Ask Kevin: Are Reverse-Cone-Megaphone Exhaust Systems Making A Comeback?


Marc Marquez race action shot Question: I have been following MotoGP more closely since the final step to all 4-stroke bikes in all classes, and I have a question. It appears that some Moto3 bikes and possibly some bikes in other Moto classes are sprouting reverse-cone megaphones. This is having a very positive effect on the audio entertainment value of the already excellent racing, to my ears, at least. Can I expect to see and hear more “proper” exhausts—that is, one reverse cone per cylinder, replacing the now-normal “some-into-one with a can” systems? Kerry Harmon Submitted via email Answer: Rob Muzzy once remarked to me that if power is the only goal, individual, one-per-cylinder megaphones can’t be beat. The universally used 4-into-2-into-1 systems on inline, flat-crank 4s may make a bit less power, but they compensate for that loss by suppressing to a useful degree the big torque flat spot that occurs at about 70 percent of peak-power rpm. The return wave from the expansion of the exhaust pulse normally helps evacuate the volume above the piston at TDC after the exhaust stroke, and this low-pressure wave then passes through the intake valve(s) and gives the intake process a torque-boosting head start, even though the piston has not yet gathered much speed on its downstroke. But at lower revs, it is a positive wave that hits the still-somewhat-open exhaust valves around TDC, and it stuffs exhaust gas back into the cylinder and can even blow back into the airbox through the intakes. This reduces torque because what the piston draws in during the early part of the intake stroke is this exhaust gas, not pure fresh charge. The result is a flat spot that tuners have tried for years to eliminate, but it won’t go away. This is why no race tuners use a simple 4-into-1 pipe any more. The answer is to make a 4-2-1 exhaust. The second enlargement that occurs as the 2 pipes join into the single collector generates a later-arriving suction wave that bucks out the positive wave that would otherwise shove exhaust back into the cylinder to create the flat spot. It’s not a perfect solution, but it has been widely used. In MotoGP, the bikes are given a 130 dB sound limit, but in AMA, the limit is only 105 dB. When you have to meet a sound level, it’s lighter in weight to do the whole job in just one or 2 places—hence the can or cans you deplore! Back when Honda and MV Agusta were racing air-cooled 4-stroke bikes, Grand Prix tracks emphasized top speed a lot more than they do now, with most races taking place on “bullring” tracks like Jerez or Valencia, which jam all their length into small (cheaper!) real estate that can be seen from grandstands, with a multitude of first- and second-gear corners. The lower an engine is pulled down in rpm by having to use lower gears (wider ratio separations), the wider must be its flat-spot-free power range. In the old days, megaphones did a fine job with MV’s 6-speed gearbox and the many more speeds in the gearboxes of Honda’s smaller-bore bikes of the ’60s. On today’s courses, those bikes would go faster with 4-2-1s. Send your “Ask Kevin” questions to cwservice@cycleworld.com. We cannot guarantee a reply to every inquiry.

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