Coolant/Thermostat/Under Cover Valence

I recently bought a very nice 2000 NB with 35,000 original miles. One owner, with every receipt from new. But it's 22 years old so I decided to bring all maintenance items up to date. I have fitted an Ultragauge, which plugs into the OBDII connector and gives all sorts of information. I was disappointed to find that the car normally runs about 205 degrees--and even tops 210 (I have a warning set) before the thermostat opens for the first time. I decided that this wouldn't do. I think Mazda selected a 195 thermostat--back in the day--mostly for emissions considerations. But it's my personal opinion that these very hot running temps are chiefly responsible for all the tan colored radiators that need replacement. Anyway, I changed all 7 coolant hoses, both serpentine belts, and installed a 180 degree thermostat from Moss Miata (part number 999-026, for $7.99). I am now running 185 degree temps, instead of 205, and I am very happy with this. By the way, the stock temperature gauge is useless. The needle shows the same for any temps from 180 to 220.

On another matter, I took off the so-called 'under cover' plastic valence which covers most of the under engine compartment. I have no idea why this is fitted. It can get caught on curbs, etc. and rip off the front fender support. This has happened to friends. It did hundreds of dollars damage. I had run it on my 1992 because I couldn't run the Ultragauge to see temps (it didn't have OBDII). But now I can tell you that it makes absolutely no difference to the engine temps. If you do break down somewhere, and have to change the bottom radiator hose, you have to take this off. Good luck doing this by the side of the road somewhere. I'm not running it anymore.

Congrats on the purchase.

The undertray is, in fact, important to the cooling system. It also helps keep water out of the engine bay electronics (the electronics in the NA and NB Miatas are not particularly good at being weather resistant), and aids to prevent lift at higher speeds. On the NB, bumper/fender struts are not utilized nor connected to it.

The plastic radiators turn brown from heat cycling. Every car with a plastic radiator and cooling components ages similarly.

Why do you say it's important to cooling if I'm not seeing any change in temps with it fitted or not?

You are monitoring coolant temps not necessarily cooling efficiency. I have witnessed testing contrary to your info on race cars and my own street cars in multiple climates.

You put in the wrong thermostat and now you're running rich for no good reason, losing power and prematurely wearing your catalytic converter. The 195F is what the factory ecu needs to run PROPERLY otherwise it takes longer to get to temperature which means it's adding fuel and will retard timing under load.

The undertray does aid in cooling which is WELL ESTABLISHED.

There's nothing wrong with a motor running as high as 220F, with the system up to pressure a motor can run safely over that. They're _designed_ to operate at these temperatures since the 50:50 Coolant mix is designed to pressurize which increases its efficiency and the boiling point changes to be higher than the 223F, at 15 PSI it's actually more like 268F. that's a touch over 40 degrees of 'headroom'. 'Don't open while hot' means something on the rad cap. Ne touche pas, Baka ! Because if you open it while hot the engine might be over the atmospheric boiling point, causing it to quickly boil the coolant when you ignored the warning and undid the cap.

As Greasemonkey2000 states, the point of the thermostat is to get the engine up to peak efficiency (temperature) as quickly as possible before then allowing cool coolant to flow into the motor, and this can show you hitting 220F or a touch beyond as the coolant pressurizes prior to the stat opening, and again, there is nothing wrong with this behaviour. The coolant system of the miata, while not as perfect as is could be (transverse engine origins..), has proven to be extremely reliably engineered for normal use. Thermostats are a service item, not a weak point in the Mazda engineering.

The undertray's cooling function is to increase/stabalize a low pressure area under the car, which helps draw air through the radiator, past each side of the motor and the transmission tunnel. Without the undertray you basically generate excess turbulence under the car. In fact there is a mod that can even lower UBTs over the stock undertray/setup by adding a small upside down gurney flap at the end of a cut OEM undertray, because it increases the size/immediacy of the low pressure area I've just described.

https://rev9autosport.com/nielex-und...r-end-lip.html

Quote:

---

Originally Posted by atlex

The undertray's cooling function is to increase/stabalize a low pressure area under the car, which helps draw air through the radiator, past each side of the motor and the transmission tunnel. Without the undertray you basically generate excess turbulence under the car. In fact there is a mod that can even lower UBTs over the stock undertray/setup by adding a small upside down gurney flap at the end of a cut OEM undertray, because it increases the size/immediacy of the low pressure area I've just described.

https://rev9autosport.com/nielex-und...r-end-lip.html

---

Thanks for the awareness. I think I’ll add something similar when I make my own custom front splitter and undertray to interface with my aftermarket front bumper. I just haven’t decided whether it’s worth the hassle to continue the flat floor aft of the stock engine under tray since I’m not building a dedicated time attack racer.

Quote:

---

Originally Posted by freedomgli

just haven’t decided whether it’s worth the hassle to continue the flat floor aft of the stock engine under tray since I’m not building a dedicated time attack racer.

---

Could just buy a flat bottom setup: https://www.rtheorymotorsports.com/s...-kit-1990-2005

I have it along with R Theorys frame rails w/backing plates and their new brace kit...... waiting to be installed. :fp:

I'm running the Moss stainless undertray on one of my cars (the blue turbo)

that R theory looks amazing, I wish we had someone offering that in povertyspec euroland.

Funny you said that; I have been working on something similar for my car. I have a complete 3D scan of the underbody which I need to clean up and then do all the CFD work, so it will not be ready any time soon though.