Results 1 to 15 of 15

Thread: Bottom End & Top end Match?

  1. #1
    dave2large
    Guest

    Bottom End & Top end Match?

    Can you bolt up a set of 1000CC cylinders & heads to a R80 bottom end?

  2. #2
    tkpinsc
    Guest
    Quote Originally Posted by dave2large View Post
    Can you bolt up a set of 1000CC cylinders & heads to a R80 bottom end?
    Yes*


    * In the worst case, depending on the year of the cylinders and and crankcase you may need minor machine work.

    What year top and bottom are you working with?

  3. #3
    dave2large
    Guest

    Already done

    They were already done by a professional BMW mechanic and I was thinking of buying the bike. It's a 1979 R80 w/1000CC cylinders so it's kind like a /7 with more low end but less top end speed.

  4. #4
    John D'oh
    Join Date
    Aug 2006
    Location
    Close to Fort Worth Texas
    Posts
    611
    Quote Originally Posted by dave2large View Post
    it's kind like a /7 with more low end but less top end speed.
    I'm curious how you came to that conclusion?
    John D'oh

  5. #5
    535IS
    Guest
    Quote Originally Posted by s09rwmb View Post
    I'm curious how you came to that conclusion?
    Same here. Stroke is the same, so adding the cylinders and pistons makes an R100 engine.

  6. #6
    Registered User
    Join Date
    Dec 2007
    Location
    Reno, NV
    Posts
    1,968
    I would think the top end is lower, because you are still using the R80 lower geared rear end

  7. #7
    John D'oh
    Join Date
    Aug 2006
    Location
    Close to Fort Worth Texas
    Posts
    611

    Checked a few on-line sources for spec's and found the following:

    A stock 79 iron barrel R80/7 was delivered in the US with a 1:3.20 final drive. It made 55hp @ 7000 rpm with a compression ratio of 9.2:1. Top speed in fifth was listed as 111 MPH. Stroke is 70.6. Listed torque: 47 Ft Lbs at 5500 rpm.

    A stock 79 iron barrel R100/7 was delivered in the US with either a 1:3.20 or a 1:3.09 final drive. It made 60hp @ 6500 rpm with a compression ratio of 9.1:1. Top speed in fifth was listed as 115 MPH with the 3.20 FD. Stroke is 70.6. Listed torque: 54 Ft Lbs at 4000 rpm.

    From experience and looking at the torque figures an R100/7 will out-pull an R80 in roll-on (low end?). They top out (top end?) within 4 miles an hour of one another (the larger displacement being slightly faster), however as you can see, the R80 is busier than the R100 at that speed. I think the important thing to consider is that at 4000 rpm, the R100 is in the lower half of its power band. At the same RPM, the R80 is in the upper half.

    Interesting looking at the charts again...I spent a sick day once calculating the top speed for each displacement using the same 5 speed transmission ratios and all the available final drive ratios...That was when I realized I was really sick :-)

    Buy it, ride it, enjoy it...more power is ALWAYS mo-betta!
    Last edited by s09rwmb; 03-27-2008 at 03:21 AM.
    John D'oh

  8. #8
    Registered User Rapid_Roy's Avatar
    Join Date
    May 2007
    Location
    Waukesha, WI
    Posts
    5,761
    It's my bro, and he has found a couple of actual 1000's to ogle now too, with lower miles.
    19 BMWMOA Nationals under my belt, and I have no idea what I am doing.

  9. #9
    John D'oh
    Join Date
    Aug 2006
    Location
    Close to Fort Worth Texas
    Posts
    611

    Lower miles is a good thing of course...

    The R80 once converted by adding the 1000cc top end effectively becomes an R100. There is no real discernable difference except when you open the throttle and at the insurance company. If it is titled as an R80, usually it is cheaper to insure due to the smaller displacement category (798.2cc's)...but, I wasn't here and I didn't tell you that and this message will self destruct presently...

  10. #10
    Registered User lkchris's Avatar
    Join Date
    Mar 2003
    Location
    Albuquerque, NM
    Posts
    6,269
    Quote Originally Posted by jforgo View Post
    I would think the top end is lower, because you are still using the R80 lower geared rear end
    That's correct, and there's more as the R100 engine doesn't like to rev like the R80 does and this can become an uncomfortable ride.

    Most R80 to R100 conversions are just half done.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  11. #11
    John D'oh
    Join Date
    Aug 2006
    Location
    Close to Fort Worth Texas
    Posts
    611
    Quote Originally Posted by lkchris View Post
    Most R80 to R100 conversions are just half done.
    What then is the other half of the equasion?
    John D'oh

  12. #12
    tkpinsc
    Guest
    Quote Originally Posted by s09rwmb View Post
    What then is the other half of the equasion?

    Better matched rear drive ratio.

  13. #13
    John D'oh
    Join Date
    Aug 2006
    Location
    Close to Fort Worth Texas
    Posts
    611

    I see...

    So, if the 1979 R80 came to the US originally equipped with a 1:3.20 rear gear set, then what final drive ratio would you recommend as a better match for an R80 to R100 displacement change?
    John D'oh

  14. #14
    tkpinsc
    Guest
    Quote Originally Posted by s09rwmb View Post
    So, if the 1979 R80 came to the US originally equipped with a 1:3.20 rear gear set, then what final drive ratio would you recommend as a better match for an R80 to R100 displacement change?
    Depends on use.

    2.91 for reduced rpms when cruising at speed.

    3.00 good for all around performance

    3.09 better acceleration and sporty riding in the twisties

    3.20 for trailer pulling

    3.36 for sidecar

    3.56 for sidecar with trailer

    each step is not that different so 1 step either way will not be all that noticeable.

  15. #15
    Registered User lkchris's Avatar
    Join Date
    Mar 2003
    Location
    Albuquerque, NM
    Posts
    6,269
    The 79 R100s came with 2.91 ratio. As did they all through 1984.

    The lower-powered R100/7 of '77-'78 came with 3.00.

    The lower-powered '88-on models came with 3.00.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •