why 90 degree's???

[dubcruiser]

Giving nearer 140C as the boiling point of the stuff in the cooling system.

Which, IIRC, above the max temp on the guage.

Yeah max on the guage is 130C...

[Egbutt Wash]

Cos if it's boiling you are stuffed.
130C max on the gauge is to scare the begeebies out of the driver when the needle hits the stop.

[datracer99]

90 is the best compromise for saftety and efficiency.
Engines run more efficiently between 90 and 100 degrees. Anything over 100 and you start getting reliability issues. Under 90 and you lose efficiency.

I have a Nissan race car which we have worked hard to get a constant 90 to 95 deg, and always tune it in that temp range. Invloved relocating the thermostat, electric water pump, and a lot of effort, but it works.

[neg]

I do know that the gauge lies.  On VAG cars the actual water temp can be about 10^o out and it will stay at 90^o.  I guess it's to stop people being alarmed.

what!? thats stupid, if thats the case then there is no point in even having the gauge in the first place!? 

Some fords and toyotas dont - they have a blue light for when cold, nothing when normal and red light if it overheats - which for most drivers is probably a good thing as you would notice it more than a gauge that is there all the time.

[dubcruiser]

I do know that the gauge lies.  On VAG cars the actual water temp can be about 10^o out and it will stay at 90^o.  I guess it's to stop people being alarmed.

what!? thats stupid, if thats the case then there is no point in even having the gauge in the first place!? 

Some fords and toyotas dont - they have a blue light for when cold, nothing when normal and red light if it overheats - which for most drivers is probably a good thing as you would notice it more than a gauge that is there all the time.

The VW Beetle doesn't have a guage, just lights as do all modern BMW's...

[Teutonic_Tamer]

Both the pressure and the ethylene glycol raise the boiling point.

Such a shame that G12 (nor the later G12+, and the current G12++) is not ethylene glycol! 

But yes, you are correct. 

[Teutonic_Tamer]

I do know that the gauge lies.  On VAG cars the actual water temp can be about 10^o out and it will stay at 90^o.  I guess it's to stop people being alarmed.

what!? thats stupid, if thats the case then there is no point in even having the gauge in the first place!? 

Not really.  Why do you think they stopped fitting oil pressure guages?  Peeps were told that the oil pressure should run at say 40psi, yet when the oil gets really hot, and you let the engine idle, the pressure can drop down to 10 psi, and the drivers arses started twitching, and kept running to their stealers.  That is why you now just get an oil pressure light, which simply warns you when the pressure drops too low.

On many cars, particularly small or city cars (the Vauxhall Corsa is a classic example) don't have a temperature guage, just a warning light to tell you when it is too hot.  Problem with doing away with the coolant guage, is that we really need to know when the engine is both too cold and too hot!

[Teutonic_Tamer]

90 is the best compromise for saftety and efficiency.
Engines run more efficiently between 90 and 100 degrees. Anything over 100 and you start getting reliability issues. Under 90 and you lose efficiency.

But the crucial issue is where you actually measure the actual temperature of the water.  Measure in the cylinder head next to the exhaust ports, and you would probably be very close to 140 deg C - or even higher.  Yet measure the bottom hose of the rad, and you are very likely to find the temp 50, 60 even 70 degrees lower!

And don't forget that you can also loose engine efficiency with too high a temperature, as well as too low.  Same with reliablity - if you thrash a cold engine, it wont last long!