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      05-21-2011, 05:46 PM   #184
bruce.augenstein@comcast.
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Quote:
Originally Posted by swamp2 View Post
Yes indeed they may have cleverly thought out the "trimming" operation and what balancing requirements might be required for a V6 derived from this V8. Although you can get by with no balancing on a 60º V6 you can't really on a 90º V6 (at least in a road car). And of course by "balancing" I mean dedicated balancing, not that on the crankshaft itself which every car has. Also a 60º V6 would require either massive modifications or an entirely different production line compared to the one making a 90º V8. Thus I highly doubt a 60º V6.
There is no need for dedicated balancing on a 90 degree V6. You can do that with standard counterweights.

What you lose is even firing intervals. Instead of a "standard" 120 degree firing interval, you get 90 - 150, 90 -150, 90 - 150. This makes for a buzzy and somewhat rough-running engine, which BMW simply wouldn't put up with.

The solution is to split the crank throws, which is what GM pioneered in 1977 (I think), resulting in an even-firing design of their 3.8 liter 90 degree V6. They sold this even-fire engine in various states of tune for at least a couple of decades, including those legendary Buick Grand National engines which ruled drag strips from the mid eighties through (roughly) the mid nineties.

One implication of splitting the crank throws was that the throws themselves would lose some strength because you'd end up with an effectively reduced diameter "at the split". A friend of mine ran his GN for years at the drag strip however, eventually running low 11s, with a best of 11.17 at around 120 mph. One thing he liked to brag about was that he had never even removed the valve covers to get those kinds of times and speeds.

He finally gave in and had the heads done, resulting in an instant 10.56 @ 124 mph, with the engine shutting down just before the traps. Turned out his chip tuner had forgotten to remove the from-the-factory 124 mph speed limiter. With that taken care of, the car would trap in the 125-126 mph range.

He sold the car with over 100K on the clock and hundreds and hundreds of drag strip passes, and never had a problem with that engine. His experience was more or less duplicated by hundreds of other GN owners, demonstrating just how bulletproof that engine was, in spite of the split crank throws.

So BMW could easily build a reliable split-crank 90 degree V6.

Would they, though? Don't think so. I'd be betting on a straight six with something innovative in turbocharging.

Bruce
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