Hartge based its H5s model on the E28 5 Series BMW by basically creating a 535i out of a 528i with improved rear suspension. The power of the 3.5-liter in-line fuel-injected six was lifted from 218 hp (160 kW) to 240 hp (177 kW) @ 5500 rpm. Torque was raised from 228.5 ft-lb (310 Nm) to 235.8 ft-lb (320 Nm), a figure which was delivered at an unchanged speed of 4000 rpm. The main emphasis of the work was on the cylinder-head, fuel distribution, ignition, and the exhaust manifold. Incorporating new springs, anti-roll bars, and shock absorbers, the car also benefited by lowering the suspension 3 cm. A long five-speed gearbox was also fitted in order to handle the extra power. Hartge had offered two different tire and wheel combinations. The classic Pirelli P7s (205/55 VR 16 front and 225/50 VR 16 rear) or 205/60 VR 15 P6s all round on BBS alloy wheels as one option. The other alternative was 200/60 VR 390 Michelin TRX tires and wheels.
As with all other cars of this breed, front and rear spoilers were standard fittings. Fully equipped, the Hartge H5s had a top speed of 142 mph (230 km/h) and accelerated from 0 to 100 km/h (62 mph) in 7 seconds. Hartge had also offered an engine kit, the H5, for the 528i that produced 210 hp (155 kW) instead of 184 hp (135 kW) and produced 199 ft-lb (270 Nm) torque at 4800 rpm.
From 1985 to 1988, Hartge produced 46 units that used the E28 535i as its base. These were the H5sp models. These models were offered with same 3.4 liter, six-cylinder engines that were used in the E28 535i but they were tuned for much greater performance using M88 individual throttle bodies. The H5sp produced 254 hp (187kW) @ 6100 rpm as opposed to the 535i’s 185 hp (136kW). 0 to 100 km/h (62 mph) was achieved in 6.6 seconds with a top speed of 150 mph (242 km/h).