Way down. Try some beautifully created German WW2 fighter addons for FSX, e.g. Classics Hangar's Fw-190s and Flight Replicas' Bf-109K-4. You'll find they perform much better FM wise and CEM wise. And they are widely aknowledged as veracious representations of their real-life counterparts.

I've read somewhere that real 109 pilots were not allowed to widely open radiators because it would cause too much drag. That indicates from another aspect that the newly introduced overheat model might be seriously flawed.

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I'm curious about "real 109 pilots were not allowed to widely open radiators because it would cause too much drag", could have been on very early 109's, I guess, but 109's were normally flown with rads set to automatik. This was done to keep the engine temp at optimum for the engine, somewhere around 90c in and 115c out IIRC. Drag was also pretty minimal, open was only 52mm wide or 2 inches max, closed was 18mm or about 3/4 in. from memory.
What circumstances would you ever want to fly with rads wide open in RL? In game there is a supposed advantage with closed rads but open? Flying me109's ingame by the book, the temp guage just about hits 40c, (just about permissable RL take off temp!), so I now fly at 100/110% throttle ingame when in combat and temps barely reach 80c. Something seriously wrong with the new temp model as far as 109's go. They are also too slow when flown by the book, F4 109 at 110% should reach 590kph, but ingame....

Oil entry temp limit for the DB 601E is 80°, 85° short time maximum. Radiators open does a lot for cooling, and it is no miracle that the oil temperature will only just go into overheating, as long as you are on cooler maps. After all, that is what the plane was designed for. Open rads were around 300mm and caused a lot of drag.

The radiators on the 109 were supposed to be on auto during climbs and closed (more correct "fast flight setting") during level flight. If you want to get the best performance out of the plane, you'll need to close radiators and if you want an overheat challenge, fly the plane on hotter maps, for instance a Desert map.

I'm getting the 109F-4 to about 600 km/h, exact figures varying with atmospheric conditions.

What I wrote about radiator was referring to MW50-equipped Bf-109K-4. In the current game, if you go into battle with throttle at 110% and MW50 on while keeping radiator fully opened. You will still get the overheat message within 1 minute. This is ridiculous especially when MW50 has a cooling effect on the engine.

Just like I said, go try these warbirds in FSX. They operate very differently both FM wise and CEM wise compared to what we have in IL2.

It's possible to fly fuel tanks dry on 110% with rads open in a 109 K-4, and that not only on winter maps. I just climbed one from 0 to 10 km at 300 km/h IAS on the Smolensk map. I left it on auto rads and firewalled it upon starting the engine. I didn't even see the overheat message, let alone had temperature related troubles.

Also, just because it overheats it doesn't mean there's any damage. In real live WEP was an 80% overload condition, and it is only logical that the plane overheats in game to illustrate this.

It is pointless to use continuous climbing as the test, since MW50 was supposed to be used only when you are most in need of energy.

Try dogfighting with some bogeys. Do some boom and zoom, barrel roll, chasing and dodging with them. My test result is that the overheat message popped up within 2 minutes since the quick mission started. And the engine was damaged within 5 minutes (Sometimes I didn't even get a audible change in engine sound. The engine and propeller suddenly changed to a full stop.)

The map I used for testing is Crimea.

Crimea is one of the hottest maps, no wonder. Also try to look at temp gauges, just because MW50 is on it doesn't mean you can ignore engine temperatures.

So the method is to go in the QMB, set up a battle, start, engage MW50 at low power and then firewall 110% for the duration of the dogfight or however long the engine lasts? What about dropping to 105%? Alternating rads closed and open depending on speed and attitude?

I'm just thinking that since the new overheat came into effect I've been having no problems as I tend to play the engine management game closely. But.. I spend more time with the 190 and that's nice and easy IMHO.

In my tests, RPMs dropped gradually, signalling the damage, even if clanking and pinging sounds weren't heard. Il-2 has a limited range of sounds, so don't rely on audio cues too much.

As others have mentioned, dogfighting low and slow with overboost is just silly. In dives, reduce power so the prop pitch can coarsen, lowering RPMs, and thus temperature. I only use the boost for extended periods when I've done everything else wrong and I have to escape in a straight line.

Does anyone really think that WW2 pilots, on any side, would fly an entire engagement with the throttle against the stop and WEP or whatever the equivalent would be on?

It's total rubbish.

Real pilots tried to take care of their mounts, as it had to get them home.

They did not have a refly button.