The general philosophy I apply to my life is to buy the best and then do everything I can to make it better. As a BMW owner—528e, 635 Euro, M6, and M1 in pro car trim—cars are my life. It is my passion to not only improve my own cars, but every BMW that crosses my path.
Presently I have been running into trouble with making my cars go as fast as I want them to, for I know they should be able to without failure. Such failures are due to unnecessary design problems.
According to the M6 owner’s manual, the reason the fuel supply is shut off in the engine at approximately 6500 to 6900 rpm is “it serves no useful purpose to drive at this maximum engine speed, since power output does not increase beyond 6500 rpm.” I find the phrase “no useful purpose” to be relative in the hands of the driver and his/her specialist. In other words, we have ways of increasing power output beyond 6500 with the installation of a computer chip.
However, it is what happens after that envelope has been punched that is the source of frustration and heartache. Driving past the “no useful purpose” zone is useless if your crankshaft self-destructs after crossing the barrier. (An event I have had the agony of experiencing three times in one year. My M6 mangled three crank hubs and three crankshafts.)
For instance, my test M6 engine has been so modified as to live in the “no useful purpose” zone often at 7500 rpm. What failed each time was the crank hub itself. It just couldn’t take the heat. All three times the crank hub oscillated on the crankshaft so much that the two parts spot welded and traded metal, tearing out the keyway. After rockwelling, I discovered the crank hub, in which I had entrusted my car and my life, was manufactured from a mild steel which only rockwells at 15 on the C scale. In layman’s terms, it had the integrity of a butter knife. Even new cranks taken right out of stock also rockwelled at 15; the metal was not heat treatable.
Armed with knowledge experience, I spoke with the head of BMW Motorsports Division in Germany. After first having to pass a verbal examination, in which I am very technically detailed my tale. He simply recited the “no useful purpose” dogma. He further suggested my predicament was inevitable for having punched the barrier. He pointed out that even if I didn’t, surely a larger safety margin than 100 rpm should have been built in for those who push their zones to the 6500 to 6900 rev limiter maximum. After pausing, he gathered all my points, promised to discuss them with his team and get back to me.
I have yet to receive a call, making the entire exchange pointless.
My solution was, as usual, to build it myself. I fabricated a crank hub out of alloyed steel which after heat treated rockwells at 38. It is still holding its own after thousands of miles of hard testing along with hundreds of other crank hubs.
Other drivers of the “no useful purpose” zone have also lost their crankshafts due to the failure of the crank hub. Merely tightening the nut to or past spec won’t prevent this problem. The hub can’t take the abuse of the extra pressure. It compresses the hub into a barrel shape limiting the shaft contact to the front and rear surfaces.
Where I think all my troubles with the crankshaft and crank hub began was with the harmonic balancer itself. In my constant efforts to find which parts are interchangeable between M and non-M motors, I discovered that the harmonic balancer in my M6 appears to be the same as the 633, 635, 535, 733, and 735 up to late the 1980’s. Also, while the part numbers are different and the prices almost double for the M parts, the crank hub and the nut are interchangeable between all the larger 6 cylinder. A groove machined on the outer diameter of the hub is the only difference. The M nut is identified with a V-90 stamp and a much higher price tag. Both nuts have the same fit, tolerance, and rockwell at the same rate on the C scale.
With a passion for fast BMW’s, I am not willing to relinquish my driving performance to the “no useful purpose” zone and I refuse to recognize the arbitrary setting of the borders. Until BMW redesigns the harmonic balancer, my survival gear of choice will be my stronger heat treated crank hub.
Every day I can count 7500 useful purposes for it being there.
“And that’s the way I see it.”
Frank Fahey
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