 The swept volume of the engine remains the same, but as the volume of the combustion chamber at top dead centre is changed by adjusting the contra-piston, the compression ratio (swept volume + combustion chamber / combustion chamber) changes accordingly.Model diesels are found to produce more torque than glow engines of the same displacement, and are thought to get better fuel efficiency, because the same power is produced at a lower rpm, and in a smaller displacement engine.However, the specific power may not be significantly superior to a glow engine, due to the heavier construction needed to assure that the engine can withstand the much higher compression ratio, sometimes reaching 30