The term "Port Timing" Refers to the relationship between the ports being uncovered by the piston and the position of the flywheel. The size and location of the ports in a given cylinder are the largest basic factor determining it's performance. Port maps provide tuners a way of calculating port timing of different cylinders by giving them an accurate depiction of the port size and location in the cylinder bore. If the ports are positioned in the cylinder to be opened sooner in the flywheel's rotation it is considered to have an 'advanced' port timing , if it is opened later it is considered to have 'retarded' port timing. This can be altered to affect the performance of a piston ported two stroke engine.