Overheating Issue at Rallycross
 

Hey guys,

I just got back from an SCCA rallycross, where I was encountering overheating issues.

The setup:
'90 Miata with stock radiator and fans and AC
'03 VVT cylinder head on '96 block (also uses VVT head gasket)
Coolant reroute at back of head using moss spacer and GM truck hose.
Brand new Gates water pump.
Coolant temp sensor located on blockoff plate on front waterneck. Gauge sender in stock location at back of head.
Fans controlled by ECU, and fan switch removed. Set to turn on at 97C (207F), then off again at 95C (203F).
No turbo (yet).

I just (last week) installed a brand new OEM plastic undertray. The car runs fine on the highway and around town - hovers around 95C (203F) coolant temp.

I did an autocross last weekend and did not have any overheating issues.

Today, during a rallycross, with two people driving back to back (3-4 minutes at idle in between 60 second runs) - the coolant temp shot up to 109C (228F) and wouldn't come down. The gauge sender also read way over on the right side of the gauge (HOT!!!). I turned on AC to get the second fan to engage, but it didn't reduce the temps. I verified that the main fan was on all the time, and that the secondary fan kicked on when AC was engaged. The only thing that helped pull down the temp was turning on the heater at full blast.

The weather was 80F and sunny, so nothing too crazy. Drove home 2 hours - no issues - stayed at 91-93C (196F) all the way home with the AC on.

No evidence of pressurizing the coolant reservoir. No milkshake on the dipstick. Power is fine.

Any ideas here?

Is the vvt head gasket screwing me??

The heater helped because it was the only heat exchanger on the car that was unobstructed. I just removed an AC unit from a '90, and I see no way that the car could stay reasonably cool with the giant AC condenser in front of the radiator.

To give you an idea, we have two local track miatas that are nearly identical. Both '90s, both have no reroute, both have a belly pan, and both have no air guide. One has a mishimoto 2" radiator and AC, the other has a stock radiator and no AC.

Guess which one had issues with overheating at the track? The one with the giant upgraded radiator. If it's not getting any airflow to it, it won't do you any good. Ditch the AC. It's also a nice 45 lb weight savings.

I'd also throw in a 180* thermostat (I'm guessing you have a 190*?), wire your fans in parallel, and set them to come on at 185*. Every little bit helps. I'd also obviously do a reroute, and of course always be ready with the heater ;)

Make sure rhat you have no air in the system, coolant rr tends to trap air at the front of the head, you can sometimes burpnit by jacking the back end of the car and runining it a bit or for an easy fix you can put something like this.
https://www.miataturbo.net/attachmen...ine=1376791085

Reroute plus head gasket combo isn't helping. Most important thing for an auto-cross car is an excellent fan setup. Both fans should be coming on when it gets hot. Your cooling system doesn't appreciate doing repetitive hard runs at slow autocross speeds with no airflow in between. Alternatively, turn the engine off while stopped with the key in the run position to let the fans cool the radiator charge via battery power. After two minutes start the car for some time to circulate coolant then shut it off and run the fan some more.

What's your coolant mix looking like? Have you tried more water and a smidge of coolant?

Stock radiator? Are you serious? Get that thing out of there.

Coolant is new as of a few months ago when I did the 1.8 swap. It is 50/50 mix, which should be plenty to keep cool.

It seems like I SHOULD have adequate cooling, but I don't. The only thing in the mix that may be hurting is the VVT head gasket, But I'm not sure if that will definately fix the problem or not.

Yes, removing the AC condensor will remove a large obstruction in front of the radiator, which may help, but I REALLY want to keep the AC. This is more of a street car, with an occasional race thrown in. Call me a pansy, but I like AC.

I'm thinking maybe do the head gasket swap, and rewiring to have both fans kick on at the same time. Maybe change to a badass SPAL fan??

Hmmm. Based on the head gasket coolant routing photos in this post: https://www.miataturbo.net/engine-pe...37/#post540791

I don't think that the VVT head gasket would make much difference. Maybe install Aluminum rad and SPAL fans?

Just the rad, fans are not needed. The stock fans are fine.

How much larger is a stock NB radiator vs my NA rad? 'Cause I should only need the same cooling as a stock NB. Of course I will be going turbo in the next few months, so the rad is probably a good idea anyway.

No. Just don't. Replace the stocker with a quality aluminum radiator. The stock NB one isn't worth crap. I was overheating with the stock one with a stock engine with the A/C on.

I believe so.

Agreed, I have a bone stock NB2 and it overheats on the highway with the A/C. Especially when ambient temp outside is over a hundred.

I still think you should try to drop the antifreeze to water ratio. I think ten percent coolant, and ninety percent water, and a touch of water wetter would be best.

Isn't the consensus that a coolant reroute shouldn't be done with the VVT head gasket. When I swapped in a 03 VVT engine into my 97 NA chasis, I skipped a coolant reroute for this very reason. Note that I haven't tried a coolant reroute + VVT head gasket, so for all I know maybe the combination doesn't cook cylinder #1 :-)

Based on the photos of the early vs late head gaskets linked earlier, it is my opinion that it won't make much difference. It is obvious that the late head gasket is severely biased towards the exhaust side of the head, which is a good thing. I'm not seeing a fore/aft bias in the holes.

I just think you're asking too much of the cooling system. Low speed, high power just wasn't the design point. Fundamental problem here is airflow.

My only recommendation would be to hit up Jegs or Summit for some more powerful fans with published CFM ratings (i.e., not the cheapo China units). Be mindful of the stock wiring limits (30A for main cooling fan, 20A for auxiliary AC fan). On top of that, use the survival techniques mentioned by Fooger.

If it's any consolation, my car's only struggle point is stuck in traffic with AC Max and OAT North of 105F. Give it any forward motion and it's "cool beans." ;)

Do we seriously have 3 overheating threads going at the same time?

It's August.

Just like how the AC idle droop issue comes up around May or June.

Is it possible that the condenser/radiator got caked with mud at the rally-x? The A/C condenser was packed with mud in my wrx the one time I was dumb enough to rally-x it. I managed to hose most of it out, but the tranny and bottom of the engine still have about 1/4" of caked on nastyness.

Nope - it was dusty as shit, but no mud at this event. My regular rallycross car (supercharged EG Civic) had cooling problems last year as well, so I put in a full width ebay aluminum radiator and a badass Spal fan. The fan cost more than the radiator, and almost stalls the engine when it turns on due to the giant current draw. No overheating now. But, this is with 2.5x the stock power the car was designed for.

The miata is only at 1.2x the stock power, so I'm surprised that the cooling system can't handle it.