Developing the Flyin' Miata crossflow radiator

Note: This testing was performed in 2011, but is still applicable today.

We do a lot of development at Flyin' Miata, and our new crossflow radiator is a good example of testing producing a better product. We tortured a series of cooling setups on our specially-built dyno rig. Here's how it was done.

The testing consisted of setting up an engine on our dyno and running it at a fixed load. This allowed us to generate a consistent level of heat for a long period of time. The engine was run until the inlet and outlet temperatures were stable for a 5 minute period. The engine was fitted with a thermostat that opened at approximately 71º, and the load was enough that the thermostat was not a factor - it was simply down to the ability of the radiator to shed heat. We instrumented the temperature of the inlet, the temperature of the outlet, the ambient temperature and the time. The numbers are an average of two runs, one with the radiator cold and one immediately following with a heat-soaked unit. All of the radiators were tested with the same fan setup.

The stock units were tested first along with a variety of off-the-shelf aftermarket and custom pieces. From what we learned in this testing, we built some new radiators for a second round of tests. This caused a delay in the testing, which added a bit of complication.

Because the tests were performed from February to May, we were not able to arrange consistent temperatures in the dyno cell. They ranged from a low of 34C to a high of 50C. Yes, testing cooling systems makes for a hot room! For this reason, we'll show the difference between the stabilized temperature and the ambient, as that should eliminate ambient temperature as a major factor. It's still a factor - the hotter it is outside, the less heat transfer takes place for a given engine temperature - but it's minimized.

We tested more radiators than shown here, but this gives you some good comparisons. The dual pass is the same construction as our crossflow, but with the coolant making two passes through the tubes instead of one. There is no "time to temp" reading for the stock radiator, as we used the stock setup to determine the appropriate power level to use on the other radiators.

All temperatures are in degrees Celsius.

radiator Ambient temp Rad inlet Rad outlet Improvement over stock Temperature drop Time to temp Outlet temp compared to ambient
stock 1990-97 35º 105º 100º   - +65º
Brand X aluminum race radiator 38º 105º 101º +1º 15:44 +63º
FM crossflow 43º 102º 98º -2º 17:09 +55º
dual pass 44º 102º 97º -3º 11:55 +53º

We're looking for a radiator that can get the coolant down as close to the ambient temperature as possible. We also want it to take a long time to stabilize at the final temperature, as that also indicates how well it can shed heat before becoming fully heatsoaked.

A couple of things are readily apparent. The crossflow and the dual pass both have lower running temperatures than the other two radiators, despite the fact that it was significantly hotter during the test. The "outlet temp compared to ambient" number really brings this into focus. The crossflow is 10º C (18º F) closer to ambient than the stock piece, and even with an ambient temperature that's considerably hotter it's still running cooler than stock. You also see that the crossflow takes longer to come up to the final temperature, so it resists heatsoaking more. It is interesting that the dual pass took less time to come to full temperature.

A nice plus on the crossflow and dual pass designs is that the radiator cap is on the outlet side of the radiator. There's some speculation that the pressure from the water being pumped into the end tank can pop the cap off early at high RPM in a stock Miata radiator configuration. Not in this case.

The dual pass design did perform very well, but it poses some other problems. For one, the radiator inlet has to move to the other side of the engine bay. This can cause some real packaging problems with your intake, never mind the fact that your thermostat outlet will be pointed in the wrong direction. Secondly, they're expensive to build. So, since the crossflow performed as well or better, we went with the simpler, less expensive solution that was easier to fit.

It's not just the fundamental design that matters, of course. There's a lot in the details. We tested another dual pass unit that didn't perform as well for several reasons. There was also another crossflow that had promising results, but the change in core design to our final specification improved it in almost every respect. As an added bonus, the final result is also the lightest aftermarket unit on test!

We're really happy with the end result. We believe it to be the best Miata radiator on the market. From a business standpoint, it's a poor choice for us because we don't make as much money on it as we did on our previous designs and we poured a bunch of money into testing!

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