[Law]

Originally Posted by Blipit_

Short answer...engine homolagation rules.

They had no plans to start a engine program based on the S55.

Originally Posted by doogee

Larger displacement = more torque and less stress.

The S55 wouldn't perform nearly as well, which is a perfectly good reason not to use the M4.

Believe it or not, BMW races to win races too, it isn't all marketing...there isn't any point running an inferior car that will sit at the back of the field.

I'm sure they chose what would work best for them. It's racing, not a parade.


Seems like you are forgetting these :

Yes & yes.
A 3.0L inline-six just wouldn't be competitive in the present day and BMW hasn't competed with its straight-sixes in this category of racing since before the E46 M3 GTR.
Not only is there a higher performance threshold with a larger engine, but also, different classes have different air-restrictor, horsepower, etc. regulations so having a 4.4L engine to work with makes for a more versatile and ultimately more reliable setup.

People want to talk about racing pedigree, but the "M6" & "6-series" moniker has more pedigree than the "M4", which has close to none.

Originally Posted by Dalko43

I guess I need to read up on the homolagation rules. Their previous Z4 used a V8 derived from the S65, even though the S65 was never placed in the Z4 on the consumer side (at least not in mass production).

Still, it's kind of disappointing. The M6 traditionally never had a racing heritage whereas the M3 did. It would be much more awesome to see a M3/M4 racing in IMSA than the M6. Maybe the M6 has a slightly more optimal wheel base, but I got to believe that the M4 would be a better handling car overall just based on size alone, right? I mean the reason most car enthusiasts watch sports car racing is because they want to see their favorite street cars performing on the track. Corvette owners watch the C7R (which I honestly think is one the best looking cars out there), Porsche owners watch the 911, ect....how many M6 owners are really going to care about the M6 racing in IMSA? Not that it doesn't look cool, but the M4 is the enthusiast's car in the M lineup....it should be the one racing in IMSA.

First of all, you're incorrect about the M6/6er.
The E24 M6(35CSi) had been chalking up victories since before the M3 even existed; European Touring Car Championship, Deutsche Tourenwagen Meisterschaft, Japanese Touring Car Championship, 24 Hours Nürburgring, just to name a few.
Here's the E24 DTM that won the 1984 championship:

Originally Posted by Blipit_

Using the P65 in the Z4 was not done under homolation rules, they had to ask for concessions. I don't think they were in the position to ask for more concessions with a brand new car. Plus using a S63 based engine in order to go GT racing with the M4 wouldn't look too good.

Regarding homologation, Blipit_ 's explanation is on the right track, but i'll elaborate.
While the E89 Z4 was never offered for sale with an S65, the homologation rules technically failed to directly state that the engine and chassis must be offered in the same road-going product (i.e., road-going Z4 with S65 engine).

To qualify, the regulations presented a quota for the engine and chassis that a car manufacturer must meet in road-going production, which the S65B40 for the P65B40 (and later S65B44 for the P65B44 via M3 GTS/CRT), and E89 Z4 chassis met.
But the rules fell short in specifying that the engine AND chassis must be the same product.
Obviously it was implied, because every other manufacturer was homologated with good faith. But BMW found a loophole where they could exploit the regulations to enter its then smallest chassis, the E89 Z4, installed with the E9X M3 engine in racing.

Now this isn't the first time in recent memory that BMW bent rules in racing.
Facing tough competition from Porsche, BMW prematurely retired the S54-homologated P54B32 and entered the V8-powered E46 M3 GTR. Per homologation rules, the car must have a street equivalent for sale on two continents within 12 months, so BMW literally did just that, put a few up "for sale" in Europe and N. America. Of course, Porsche wasn't oblivious to this loophole and neither was the FIA, so rules were amended the following season, raising the minimum to 50 units, and BMW pulled out. It was a great car and a great season, but BMW was borderline cheating & they knew it.
In fact, only 3 M3 GTRs are known to have been "sold", all of which were magically returned to BMW M's collection in Germany in the years following




This time around, perhaps under pressure, BMW is actually playing by the rules of homologation, entering a chassis and engine combo that one can actually buy as a street car.
And that is what homologation is all about.