Does a cool head protect oil viscosity?

My last post got me thinking. Cylinder head temperature can be very high even if engine oil temperature is moderate.

That means the engine oil does not do a very good job of cooling the cylinder head. If you have a look at a Harley oil flow diagram you can see why. Only a small amount of oil travels up through the push rods to the top end to lubricate rockers etc. The oil flow is not designed to cool.

But while that small amount of oil is flowing around the top end, it may be getting super heated. It may be picking up head temperatures of close to 300°C.

We know that high temperatures degrade oil. In fact, lubrication capacity decreases rapidly over 120°C. (see the info on Cooler engine oil)

So, oil may be overheated and damaged as it moves around the head, and then return to the sump where it cools – but the damage is not fixed up. Over time, all the oil in the engine will be affected. Maybe that’s one reason why Harleys need frequent oil changes.

If I’m correct, keeping the cylinder head a little bit cooler by using engine cooling fans in traffic could be very beneficial.

Head temperature and oil temperature are different things

It only hit me the other day – just because your oil temperature is OK does not mean your cylinder head temperature is. (maybe you already knew this, perhaps I’m a bit slow)

It’s winter here in Sydney, and last week I rode into the city on a miserable rainy day. I kept an eye on my oil temperature (measured at the sump) – it was staying around 80°C, mainly because water was splashing on the oil cooler.

I didn’t have my fans on. I thought I didn’t need them because the engine oil was staying so cool.

But I was wrong! After sitting in traffic for a little while, my engine went into parade mode. That means the cylinder head was in danger of overheating – around 350°C.

I immediately switched the fans on and was out of parade mode by the next set of traffic lights. But a good lesson – run the fans in traffic even if everything else is cool.

Oil temperature 17 degrees cooler

Two near identical trips on consecutive days show cooling fans keep engine oil temperature down.

Day 1: Parramatta to Sydney city along Victoria Rd. Start 7am, ambient temp approx 18°. Heavy traffic and slow speeds.Fans turned on 10 minutes into the trip. Maximum oil temperature – 100°

Day 2: Parramatta to Sydney city along Victoria Rd. Start 7am, ambient temp approx 18°. Heavy traffic and slow speeds. Fans not turned on at all. Maximum oil temperature – 117°

17° difference – does it matter? The motor company says oil temperature should reach about 110°, so 117° is on the high side. See cooler engine oil for some info about oil temperatures.

These two trips demonstrate these cooling fans have a definite and beneficial impact on engine temperature, especially in heavy traffic condistions.

I’ll run this test again once the weather warms up more.

Surprised by engine heat

Yesterday I rode down the M2 in Sydney, about 20km at 100km/h.

I took the Pacific Hwy exit and immediately was caught in slow moving traffic. I didn’t turn my cooling fans on – it was a cold day (about 15°C) with light rain. I figured the engine wouldn’t need forced air cooling on a day like this.

After 10-15 minutes of slow moving traffic and trying to find a parking spot, the engine went into ‘parade mode’. The engine heat sensor sent a signal to the engine control unit that the cylinder head was too hot; take action to keep it cooler.

I was mighty surprised given the weather conditions and short time in traffic. It reminded me that I should use my engine cooling fans whenever I’m moving under 60km/h.

Do you need engine cooling fans in winter?

Clearly your Harley’s engine will run cooler in winter than it does in summer, but not as cool as you may think. Most Harley Davidson models run air-cooled engines; cooler air means a cooler engine, all other things being equal.

But the amount of cooling depends on the difference between engine temperature and air temperature (remember that from high school science?)

Some calculations:
Summer: head temp 180°C, air temp 40°C: difference = 140°C.
Winter: head temp 180°C, air temp 20°C: difference = 160°C.

So the cooling effect in winter is only 4-5% better (calculated using the Kelvin temperature scale).

Riders feel a big difference between summer and winter, but your engine won’t. I’ve been using my cooling fans in Sydney traffic even in these cooler months.

Engine cooling fans keep oil temperature within range

Two test runs in near identical conditions demonstrate how Cool My Hog engine cooling fans work to control oil temperature. The forced cooling is directed at the cylinder heads but the entire engine benefits.

