Big-Bore or Stroking the Engine, or Both?


Lots of people are asking themselves how to increase power, for some increasing the bore of the cylinder and cylinder head is the answer. It is true big-bore pistons can produce more horsepower at a very low cost. But sometimes taking doing a bore-up means the thickness of the cylinder walls become unsafe.

What are the alternatives for increasing the combustion volume? The answer is to stroke the engine. Once the piston is in motion, it will do more work if you let it more farther.

Instead of increasing the combustion dome area more and more, stroking an engine allows you to increase the depth of combustion.

Stroking an engine isn't easy. Moving the big-end crank pin off-center by as little as 0.5mm increases stroke 1mm. That means the piston travels up the cylinder an additional 0.5mm and comes down an additional 0.5mm closer to the crank. Obviously, it will hit stuff at both ends unless compromises are made.
On a four-stroke, that usually means running a longer rod to provide adequate piston-to-crank clearance and then spacing the cylinder up to accommodate the stroke increase. On a two-stroke, the cylinder head can be re-chambered to make room for the piston to come up higher into it.

Stroked engines are difficult to build, entail more parts and require machining. They also enter whole new tipping-point equations. To get the most from a stroked engine, you will often need to run an aftermarket cylinder that allows for an over-bore larger than 2mm. A properly stroked engine will cost five times more than a basic big bore kit, but it can double the power increase you get from just a big-bore alone.
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