An existing 4 stroke-cycle gasoline engine has been partially modified without dynamically changing its mechanism for the purpose to utilize the compressed air as an alternative energy source. The principle is to mechanically control the compressed air flow through the intake and exhaust valve every revolution of a crankshaft by modifying the camshaft cam's lobes, which changes the engine operation from 4 strokes to 2 strokes cycle mode. In the previous investigation the principle was verified with a current 50 cc motorcycle modified engine and the pressure behavior inside the cylinder of the compressed air engine was evaluated. It turned out as a most promising result that the back pressure, which is defined as the positive pressure left in an expansion chamber while a piston is moving back from BDC to TDC, has a high influence on the performance of the compressed air reciprocating engine. Therefore, the pressure measurement was conducted for the modified engine assembled the cylinder with side holes drilled to relieve the back pressure so as to verify an effect on the performance. In this investigation the same pressure and performance measurement were conducted for the same engine assembled the camshaft of different cam profile in reverse air-flow direction, thus confirming an effect of the valve size on the back pressure reduction to evaluate the performance of the compressed air reciprocating engine.