I think I have mine set for 35 or 45mph. I believe I read somewhere years ago that about 35mph is where the cut over happens of the air going in is stronger then the fans.

 

That depends on a lot of variables. Which fans, shrouds, radiator orientation, what kinds of obstructions, etc?

I once tested that with my RX-8, and I found both fans running on low speed (3 speed dual fan setup with independent speed controls) contributed to cooling at speeds upwards of 60mph, which surprised me. I figured it would be closer to 35 or 40mph. I still have that data somewhere... Anyway, the Miata is a totally different animal, so those numbers do not apply.

 

Disclamer: I havne't actually tried this, but it should work in theory. Please let me know if you do try it and what results you get.

The best / cheapest way to test this would likely be to make yourself a manometer. They are generally used for balancing carbs but will easily show a pressure differential.


That should give you an idea. Just run one tube to the front of the rad, the other to the back. Run the car at different speeds with the fans on and off, see where the crossover is.

 

Fully familiar with manometers. Not a bad idea at all.

More likely, I'll set them to turn off at a low speed and see when temps climb.

Lars, I had also seen the 35 mph, but that was on a 'Vette forum.

On the track, I was running about 208*, and my fans are set at 206* turn on, which means they (or the main one) was running at 80 mph, which is were I had it set at the time. After-thinking said that was likely too high; thus the question.

 

Airflow through rad should overcome fans by 10-15mph. If not, work on your ducting and hood vents instead of adding more fan.

 

Any reason my car should be running so hot? oem undertray installed, SM Crossflow radiator w/ a 11" and 9" spal fan pulling configuration. A metal shrod was fabricated so air isn't pulled from the side (similar to Flyin Miata's fan shroud). AC off, I stay at about 195-200f on the highway. The moment I turn AC on, I start to slowly creep up to 220f. Also have the qmax coolant reroute running the provided 195f thermostat. Couldn't find any pinhole coolant leaks either...I'm guessing a slow leaking headgasket? Haven't compression/leak down tested yet.

 

Ditch the shroud and second fan.
Seal up any gaps between bumper skin and radiator core. NO gaps.
Make sure the Qmax is bled properly.

If it's a turbo, add hood vents. This part is not optional. It simply won't get enough airflow with an FMIC without hood vents.

 

In addition to blocking airflow to the radiator, AC is dumping hot air in front of it. The car is going to run noticeably hotter when the AC is on, that's just kind of the way it is. High-power track Miatas usually delete it for that reason.

Also, if by "fabricated metal shroud" you mean something like this:



Then while that may improve cooling performance when idling at a traffic light (because it makes the fans work better), at track speeds I suspect it's actually going to hurt (because it blocks airflow through some percentage of the radiator core).

--Ian

 

FWIW, my 340 rwhp turbo car will happily do 95F track days without hood vents. Trackspeed rad, reroute, ducting, no shrouds, one fan, no AC.

I haven't tried it in 100F+ track days -- by that point the driver is overheating, rather than the car.

--Ian

 

Thank you for the advice. I'll drop the 9" fan to see if it helps. So I understand the point of the shroud is to prevent the fan from sucking air in from anywhere besides the radiator such as the gap between fan and radiator. Wouldn't this make the one fan I keep effectively useless? Codrus, you are exactly right. I DIY'd a shroud that's funtionally the same as that.

I don't have a way to mount the fan w/o the shroud (I hate the zip tie through radiator method) so I'll start with simply removing one fan. I'll go through my bumper and undertray and ensure all holes are sealed. Thanks for the quick reply!

 

Simplest way to mount the fan is to get a length of aluminum bar from the hardware store, cut it into pieces the right length, and drill two holes in it. Like this: (picture is of a Civic rad that I was putting in a Locost, but principle is the same):



--Ian

 

You are missing the point. The car moving the the air over 15mph pushes far more air through the radiator than any fan will. The shroud does just that, it shrouds the airflow. Meaning less air through the rad when the car is moving.

You asked a direct question and got a direct answer. Ditch the shrouds if the car overheats when speeds increase.

