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Old 01-04-2012, 05:33 PM   #1
bikecat OP
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R80 v R100 engines ... how are they different?

Hi,

Apart from the cylinder sizes, there seems to be some views that the post-1985 R80 engines are smoother than the R100.

Is that really so and if so, why?

Appreciate the input, especially from riders who had ridden both.

Cheers
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Old 01-04-2012, 05:47 PM   #2
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Quote:
Originally Posted by bikecat View Post
Hi,

Apart from the cylinder sizes, there seems to be some views that the post-1985 R80 engines are smoother than the R100.

Is that really so and if so, why?

Appreciate the input, especially from riders who had ridden both.

Cheers
I have a '77-8 R100. I have an 85 R80. I also have a newly acquired 91 R100.

The 77 R100 is the roughest ... but it's still very smooth by most standards. The 85 is pure frickin' silk. The engine and tranny in it is matched and perfect. It's crazy how smooth it is.

The 91 is extremely smooth. Close to the R80 but not quite.

I've heard that the reason is in the weight matching/balancing of the pistons and connecting rods. But I really don't know. It's quite possible that the manufacturing process for the R80 stuff was tighter than the R100 stuff. I do know that my 91 R100 was 'gone through' and the guy who did it made sure that the pistons and connecting rods weighed within a gram of each other ... and he did say that he had to take off some weight from the right one.

This really is hearsay, FYI. I've heard a lot of things, but have very little first hand evidence of any of it. These litterally are my first three airheads, and I'm still climbing up the learning curve.
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Old 01-04-2012, 06:44 PM   #3
bmwrench
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Almost all R80s are smoother than R100s due to less reciprocating weight. Interestingly, I've found far more R80s with significant imbalance than R100s, leading me to believe that BMW tried harder on the big motors.
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Old 01-04-2012, 06:56 PM   #4
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I've only had one 800cc motor and it was very smooth. On the other hand I've had three 1000cc motors and they seems less smooth, but still fine. I mean, these are air cooled opposed twins. Want smooth? Ride an inline-4 Yamaha R1. You can be going 150 and the engine sounds like an electric motor.

To me, IMHO only, smooth on a BMW is sort of an oxymoron. Even comparing a K75 to any boxer is strange. Smooth is not the word that comes to mind when I ride a boxer.

I do like it because it has "character". I think tuning (carbs, timing, etc..) on an airhead can make things a lot smoother. I've ridden a few poorly tuned airheads (mine included) and they felt nasty.

Tom
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Old 01-04-2012, 07:42 PM   #5
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I just do this for a hobby....but I can make the R100 just as smooth as an R80. From the factory, I've seen as much as 8 grams difference between the sides. Yes, less weight on the R80 top end means less mass to vibrate..but if you can find a good speed shop and tell them to get both sides with 1/10 of a gram and balance the carbs properly, both can be as smooth as possible for an airhead. Still, even after all that, the first time you fire up a K75 will shock you with the lack of vibration..even over an inline 4.
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Old 01-04-2012, 07:50 PM   #6
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Old 01-04-2012, 08:28 PM   #7
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Clutch + Flywheel

Since the BMW airhead clutch is a single plate dry clutch w/ a relatively large flywheel (like a car) I'd bet it'd be worth the extra bucks to get this assembly balanced when doing a rebuild.

Years ago I had my local machine shop balance the clutch & flywheel assembly on my '64 Volvo 122S...and it made a world of difference in how smooth the car felt at idle.

Claytonroy screwed with this post 01-04-2012 at 08:42 PM
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Old 01-04-2012, 11:02 PM   #8
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I guess it goes to show how it depends on what you're comparing it to. I had a KLR and a Sportster before the G/S. It was like going from a paintshaker to a sewing machine. The r100s I've ridden do shake more. I've heard the 980cc Siebenrock kits are super smooth since the pistons weigh so much less than stock.
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Old 01-05-2012, 06:10 AM   #9
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My '84 R80 is extremely smooth as well.

