The Story of the Aprilia V-Twin SXV Engine

<img width="200" height="159" class="floatleft" src="" alt="" />Aprilia was facing gloomy times at the end of 2003. The only bright spot among financial woes and an otherwise foundering racing program, was the progressively better showing of the SXV 4.5 supermoto bike. In its first racing season, the 450cc, 77-degree V-twin remarkably won the world title in the S2 class, exceeding even the most optimistic expectations of the bike's creator, Ampelio Macchi.

Ampelio Macchi, chief engineer on the project, had long been associated with Cagiva and Husqvarna, then quit the Varese-based company back in early 2003, taking all his assistants with him. He established his own independent engineering team to develop a V-twin to go against the most celebrated 450cc singles from all the major manufacturers.

The new four-stroke engine's concept began with the basic design goals of revving to 14,000rpm and producing 80 horsepower in supermoto applications.
Interest piquid by this, Aprilia eventually hired Macchi's team to complete development of the engine. With the Noale-based factory's involvement, the project was expanded to include variations to power motocross and enduro bikes, too.

Macchi and his assistants started with a clean sheet, which explains, among other things, the unusual 77-degree V-angle, chosen as the most promising compromise between acceptable engine balance and compactness. The latter was a primary goal, along with maximum weight reduction. Every detail shows it, for this is a very small, very light engine. Ready to race, it weight just under 30kg, including the electric starter and high-power alternator.

The pair of cylinder heads are similar to Honda's Unicam setup, with four valves per cylinder actuated by Hyvo-Morse-chain-driven single overhead camshaft. The intake valves are operated directly off the cam lobes, while rocker armms operate the exhausts. As a result, the heads are very compact and the valve included angle is a very narrow 23 degrees. This makes a compact, detonation-resistant combustion chamber.

The first iteration featured 72 x 55mm cylinder dimensions for a somewhat oversquare bore/stroke ratio of 0.723:1. But the projected 14,000rpm redline resulted in excessive piston speed, so a second engine was developed with a 78mm bore and 47mm stroke. The latter value allows as much as 15,300rpm.

The port shapes, obviously, received great attention in order to achieve maximum volumetric efficiency over a wide range of revs. The first engine was fed by a Marelli integrated fuel-injection/ignition system sporting 38mm throttle bodies, but 40mm units were adopted after testing showed better results.

The compression ratio is a very high 12.3:1, while the conrod center-to-center length is 96mm, which produces a rod ratio of 1.745:1 in combination with the original 55mm stroke, and a healthy 2.04:1 with the 47mm version. This is very close to what is prevalent on modern sportbikes, the main benefits being reduced piston side thrust for lower friction and reduced vibration.

Power and torque figures have been kept secret, but the target figure of 80 horsepower was likely achieved with the first version and exceeded with the second. Both short- and long-stroke versions have been used.

In addition to the original 450cc, Macchi and his assistants also developed an equally successful 550cc version
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