The following is from the
www.thunderproducts.com." The TPI Valve is designed to increase air flow into your air box and carburetors.
Racers and tuners as well as many trail riders have known for many years that opening up the air box in order to flow more air makes more horse power. This is usually called “ventilating” or “gutting” the air box. They also know that more air means that the engine will be running leaner exactly the same as installing a smaller main jet.
Only one thing was missing, and that was the ability to go back to the richer jetting required for colder weather or lower altitudes. What did they do about this? They tried to tape or stuff foam rubber in the openings they had created. All of this was very messy and imprecise. Much of the time they were either too rich or too lean or they had to start changing main jets in order to be jetted correctly.
There must be a better way? There is!! It’s called a TPI valve. The TPI Valve can be opened completely to the leanest position or closed completely to the richest position or any position in between.
The TPI Valve gives you a range of two to four jet sizes of adjustment providing you use the recommended number of valves in your air box.
For more information on airbox modifications, read: Airbox, Friend or Foe?
The TPI "Knobby" Valve is 1 1/2" diameter and 1 1/2" high.
The TPI "Low-Boy" valve is a flush mounting version which is 2 5/8" square overall.
TPI Valves come with filters. "
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