DK Custom Products

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Long Ignored - Little Known Performance Mods

Long Ignored/Little Known Performance Mods



If you ride in cold weather, you mostly likely know that your Harley runs stronger in cold weather than warm weather. The throttle is more responsive, it has significantly more acceleration, and overall it just feels stronger.

That is because Harley engines, like all other internal combustion powered engines, run better in colder temperatures.


This is common knowledge outside the Harley World.  In automobiles, a cold air intake is a popular modification, providing one of the best power increases per $ spent.

Cooler air bring more oxygen (denser air) into the combustion chamber, and that means more power.

It makes such a big difference, in fact, that the simple process of redirecting the filter to draw cooler air is good for a horsepower gain of about 5 to 20 ponies in most cars.


Back to Harley’s-  


A cold air intake on a Harley will not add 20 horsepower, but it will add a 3-7% bump in small amount!

Additionally, our Harley’s will also experience a power increase if the actual metal is maintained at stable (& lower than typical) temperatures.

Setting aside rider and passenger comfort regarding heat for a moment, and looking only at engine temperatures, & at what temperature produces the best power in a Harley, is a complicated subject.  

Since heat is how our engines generate mechanical power, heat is essential to producing power.  When an engine is “cold” it will not produce good power, when it is too hot, it will not produce good power.

BUT, to produce the best power, not only is there an ideal temperature for the engine, Different parts of the engine have different ideal temperatures:  


  • The intake, from the air cleaner, all the way to the intake valves need to be as cold as possible.  


  • The cylinder walls should not heat up the air before compression, BUT also not cool down the gas at combustion.  The compromise temperature at the cylinder walls, for best performance, is 190 degrees F.


  • The exhaust should be hot, but should not be made hotter by excessive restriction or reversion.


Keeping the engine metals at their ideal temperatures for producing maximum power will also prevent the metal from getting so hot that the longevity of the engine components are compromised.

 ADDITIONALLY, by keeping the engine metals at the ideal temps for best performance, there are three more benefits:


  • The engine temps will not be so hot that they will cause rider or passenger discomfort.


  • The built-in safety mechanism, installed by Harley in the Delphi ECM, is designed to retard the timing when the ET sensor in the cylinder head hits around 250* F. (the timing retard is actually based off a calculated CHT #, and that algorithm derived # is almost always hit when the ET sensor crosses the 245-255* F threshold)   This is there, partially, as a strategy by Harley to prevent the engine from getting too hot for its’ own good (read engine longevity/warranty claims).  Retarding the timing will help prevent the heat from climbing higher, but it also drastically reduces power/performance.


  • By keeping the engine temps at the ideal level, the oil is not being subjected to the excessive heat, which breaks down the additive packages in the oil.


NOTE:  Since there is no easy way to measure cylinder wall temperatures, it is good to know that the oil temp will generally be between 200-230 when the cylinder walls are at the optimum temperature for Best Performance.  If you see your oil temp going over 230 F, then you can know that your NOT getting the best performance possible.



The vast majority of Harley owners look at doing at least a “Stage I” upgrade to get some performance gains out of their bikes, and while a proper Stage I upgrade is important, it is only part of the basics in getting a performance boost out of a Harley.


Important, but mostly ignored, Performance Mods:


A cool Air Intake must include an External Breather System.  Most “Stage I” air cleaners are a compromise, at best.  To get the most performance out of a High Flow Air Cleaner, it must have an External Breather System which re-routes Hot, Oily, Oxygen Depleted air Away from the intake.  All DK Custom Products Air Cleaners have this built in.

Read more about External Breather Systems (EBS) at this LINK.  The info at the link is applicable to all DK Intakes.  If you already have an aftermarket air cleaner, it is likely that there is an EBS that can be fitted to it, see link above.


Keeping the cylinder walls at the optimum temperature of 190 F requires a multi-pronged approach.

Except in the coldest weather (50* F and cooler) most Twin Cams and M8’s will quickly reach 190* and pass that ideal temperature within the first 15-20 minutes of operation. (special notes on the Sportster at the bottom)


What can be done to tame this heat, and get the best performance?


First, a Cold Air Intake, that is pushing the coolest possible air into the cylinders at a rate of up to 300 CFM.  The air flowing thru the A/C can have a very significant impact on cooling (or not cooling) the cylinder walls.


Unrestricted Exhaust that limits reversion.  The faster the exhaust can exit the headers and mufflers, WITHOUT experiencing reversion, the faster the cool air can flow through, helping cool the cylinder walls.  See more info on exhaust at this LINK.


Other, mechanical, methods of keeping the engine at the optimum temperature for Performance (which will also enhance longevity and Rider/Passenger Comfort):


1. Get rid of anything that is blocking or trapping heat- fork wings, thigh protectors, etc.

2. Lift the tank up off the engine.  Harley has done that on all their 2018 Softails.  They finally fixed the design problem of the tank trapping heat/blocking air flow.

3.  Use CDW’s, if you can, to redirect more air onto the engine to cool it down.

4.  On Softails and Dynas, use a Coil Relo open up the air flow around the rear cylinder.  (this is another thing HD finally did on their 2018 Softail models)

5.  Relocate the oil filter out in the wind.  Not only will this cool down the oil, it makes oil change Much Easier.

6.  Use a Permanent High Performance (High Flow) oil filter.  This will provide superior lubrication, less parasitic drag, & better filtration, all resulting in better cooling.

7.  Make sure you have a good tune..this is important for performance (aside from heat) and it can also result in as much as a 15* drop in temps.

8.  Install a good oil cooler, in many cases, a secondary oil cooler works wonders, especially on heavier bikes like the baggers and trikes.  On Twin Cooled models, that have no oil cooler, there is an option.  More details on various options in #14 at this LINK.

9. Cooling fans are particularly useful in slow moving traffic, and even at highway speeds they provide a benefit, especially for the rear cylinder.




Sportster Notes:  Unlike the Twin Cam & M8 (which all need help cooling the engine metal & oil), the Sportster Evolution engine generally does not have a heat issue.  It will still run stronger in cool weather, and will benefit greatly with a cool air intake and unrestricted exhaust.  BUT, most do not need help cooling down the cylinder walls.  

The heat sensor on the Sportster is in a different position (top of rear cylinder head), and as long as it is reading between 310 and 375, the cylinder walls should be close to 190, which is the optimum for power. 

If you are getting more heat than that, or if your oil is going above 230, then a few effective options are relocating your oil filter, using a permanent performance oil filter, and/or using an oil cooler.


See a complete Report on How & Why to Cool Down the Twin Cam & Milwaukee-Eight Engines HERE.


Excessive Heat is Not only Uncomfortable, it Robs Performance!


Controlling Heat is the “Sleeper” Performance Mod!