There will be a reduction in the performance of the engine if there is a restriction in the air inlet system or the exhaust system.
The air flow through the air cleaner may have a restriction. The pressure at the restriction of the air flow must not exceed 6.25 kPa (25.0 inches of H2O).
Back pressure is the difference in the pressure between the exhaust at the outlet elbow and the atmospheric air. Back pressure from the exhaust must not be more than 5.0 kPa (20 inches of H2O).