[T3] temp gauge

[Jim Adney]

On 23 Mar 2013 at 11:17, Dave Pallo wrote:

> As far as VW not installing gauges - I believe the number one reason was
> probably cost, but 1A was the fact that we customers should not see what was
> actually going on (temp. wise)  with these air cooled motors. You could get
> many cars back then with gauges (Mopars were known for factory gauges in
> most all of their cars) and people were used to seeing water gauges hitting
> a position and staying there - summer or winter. 

There's another difference that most people don't think about, and 
that's temperature uniformity. Water is so much better as a coolant 
and heat transfer medium, probably 10,000 times better, so a water 
cooled engine will have pretty much the same temperature everywhere. 
The max temp variation would be between the water inlets and outlets 
from the engine, and that difference is probably less than 20 deg F. 
And once a water cooled engine is warmed up, that temp difference, 
and all the temp differences throughout the water jacket, will stay 
pretty much the same, until the cooling system is overwhelmed and the 
engine overheats.

On an aircooled engine, temp differences of 100 F are common and can 
probably reach 200 F under extreme circumstances. There will be wide 
variations in "normal" running temps depending on air temp, road 
speed, and load.

In a water cooled car, if one part gets warmer, most everything else 
gets warmer by the same amount. This is not always true for aircooled 
engines. So, with our cars, the big question always is, "Where are 
you measuring, and how are you measuring."

I agree with Keith that the best place to measure a sort of "average" 
temp is at the pressure port on the oil cooler, but to do this with 
some reasonable accuracy and response time, you would need a probe 
that sticks down into the flowing oil inside the body of the cooler.

If you measure at the top of a Tee there, you're depending on the 
thermal conductivity of the Tee, because the "oil" space inside your 
fitting is probably filled with a bubble of air. That doesn't change 
a pressure reading, but the air insulates the temp sensor, making it 
partly dependent on air temp and road speed. And if you don't have a 
good foam seal over the top of the oil cooler, you'll also get extra 
cooling air blowing out on your temp sensor and cooling it.  

Measuring at the heads is another good place to measure, because 
that's where any damage is most likely to occur, but which head, 
which cylinder, where? Each of these places can give you a good 
reading at that spot, but will ignore everything else.

I think it was EMPI that used to sell temp sensors that fit under the 
spark plugs. That must have been annoying, but they also offered 
these in a set of 4, with a switch, so you could switch between 
sensors and check each cylinder in turn. Not a bad idea, but I never 
saw one and I haven't seen them advertized in decades. Still, that's 
something you could do yourself.

Finally, we come back to the real problem, which is that none of us 
actually KNOW where the danger area begins. Even given perfect data, 
we don't know what it means. We may think we know, but we're just 
guessing.

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