[T3] cylinder head temps

Jim Adney jadney at vwtype3.org
Fri Apr 15 06:48:39 PDT 2022


On 14 Apr 2022 at 20:16, Keith Park wrote:

> Question:
> 
> When would cylinder heads run hotter, on a 30 degree day or a 70 degree
> day? (trick question)

Since you say this is a trick question, I have to look for the trick. I'm 
assuming a stock engine, with all the cooling system parts in place and with 
all 4 cylinders working correctly. The obvious answer to the question is: 
Hotter on a 70 degree day, but when might that be wrong?  

Well, it's obviously right on startup, but how about after warmup, when 
everything has reached thermal equilibrium? The thermostat's job is to let 
each part rise to its same equilibrium temperature, but the thermostat only 
knows about its own temperature.

According to Bob Hoover, the thermostat opens and closes the cooling flaps 
frequently, so the temperatures must vary up and down slightly with those 
changes, but I'm going to guess that Keith doesn't have that variation in 
mind.

The temperature of the thermostat depends on the airflow around it, which 
comes mostly thru the head and cylinders above it, but, while driving, some 
outside air may leak in around the gaps around the lower cooling airflow tins. 
And there would also be some extra radiative cooling from the lower cooling 
tins that have cooler air on their outside faces.Those would cool the 
thermostat more on a 30 deg day, making the thermostat tend to run slightly 
cooler on a cool day, leading to less cooling air for the heads and cylinders 
on a cool day. But that cooling air would be colder, so that seems like it 
might even things out. So that seems like a wash.  

There's one other "trick" that Keith might be up to, and that would be if he's 
asking about what happens at FULL WOT power. On a cold day, the air is 
denser, so we should get more FULL power on a cold day, so that might 
make the heads (also pistons & cylinders) run hotter on a cold day.

That's the best I can come up with. Watcha got, Keith?

Sorry, this was so long, but this is an interesting question.

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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