 

Hi guys, new to this form so bear with me here. Still trying to figure out this forum but I figured this thread to start asking..

I am about to type out the history of my situation so I apologize if its lengthy, figured it'll only help pin point the issue better.

Boosted NA6,104xxx miles, FMU, 7-8 lbs.

So I did a coolant flush and new lower temp thermostat change back in March-April. I ran some water/radiator flush for the next week or so to clean out my cooling system further. During that week I decided to go to the lake (hotter temperature day). On the way back, my thermostat gauge was TAPPED out to the max with smoke coming from the engine bay. Of course as expected, I was boiling the WATER/radiator flush mix which must've also fried my water pump (Water dumping from the crank). Month or two later of the car sitting I install a new water pump/timing belt/all the recommended seals and components etc they recommend while you're in there and of course I put actual coolant in at this time. Car seems to be fine again but I noticed that after about 30-40 minutes of normal driving, the thermostat gauge would start to creep up and stay at about the 3/4 mark of the gauge and my 60/40 coolant to water mix would start to boil. After ignoring this issue (I know, stupid, I still shouldn't even have been driving it knowing that), I decide to go on a longer cruise. About an hour into it, my thermo gauage is tapped out, engine bay smoking, coolant boiling, and then my radiator decides to blow up at the plastic trim on top of the radiator. So then I go grab the radiator out of my parts car to go swap it in real quick and drive it home. Next thing I knew, same darn thing happens again and I blow up my spare radiator in the same spot. I know, crap.

I should mention BEFORE any of this happened, the car had NEVER overheated. Hard driving, drifting, donuts, whatever, it never overheated before all of this.

So my question is, what the hell is going on? Did I blow the head gasket with all the overheating? Did I warp/crack a head? Some other underlying issue? Bad thermostat?

Summary of the symptoms I know:
- The car tended to stay cooler longer/not over heat as much when the outside temperature was cooler
- The car tended to stay cooler longer/not over heat as much with "normal driving"
- The car tended to stay cooler longer/not over heat as much with the hood completely off
- The car didn't overheat one bit before I did the radiator flush and blew out my water pump running water in the cooling system
- I've opted out lack of air flow or the aero or undertray issue because like I said it never did this beforehand
- Car doesn't overheat till about 30-40 minutes of driving
- Coolant is boiling even with 60/40 mix
- Coolant boiling is causing too much pressure and keep blowing up my radiators
- I should mention, I did expand my exhaust to a 2.5" straight pipe all the way out to the rear after the downpipe during the week of my radiator flush. I don't know if that makes a difference but figure I'd ad that in there.

I'm currently thinking its either a blown head gasket, cracked or wrapped head, bad thermostat, gunky cooling system, or I don't even know what else.. What do you guys suggest? Thoughts?

Thank you

 

Start with a leakdown test to look for head gasket problems or cracks.

--Ian

 

This is starting enough from OP to warrant a new thread. Post link to that thread here if you like.

 

Is there a better thread to post this in? Or are you saying to start my own thread with this?

 

New thread in engine performance section for your street car

 

a 3 year old post just answered a nagging question of mine. My MSM also has this gap and when looking at it yesterday, the gears started turning about this potentially being a relatively simple thing to tackle (along with a different water/coolant mix) to bring down my temps. South FL is starting to heat up again and want to start addressing cooling since this car runs pretty hot.

Did you have enough adjustability in the stock rubber hood stops to effectively bring the hood down a bit and kill some of that gap?

 

Not track related - a quick ducting question. I seem to recall reading years ago that you typically want to size your inlet 1/5th-1/6th the ^2 in of your radiator core size. I've got an racing beat type2 nose and it's got a significantly larger mouth, so I was planning on necking up from the inlet to radiator portion, redirecting the excess around the shrouding - maybe using some aluminum honeycomb grid in the entire mouth to slow the air/prevent reversion - no intercooler. Not really concerned about aerodynamics. Is this "because racecar" advice? Or should I use the whole mouth?

 

I've seen 1/4 to 1/3rd thrown around as a rule of thumb. Obviously the needs will be different at 30mph and 130mph.