For the best results the gas must be completely ignited at the point of maximum compression, and the pressure must be established on the dead center, so that the indicator card will show a straight and vertical combustion line. As all gases require a certain length of time in which to burn, the ignition should have =LEAD=, that is, should be started before the end of the stroke so that combustion will be complete at dead center. The amount of ignition lead required depends on the fuel and the compression. In Fig. 10 the point of ignition (I) is shown as occurring before the end of the compression at (C), which insures a straight combustion line CD. With a lean or slow burning gas, that is, a gas slower than used on the diagram, combustion would not be complete at the end of the stroke if the same point of ignition were used. This effect is shown by Fig. (11), in which the full line diagram BCDE represents the ideal diagram (Y), and BCFG represents the slow burning mixture with the same point of ignition (X). The compression curves of both diagrams are coincident as far as C, the ideal diagram shooting straight up at this point and the weak mixture diagram staying at the same level. When under the influence of the mixture (X) the piston starts from left to right and reaches the point F before the slow burning gas reaches its maximum pressure. During this part of the stroke there has been very little pressure on the piston and it will be noticed that the maximum pressure is far below that of the ideal diagram. This low maximum is due principally to the reduced compression under which the gas has been burning, from C to F. [Illustration: Figs. 13–14. The First Diagram (13) Shows a Two Port Two Stroke Diagram, the Second Shows a Typical Diesel Card. ] As the gas has but a small part of the stroke left in which to expand, the pressure at the point of release is much higher than the release pressure of the ideal diagram, which means that a considerable amount of heat and pressure have been wasted through the exhaust pipe. Besides the heat loss, the high temperature of the escaping gas has a bad effect on the exhaust valve and passage. The great volume of gas passing through the exhaust valve also increases the back pressure on the scavenging stroke.