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Why & How to Cool Down the Twin Cam & M8 Engines - Part I

Cooling down Your


Twin-Cam & M8 Engine


 (Temp Testing & Before & After Testing Videos with Results on the next page, Part II)


Why bother cooling down the engine on your bike?

  1. It will last longer

  2. It will be more comfortable to ride (less felt heat)

  3. It will have more power (enough that you will feel the difference when you twist the throttle)

  4. It will get better MPG


Twin Cam engines have always been hot, but starting in 2007 they have been much too hot for their own good.  The M8's are running just as hot.

On a warm day, and after 45 minutes of riding, It is not unusual to see oil and Engine temps well above 270*, and on the heavier bikes/trikes,  300+ degrees is considered “normal”, “they all do that”.

It is in the fundamental design, plus the EPA requirements that continue to rain down on The Motor Company.

Whether it is the air-cooled, oil-cooled or the Twin-Cooled (aka- wethead or waterhead), they are designed to run hot, and, for the most part, they will continue running (albeit poorly) for quite a long time.  But it is at the sacrifice of:


Rider & Passenger Comfort

Reduced (sluggish) power

Poor fuel economy

Component failure


This article addresses a little in the understanding of Why they run so hot, and More on what can be done to cool them down...

For the Air-Cooled, Oil-Cooled & Twin-Cooled Twin Cam & Milwaukee-Eight Engines.

Which Heat Do You Want to Get Rid Of?

Reduce Actual Heat for the engine, to help it run better, or reduce Felt Heat by the Rider and/or Passenger for comfort?  They are not necessarily the same.

I prefer to keep the heat for the engine in safe temps...what is good for the engine.  That will have a significant effect on reducing felt heat to the Rider & the Passenger.  There are ways to get rid of felt heat, at the cost of increasing actual engine heat.  We will not be discussing those.

Why do these engines run hot?

The basics are:

1. They squirt oil at the underside of the pistons.This is done to cool the pistons, but it transfers that heat to the oil that then circulates the heat thru the entire engine.

2. EPA compliance dictates that the Twin Cam run lean (too lean, which creates excessive heat and reduced power) for reduced emissions.

3. EPA compliance dictates that they are quiet. This excessive baffling works against the engine, creating more heat. TC 7 M8

4. EPA compliance regs on emissions are partially met by a catalytic converter. This creates significant felt heat. TC 7 M8EPA

5. EPA compliance regs on emissions requirethat oxygen depleted, oil laden, hot air be pushed into the "fresh air" intake (air cleaner), resulting in less than optimum combustion, which creates even more heat TC & M8

6. EPA compliance regs on emissions are partially met by "epa cams". These cams are designed to create even more heat, in an effort to reduce pollutanta exiting the exhaust. TC & M8


Unique to the Twin-Cooled (air-cooled/liquid-cooled hybrid), also know as

waterheads or wetheads:


The basics are the same on the Twin-Cooled as the Air-Cooled & Oil-Cooled, with the addition of the following:

  1. They have higher compression, which creates more heat.
  2. They Have slightly "hotter cams" which create more heat.
  3. The water pump is in the location where Harley normally puts the oil cooler, resulting in NO oil cooler. So the oil and all portions of the engine, except for the liquid cooled heads, are even Hotter on the Twin-Cooled engines than the air-cooled.


 Bottom line is the TC & M8 Twin-Cooled has Twin Problems

  • While the heads are cooler, the rest of the engine is hotter.
  • The oil gets hotter with no oil cooler to dissipate the heat.


 How  to Cool Down the engine?

We have been working on this for years.  Thousands of hours, tens of thousands of miles of testing various solutions.

Up front, it is important to recognize there is No One Magic Bullet.  There are many things that each contribute varying increases in cooling.  Only when combining all, or most, of them is an acceptable level of cooling obtained.


Our target  temperatures are:

Engine Temperature, as measured by the factory sensor & then displayed by the PV as "ET". 

230* F everyday riding, with no more than 250* F in extreme circumstances.  

(at 270* F you will feel the sluggishness in power, and accelerated engine wear begins)


Oil temperatures, as measured in the oil tank or the sump:

230* F everyday riding, with no more than 260* F in extreme circumstances.  

(at 270* F you will feel the sluggishness in power, and damage is occurring to the oil and there is accelerated engine wear)


Steps to Cool The Twin Cam & M8 Engine Down

Outlined in Part II at this LINK


Live Free - Ride Free!