R80 v R100 engines ... how are they different?

Discussion in 'Airheads' started by bikecat, Jan 4, 2012.

  1. bikecat

    bikecat Long timer

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    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
    #1
  2. Beater

    Beater The Bavarian Butcher

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    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. :eek1

    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. :lol3
    #2
  3. bmwrench

    bmwrench Long timer

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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.
    #3
  4. Infracaninophile

    Infracaninophile Finding My Way..

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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
    #4
  5. Beemerguru

    Beemerguru Beemerguru...G/S guy

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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.
    #5
  6. bereahorn

    bereahorn Long timer

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    :lurk
    #6
  7. Claytonroy

    Claytonroy Been here awhile

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    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.
    #7
  8. bgoodsoil

    bgoodsoil Dare to be Stupid

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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.
    #8
  9. 97oilhead

    97oilhead Been here awhile

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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?
    #9
  10. subagon

    subagon Hopelessly lost

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    +1, what he said :thumb
    #10
  11. Grayghost66

    Grayghost66 Been here awhile

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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?
    #11
  12. Beater

    Beater The Bavarian Butcher

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    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.
    #12
  13. Arkwright

    Arkwright Grumpy ole Git

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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!
    #13
  14. supershaft

    supershaft because I can

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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.
    #14
  15. wmax351

    wmax351 Been here awhile

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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.
    #15
  16. puncar thogoole

    puncar thogoole Been here awhile

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    Don't forget the pre 81 /7s had heavier flywheels, it's often stated the later lighter flywheel bikes had more vibes (& faster acceleration).
    #16
  17. bikecat

    bikecat Long timer

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    Hi,

    Thanks for the feedback. I was prompted to ask after reading this on UKGSR:

    http://www.ukgser.com/forums/showthread.php?t=88688&page=4

    "I learned that only the most experienced mechanics worked on the Basic production line, indeed many of them wanted to be a part of the final limited run... I met few of them later over a couple of beers and it was great to hear them still talk with a very real fondness about that last air-cooled motor, the simple elegance of the steel frame and the racing heritage that final basic represents.

    I asked why the decision had been made to make it an 800 and not a 1000 motor and the unanimous answer I got was its the smoothest air-cooled boxer they ever made - any power advantage was negligible - I've ridden a couple of GS 1000's over the years and would have to personally agree. Anyone who's ridden one knows it would be hard to accuse even a 1000cc air-head of being a quick bike by todays standards. .."

    It piqued my interest in how the 800 is smoother than the 1000 boxer. I agree that the K75 is really smooth.

    #17
  18. Hawk Medicine

    Hawk Medicine Coyote's Brother

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    I've owned the 600, 750, 900 and 980 Airheads, so heres my $.02.

    The R60s are OK. I never paid attention to the vibes because I was usually wringing it's neck, trying to get it out of it's own way. The R75 is tons smoother than an R90 and the R90 is smoother than an R100. I also remember that out on the highway, riding the R75 I had to plan my passes, downshift and let the bike spool up before pulling out. With the R90 I could just roll-on and go.

    Of all of em, my all-time favorite engine is the R90. When BMW built that one, they really got it right. Smooth enough, powerful, great mileage... For me, that engine just has a majick to it and I could happily own one as my only bike.

    R100s are kind of in a class of their own. They vibrate, run hot and suck gas but they also generate significantly more HP and torque than the smaller engines. When I had to decide which engine to put into my latest bike I went for the big one and I'd do it again. It's the same choice you have when you're building a Hot Rod car. "Do I build a 283, a 327 or a 350?" Well, you're almost certainly going to build a 350 because it turns out more HP and torque for the same $$$$.

    I remember riding a San Jose'd 1050 77-S Airhead and it was smooth but it wasn't as smooth as an R75. The difference? On the built-up R100, you could feel those big slugs going up and down and the cylinders firing !! Talk about good vibrations! :lol3

    I'm almost 100% sure that magick surrounding the R80 GS is that the engine has the right power output and smooth power delivery thats right for it's gearbox, frame, suspension and weight. Add in it's inherent smoothness and it's the perfect little Airhead!
    #18
  19. Grayghost66

    Grayghost66 Been here awhile

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    I have to ask (not trying to hijack the thread)....I have always been interested/curious about the R100 Siebenrock pistons and big bore kit. Can you comment on the difference in power, any difference in vibration, and in your opinion....worth the investment?:D
    #19
  20. supershaft

    supershaft because I can

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    Mine already was a 1000. The rings look different but it will take some years to know how they hold up. I hate the finish they put on the cylinders. It looks like silver paint. I hate engines painted silver. I thought the inside of my wrist pins were crudely machined and the chrome wasn't as good as BMW's but BMW has damn good chrome. Robtg balanced my rods so I won't be able to compare vibrations.
    #20