When it comes to engine performance there are many formulas that engine builders use to get the most out of their power plants. The most common engine modifications are stroking or boring the motor .These two modifications are...

the best way to increase the horsepower and torque of the stock engine .Horsepower of a stock Twin Cam is around 62hp, this might not sound like a lot of power but compared to the Evo motor at 45 hp it is quite a jump. Now some people might say that stock is fine for the way they ride. I’m not one of those people, I need power and torque, now this doesn’t come without a price. I’m not talking dollars, I’m talking engine wear.

 Increasing the engine displacement allows the motor to breath in more air/fuel mixture during each cycle for greater torque and horsepower. The engine will produce higher torque at a lower RPM .There are certain mechanical limitations to an engine. The engine has what is called frictional losses; the largest source of engine friction is attributed to piston ring drag on the cylinder walls. There is also a loss due to push rod angle.

 Let’s look at the Stroker motor first. The term Stroker means increasing the pistons stroke by changing the flywheels so the crankpin is closer to the outer edge of the flywheels. This will cause the piston to travel farther up and down the cylinder. Stroker pistons travel at a faster rate than the stock pistons, this faster speed leads to increased heat. 

 Using larger bore cylinders and pistons is what the term “Big Bore” means. This is a good way to increase engine displacement. The thing to look out for when boring a motor is the possibility of weakening the stock cases. The maximum bore for stock Twin Cam cases is 4in. Any bore larger than this would require aftermarket cases. 

Positive and negative effects of...

BIG BORE 

+Increase displacement
+Larger piston area
+More squish band
+More high RPM hp
-Weaken stock cases

STROKER

+ Increase displacement
+ Increase leverage factor, more torque thru out RPM
- Increase piston speed
 -Increase piston &ring wear