BMW M has built some wild prototypes over the years, among them the E30 M3 pickup and a McLaren F1-powered X5. BMW is displaying old CSL prototypes never before seen as part of its M4 CSL celebration, and the Road & Track staff, largely comprised of hopeless BMW buffs, is losing it. This M5-powered M3 CSL prototype demonstrates why.

Han Rahn, head of prototyping at BMW M, says that the idea was to take the E46 M3 CSL a step further by installing the 5.0-liter S65 V-8 from the E39 M5.

There was a second hole in the grill to allow the V-8 fuel to pass through, but otherwise this is a stock E46 M3 CSL. Rahn explained that the V-8 made this vehicle heavier than a six-cylinder M3 CSL, however he did not seem to mind it, grinning from ear to ear the whole time.

We don't know for sure but we dare not ignore the possibility of this big, torquey V-8 replacing the E46 M3's rev-mad six-cylinder engine, which may or may not have substantially changed the nature of the car, but even then, we wouldn't refuse the chance.

This is especially true knowing how special the later V-8-driven E92 M3 feels in comparison.

The Rahn product line also includes CSL versions of the M5 and M6 from the E60 and E63 generations. Its purpose was to commemorate the 25th anniversary of the model by producing a new version of the famous 5.o-liter V-10 that ran at 9000 rpm. BMW M has equipped the V-10 with a full carbon fiber intake system, and it looks amazing.

Also of note is the fact that this M5 ditched the standard single-clutch automated manual gearbox for a prototype of the dual-clutch Getrag that later appeared in future M cars.

My bet is that E60 M5 owners would love to know what the car would feel like with a newer transmission, since the single-clutch 'box has been known to be slow and unrefined in its performance.

The M5 looks like it was built just for fun, but the E63 M6 CSL looks like it would be a very good production car.

The car was developed by BMW as a means of exploring active aerodynamics, the car has a deployable splitter and rear wing, and an interior that has been completely stripped. If this had been added to the original CSL 3.0, it would have been a perfect tribute to it.

At the very end of this video, BMW M teases a coming clip of more CSL prototypes, including a version to the old M2.

The three cars shown to me never made it to production, but being shown a never-built M2 CSL seems cruel to me. I suppose it is good to know there are more CSLs coming from BMW.