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Testing Air-filter Assemblies fitted to Fuel-Injected Twin Cam Engines

Rider Contributor
July 28, 2006
Filed under Uncategorized

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In our August 2001 issue, we flow-tested air-cleaner kits on the K&N Powersports flow bench. Those filters were designed to fit carbureted Evolution engines. Now the Twin Cam is king, and more than half of the bikes currently rolling off the H-D assembly line are fuel-injected models. So, for this test, participants were asked to send an air-filter assembly to fit a fuel-injected Twin Cam engine.
When we started calling companies to get products for our testing, we found that some of the participants from the 2001 test either didn’t want to be involved this time around (Arlen Ness, Rivera Engineering, Custom Chrome), or were no longer in business (Force Motor Products). Fortunately, most did want to be involved again, along with newcomers Küryakyn and Doherty Machine, and willingly sent their products in time for testing.
We all know that a Harley’s inlet system is restricted from the
factory. Factory H-D air-cleaner assemblies are required to not only filter the air going into the engine, but to limit evaporative emissions from escaping into the atmosphere. Additionally, stock exhaust systems are choked by EPA requirements for noise that reduce the throaty rumble of a V-twin to a raspy whisper.
The vast majority of riders choose to make modifications to their stock bikes that result in better performance. Central to these modifications, the lowly stock air-cleaner assembly is typically one of the first items thrown into the take-off parts bin, followed closely by the stock exhaust system.
Does an air cleaner’s CFM rating have that great an impact on performance? Well, yes and no, depending on how much air your Harley’s engine is capable of pumping. Given that many things happen to the airstream after it passes through the air filter, a high-CFM air-cleaner rating on a bike with a small carburetor, intake manifold, or throttle body, or an inlet port that flows very little, will probably not help much.
There are also other things to consider that a simple flow-bench test can’t tell you. For instance, some backing plates use a venturi design that increases flow efficiency as engine speed increases. Also, many air-cleaner assemblies are designed to draw air from behind their backing plates. This air, close to the engine, is usually warmer and doesn’t produce power as well as fresh, cool air coming from the airstream. Translating any results simply by looking at a single CFM measurement is risky. Where the CFM ratings really come into play are when deciding what level of performance your future modification plans entail. Increased displacement, more radical cam profiles, free-flowing exhaust systems, and head and port modifications demand more than a marginal airflow. Keep this in mind when making your choice.
Given that engines are basically made up of finite dimensions (bore, stroke) and operating characteristics (rpm range, gearing), there is a mathematical limit on how much airflow an engine needs to operate efficiently. After all, the engine will only use what it needs regardless of how much air the filter is capable of passing. However, the fact remains that if improved performance is your end goal, there’s no room for a stock air cleaner in the calculations.
With that in mind, we once again prevailed upon the experts of airflow at K&N Engineering to assist us in our testing. Using a SuperFlow flow bench in the hands of Tom Brown, Director of Special Products, we came to the conclusion (just as we did in 2001) that almost anything is better than stock in terms of airflow. Even the lowest-ranking air cleaner, the Küryakyn (a filter designed more for aesthetics than performance), flows a whopping 57 percent better than stock.
If the only modification you’ll ever make to your bike is a new air cleaner, don’t expect to see much improvement (maybe 3 or 4 horsepower). For a mildly modified street bike (slip-on mufflers, mild cam), choosing a filter by appearance or style is not likely to hurt performance, either. But if your future hop-up plans include an efficient performance exhaust system, port work, a high-lift cam, or displacement larger than 95 inches, a high-flow air cleaner is a definite must.
Zipper’s Performance Products
The Zipper’s kit (Part No. 117-296; MSRP $62.49) includes a chromed backing plate, a new flange gasket, and all mounting hardware. It is easy to install, and is designed to use the stock cover and breather vent system. The element is made up of a pre-oiled material that is washable and reusable. The airflow performance of the Zipper’s kit measured 7 percent better than its nearest competitor, but you’ll likely sacrifice some legroom for it.

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