion, allow the car to run cooler all the time after warmup, all other parts remaining the same?"

a one line answer without dissertation would be unfair! Putting a lower crack point thermostat in a normal cooling system (not a 928) will most likely change the operating point down due to the points you raised earlier

But the one word answer when applied to a 928 would be "unlikely" and there is much circumstational evidence here to support that - ask Ernest!

I have never seen the operating characteristics of the 75C thermostat published for a 928 - what is the actual crack point and the fully open point for instance?
Most thermostats have an opening (or "crack") point and a fully open point. These can be up to 20C apart by design and in the 928's case it is 15C. An engine will run anywhere from the crack point upwards to a possible overheat point. This will depend on many things - but you know that anyway! A well engineered cooling system will maintain the engine's core temperature where the designers intended it to be

IMHO the critical thing to remember with the 928's cooling system is that it was concieved as "an intelligent water cooling system"(Jorg Austen). In its entirety it certainly is that and understanding the inter-relationship of all the components is not easy for many people.
Most people should not even try to!
Many people get confused by the simple fact that if you remove a 928's thermostat the engine will overheat. In a "normal" engine the reverse is most likely! This tells you a part of the 928 cooling system's overly complex story

The 928's OEM thermostat operates from 83C to 98C (all +/- about 3C) and the 87C crack point of the oil cooler thermoststat will endeavour to manage the engines "core" temp to an almost stabil point and within a narrow band. This seems to be normally around 88-96C with the thermostat modulating the coolant's return flow, but according to my testing it appears that the thermostat may remain almost closed for a lot of the time

Remember that the 928's thermostat operates on the return hose from the radiator too and NOT (only) on the outlet hose from the engine like most vehicles! Lowering the crack point may be counterproductive in normal use especially in very cold weather (at perhaps <5C) due to the considerable external heat loss from the all alloy engine as you correctly highlighted in another thread on here. Keeping the engine's core temperature above about 88C and below about 110C is what it is all about

No one liner - sorry. But MHO!

Regards
Doug

Originally Posted by WallyP

Any auto manufacturer can select any operaing temperature that they want.
Every auto manufacturer that I know of currently selects a core temperature of about 85-90 C.
No auto manufacturer that I know of uses a core temperature of 75-80 C.

As compared to a 75 deg C thermostat, an 83 deg C thermostat on a 928 will:
Give better fuel economy.
Give lower emissions.
Keep the engine oil cleaner.

The fact that you don't see any ill effects from running too cool doesn't mean that they aren't there...

But - your car, your money, your choice.