Chainsaw Repair

Intake port duration

Controlling factors: crankcase compression, carb size and type, intake manifold, air cleaner

The intake port consumes the fresh fuel air mixture the engine uses to produce its power. The port duration determines the power and RPM potential of the engine. The problem with piston port intake systems is the timing is symmetrical. The sooner you open the port, the later it closes.

An example of this is an old piston port 125cc race motorcycle that may have a port opening around 90' BTDC (the piston is on the up stroke) and has had plenty of time to build up vacuum to suck in the fuel air mixture. The problem is on the down stroke. The engine has had 90' of time to build up pressure inside the crankcase and it wants out. With the intake port still open, a back flow through the intake port at low and possibly the mid RPM range. The back flow will stop when the incoming fuel air mixture velocity is greater than the back flow. Any engine with that much intake timing would have terrible bottom end power, some engines may not even be able to start running.

The later the engine closes the intake port, the shorter the time (in degrees) the engine has to build up pressure to send the fuel through the transfer ports. This is "Crankcase Pressure Time". Increases in intake port timing decreases the CPT. Some engines have such low intake port timing and high CPT that small changes with intake port timing can make a significant difference. The intake port must be a balance of the need for power @ a given RPM vs. torque required for a satisfactory low & mid range power.


As a general rule: timings with openings less than 68' BTDC make good torque, openings from 68' to 75' BTDC have good overall power and openings from 76' to 82' BTDC can have substantially better top end with a corresponding loss of bottom end.

Joe aka 1manband is exploring this torque vs. high-end right now in another thread.

So depending on where in the rev band you are seeking advantage will either increase or decrease the impact of Intake Port Duration on the performance of the engine.