The function of a 2 stroke power valve

The power delivery of a 2 stroke dirt bike is completely different to that of a 4 stroke dirt bike. This is partly due to the power valve.

How a 2 stroke motor works

A stroke is the action of the piston traveling the full length of the cylinder. In a 2 stroke engine, one of the two strokes combines the intake stroke and the compression stroke, while the other stroke combines the combustion stroke and the exhaust stroke.

As the piston travels upward in the cylinder, it creates low-pressure area in the crankcase, which draws fresh air and sprays fuel from the carburetor through a hole in the cylinder wall. While the piston continues traveling upward, transfer ports and the exhaust ports are closed off, which traps the combustible mixture in the combustion chamber. As the piston reaches the top of the cylinder, the mixture in the cylinder is compressed to the point of ignition.

The second stroke begins once ignition has taken place. The power stroke begins after the air/fuel mixture is ignited. The burnt fuel creates pressure in the cylinder above the piston and forces it downward. As the piston passes the middle point of the downstroke, the exhaust port to the side of the cylinder is uncovered and initiates the flow of burned fuel out into the expansion chamber through the manifold.

The piston then moves downward, where the air-fuel mixture remains from the previous intake-compression stroke. Shortly after the exhaust port is uncovered by the downward travel of the piston, the transfer ports begin to open. The transfer ports act as a passage through which the air/fuel mixture moves from the crankcase into the cylinder above the piston. Once the piston reaches the bottom of the stroke, the second stroke is complete and the process gets repeated.

What is a power valve

A power valve is a movable flap located on the cylinder exhaust port of a 2 stroke engine whereby the burnt gasses are flushed out of the cylinder. When the bike idles at lower RPM, the power valve flap is held partially closed to make the exhaust port smaller, which in turn helps the engine generate more torque at low speed. When you start to open the throttle and the revs build up, the flap gradually opens, expanding the size of the exhaust outlet, which creates greater top end power. This change in exhaust port size is what gives your 2 stroke engine more broad, manageable power.

Adjustable power valve

The power delivery characteristics of a 2 stroke engine get determined by the speed and timing at which the power valve opens.

With more modern 2 stroke bikes, specifically KTM bikes, you can adjust the power valve to suit your needs. You get various springs, which comes in different lengths and thicknesses, to control how quickly or slowly to power valve opens.

The power valve is held closed with two springs and exhaust gasses force it open when you twist the throttle.

If you change the auxiliary to the red spring, the exhaust flap will open quickly (at lower RPM). Yellow is the stock spring, which will make the exhaust flap open more gradually. The green spring will open the exhaust flap quite slowly (at higher RPM).

KTM Power valve springs

 

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