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Exhaust valves and servo motors explained

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What does an exhaust servo motor do?

exup systemAn exhaust servo motor is one component of the exhaust valve system (sometimes called the ex-up or power valve system) found on almost every modern sportbike including all Honda, Kawasaki, Yamaha, Suzuki, BMW, Ducati, Triumph and a few others. The system consist of a few parts, the first is the valve assembly inside the exhaust pipe. This is a simple butterfly valve that can open or close to change the amount of exhaust flow. Connected to this valve is a set of cables that run to the other part of the system, the servo motor. This motor, which is controlled by the engine control unit, has a pulley on top that rotates the cables to open and close the valve in the exhaust. The cables are used to isolate the motor from the hot exhaust and allow it to be placed in a more convenient place on the bike.

The purpose of this system according to the motorcycle manufacturers’ marketing departments is to create back-pressure at lower RPM’s to increase torque. Unfortunately, this probably isn’t the real reason; the true purpose of the exup valves is to meet noise and emissions regulations. The exhaust valves are partially closed at idle and low rpm to reduce noise, and closed again at the upper RPMs to meet peak noise and emissions regulations. The proof of this has been shown on the dyno where removing the valves and retuning the engine creates a flatter better torque curve. Additionally, in the USA, most bikes close the valve again at the upper RPM range, but in Europe they do not (different regulations) and the European bikes typically create a few more HP on the top end as a result.

Why should I eliminate the exhaust servo motor?

The exhaust valve system is typically replaced for several reasons including the installation of an aftermarket exhaust that does not have the valve (most full systems and some slip-on exhausts), the valve or servo motor fails and needs to be replaced or removed, or the owner just wants a little more power.

Removing or bypassing the valve is quite easy. It can be accomplished as easily as removing the cables that connect the valve to the motor. The valve typically is spring loaded so if disconnected it will hold the valve in the full open position. Additionally, the purchase of a full or slip on exhaust system often eliminates the exhaust valve.

But now that the ex-up valve has been removed what do you do with the motor unit? The motor is connected to the ECU or engine control unit and as such the ECU will know when it is disconnected or not operating correctly. When this fault is detected a “code” will be thrown; usually this results in an FI light (fault indicator, or fuel injection system fault) being illuminated on the dash. The exhaust code varies by manufacture but general code descriptions can be found below:

  • Kawasaki ZX6R and Kawasaki ZX10R – Code 34 or 63
  • Yamaha R1 and Yamaha R6 – Code 17 or 18
  • Suzuki GSXR-600, Suzuki GSXR-750, and Suzuki GSXR-1000 – Code C46
  • Honda CBR600RR and CBR1000RR – Code 34 or 35
  • There are many more of course, but these are the most common.

What is the Servo Buddy, and how does it work?

Servo BuddyThe Servo Buddy®, by SkutrNet, is a small electronic device that replaces the servo motor. By interfacing directly with the engine control unit the servobuddy can emulate a fully functional, stock, exhaust valve and motor system. By doing so the ECU has no idea anything is different from stock and will happily operate as intended without any FI lights or faults being triggered. This is important because if you have retuned your motor it ensures the correct fuel maps are being used and the bike does not go into any sort of “limp home mode”.

The factory motor unit really contains two components inside, the motor that spins the pulley, and a sensor that reports the motor position. The fuel injection system detects faults by monitoring the sensor feedback while sending signals to the motor. When a command is sent from the ECU to spin the motor in the direction that opens the exhaust valve, the sensor is monitored to make sure it spins in the correct direction, and stops in the correct place so that it can confirm the valve is fully open. The system operates similarly when closing the exhaust valve.Servo Buddy PCB

The ServoBuddy uses clever electronics to monitor the servo motor control lines, and output the correct sensor feedback to the ECU. The Servo Buddy is unique as it has been intelligently designed to so exactly replicate how the real motors work that the same circuit board can be used for almost every motorcycle manufacture on the market! The differences between models come down to the wiring schemes and connectors used. Additionally, the servo buddy has been carefully designed to be safe for your precious bike with diode protection, feedback protection, ESD protection, motor and sensor side isolation and a completely hardware implemented digital algorithm to make sure it is the most reliable option available. If you don’t understand what that all means, that’s ok. It basically means we’ve designed our circuit to be as safe as possible to your bike’s sensitive electronics, and be reliable for the life of your bike. Our components are automotive or military grade for durability and long life, the connectors are made to OEM specifications, and the wiring systems are automotive quality.

Are there other options?

Well, there are a couple. Most full exhausts or slip-on exhaust systems that eliminate the valve assembly need a way to trick the motor, often times they do this by including a specially designed metal plate that replaces the pulley on the exhaust motor. The purpose of this plate is to restrict the motor movement to simulate the valve and cable assembly. However, this has a couple drawbacks. First, the motor assembly is still in the bike spinning around doing nothing! This is extra weight and space being taken up by a component that isn’t really performing any function other than tricking the computer. We’ve also seen lots of servo motors burn up by poorly designed or installed plates that do not have the stops set correctly resulting in the ECU trying to spin the motor past a hard stop, the motors fail and start making annoying ticking or buzzing sounds. To be fair, a lot of servo motors fail just under normal use as well, but these plates seem to exacerbate the problem. The other option we’ve seen is even worse, “homemade or DIY” servo eliminators. These are usually sold on ebay or on web forums by people who have no electronics skill or knowledge. You can pick these out pretty easily: they are big, wrapped in electrical tape balls, or sealed in globs of epoxy, etc. The people making and selling these usually don’t have access to the connectors so they come with bare wires you are supposed to solder up to your wiring harness or even worse paperclip into the factory connectors (yes seriously, we’ve seen it!). However, the truly scary thing is the “circuits” they use, usually just a couple of resistors and a capacitor. They feed motor control voltage (+ or – 12volt) directly into the ECU sense pin (spec’d at 0-5volt max) with only a current limiting resistor. These often times do not work, but even when “working” could be slowly damaging the engine control unit by repeatedly sending unsafe voltage into the sensitive computer over and over and over again. Unfortunately, once the damage has been done it can often only be fixed by purchasing a new engine control unit which costs a fortune. Please, please, please do not go this route. You are rolling the dice and eventually will be replacing your bike’s electrical system. The good news is newer bikes are combatting this by making the servo motor checking algorithms more complex requiring a better design, which fits perfectly into the Servo Buddy’s capabilities and eliminates the market of these hack jobs.


Let’s wrap it up:

Well I hope you found this information helpful and informative. We’ve tried to hit on most the questions we get from our customers and others curious about how this often misunderstood system works. However, if we’ve missed something or you have any questions please feel free to reach out to us, we’re always here to help. If you are interested in purchasing a Servo Buddy for your bike, we have a large dealer network that sells in brick and mortar shops, dealer websites, ebay, amazon, and we have our own webstore at www.shopskutr.com where you can order anytime.

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