Excess heat in your Harley’s engine can cause serious problems. The cylinder head is where most heat is generated (the combustion explosion). Fans can keep the head cooler; the entire engine benefits.
More power often equals more heat. If you’ve modified your bike to produce more power, chances are it’s generating more heat too.
Excess heat robs engine power
A Harley Davidson press release (Jan 2014) says “Heat is the biggest thief of engine power. The thermal dynamics of an engine change considerably when it gets hot especially in stop-start traffic and can, at worst, cause engine damage. This is the reason why the ECM (Electronic Control Module – the bike’s electronic brain) retards the ignition to reduce maximum cylinder pressure from the resulting fuel detonation and temperature.” http://www.harley-davidson.com/content/h-d/en_ZZ/home/events/press-release/general/2014/news19.html
Excess heat wastes fuel
The ECM also pumps more fuel in when your engine gets too hot in an attempt to cool it down. I reckon that’s wasteful and expensive.
“The last thing on the agenda to talk about is the “Heat Management System” incorporated in the Harley ESPFI systems. This system is used to control excessive heat, and operates in three “Phases.” In Phase 1, if the ECM sees engine temperature above 300 degrees F (149°C)., while the bike is either moving, or standing still, it will reduce idle speed. Theory being that a lower idle has less sparks, producing less heat. In Phase 2, if the ECM sees an engine temperature that is still climbing from Phase 1, it will richen up the AFR. Richer mixture has a cooling affect. In Phase 3, if the ECM sees that the temperature is still going up, and the bike is sitting still, it will go ahead and skip-pulse the injectors, not delivering fuel on each intake stroke. Again limiting combustion and producing less heat. Phase 3 is only active when the bike is sitting still. These 3 Phases pass from one to another without pause, and you may not even feel or notice it.” http://www.harley-performance.com/harley-fuel-injection.html
Excess heat can put lubrication at risk
And of course, excess heat is passed from the cylinder head to other parts of the engine and the lubricating oil. I’ve regularly measured oil temperatures (at the back of the oil filter) on stock 103 twin cams of between 120° – 150° Celsius. The lubricating effectiveness can start to reduce at these temperatures.
See notes about the impact of high engine oil on the Cooler engine oil page.
Excess heat can burn you