Power Engineering and XR4x4's went hand in hand during the early days of the model. They were responsible for the Sprintex supercharged version, and when the supply of superchargers dried up, the Ultimax 3.7 litre conversion.

Supercharged
The Sprintex supercharged version of the XR4x4 produced about 250bhp and 305 lb/ft torque @ 3000 rpm. Superchargers were driven by a belt attached to the crankshaft pulley on the engine. This had the advantage over turbocharging in that the heat produced is significantly lower meaning the compression ratio can stay the same. The EEC-IV management system is also reworked, on the 2.9i version. Koni shock absorbers and revised spring were also fitted, along with upgraded brake pads. 0-60 time is reduced to about 6.2 seconds going on to a maximum of 145mph. In 1990 conversion of a 2.9i would cost about £4,000 for all changes mentioned above.

In all somewhere in the region of 80 cars were converted, with all but about 12 being 2.8i examples.

Ultimax Conversion
The Ultimax conversion came about due to the manufacturers of Sprintex superchargers going out of business. This left a gap in performance mods for the Cologne V6. Power came up with changes that would ultimately take the car up to a 3.7 litre capacity. This was achieved by taking the bore out to 95mm and lengthening the crankshaft stroke to 86mm giving an overall capacity of 3660cc. Pistons were replaced with Mahle versions, these had a revised compression ratio of 9.8:1. The engine was then balanced and blueprinted. Revised gas flowed cylinder heads which allowed better breathing but used the standard valves were also added. These were matched to the manifold port faces (the biggest problem with the Cologne had always been the siamesed ports). A reprofiled camshaft basically finished off the engine work. To this was added a free flow stainless steel exhaust.
Other improvements were also offered with the engine package. These included Koni dampers and progressive rated springs, polyurethane bushes and finally grooved disc brakes coupled to kevlar compound pads and braided steel and PTFE flexible brake lines.
Power Engineering offered the Ultimax conversion in four different stages. Stage 1, just the basic block assembly with standard heads, followed by Stage 2 which included fitting of the previously mentioned engine, offering 195bhp coupled with 240lb/ft torque. Stage 3 added a high torque cam coupled with standard heads, this gave 205bhp with 250lb/ft torque. Stage 4, also known as the Mach 1 Ultimax offered a hotter cam and modified cylinder heads with larger valves, three angle valve seats, special valve guides and modified ports and throats. This package gave 215bhp and 255 lb/ft torque. This engine first appeared in modified XR4i's but was more suited to the 4x4 as the power could be handled better. 0-60 was improved to about 6.6 seconds, acceleration through all gears noticeably quicker.

Power Engineering also offered an Ultimax conversion for the 2.9 24v Cosworth engine as fitted in the Scorpio. As this engine is becoming a popular conversion for the XR4x4 it is worth mentioning. On this engine the Ultimax conversion would give 265bhp coupled with 280lb/ft torque, an impressive figure for a normally aspirated engine.