Thermostat question

[sirstude]

This has been talked about before, but now it is my car, so a few more questions.  I am going to change the rear gear in my Impala from a 2.56 to a 3.42.  This weekend coming home from Spokane I ran in 4th gear to simulate what the gear change will do.  4th is almost exactly what 5th will be with the 3.42s.  Anyway, it ran much better up in the rpm range, about 2700 or so instead of 2000.  The thing I noticed was the temp gauge dancing.  It would go up to almost 190 and then drop down to 170 or so.  I have a 185 degree thermostat in it.  Probably time to replace the thermostat.  The question I have is since I only drive in the summer, should I put a 160 degree thermostat in it and let the radiator control the temp, probably will run 170+ on the radiator.  Or should I have the 180 degree and run on the thermostat?  Heating is not an issue with this car, just wondering what everyone thinks.  The motor is a ZZ502 for information.  The gearing change seems to drop me down from 15 mpg to about 13, so not a problem there either.

Doug

[enjenjo]

My opinion. The hotter you can run the engine without overheating, the more HP you get, and the better mileage you get. In this case I am defining overheating as uncontrolled rise in coolant temp. Most modern cars have coolant temps in the 220 to 240 range as normal temperatures.

I have never been able to trace an overheating problem to  the thermostat, unless it was not functioning at all, and blocking the coolant.

[HOTRODSRJ]

My take is a tad different.  Actually, the hotter the "peak combustion temperature" the more hp you make. The engine coolant temps may or maynot have something to do with that.  

If your nominal engine temps are running so hot as to affect the incoming fuel/air charge temperature (namely increase the temperature), the less dense the charge will be resulting in loss of power (this is why outside air induction works to make hp).  So, keeping fuel and air cool is important if you are interested in power.

Today's modern engines are designed both from materials and function to run a hotter thermostat. This has to do more with pollution issues but combines for more effiicency as well.

Most of the crate motors of yesteryear's design (such as your 502) are made for the 175 to 210 degree range. In fact, running less than 175 reduces not only power but also is not hot enough to work correctly with most oils (can't burn off all the moisture etc).  

I would recommend a "balanced" (AKA high flow) 180 degree thermostat from Mildon, Mr Gasket or Robert Shaw.  These are highly effective at controlling temperature, higher flow and won't fail in the closed position.

Here's a great discusion page for more... http://www.performanceunlimited.com/illustrations/thermostats.html

[Dave]

This has been talked about before, but now it is my car, so a few more questions.  I am going to change the rear gear in my Impala from a 2.56 to a 3.42.  This weekend coming home from Spokane I ran in 4th gear to simulate what the gear change will do.  4th is almost exactly what 5th will be with the 3.42s.  Anyway, it ran much better up in the rpm range, about 2700 or so instead of 2000.  The thing I noticed was the temp gauge dancing.  It would go up to almost 190 and then drop down to 170 or so.  I have a 185 degree thermostat in it.  Probably time to replace the thermostat.  The question I have is since I only drive in the summer, should I put a 160 degree thermostat in it and let the radiator control the temp, probably will run 170+ on the radiator.  Or should I have the 180 degree and run on the thermostat?  Heating is not an issue with this car, just wondering what everyone thinks.  The motor is a ZZ502 for information.  The gearing change seems to drop me down from 15 mpg to about 13, so not a problem there either.

Doug

The bouncing of the temp gauge may or may not be a bad thing. It may just be working good enuff(the temp gauge) that its actually reacting to what is really happening. Ive experienced this before and I never worried about it unless the engine over heated.
I like a 180 tstat on the older engines......
Dave

[C9]

Sounds like you have a mechanical temperature gauge.
They're good at indicating small changes.

After the startup and once oil pressure is up I take off.
Easy driving till things get warm, but I seldom do the warm it up in the driveway bit.

After driving off, with the engine not fully warm, oil still cool etc., the temp gauge swings up to 192 and after a short bit goes back down to 140.
It will swing back and forth - slowly, in an ever decreasing arc - until the temp settles out at 182.

After that, the temp gauge stays pretty much at 182.
On hot days it may work it's way back up to 188 - 192, but the gauge needle is no longer swinging.

I run a 180 degree thermostat year round.

To combat air lock which isn't a big problem with my car, I drilled a 1/8" hole in the flat area of the thermostat so air could escape while the radiator/block was being filled with coolant.
A single 1/8" - 3/16" hole is all you need.

I had two holes drilled in the stat a while back and warmup took forever.
It was four miles across town until the temp gauge hit 180.

With a new stat and only one hole, the temps came up as they always did prior to the hole drilling.
Which is to say, about one mile at an easy 35-40 mph had the temp gauge at 180.

This in 16 degree weather - in a roadster....

[Leon]

I always drill my thermostats, also.  They have a little notch to do the same thing but I haven't had the same success bleeding the system with that notch as when I drill a small hole.  I have been using about a 1/16" and haven't had any difference in the operation of the thermostat.  I have noticed that some thermostats do cycle more around their operating temp than others, as long as it doesn't overheat I don't worry about it.