?? Variable Valves are Bad for the BMW R1250GS - Review ?? - BMW Luxury Touring Community
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post #1 of 5 Old Mar 30th, 2020, 9:38 pm Thread Starter
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?? Variable Valves are Bad for the BMW R1250GS - Review ??

Hi all. Does anyone have a comment on the recent post by Ryan @ FortNine

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post #2 of 5 Old Mar 30th, 2020, 10:24 pm
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Re: ?? Variable Valves are Bad for the BMW R1250GS - Review ??

IMO he was saying he was disappointed that BMW is using old technology from another company and not coming up with something more advanced. He did like the low end torque and the high end but not the surge in the middle. An honest review I thought. He does ride an 850gs.


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post #3 of 5 Old Apr 1st, 2020, 1:15 pm
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Re: ?? Variable Valves are Bad for the BMW R1250GS - Review ??

He does a good job of explaining why overlap should change as speed increases. He even illustrates that changing overlap (duration) means changing the cam profile. Then he goes on to diss BMW's choice of having two separate profiles and implies Suzuki's solution is better, even though it only changes valve timing. What?

Cam profile controls timing (when it opens), duration (how long it's open), and lift (how far it's open). Two cams on the same shaft can have two profiles optimized for two different speed regimes. Is it as good as optimizing for every possible combination of speed and load? No, but doing that would require doing away with a camshaft altogether in favor of direct computer control of the valve actuators. Imagine the grief BMW would get from the "I'd rather have mechanical <xxx> than computer controlled <xxx>" crowd if they did that!
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post #4 of 5 Old Apr 7th, 2020, 4:40 pm
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Re: ?? Variable Valves are Bad for the BMW R1250GS - Review ??

Personally I think the the guy is full of crap. Technically he right about many things, but who cares? The bottom line is, does shift-cam make the the motor better? The answer is a resounding YES. Why?

- Much smoothing running at low RPMs because of more complete, stable combustion.
- Great low-end and mid-range torque where 90% + of riding is done.
- Better fuel economy. My R1250RT averages 47MPG vs the 42 my old 2005 R1200RT got.
- Hauls ass when you want it to.
- One of the cleanest running motorcycles you can buy.
- Shift-cam is relatively simple when compared to other VVT systems and it's all accessible in minutes. Easily maintained by lay-people. Even having a valve adjustment done by a dealer is less than $200. A K1600 is valve CHECK is $1500 to $1600.
- Drive-ability. The large amount of torque everywhere makes riding my RT in town or in the mountain twisties a joy. Less shifting and that satisfying rush of power as you exit a turn.

Shift-cam is a compromise between complexity and simplicity/lower cost. Sometimes compromises are wonderful

Scott T
McKinney, Texas
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post #5 of 5 Old Apr 8th, 2020, 5:16 am Thread Starter
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Re: ?? Variable Valves are Bad for the BMW R1250GS - Review ??

Quote:
Originally Posted by taran1900 View Post
. . . . . . . . Shift-cam is a compromise between complexity and simplicity/lower cost. Sometimes compromises are wonderful
You made some useful points there . . . thanks
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