Spying a throttle-controlled gate on an exhaust pipe for the first time in pit lane, I asked Yamaha mechanic Alex Briggs about Aprilia’s use of such. Alex generously explained many the finer points of adjusting exhaust back pressure, with a mechanical gate like the one shown above being one of the solutions.

While different engine designs approach the issue of back pressure in different ways (to me Honda’s is the most interesting, as it has led to many photos of their solution), each design must do something to manage this pressure for maximum performance.

Basically, back pressure keeps the fuel mixture from escaping the combustion chamber before it can be efficiently ignited. This pressure can be accomplished by a wide variety of exhaust pipe designs, the general principle being to make a section of the exhaust path smaller in diameter than the rest in order to change how freely exhaust gases can pass through the pipe.

Engine layout is a determining factor, as seen when comparing the Honda’s V design with the Yamahas inline layout. Yamaha sends exhaust though a single outlet, which Honda has two paths, one from the front cylinders and the other from the rear cylinders.

Because the pressure must be even on all cylinders, Honda has a rear pipe that twists around to end up the same length and have the same back pressure as the front pipe.

You may have noticed the distinctive designs of 2-stroke engine exhaust pipes, specifically the large expanded area just beyond where the pipe attaches to the cylinder. This shape directs a sonic wave (created by combustion) back toward the cylinder to produce back pressure. No kidding!

Aprilia is currently taking the gate approach, as seen in the top photo. As the rider adjusts the throttle, cables connecting the throttle and the gate open or close the mechanism, thus adjusting the back pressure according to the amount of fuel mixture heading into the combustion chamber.

Any errors above are not Alex’s but my own for not understanding what he was trying to explain to me. Thanks very much to Alex for taking the time to chat about this.

Photograph: ©2015 by Scott Jones / PHOTO.GP – All Rights Reserved

Camera Info: Nikon D4 with Nikon 70-200mm f/2.8

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Aprilia MotoGP Bike