1997 Mazda MX-5 Miata: Curing The Overheating
August 30, 2011
Perhaps the most weaksauce portion of a first-generation Miata is its cooling system. Even stock, the fundamentally compromised cooling system layout can mean overheating in adverse climates. That, and a sweaty crotch.
If you then stick an intercooler and a heat exhanger for the supercharger's magic fluid in front of the radiator and make a bunch more power, well, you can guess how much that helps.
The best approach to licking Project Miata's overheating conundrum comprises many facets. Hit the jump.
Rather than muck about with halfhearted solutions, we opted for the 'big gun' approach -- this is Flyin' Miata's crossflow radiator. Overkill? That's the idea. After the recent near-constant-overheating escapade, all we want is simply the ability to run the car as hard as possible, not overheat, and still have the a/c blow cold. When it comes to modified Miatas, this is unfortunately a tall order.
The crossflow radiator is so named due to the side-to-side path taken by the coolant within it. Stock Miata radiators are a 'downflow' design, where coolant enters at the top and travels downward.
To understand the difference between the two layouts, it helps to first realize that the cooling system's job isn't just to regulate the temperature -- it also controls detonation. When coolant boils, it cannot transfer heat effectively and you end up with localized hot spots in the head, which leads to a craptacular cascade of further overheating and detonation. Thus, we want to prevent boiling, both for thermal and detonation prevention reasons.
One very effective way to prevent boiling is high cooling system pressure -- this directly increases the boiling point of the coolant, staving off the cascade. So we want as high a cooling system pressure as our radiator can tolerate.
Picture what happens as the coolant flows through the tubes in the radiator. Its pressure drops. With a downflow layout, you're regulating pressure (via the cap) on the high-pressure side of the radiator, so you're needlessly sacrificing system pressure.
A crossflow radiator regulates pressure on the low-pressure side. As a result, the rest of the cooling system operates at a higher pressure, which is goodness. Also, unlike a downflow layout, a crossflow layout makes the cap immune to being blown open by the velocity of the coolant flow.
Now this is cool. In determining the particulars of their crossflow radiator -- fin geometry, tube size, et al -- Flyin' Miata took a Miata drivetrain, lashed it to their dyno and got busy. They tested competitors' radiators, their existing offering, and variants of the crossflow design. See more details and results here.
A fancy radiator is a good start, but without airflow it's only a matter of time before it succumbs to overheating. As such we also picked up Flyin' Miata's Stage 2 Airflow Kit. This comprises twin Spal high-flow fans, a gorgeous laser-cut aluminum shroud plus all the necessary spacers and hardware necessary for installation.
All the crap stuck in front of our radiator poses a large resistance to airflow. Fans, it turns out, are sensitive to this resistance. It's simple -- the higher the resistance, the less flow they can pull. The dropoff in performance can vary dramatically depending on fan design. Fortunately, the folks at Flyin' Miata know this, and selected these Spal fans specifically for their ability to pull big airflow at high resistance.
The shroud is beautifully made. All features are cut with exacting precision, and it fits the radiator like a glove. Those little flaps? They facilitate airflow through the radiator when the fans are off.
Installation was fairly straightforward. Once I 'tuned up' the fan shroud and mounting peg to resolve a territory dispute, the radiator and fans dropped right into Project Miata. The big fans clear our Racing Beat stabilizer bar with about a millimeter to spare, which is perfect in my book.
You'll also notice in the lead shot a bright red silicone coupler on the throttle body. This piece, graciously supplied by Hose Techniques, addresses a conflict between the Kraftwerks intercooler plumbing and the upper radiator hose. A little trimming of the silicone coupler's end and now both systems coexist peacefully. Sometimes, the mods need mods.
Speaking of the upper radiator hose, the crossflow radiator necessitates a different upper radiator hose than stock, and the one FM supplied didn't fit our car quite right for some reason. It'll do for now until I do a coolant reroute, which I'll whip up in the near future. Also, it's important to encourage air to go through the radiator rather than around -- various leak paths were sealed up with bits sourced from my garage. That includes the space just forward of the radiator in the lead shot (pic was taken before the piece was put in place).
With the Flyin Miata crossflow radiator, Stage 2 Airflow Kit and a reroute, we should then have enough cooling system headroom in Project Miata for anything we want to throw at it. And we do.
--Jason Kavanagh, Engineering Editor