<VV> Snap Switch Temperature [long] - was: Engine Cooling

[Dale Dewald]

Hello Folks,

For the past 30 years I had made the same assumption as Dave (see message 
below) concerning the 575F snap switch used in turbo/140 cars, but I 
recently noticed a difference in the head design that may offer an 
engineering-based, rather than marketing driven explanation for why they 
are different.

But first some background; I have been trying to set up an aftermarket 
cylinder head temperature gauge that would reproduce temperature readings 
in line with the factory thermistor gauge, but without having the T/C 
rather crudely mounted under a spark plug.  My first attempt was to use a 
ring terminal configured T/C placed under the snap switch.  At this 
mounting position I discovered that the readings are always about 1/2 the 
measured temperature of the factory gauge; 150-200F for most driving.  This 
was the same for both VDO and Westach gauges using type J thermocouples 
mounted to 110 heads.  It seems that the top surface of the snap switch 
boss runs quite a bit cooler than the metal immediately under the quench 
area or combustion chamber that the switch or thermistor senses.  I then 
decided that an aircraft type thermocouple probe inserted in a hole drilled 
somewhere in the snap switch boss would more likely yield the measurements 
I was seeking.  I considered this route because I intend to use a dual 
gauge on my new FC engine, yet I want to retain the snap switches [under 
each head] both wired to the idiot light.  I looked at a 140 head that was 
handy and noticed that a hole for such a probe could easily be drilled 
parallel to the thermistor hole and that it would end up just under the top 
edge of the quench area.  I placed a call to Ken Hand concerning this 
idea.  He immediately pointed out that the switch/thermistor  boss on 
140/turbo heads is different than all other heads.  Back in my shop I 
located a spare 102 head and also looked at the head on a core 95 
motor.  Sure enough, the boss designs are different!  In particular, the 
turbo/140 boss is further inboard and higher up behind the end of the 
combustion chamber with the mounting surface nestled between the fins, 
whereas the standard [98/102/110] head boss extends lower down behind the 
quench area with the snap switch mounted about even with the lower edge of 
the fins.

While thinking about how to drill a probe hole in the 110 head switch boss 
it suddenly occurred to me: A snap switch mounted on a turbo/140 head boss 
would be closer to the combustion chamber, and therefore likely measure a 
higher temperature than if mounted to a standard [low HP] head boss. This 
makes perfect engineering sense to me; the different snap switches may 
measure essentially the same overheat condition, but are calibrated to 
their individual mounting locations.  I think that a snap switch mounted on 
a turbo/140 head sensor boss, being nestled up between the adjacent fins 
and closer to the top of the combustion chamber, could easily measure 50F 
hotter than a snap switch mounted on a standard head [low HP} sensor boss 
when either engine reaches an overheat condition.

Dale Dewald
Hancock, MI

At 11:21 5/23/2012 -0400, Dave Keillor wrote:
>It's clear that GM knew that the high performance engines (turbo and 140)
>  had a tendency to overheat.  They discovered this, no doubt, during
>pre-production testing.  The logical thing to have done would have been to
>improve the cooling, but that would have cost time and money.  Instead,
>they took the cheap and easy way out, and simply increase the actuation
>temperature of the snap switch from 525F to 575F for those engines (and AC
>cars).  Out of sight, out of mind.  If 575F is a safe operating limit for
>the high performance and AC cars (it isn't!), why isn't it safe for the
>other cars?  It costs money to use two different part numbers.
>
>The use of a 575F snap switch is, in my opinion, the single biggest reason
>for the high rate of dropped seats in the 140 engines.  You can run your
>140 at 570F (which, in my opinion, is quite overheated) for long periods
>and for multiple cycles and never know it.  The turbos at least had a CHT
>gauge.  525F is unreasonably high, but 575F is ridiculously high.  Clearly,
>GM knew that the high performance engines ran hotter, but felt that setting
>the over-temperature indication just short of meltdown was a satisfactory
>solution.