Back in the 1980s and the 1990s, tuning a car was a lot harder than it is today. Most of the time it required some actual knowledge of how to mechanically squeeze more power out of an engine, without simply changing the ECU map. When it came to BMW models, few did it as well as the guys from Hartge. They created some really nice kit for a wide variety of BMW models and some of them are one of a kind builds, like the Hartge F1 we showed you not long ago.

This BMW 635CSi was yet another one of their rare builds. The 635CSi was just shy of the M6, or M635CSi as it was called in Europe, in terms of power and refinement. It used a different engine, the M30 inline-six, not one derived from the great M88 used by the M1.

The 3.4liter unit developed different amounts of power, depending on the year. These values ranged from 208 HP to 217 HP on the most powerful units, built between 1987 and 1989. In this case though, the car we’re looking at was built in 1983 which means it was using the M30B34 engine good for 215 HP and 229 lb-ft of torque.

Thanks to the Hartge treatment its power output was probably raised to about 250 HP and some 335 lb-ft of torque as it was capable of reaching a top speed of 240 km/h.

The complete kit included not just the engine upgrade but also a new exhaust, new wheels, a new suspension and even an add-on strut bar in the engine bay. Inside the cabin you can notice all the small touches Hartge brought to the table, like a new steering wheel, new floor mats, new gearshift knob and so on.

This car is rare and I do mean rare in the strongest way. Finding any info about it is borderline impossible. It also looks in decent shape and has nearly 76,000 km on the clock. The best part? It’s going under the hammer at RM Sotheby’s Essen auction this summer.