Overheating 70-80mph
 

Car runs normal under those speeds. Once I reach 70-80 the temp rises, holds for about 5 minutes, then goes back down for a while. Rinse, repeat. Fluid levels are topped and I'm almost certain there's no bubbles in the system. Things to consider:

-Turbo'd obviously
-Stock thermostat
-no belly underpan

Any ideas?

you should make sure air coming through the mouth has to go through the ic and radiator. IOW block off the sides and bottom and make sure you have the rad cowl cover in place so air can't just go around like it wants to. Johndoe did a really nice setup, search for that.

I've noticed that at around 70 i see my highest CLT's. has to do with air just building up at the nose, miata sucks aerodynamically . same RPM in 4th gear at the same load sees much lower temps

My 99' did something similar. Overheated when the A/C was on and going +75mph. Kinda like you say. On and off. It would overheat, then "catch" itself. That is, the thermostat finally opened up all the way. Anyway stock old thermostat wasn't opening all the way. New mazda thermostat and that was the end of that.

And I'd suggest you put the belly pan undertray thingy back on. And 70/30 water/antifreeze. Hell, your in FL, 110% water/ -10% antifreeze.

Would a garage vary lip help with this at all? I'd love a good excuse to get one of those soon. I'll probably end up replacing the thermostat before my interstate trip this Friday anyways so we'll see. What about those 180 deg. thermostats? What difference would that make?

I think mine helps a lot but then again with my coolant reroute that car has trouble maintaining operating temperature without blocking half the radiator in temps under 70F

IMO a lower temp thermostat is a BAD idea. The engine is designed to run at a certain temperature. It's machined such that when operating at normal temperature, exacting clearances are kept to minimize wear. If the operating temperature of the engine is reduced, the clearances are increased. This causes accelerated wear which just adds to the problem.

Little example: Dad was a shrimper for 27 years. He's rebuilt and helped rebuild MANY diesel engines. It's well known that once an engine starts to wear out, they go fast. Most engines would last 3-4 years of good service, and then over a short period of time (say 400 hours of use), the engines oil consumption would double. Power drops, engine oil pressure would decrease, etc. End of the season you'd have to overhaul the engine.

Awe hell, my example's going nowhere. It'd take forever to develop into something meaningful. Point is you run a motor colder than it should it will wear out really fast. Racing they use lower temp thermostats because a loose motor makes more power AT THE EXPENSE OF EXCESSIVE WEAR. Also running the engine cooler gives you more of a 'cushion' for the cooling system as when you boost you can boost longer before you overheat. Nother reason it's popular. But the fact remains it's bad on the engine.

Ideally you'll have a cooling system that can maintain the correct temperature ALL the time whether at idle, cruise, or boost.

now that it's getting a bit cooler I'm worried that my cooling is a little too effective as well. During the day when it's 70F+ I maintain normal operating temps but last night while tuning, even after my 16th full boost (14psi) run on the highway in a row my CLT temps would quickly drop to 170F when I let off and cruised for a minute.

and you run the stock radiator still right Jan?

yup, with an a/c condenser in front of it.

Do you have that BEGi t splice going into the upper rad hose? Kinda weird that your temps are that low with a stock rad.

yes I do have their t-splice. think that's part of it? I think I'll try going to a 50/50 coolant/water ratio instead of the 30/70 I'm doing now.

johndoe, wasn't that you with the super-d-dooper shrouding job?

EDIT: Uh, yeap, that was you. I fail at reading comprehension.

Oh definitely. If it's in the upper rad hose, water is constantly bypassing the thermostat and going through the radiator. You should move it to the lower hose.

yes it was. Thanks for the tip paul, I'll try switching that splice over the weekend and see what difference it makes.

made a 20-30 degree difference for me BEFORE it got this cool out.

well that would put me back where I should be. I guess that splice was made with Texas weather in mind.

Yes. At that speed you're get high pressure under the hood w/o the belly pan. All the air being compressed and squeezed under the front lip is expanding into the engine bay, making it much hard to force air through the exchangers. A lip spoiler will help, but not as much as the undertray.

do your fans work?

I'd do a coolant flush. If that does not work, try a new thermostat.
Stephanie