August 26, 2017 Dustin Porter

Undertail Exhaust, Pretty But No Performance!

It seems the trend of taking MotoGP inspired bodywork and styling has become more and more obvious in the past few years. Now almost all of the sportbike manufacturers are taking some aspect of MotoGP and incorporating it into their production sportbikes. It’s pretty nice to see that the big four of sportbike manufacturers are taking notice that we, the sportbike rider, are eager to get our hands on anything that is race inspired. For instance Honda and its new breed of CBR-RR models all come equipped with undertail exhausts and the marketing ads shown in magazines like Sportrider and Road Racer X have tag lines pointing out the new features. The ads run a main heading saying: “DNA DONOR: RC211V, RECIPIENT: CBR600RR” Honda has lead the charge to bring MotoGP inspired performance accessories over to the new CBR-RR’s, but they aren’t the only ones. Kawasaki, Yamaha and Ducati have added underseat exhausts to a select few sportbikes in their respective lineups. The only motorcycle manufacturer not going in the trend of a stock undertail exhaust is Suzuki, which obviously doesn’t seem to be a problem for motorcycle buyers as the Suzuki GSX-R1000 was the sportbike of choice for the serious racer and certainly hasn’t hurt their road race effort, just ask Matt Mladin.

Personally I think anything that can make a sportbike more race inspired is cool. It cuts back on aftermarket needs and saves money for gas and track days. Undertail exhausts are a tough one though because after reading the May 2016 Sportrider where Kent Kunitsugu rips on underseat exhausts trend for being more fashionable than performance enhancing, I may have to change my thinking. Here is a quick run down of the main points of the article. First off an underseat exhaust doesn’t seem to be saving any weight. These undertail exhausts usually only add five or six pounds but that’s till some additional weight that has to be accounted for. With the increasing number of inline-four-cylinder sportbikes coming equipped with undertail exhausts, motorcycle manufacturers are trying to find other ways to shave off weight, which may mean more titanium, which is great, but since titanium means equals expensive it’s kind of a double edged sword.

The article goes on to explain that in this new millennium of mass-centralization designed engines and chassis it seems odd that these new undertail exhausts are being run. It is a fact that by placing these new sportbike exhausts higher and farther away from the motorcycle’s roll axis the manufacturers ideas are running counter to the engine and chassis design.

The last and most important part of this underseat exhaust article is that these underseat pipes have less power potential than a traditional under engine exhaust. Two undisclosed European motorcycle exhaust manufacturers said that in the particular case of inline-four-cylinder motorcycles running underseat exhausts are at a disadvantage versus traditional under-engine exhausts. An under engine exhaust has two main advantages over undertail exhaust:

  1. Midpipe – The section that connects the four head pipes to the muffler must be routed differently so that it may travel far enough to reach the exhaust muffler. The mid pipe is also bent in certain places to allow for clearance. These bends restrict exhaust flow and volume which is critical in power output.
  2. On traditional sportbikes the exhaust canister is much larger which allows for some baffling to take place and reduce noise without hurting power. Underseat exhaust have the opposite problem, the exhaust canisters have to be smaller to fit under the seat while still having to account for noise. The internal baffling within the muffler on an already small exhaust canister means even less exhaust flow which in turn hurts performance.

I think racing inspired designs are what make sportbikes so appealing to motorcyclists. Since every other race team except for a MotoGP race team has to use some stock parts it seems that the race teams are influencing motorcycle engineering more than the consumer desires. I think it’s great we as enthusiast and consumers just get the benefits in the new sportbikes every year. The only problem I see is when these new additions make the price tag increase and we don’t see any real benefit in the sportbike. Don’t get me wrong, if the new 2017 Yamaha R6 comes out with an undertail exhaust it won’t make me want one any less, it will just make me spend money on a better aftermarket exhaust to try and save weight and increase performance. For now I’m fine with my traditional under engine exhaust and besides I know my girlfriend wouldn’t want to ride on my bike if here ass is being cooked every time we ride. Earlier I said that I think race inspired sportbikes are cool but I don’t think it’s worth it if you have to go out and spend $400-$2100 on an after market exhaust just to make them as functional as last years traditional under-engine sportbike.

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