The straight-4 engine is not a balanced configuration. While a pair of pistons is always moving up at the same time as the other pair is moving down, piston speed - as with all internal combustion engines - is higher through the top 180 degrees of the stroke than the bottom 180 degrees, which leads to a secondary harmonic imbalance. While this is tolerable in a small low-displacement low-power configuration, the vibrations get worse with increasing size and power. Most straight-4 engines below 2.0 L in displacement rely on the damping effect of their engine mounts. Today, most engineers will make use of balance shafts above that limit. A 4-cylinder engine needs two balance shafts, rotating at twice the crankshaft frequency, to be smooth. Nonetheless there were several samples of larger straight-4s in production using no balance shafts, such as the Citroën DS 23 2347 cc engine that was a derivative of the Traction Avant engine, and the 1948 Austin 2660 cc engine used in the Austin-Healey 100 and Austin Atlantic. These engines were generally the result of a long incremental evolution process and their power was kept relatively low regarding their capacity.