low temp thermostat

[sambb]

When I do my radiator, I'm going to fit the 82 degree (rather than 90 degree stock) thermostat. My question relates to the interplay of this thermostat with the fan switch. There is a 91/100 degree stock one and an aftermarket 85/90 degree one.
The guys at Awesome GTi said that on our Polo the fan control actually looks at the engine temp and the radiator temp as seen by the fan switch.
So to my mind if I change to the lower temp thermostat, I'd pretty much have to be running the lower temp radiator fan switch too wouldn't I so that the two marry up?
anyone gone down this path??

any advice welcome as I need to get onto ordering bits before my radiator has a haemorrhage

sam

[Eddy]

Hoyhoy.

Interesting subject Sam.

[louis19]

I believe Sean's car runs a 71C thermostat. Might be worth a chat with him or Gav as he might know a bit more info on the subject

[sambb]

Yeah I'm just not sure how it will play out if I run the cooler thermostat but retain the standard fan switch. Awesome thought that the ECU would probably prioritise the engine temp sensor reading and override the fan control even though the switch should theoretically only turn on at 100 degrees and back off at 91. I'd just hate for it to not work out that way and the fans end up hardly being on.
I was surprised that the our fan switch could input to the ECU but apparently its not a dumb/local switch like in the golf mk3/4 & bora. Actually after re reading their return email I'm going to have to get clarification on whether it may be more than just incompatible with the fan control system - I'll have to check if the 85/90 fan switch will even fit!

[Stuwey]

Why do you want it to run cooler?

[sambb]

Just got a return email from Awesome. He thought that running the one step cooler thermostat would be effective at keeping the engine temps more consistent with the standard fan switch retained. I'll have to clarify again but he seemed to intimate that doing the switch too might be a bridge too far and cause the ECU to enrichen the fuel trims.
All I want is to smooth out any big temp differentials between the engine and radiator and the existing thermostat doing big temp dumps in and out of the engine. The whole sequence of events at hilllclimbs absolutely cooks the engine - sitting in the line, launching hot and then having to shut it down hot with no cool down possible. I've learnt a few tricks like turning the A/C on to keep the radiator fans running which may help but I'm afraid I'm going to warp/blow something that I can't afford to fix. I'm not keen on any detrimental effects on trims though

[Martin]

I've never been keen on lower temp thermostats - the engine is designed to run at 90 degrees (tolerances and ECU)
(5 degrees lower is not a big deal but ~20 degrees is way too much, the ECU will notice that and intervene)

There should not be any big temperature differentials between the engine and radiator - no issue there

Certainly going hard on a low speed hill climb may not give enough airflow over the radiator,
what temp does the engine reach after the finish line?
As long as it's not over 100 you don't have a problem (imo)
Does idling for a few minutes (with the A/C on) not bring the temperature down?

Turning the A/C on will turn the A/C fan on which will give airflow over the radiator
It will also be robbing some power from the engine to drive the pump

This is a street car you use at hill climb events?

You can add manual circuitry to turn both the radiator and A/C fan on
(despite the ECU not turning the fan on and the A/C being off)
When turned off the ECU and A/C will be free to control the fans as per normal

A larger radiator (normally thicker) would be the next thing to look for

More shrouding around the radiator to block gaps will also help
Air always takes the path of least resistance - cool air will escape through gaps rather than go through the radiator

[Stuwey]

I think this is all a bit storm-in-a-teacup. Modern engines run at peak efficiency when hot. In the bad old days 82C was the aim - these days running at 95C+ isn't just not a problem, it's actually what they aim for.

Until it's boiling and expelling the coolant from the header tank, it's fine.

[sambb]

That's exactly it. Modern engines are designed to run hot (and lean) because hot lean engines produce the best fuel economy. Fuel efficiency not power efficiency is the aim of the game when they do that. If VAG engineers had designed our cars specifically for what we are doing to our cars and in our climate, I think its reasonable to assume they'd have slightly uprated the cooling spec. I don't see anything twisted about looking into the cooling system when a motor with 100hp more than stock, is being baked on and off the line for 2 days straight in Australian 36 degree heat, when it was designed for deutsch shopping malls.
Going to the lower thermostat is just going to mellow out the extremes so that engine temp won't cycle as much. Why have head temps in the high 90's and then climbing beyond that when you shut it down hot, and a radiator full of cool water that the thermostat won't let through. It'll be more steady state when being abused this way.
Remember we still have an iron block and alloy head. The turbo will heat its cooling water in an extreme way when shut off like this too- bolts stretch etc etc
At any rate Awesome gti just confirmed that the thermostat will do just that but won't drop temps to any degree where the ECU will assume a cold engine state and start to mangle fuel trims. They did confirm that the fan switch system should be left alone though.
I was going to do what Martin suggested and have a good look at the shrouding too.

[Eddy]

Hoyhoy.

I found if ya having a serious go @ the track the thermostat won't make too much difference because the temps are way up anyway.
I found that an extra oil cooler, water-meth & extra engine ventilation helped a lot.