Sidewinder Thundercat/9000/King Cat Exhaust Systems

Oct 22 2016


EVP is once again on the cutting edge of design and development for the Yamaha Sidewinder / Arctic Cat Thundercat.  The OEM muffler weighs 15 lbs and is extremely restrictive.  This is evidenced by the 11+ horsepower gain realized just by replacing it with our free flowing exhaust.  We have two exhausts  – our “Bullet” and “Magnum”. The EVP Bullet is designed to be lightweight and has a sport tuned exhaust note.  Although the silencer portion of the muffler is not very large, the use of our dimpled “dB Killer” flow divider keeps this exhaust from being obnoxious.  Like all of our exhausts, the EVP Bullet is made from 304 Stainless and TIG welded for quality and durability.   The pipe diameter has been increased to 2.5″ which gives enough flow for well over 300 hp.  It uses the stock mounting brackets and rubber grommets and weighs less than half of the original muffler.

The EVP Magnum exhaust is constructed similarly, but is designed to be much quieter while still being free flowing.  The Magnum give the Yamaha 998cc a nice low growl on the bottom end and lets the engine sing on top while being only slightly louder than stock.  Our “Magnum” has been tested by the WI DNR and has passed the dB rating, so no sound violation tickets!

Both exhausts reuse the factory heat shielding!

Sound clip of our Bullet Exhaust in the field

A note on Sidewinder/Thundercat exhausts: There are many others claiming that by putting their “magic” exhaust on that the boost pressure will increase by 2-3 lbs and wild horsepower gains are achieved.  This is simply false.  The ECU programming controls the boost electronically.  When the programmed value is deviated from, (stock is 9.5-10lbs) there is a correction table which will actively move the boost back to the target value.  Flow dividers or any other device after the turbine outlet does not change the fact that this table exists and will move the boost back to the target pressure.  Anyone who is actually tuning this ECU knows this.   If the wastegate flapper valve travel were limited (this is called locking the wastegate), this would allow the turbo to produce more boost, but the variables in turbo efficiency depending upon altitude or temperature would make this completely unworkable in addition to fighting the ecu correction and overboost limitations.

Without additional boost/fuel, it is impossible to make 30 more horsepower by just changing an exhaust.  So why does the mixture lean out and where does the extra horsepower come from?  The stock exhaust has a lot of back pressure.  This resistance to flow contaminates the cylinders with gases from the previous combustion event which RICHENS the air fuel mixture because there is less available fresh air to burn.  When you replace the factory exhaust with a free flowing one, now there is more fresh air to burn and the mixture LEANS out.  The ECU does not run closed loop fueling, so it cannot compensate for this.  Here is a video showing the stock tune with the EVP 2.5″ Bullet free flowing exhaust – it makes the exact same boost as with the stock muffler.  


  • The EVP “Magnum” has been WI DNR tested and approved to meet sound regulations!
  • Both reuse the factory heat shielding
  • Both exhausts net +11 hp over stock
  • 304 TIG welded stainless construction with lifetime packing material
  • Bullet weighs 8lbs less than stock, Magnum 4lbs less
  • A fuel correction tune or one of our other performance tunes are HIGHLY recommended



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