AMG started in 67', and offered engine tuning beginning in the mid 70'.
The Daimler Benz "middleclass" Limousine of those days was the W 114/115 called /8, the W 114 was the line with the more powerful engines.
In 67' Daimler Benz offered the W 114 with three old-fashioned carburettor engines, the inline six cylinder types M 180.954 {2.292 cm?}; M 114.920 {2.496 cm?} and M 130.923 {2.778 cm?}.
AMG especially offered engine tuning on the high end, means the M 100/116/117 {V 8 3 ½ - 6.8 Litre} engines.
In Spring 1972 Daimler Benz started the production of the W 114.060/.062 and C 114.042/.073 with the new M 110 2.746 cm? twin cam inline six cylinder engine.
These new inline six cylinder engines, designed like older famous competition engines like Maserati, Jaguar, Osca and Alfa, have had enough potential to be tuned as well.
AMG based their medium sector activity on those new twin cam 2.8 Litre engines and heavily modified the carburettor sohc M 130.
They used the same two types of tuning for the M 110.921 and M 110.981 engines, used Bosch D Jetronic indirect fuel injection on the M 110.921 instead the Solex 4 A 1 carburettor originally mounted.
The "first tuning stage" was to replace the cams by AMG cams and to modify the valve levers.
The ignition point was changed.
The power was increased to 195 hp @ 6.000 rpm.
The "second tuning stage" reflected much more impact on the basic design. Using the "stage one tuning", the cylinder head was removed an the intake manifolds and exhaust passage extended and polished, valves and combustion chambers have been reworked.
The exhaust manifold was replaced and the inlet pipe modified.
Such engines reached 210 hp @ 6.300 rpm, the torque was increased to 250 Nm @ 5.000 rpm.
Later a "stage three tuning" was offered {in my opinion after the W 114 has stopped and the W 123 was offered}, increasing the compression ratio, replacing the exhaust and inlet pipes, using larger valves and valve springs those engines reached >225 hp.
The M 130.923 was tuned equal to the M 110 "stage three tuning". Carburettors have been replaced by the Bosch fuel injection.
These single cam M 130 reached the power of the serial M 110.981 185 hp @ 6.000 U/min.
AMG offered suspension kits and used larger rims on those cars, it was essential to rework the suspension after engine tuning, but those kits have been sold independent from engine tuning. An the other hand the seats sometimes have been replaced by Recaros.