Performance tests: effective engine cooling

The fans deliver effective engine cooling. I’ve tested the performance of these fans in real world riding conditions – largely as I ride my Harley-Davidson Road King® from Parramatta to Sydney city and back.

Riding in this sort of traffic generates lots of engine heat. Blast away from traffic lights (high power, high heat) and then stop at the next lights for a few minutes (no air flow, no cooling). Excess engine heat is not good.

The demonstrated temperature reduction depends on where it is measured.

Effective engine cooling: Temperatures measured under the spark plug on the rear cylinder

effective engine cooling: temperature sensor on sparkplugMeasuring cylinder head temperature at the spark plug is a common method. It is the hottest place on the engine and the favoured location among many engine builders and testers. It’s where most other fan suppliers measure their cooling performance data.



effective engine cooling: sensor fitted to sparkplugThe rear cylinder is hotter than the front on Harleys – it gets less air flow. When measured here, the fans show an impressive temperature reduction – effective engine cooling.




Watch this video to see how quickly the cooling fans drop cylinder head temperature.

Drop 30°C in the first minute, a further 15°C in the second minute. Total drop: 70°C in 7 minutes.

Test method for video above: Ride the bike for about half an hour to get to operating temperature. Leave it idling in my carport for about 5 minutes for head temperature to stabalise. Turn on the fans and watch the temperature fall.

C° to F° conversions to keep in mind if you are comparing products from the USA

50°C = 122°F
70°C = 158°F
100°C = 212°F
130°C = 266°F
150°C = 302°F
180°C = 356°F
200°C = 392°F
220°C = 428°F

Here’s another video showing a temperature drop from 221°C (430°F) to 140°C (that’s a drop of 81°C!) in just 12 minutes on a 35°C day. It plays at 2x speed to reduce viewing time. The first minute shows temperature has stabilised while idling, then the fans are switched on and the temperature drops.

effective engine cooling temperature graph

Effective engine cooling: Temperatures measured on the right side of the rear cylinder head

effective engine cooling: temp probe on rightMeasuring from this location demonstrates effective engine cooling. The fans cool the entire cylinder head – from one side to the other.





When the temperature sensor is fitted on the right side of the head:

  • temperature is much lower than that at the spark plug, by about 100°C.
  • temperature change is much slower. That’s to be expected – cooling needs to travel across the head.
  • road speed has a much larger influence because the sensor is closer to the air stream.

Test procedure:

  1. let the bike idle for a few minutes after a ride so the head heats to a stable temperature
  2. switch the fans on
  3. watch the temperature change.

Initially the temperature increases before falling. That makes sense – heat from the left of the engine is blown to the right side by the fans. But then the entire head begins to cool and the temperature falls.

This is good evidence that these cooling fans work – they cool the entire cylinder head, not just the temperature sensor. (some critics have commented that the measured fall in temperature is because air is blown on the sensor, not because the head is being cooled – this test shows that thinking is wrong)

graph of effective engine cooling on right side of head

Effective engine cooling: Measuring oil temperature

Oil temperature is a good indicator of overall engine temperature. The cooling fans help keep engine oil cooler by about 10 – 15°C.

Test bike: 2010 Harley Davidson Road King with Screamin’ Eagle Stage IV kit, fitted with Jagg 10-row oil cooler. If you don’t have an oil cooler, the oil temp on your bike could be higher than these test results.

Oil temperature measured by K-type sensor in oil sump.

Test 1: about 26°C ambient temperature

Run 1: Engine thoroughly warmed up. Starting oil temperature 100°C.  Ambient temp about 26°C. Cooling fans off.
Ride around Sydney CBD for about half an hour.
Maximum oil temp: 126.7°C

Run 2: Starting oil temperature 105°C. Ambient temp about 26°C. Cooling fans on.
Ride around Sydney CBD for about half an hour – same route and riding style as run 1.
Maximum oil temp: 116.8°C

10°C cooler – that’s a good thing

(Harley states normal operating oil temperature is 110°C – see oil temp gauge fitment guide)

Test 2: about 22°C ambient temperature

Run 1: Cooling fans off.  Ride from Parramatta to Sydney City in morning peak traffic.  Oil temperature sits at over 120°C for the last part of the journey, peaking at 123.4°C.  (On a warmer day and without an oil cooler this could have easily have been over 130°C.)  The Engine Idle Temperature Management System (EITMS) kicked in frequently, at just about every red light.

Run 2: Cooling fans on.  Ride from Parramatta to Sydney City in morning peak traffic.  Oil temperature sits between 105°C and 110°C during the last part of the journey, peaking at 110.8°C. EITMS did not activate at all.

Preventing oil temperature going over 120°C – that’s important.

Info about engine oil temperature


  • Add 10% to the oil tank reading to get a better indication of engine temperature.
  • Oil oxidation rate increases explosively above 250°F (121°C) for fossil oil like that recommended by Harley. At 270°F (132°C), a mere 20 degrees above the 250 degree limit, the oxidation rate doubles, thereby reducing the life of the oil by half. Sustained riding at over 300°F (149°C) reduces oil life by 80 percent. (maybe that’s why many Harley dealers offer oil changes at twice recommended frequency – at 4000km intervals)


  • If the operating viscosity of the oil goes below 10mm2/s, then the oil is too thin to lubricate all components within the engine adequately. At 120°C many oils fall below this viscosity level.
  • Once the oil temperature goes over 125ºC the oil is so thin that it starts to bypass the rings and gets burned in the combustion process. The oil is in a very low concentration compared to the fuel and generally is not visible as combustion smoke. In a situation such as this, oil consumption will increase