It is smoother than any of my various R100's. I have never had the chance to ride a R65 how do they compare to the R80 and R100? Does the R80 just physically just come together better?
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Old 01-05-2012, 01:05 PM   #10
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Quote:
Originally Posted by Infracaninophile View Post
I've only had one 800cc motor and it was very smooth. On the other hand I've had three 1000cc motors and they seems less smooth, but still fine. I mean, these are air cooled opposed twins. Want smooth? Ride an inline-4 Yamaha R1. You can be going 150 and the engine sounds like an electric motor.

To me, IMHO only, smooth on a BMW is sort of an oxymoron. Even comparing a K75 to any boxer is strange. Smooth is not the word that comes to mind when I ride a boxer.

I do like it because it has "character". I think tuning (carbs, timing, etc..) on an airhead can make things a lot smoother. I've ridden a few poorly tuned airheads (mine included) and they felt nasty.

Tom
+1, what he said
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Old 01-05-2012, 01:31 PM   #11
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Going the other way.....are the 600cc boxers any smoother?

I've never owned one (or ridden one) so I don't have a point of reference. I would bet the same crankcase, crank shaft etc. was used on the 600cc bikes as on the 800cc ones, but the pistons would have less reciprocating mass = less vibration.

If memory serves me there was also a 450cc Boxer wasn't there?
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Old 01-05-2012, 01:35 PM   #12
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Quote:
Originally Posted by Grayghost66 View Post
Going the other way.....are the 600cc boxers any smoother?

I've never owned one (or ridden one) so I don't have a point of reference. I would bet the same crankcase, crank shaft etc. was used on the 600cc bikes as on the 800cc ones, but the pistons would have less reciprocating mass = less vibration.

If memory serves me there was also a 450cc Boxer wasn't there?
The 600's are smooth ... about as smooth as the 800's ... or at least I don't remember them being so different to notice. The 450cc was the smaller displacement version of the 650cc. I have no experience with either of those beasts.
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Old 01-05-2012, 01:59 PM   #13
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My R80 G/S Mono has a Siebenrock 1000cc kit on, I've just machined the squish band steps off the pistons and opened the heads out to 1000 cc and machined the squish steps off, then shortened the barrels to put the compression ratio back to 9.5 :1. The difference is quite surprising, it ticks over like a sewing machine, the mixture screws are now very sensitive and sooo much smother through the rev range..........................time well spent!
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Old 01-05-2012, 03:04 PM   #14
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800's have around 9mm smaller bores and 2mm smaller exhaust valves. Some say they are smoother for their smaller pistons but I never hear that about 750's, 650's, 600's, 500's, and 450's. Maybe the R80 pistons work out right with the crank weights at the speed most cruise at? I have test ridden tons back to back. I can't tell that much difference in vibration compared to the difference in power but I suspect I cruise at a higher rpm than most. There is a big diff in power. Overall I guess they are a tad smoother but I miss the power of the liter bikes too much to care. The engines are the exact same size on the outside so I want the biggest bore that works well and I think that is 94mm and 980cc. I like R65's but the cylinders and heads are much smaller externally making for a much smaller feeling engine. There are hogged out 450's to the exact bore to stroke ratio of a R100: 1.33:1 which are what I call a hogged out 900. A R80 is 1.2:1.

The Siebenrock pistons I put in my R100 are barely lighter than the stock pistons. Most of the advertised weight savings is in the shorter wrist pins and then they take that not very impressive figure and add both sides together. You can go a LOT lighter than the Siebenrock setup with Venolia's and wrist pins like Luftmeister use to sell.
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Old 01-05-2012, 03:59 PM   #15
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The R65 block bikes can be quite smooth. Shorter stroke, smallish pistons.

There really isn't anything that come close to a K75 in terms of smoothness, except perhaps the K1600 or a CBX1000. I-4 engines need a balance shaft for second order vibrations (the type that fuzz your mirrors and make your hands go numb). V4's are quite smooth as well. Boxer 4's are good, and boxer 6's are perfect. The K75 is half of an H6, with the other half simulated with a counterbalancer.
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