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Colder Thermostat Increase Power - Fact Or Fiction

Debunking Tuning Myths

Luke Munnell
Nov 30, 2009

Think back to building your first car. If you were turning wrenches before the aftermarket was plentiful, chances are you made some pretty funny mods in the name of performance. From DIY Cherry Bomb exhausts and dryer-ducting intakes to cutting A/C belts and swapping to multi-electrode spark plugs, we've seen it all. When adding power to your ride isn't a luxury, making the most of it is a necessity. This month we explore one such "trick" mod from our power-desperate younger days that just won't seem to disappear.

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Will switching to a colder thermostat increase power?
Like spark plugs available in varying heat ranges, many aftermarket auto parts manufacturers offer replacement thermostats in a range of operating temperatures-most commonly, those that open at the OE-specified temperature, and some that do so several degrees cooler-for those of us who live in hotter climates or stress the capacities of our car's systems. Thermostats that open earlier allow coolant to circulate sooner, ideally keeping engine temperatures lower. The debate occurs when certain aftermarket producers of cooler-temperature thermostats market them for "power increasing" benefits . . . not naming names, of course.

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To test, we and our project Miata visited MD Automotive, SoCal's premier two-wheel-drive dynamometer rental facility, and hit the Dynojet for back-to-back power testing with a 160-degree aftermarket replacement thermostat installed in the roadster, then again with it replaced by a 192-degree, OE-specified unit. Temperature of coolant (50/50 glycol/water mix) was monitored at the radiator inlet throughout testing, to ensure dyno runs for each thermostat option were conducted at consistent starting points, once the engine had reached a base idle temperature for each given thermostat. Ambient temperature remained within 1-2 degrees throughout testing.

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The Verdict:
Hotter engines make more power. Sure, cooler air and fuel is denser, meaning more of it can fill combustion chambers to make more power, but once detonated, less energy (heat, in this case) being absorbed by engine material and coolant means more of it can be put toward forward motion. Also, many of today's knock-sensor-equipped cars are programmed to run optimally at a pre-set engine temperature; lowering that temperature might keep your engine from running on its optimal fuel/timing trims. "WRXs and STIs that aren't driven hard lose power with cooler thermostats," explains Tuning Technologies' Alfred Beltran, "usually as much as 10-15 whp."

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MD Automotive
Westminster, CA 92683
Tuning Technologies
Colton, CA 92324
By Luke Munnell
278 Articles



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