Re: Centre heater & head temps

[rdh@sli.com (Robert D. Houk)]

   With all this talk of heaters, and people commenting that their
   engines aren't running hot enough for their heaters to put out much
   heat, and the fact that I can't seem to get any *cold* air out of my 

Trust me, the engine is running *plenty* hot. It's just keeping all
that heat to itself to bake the injection system, and not giving up
much of it for the passenger compartment.

   system, leads me to ask, what is a "normal" temperature for the five
   cylinder? (mine's in an '81 4k auto) Probably an ignorant question,
   but does the centre mount temp gauge show head temp?  Mine cycles up

I think that is actually block mounted, but I'm not sure. The stock
UrQ's don't have temp gauges. They only have a blue "it's cold you
turkey" light that turns off after a few seconds (before my temp gauge
even starts registering at 100F minus a few), and a red "you've just
warped your head" overtemp light, and they're block mounted sensors
(actually, the overtemp sensor is in the hose from the expansion tank
back to the block...so by the time it overtemps, you are seriously
overtemp!!!)

   and down between around 190 and 215 (F) as the fan cuts in and out,
   and the engine bay always feels like it's cooking.  Ambient temp 
   here is in the low 70s at the moment, but I'm worried that I'm going 
   to pop the exhaust studs again or something when the ambient starts 
   to creep over 100 F in the coming months.

   Any comments?

Yeah, Audi s**ks...

The design, placing the thermostat in the water intake from the radiator
rather than the head outlet leads to rather wide temp swings (and is
downright crinimally stupid and incompetent, in my opinion).

For steady-state driving (constant air temp, load, etc.) the system regu-
lates the head temp at about 20F above the setting of the thermostat.
The regulation I have seen is very consistent, the temp needle hardly
ever moves once it has stabilized and the load is constant.

If you then really work the engine (go from, say, steady 60mph
cruising drawing about 5 - 10 InHg manifold vacuum to floor it at
14PSI boost for 30 seconds or so up a nice convenient steep Route 2,
just for example) you can see the head temp zoom up to 240F or so be-
fore the thermostat condescends to start cooling off the extra heat
load by opening up and letting more coolant flow.

At idle, the engine pumps little enough coolant that the head temp will
climb another 20F or so before the thermostat decides to grudginly ad-
mit more cool coolant.

When you shut a hot engine off, the head temp "heak soaks" up another
20F or so.

So -- with a lower-than-Audi-stock 180F thermostat, steady-state highway
temps will be right around 200F; idling in town will be around 220F, and
shutting the engine off will kick the head [coolant] temp up to around
250F. Leaving the engine to idle does not decrease the heat-soak temp
rise -- if anything it increases it! In other words, if you are driving
around and stop and shut the engine off, you will heat-soak the head to
around 200+20orso or about 220F-230F (typically observed); if you let
the engine "cool off" by idling for awhile, you can be at 220+20orso and
hit around 250F when you shut the engine off.

All "head coolant temperatures" measured in the coolant flow leaving
the head; sensor and gauge verified "dead on" at 180F, 200F and 220F.
I have no idea what "hot spots" in the head might actually be reaching,
just the gross average coolant temperature leaving the head for the
radiator.

The stock 195F thermostat pegs my 250F temp gauge, so I don't know how
hot it really gets, I can only extrapolate . . . [hint hint]

Also, all measurements on a '83UrQ running with a increased (to 14PSI)
boost limit, so the instantaneous peak heat load ("floor it") is sub-
stantially higher than the average Audi will ever see. I suspect all the
other numbers apply handily to the whole 5-cyl lineup.

					-RDH