1994 Mazda MX-5 Miata: Thermostat Walkaround
September 07, 2010
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