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.

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

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.

Read about the importance of controlling engine oil temperature at

Where to measure Harley engine temperature – oil or cylinder head?

The best place to measure your Harley Davidson’s engine temperature is at the cylinder head. That’s where the Harley design engineers placed the Engine Temperature Sensor – in the fins behind the front cylinder head. If they wanted to monitor engine oil temperature they could easily have done that. But they choose to monitor head temperature.

A surprising thing happened as I got caught in traffic on Pennant Hills Rd after riding down the M1. I was monitoring oil temperature (with a sensor fitted in the oil sump), and had my engine cooling fans off. I was wondering how hot the oil would get with the fans off.

But surprisingly the Engine Idle Temperature Management System (EITMS) kicked in when the oil temperature was only 105°C. (EITMS shuts down the rear cylinder when the engine gets too hot.) So why was the engine too hot while the oil was OK.

Finally I got it! (perhaps I’m a bit slow). Engine temperature and oil temperature are not exactly the same thing. The engine – the cylinder head – can be too hot while the oil is not. That’s why the Harley engineers placed the sensor where they did, and that’s why they’ve built in some corrective action when the engine runs too hot.

I quickly flicked on my cooling fans and the EITMS turned off in under a minute.

Harley engine’s are air cooled, not oil cooled. Reading the description of the oiling system confirms this. The oil system is about providing lubrication not cooling, although there is some cooling as a side benefit.

The cylinder head is the hottest part of the engine. It’s the area that needs cooling. The design engineers have played particular attention to the cylinder head area when designing the new M8 engines.

So, cooling fans that blow air directly on the cylinder head drop the temperature in this critical area.

I’ll continue observing engine oil temperature, but won’t be so obsessive any more.


Stop extreme engine oil temperature

It’s more important to protect your engine against extreme heat rather than having it run just a few degrees cooler. There’s little advantage in keeping your engine oil at 100°C rather than 115°C. Both are well within your engines operating range and the oil’s lubrication range.

But there is an advantage in preventing it getting as hot as 130°C. That’s not good – even for short periods. See notes on cooler engine oil.

Cool My Hog engine cooling fans for Harleys are designed to fight extreme temperatures – to take the edge off extra high temperatures.

Cooling fan performance impresses

I demonstrated the effectiveness of the Cool My Hog Harley engine cooling fans to the crowds at the Bathurst Bike Show on the weekend (4 Feb).

Onlookers were amazed at how quickly the fans cooled cylinder head temperature. One bloke said “I thought these fans were just novelty value, but they really work.”

I had connected a temperature sensor under the front spark plug and had the digital readout attached to my handlebars. I left my bike idling and let the cylinder head temperature rise to about 180-190°C. Then I turned the fans on.

The temperature falls rapidly – a drop of 20-30°C in just a few minutes. You can see similar results in the video on the performance tests page.

It was stinking hot at Bathurst, 35+°C. And we were on the asphalt in Russell Street, just outside the court house. A good replication of sitting in traffic.

The Bathurst Bike Show was the biggest ever, with lots of impressive machinery on display. Well worth a visit – first Saturday in February every year.

Cooling fans lower oil temp by 10 degrees

Oil temperature is a good indicator of overall engine temperature. I know for sure that the Cool My Hog cooling fans lower cylinder head temperature (see previous blogs), but I was keen to investigate impact on oil temperature.

The tests:

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

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

10oC cooler – that’s a good thing.

Looking forward to repeating this test on a hot day.

Cooling fans useful even on highway runs

On a recent trip from Tumut to Cooma to Queanbeyan to Batemans Bay back to Sydney – what fantastic roads – my cooling fans were unexpectedly useful.

I normally don’t have the fans turned on for highway work; they don’t do a lot over about 60-70 km/h.

But I’m glad I had them for this run. On a few occasions I had to stop at road works. When that happens you have an engine that’s been working reasonably hard and well warmed up – and then the air flow stops instantly. My fans helped keep all that heat in check.

And it always seems just as you’re enjoying a twisty bit of road you find yourself stuck behind a semi trailer – down to 40km/h and no way around. Again, my fans helped make up for the lost cooling due to low air flow.

