This won't be nearly as cool as one of Dan's Suspension Walkarounds, but what the heck. I had both of Project Miata's thermostats (the old and the new one) in hand during my cooling system festival and saw some notable differences.
Maybe you'll learn something from this. Perhaps not. In any case, here goes.
Right away, it is clear that the old aftermarket t-stat is missing a sub-valve. But what the heck does this do?
A sub-valve is a mini thermostat. Its purpose is to compensate for any lag in the main valve's ability to respond to temperature. The main valve can't open and close instantly. By being smaller than the main valve -- and opening at a slightly lower temperature (185-190 deg F) than the main valve (192-195 deg F) -- the sub-valve irons out any spikes in the temperature of the coolant in the engine that may occur before the main valve can react. Basically, the sub-valve allows an additional degree of temperature control as the engine warms up.
Not all aftermarket t-stats have a sub-valve. This is not the end of the world, but it's a good idea to use one thusly equipped if the OEM unit sports one. The Miata's OEM t-stat uses a sub-valve.
Also notice the pee hole. This is simply a bypass hole that allows coolant to pee out of the engine when the t-stat is closed. Without this hole, coolant flow to the t-stat is stagnated during warm-up, which isolates the t-stat from the hottest coolant in the engine. The little flow through the pee hole ensures that the t-stat is seeing a continuous supply of hot coolant, and this is goodness. A secondary benefit to the hole is that it introduces hot coolant to both sides of the t-stat, warming it up a bit more evenly.
The pee hole also allows air to be more easily purged from the cooling system when you refill it with coolant. This works best when the hole is the high point of the t-stat since the liquid will push the air upwards as you fill -- orient the pee hole at "12 o'clock" (left) when you plop the t-stat into the engine to ensure that no big air bubbles are trapped in the engine.
Finally, don't go installing a low-temp t-stat thinking it will give any performance benefit. It won't. Nor will it solve an overheating problem. All a low-temp t-stat will do is prolong the engine's warm-up period, so you'll use more gas as the engine will be operating in 'cold start enrichment' mode (i.e., running rich) for a longer period of time.
And if the engine is experiencing an overheating issue, a low-temp t-stat will only delay the inevitable.
Jason Kavanagh, Engineering Editor
cello_one says:
04:26 PM, 09/ 7/10
I think a caption contest for the 2nd, notated picture is in order...
Thanks for the thermostat walk around. Put in a picture of a jack stand and Dan will have a run for his money...
felonious says:
05:02 PM, 09/ 7/10
You'll never convince me that you wrote an entire post about a pee hole without giggling like an idiot half the time.
saxdogg says:
06:28 PM, 09/ 7/10
Excellent posts lately...keep them coming.
church123 says:
06:49 PM, 09/ 7/10
On some of our race cars, with lots of extraneous cooling system parts removed, we'd have a hell of a time bleeding them. We used to drill out the pee-hole (never heard it called that before today :) with a much larger drill bit just so all the air bubbles would easily pass through and the temps would equalize on both sides of the t-stat more rapidly. If we didn't do that, engine temps would get dangerously hot before enough heat penetrated the t-stat and got it to open.
Just one comment on low temp t-stats. On cars that tend to run normal operating temps well above their thermostat opening point, yes, a low temp unit is pointless - the cooling system isn't capable of operating below the stock t-stat opening point unless you're near the artic circle (or Minnesota :). This would include most of your modern imports made in the last 20 years.
However, on a lot of GM vehicles, the t-stat is set to fully open pretty high (180-190) and the cooling system is capable of keeping the operating temps noticeabley lower under most conditions. This may also be the case on Fords, but I don't have much experience with them. Going to a lower temp t-stat on a GM car/truck, plus lowering the fan turn-on point can lower average operating temps by 10-15 F easily. This can result in a small boost in power as you can run a couple degrees more ignition advance (the ECU will actually do it for you on modern GM vehicles). This is especially useful for drag racers, track or stoplight. The draw back is that a cooler operating temp tends to be worse for fuel economy and emissions (laws of thermodynamics I'm told), which is probably why GM seems to want to run all their vehicles at 205-210F coolant temps. The same sort of logic generally applies to vehicles with aftermarket upgraded cooling systems - provided the radiators aren't sourced from a subpar Chinese supplier ;).
vt8919 says:
07:51 PM, 09/ 7/10
Now I feel enlightened, AND a little bit dirty. Thank you. :-)
kevm14 says:
07:09 PM, 09/ 8/10
Part of that advanced timing when cooler is because it NEEDS to be advanced to get the same burn time. The heads are less efficient at a cooler temperature. The cooler the engine, the more heat you are removing via the radiator. Gas mostly goes straight out the radiator and tail pipe, with some of it actually making motive power. Don't make that worse by removing even more heat...