Sacrilege! This Mercedes-Benz has a BMW engine...

At first sight, it appears that this is a bog-standard W124 generation Mercedes-Benz 300E. But, it is anything but ordinary because instead of featuring a Mercedes engine under the bonnet, it is in fact powered by a motor from its biggest rival - BMW...

Engine swaps are very common and are generally done to improve upon the recipe a manufacturer had originally put together. However, what we have here might just be the most contentious we have ever seen - and it isn't even new...

It was done by BMW tuner Hartge who were known for several ambitious projects back in the day. They wanted to test the chassis of the 1988 model 300E which made 177 horsepower from the factory. So, they dropped in a BMW straight-six engine into the bay to create the Hartge F1...

It got the the legendary BMW M88 engine that was used in various specs on the BMW M1 and E28 M5 but was bored from 3,452cc to 3,535cc to deliver 325 horses. It was mated to manual gearbox from an E24-generation 6 Series to direct power to the rear axle.

It was also fitted with new wheels, brakes, bushings and a Bilstein sports suspension to help turn the Hartge F1 into a properly capable performance saloon.

Cross-swaps between the two German giants are an uncommon notion and this Frankenstein-like monster sure raised eyebrows in the Eighties and continues to do so today. It has now come up for auction with RM Sotheby's and if you have always wanted classy Mercedes looks with BMW power then now is your chance...

But remember, when you pop open the bonnet at car meets, you'll have some serious explaining to do; in the BMW world, the M88 engine is iconic - which makes it even more strange to find it in the bay of its biggest rival. It will go under the hammer in Essen on June 24.