Heavy breathing the key to optimum performance

Okay, so you’ve added a set of new pipes or slip-on mufflers. But are you getting the full benefit from this upgrade?

Chances are you’re not unless you compensate for the freer-flowing exhaust by allowing a greater inflow of air into the combustion chamber. This is done with the addition of a high volume air cleaner. In other words, an air filter designed expressly to admit more air (and oxygen) into the combustion mixture.

It follows therefore that if you have more fuel/air mix being drawn into the chamber, you should achieve an increase in power and torque. This presumes that you have all your ducks lined up (so to speak) by ensuring the pipes, the air cleaner and your ignition are all working in harmony. Any one of these factors will decrease your potential gain - or even makes things worse.

Screamin' Eagle Stage I
Air Cleaner Kit
Part No. HD29400356

“The choice of the correct air cleaner is important,” says Blair Freeth, dealer principal and factory-trained technician at Adelaide Harley-Davidson Bike Works, “not only do high-performance air cleaners offer another point of style, but better air quality and air volume. All this increases fuel efficiency and improves acceleration."

Unfortunately, the optimum result will not be obtained with just bolt-on parts. With major changes to the airflow and mixture, remapping of the fuel injection is required and this means changes in the electronic tuner unit. Now be careful here, because changing this during the warranty period runs the risk of voiding your warranty, so my advice is to get this done at your dealer - or don’t touch it and be satisfied that you’re not quite at optimum performance, but still sounding great.

“Changing the way your Harley-Davidson engine draws in air and pushes it out means it’s highly recommended to retune,” advises Blair, “Modern tuners allow us to change the parameters (mapping) of your bike by changing the air/fuel mixture and ignition timing thus allowing your bike to run optimally with its new components.”

Our 107ci FXBB Street Bob with TBR pipes now has a Screamin’ Eagle Stage 1 air cleaner kit (Part No. HD29400356) and recalibrated ECM.

You can find Tim's hardcore M8 mods on p.118

If you're looking to take your Milwaukee-Eight further, you can see Tim Munro's similar Street Bob (FXBB) getting a workover in Issue 180 of Heavy Duty Magazine.

Tim goes APE (Alpha Performace Engines) and has added a CR480 cam kit that drops in on stock valve springs and he reckons it delivers "loads of low-end torque and a very broad power curve from 2000-5000rpm" which suits our everyday riding.

Whether you choose factory Screamin' Eagle or aftermarket, there really is no limit to where you can take your M8.