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.