Downdraft induction - Harley XR

By Kevin Ash

Pictures: Harley-Davidson Press

If you’ve read the test of Harley-Davidson’s XR1200 and seen the pics you might have noticed how the usual Harley right-side airbox has been deleted on this bike. It also produces some 20bhp more than a stock Sportster 1200, which comes partly from its Buell-type cams and a higher compression ratio. Some of that power increase, or at least the ability to achieve it while meeting Euro 3 emissions laws, will have come from the bike’s upgraded fuel injection system. But what’s especially interesting about this is how fine an example it is of where engineering and styling have to work very closely together – very much a motorcycle thing. XR1200 downdraft system leaves the engine profile visible - Click on imageA car design chief can leave the technical bits to engineers dressed in grey knitted tanktops, polyester slacks and canvas deck shoes and not have to worry about what the fruits of their labours will look like, as they’ll all be hidden by bodywork or plastic engine covers anyway. But on many bikes, especially unfaired ones, and especially again on ones such as the XR1200 which attempt to evoke a history or particular style, the shape and look of the heavy, lumpy bits is as fundamental to the bike’s success as how well they work.

This is why the XR1200 is the first Harley-Davidson to feature a downdraft fuel-injection system. This means the inlet tracts are angled steeply downwards from an airbox repositioned above the engine – instead of that typically Harley side-mounted one. The technical excuse is that a downdraft design improves cylinder filling by making use of gravity to help accelerate the flow of gas down into the engine. Harley Nightster with traditional side-mounted airbox - Click on imageThis its does, although it’s the short, straight path through the cylinder head and past the inlet valve which is the most beneficial. On the XR it also means the airbox design can be considerably larger than on other Harleys, which also encourages higher power levels – engine designers are constantly battling to fit the largest airboxes they can to help maximise power within increasingly strict noise emissions requirements, although very often this eats into the space once reserved for fuel storage.

But something else matters here: if you look at the XR from the right side, you can see how exceptionally clean and uncluttered its lines are, and importantly, how elemental the engine looks. The reason is that without the traditional sidedraft Harley airbox, the famous 45-degree twin is showed off in full. And according to XR project chief Frank Savage, that as much as for any technical reasons is why the XR has a downdraft intake system – so you can see the motor better. Engineering and styling entwined.

And why not?

* Harley-Davidson XR1200 test

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