Why my Harley runs so hot in traffic

I’ve been watching my digital temperature gauge (attached to the rear sparkplug) as I’ve been riding around. In my observation, there are a few (obvious) things that impact engine heat.
<li>ambient temperature – the bike runs hotter on hot days. Of course, but the effect is less than I first thought</li>
<li>engine revs – the faster the engine is running, the hotter it gets</li>
<li>engine load – going up a hill or accelerating generate heat, going downhill or decelerating tend to cool the engine</li>
<li>air flow – the less the airflow, the hotter the engine, all other things being equal.</li>
Now, when I ride around town at an average of 50km/h I’m usually in 3rd gear, so the engine is revving at 2017rpm. At 100km/h on the highway it revs at 2349rpm in 6th gear. So the revs aren’t too different, but the airflow in the city is half that on the highway. Add to that frequent high load as I accelerate between traffic lights.

No wonder it runs hot in town!

42 degrees, traffic snarl and no fans equals extreme heat

Thursday last week was extremely hot in Sydney, over 42°C in the suburbs. I got caught in traffic on the M4 going towards Parramatta after a truck breakdown. Traffic was crawling.

Unfortunately I’d removed my cooling fans for some adjustments, so I was just relying on the cooling fins and my oil cooler, but neither work well without air movement.

I still had my temperature gauge fitted. The cylinder head climber to over 220°C in the slow moving traffic, and stayed there. It even hit 230°C, that’s 446°F, at one stage. I reckon that is way too hot, and the engine was pinging and struggling big time. Even when the traffic started moving again it stayed well over 200°C until I got home.

So I’m convinced. I’ll make sure I have these fans fitted until the end of summer. Who knows when I’ll get another day like that.

Running fans from start-up keeps Harley engine cool

I used the cooling fans differently today. I let the engine warm up a little and when the head temperature was 100°C I switched the fans on. I rode around town for a little over an hour (light traffic) keeping an eye on the temperature. Mostly it was about 130°C – 140°C but did peak at 154°C going up a hill.

It was a pretty warm day, about 34°C. Normally the engine would get very hot with this sort of riding – around 190°C.

Using the fans in this way is probably the best way to run them – it’s much easier to keep something from getting hot than to try to cool it down once it is.

10 minutes from home I switched the fans off. Engine temperature rose quickly and peaked at 203°C, settling around 190°C.

Yes, these fans work, and work well.

40 degrees C cooler Harley even on a mild day in gentle traffic

I rode the thirty or so kilometres from the Sydney CBD to my home this morning. It was a fairly mild day for this time of year – about 26°-28°C. The traffic wasn’t too bad coming out of the city so I maintained the signposted limit for most of the journey, about 60-70 km/h. But of course, there were multiple traffic lights.

Cylinder head temperature with the fans off: 185°-195°C
Cylinder head temperature with the fans on: 145°-155°C

I’m waiting for a stinking hot day to see what these fans can really do.

10-15 degrees cooler even at highway speeds.

I’ve fitted my bike with a sensor that allows me to monitor head temperature while I’m riding; much more convenient than the hand held infrared device I’ve been using.

I know these fans work well at low speed and in traffic, but surprisingly, they also work well on the highway. They drop the head temperature by 10-15°C.
I wasn’t expecting that because at 100-110 km/h there is already good airflow over the heads – I didn’t think the fans would improve that much. But clearly they do. A 10°C drop is not huge, but every bit helps I think.
(tested at 27°C ambient temperature at constant speed on the M4 in western Sydney. Waiting for a very hot day to repeat the test)

Cooling fans work OK in rain

Tested these fans in torrential rain last week (22 Nov). They ran fine.
You don’t really need them when it’s raining – water is an excellent coolant. But I wanted to check the fans were sealed up ok and water resistant. No problem.

75 degree temperature drop this morning

Victoria Rd was very congested this morning (16 Nov); accident on iron cove bridge.
Head temperature reached 205°C even though ambient temp was only about 26°C. Switched on the fans and temp had dropped to 147°C in two sets of traffic lights. Down to 130°C by end of journey.
Very happy with the way these fans are working.