When you move beyond the "bolt-on" basics of Stage 1 and 2, you enter the territory of internal engine surgery. These upgrades are designed to turn a cruiser into a high-performance machine by physically changing the size and breathing capacity of the motor.

Stage 3: The "Big Bore" Upgrade

Stage 3 is all about displacement. The goal is to make the engine larger so it can move more air and fuel in every cycle. This is generally considered the "sweet spot" for riders who want massive torque for passing power and two-up riding without sacrificing reliability.

• Key Components: * Big Bore Cylinders & Pistons: Larger diameter cylinders and high-compression pistons that increase the cubic inch (CI) displacement of the motor.

   

  • High-Lift Camshaft: Usually a cam designed specifically to handle the increased displacement.

  • Heavy Duty Valve Springs: To handle the more aggressive cam profile.

     

• The Result: A significant jump in torque (roughly 15–25% over stock). It provides that "arm-stretching" feeling when you roll on the throttle at highway speeds, making the bike feel much lighter than it actually is.

   

## Stage 4: The "Maximum Flow" Upgrade

Stage 4 is the highest level of factory-supported performance. While Stage 3 makes the "lungs" bigger, Stage 4 makes the "throat" and "nose" bigger too. It is designed for riders who want maximum horsepower and like to run the bike hard toward the redline.

• Key Components:

  • CNC-Ported Cylinder Heads: The stock heads are replaced with high-flow versions that have been precision-machined for better airflow.

     

  • Larger Throttle Body: A massive intake (often 64mm or larger) to feed the engine more air.

     

  • High-Flow Fuel Injectors: To keep up with the massive increase in air.

  • Highest-Lift Camshaft: A racing-style cam that pushes the power band into the higher RPM range.

• The Result: This is where you see the biggest horsepower gains (up to 40–50% over stock). It turns the bike into a "hot rod." Note that because the power is shifted to the higher RPMs, you may actually lose a tiny bit of that "low-end grunt" compared to a Stage 3, but the top-end speed is unrivaled.

   

### Quick Comparison Table

A Note on Reliability: Both stages typically require a High-Capacity Oil Pump and a Heavy Duty Clutch to handle the extra stress. Also, because these are internal modifications, they require a professional tune to ensure the engine doesn't run too lean and overheat.