Technical Information

Shear Plates – Why Do They Work?

Shear plates, tapered spacers and insert designs are tuning tools. Some engines will respond to them better than others and often they will broaden the power band. The air/fuel mixtures transition from the carburetor to the plenum is not the same on all engines. There are many variables that affect this transitional area:

  • Venturi air speed
  • Carburetor  design
  • Fuel droplet size
  • Pulse strength
  • Firing order
  • Plenum size and design
  • Carburetor distance from runner

The pulsing air from each runner creates chaos in the plenum. This affects the fuel leaving the carburetor and entering the plenum, causing fuel separation and air flow disruptions. The combination of taper and the sharp edge helps to separate these pulses from the air column entering the plenum. Typically, over carbureted engines respond better to shear plates because of the slower air speeds through the venturi.