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Old 04-19-2012, 10:09 AM   #31
lkchris
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Quote:
Originally Posted by Wirespokes View Post
I've run sans cross over and noticed no difference except for a more defined exhaust note. Not particularly louder, just undiluted. Sure makes it easier installing and removing the exhaust.

It's true that BMW is good at designing engines, but they also have contorted to environmental concerns and conformed to DEQ regulations. One of the tests they had to pass was noise, and the second crossover could very well have been due to that one.

Frankly, I like ditching the crossover.
BMW had crossovers in the 1960s--way before any "environmental" concerns.

Heck, it wasn't until 1981 that they got electronic ignition ... the best "environmental" improvement they ever did to an Airhead.

2nd crossover makes a 38mm system equal (or larger) in volume to a 40mm system

Guys just need to understand that it is 100% hype that aftermarket exhausts are an improvement worth the money. It's astounding how many experts have been born just by reading some advertising. It usually entails blaming the factory and blaming greenies and this thinking is as "conventional" and "me too" as the clothing found at Harley dealers.
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Old 04-19-2012, 12:53 PM   #32
robtg
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Quote:
Originally Posted by lkchris View Post
BMW had crossovers in the 1960s--way before any "environmental" concerns.

Heck, it wasn't until 1981 that they got electronic ignition ... the best "environmental" improvement they ever did to an Airhead.

2nd crossover makes a 38mm system equal (or larger) in volume to a 40mm system

Guys just need to understand that it is 100% hype that aftermarket exhausts are an improvement worth the money. It's astounding how many experts have been born just by reading some advertising. It usually entails blaming the factory and blaming greenies and this thinking is as "conventional" and "me too" as the clothing found at Harley dealers.

BMW "improvements" over the years to the exhaust system, intake system, and cylinder heads on the R100RS have netted a loss of at least 10hp, lower comfortable cruising speed, and a rougher motor.
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Old 04-19-2012, 01:17 PM   #33
bmweuro
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Quote:
Originally Posted by lkchris View Post
BMW had crossovers in the 1960s--way before any "environmental" concerns.

Heck, it wasn't until 1981 that they got electronic ignition ... the best "environmental" improvement they ever did to an Airhead.

2nd crossover makes a 38mm system equal (or larger) in volume to a 40mm system

Guys just need to understand that it is 100% hype that aftermarket exhausts are an improvement worth the money. It's astounding how many experts have been born just by reading some advertising. It usually entails blaming the factory and blaming greenies and this thinking is as "conventional" and "me too" as the clothing found at Harley dealers.
"2nd crossover makes a 38mm system equal (or larger) in volume to a 40mm system" Please explain the relevance?
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Old 04-19-2012, 04:16 PM   #34
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One of the 'improvements' robtg didn't mention was lower gas mileage as well. Go figure - EPA improvements should decrease fuel consumption, not increase it!

I'd also like to understand how crossovers increase the effective diameter of the exhaust system. With exhaust velocities what they are, I'm sure above 2K RPM all the exhaust 'sees' is added turbulence - especially since the Xover is at right angles to the flow.

I thought the /2 didn't have a crossover.
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Old 04-19-2012, 05:25 PM   #35
ericrat
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Various thoughts

I haven't seen a pre-70 BMW with a crossover pipe.

I would think the primary performance advantage to the crossover pipe would be to aid in the scavenging effect. I am sort of thinking on the fly here, but bear with me (sometime this gets me in trouble). Cylinder 1 is on the exhaust stroke. Fast moving exhaust is steaming past the crossover pipe, this creates a slight vacuum in the crossover pipe. Just as blowing over the top of a straw in a glass of water picks up water out of the glass. Cylinder 2 is coming up on the compression stroke. Cam overlap still has the intake cam open just as the cylinder is begining to rise. Exhaust velocity is probably relatively high at this point since clyinder pressure is high as the begining of the exhaust valve opening.

At this point the vacuum effect of the crossover pipe helps draw more fresh mixture into the other cylinder, scavening the exhaust out of the combustion chamber. This increases volumetric efficency making MORE POWER.

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Old 04-19-2012, 05:50 PM   #36
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pre-1970 crossover pipe

"I haven't seen a pre-70 BMW with a crossover pipe."

Sorry to rain on your parade, but my 1954 R-68 with the original exhaust system has a crossover pipe.
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Old 04-19-2012, 09:32 PM   #37
ericrat
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Quote:
Originally Posted by slmyers View Post
"I haven't seen a pre-70 BMW with a crossover pipe."

Sorry to rain on your parade, but my 1954 R-68 with the original exhaust system has a crossover pipe.
No parade extent. As a midwest rider, rain perfectly tolerated. I have a certain limited, but deep, experience with 55-59 models, mostly applicable up to 69 which may not be useful and is certainly not complete.

Huh, I may have a lot to learn about what years have crossover. Remaining pro crossover.

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Old 04-19-2012, 10:02 PM   #38
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For a better explanation of the crossover by Professor Wizard (David Visard) see his explanation in the Mustang forum:
http://mustangforums.com/forum/2005-...explained.html
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