More mid range for R100?

Discussion in 'Airheads' started by Nortryder, Sep 8, 2010.

  1. Nortryder

    Nortryder n00b

    Joined:
    Sep 4, 2010
    Oddometer:
    4
    It's probably been covered before but I can't find it. I have a 1984 R100RT in S clothing. It's pretty much stock and I'd like to get some of that shoulder dislocating mid range acceleration. I've seen interesting stuff on 336 cams etc but that seems to be more top end. I live in the wilds of northern VT and we don't have any roads long enough or straight enough for top end.
    #1
  2. nella

    nella Been here awhile

    Joined:
    Apr 16, 2010
    Oddometer:
    403
    Location:
    Raleigh, NC

    Me too!

    But I think we both started with the wrong bike to realistically achieve that. I think you should just go for arm-straightening mid-range acceleration and be happy, I am. Spend the money you save on a road trip.

    If you really want to mod, look at some of the recent threads on cylinder and pistons swaps. Luftmeister made a turbo kit. Krauser made 4 valve heads. SanJose BMW makes a 1050 kit, an Israeli company makes a piston kit. Motobins sells performance parts in England. Some years R100 have better cylinder heads.

    Enjoy the ride!
    #2
  3. zenben

    zenben all roads are one

    Joined:
    Feb 27, 2007
    Oddometer:
    595
    Location:
    Portland Oregon
    Smaller valves, carbs, and more restrictive exhaust keep the torque curve down in the low and mids.
    Wide open everything can generally only be driven wide open with good results.
    I've built them both ways, and mild tuning is always nicer to drive all around.
    #3
  4. bmwrench

    bmwrench Long timer

    Joined:
    Nov 10, 2006
    Oddometer:
    3,460
    Boost the compression-your 84 is set at 8.2:1. You can get a small C.R. increase via head milling, which will also tighten the squish area, do a really good valve job, clean up the ports and push the ignition timing ahead a bit.

    And/or, install a 1050 kit. It won't last as long as the Nikasil cylinders, but it is a lot of fun.

    You might also consider a shorter rear drive ratio. The stock R100RT ratio is 2.91:1. A 3.20 will really make it scoot. This is probably the biggest bang for the buck.
    #4
  5. east high

    east high Been here awhile

    Joined:
    Oct 13, 2009
    Oddometer:
    955
    Location:
    Portland, Oregon
    Mikuni conversion?
    #5
  6. ericrat

    ericrat Long timer

    Joined:
    May 18, 2007
    Oddometer:
    1,314
    Location:
    Cincinnati, OH
    I don't think the 336 cam is going to help much at mid-range. The increased duration is more of a bennefit at high RPM. zenben is right about smaller valves, but this is exspensive to do and to reverse.

    Which final drive do you have? I am guessing an 11/33 or 11/32. Those are long legged highway gears. I would try something in the 3.3 to 3.5 range, perhaps a 37/11 maybe from an R80. Should be plenty of those around and it is an easy swap.

    I can help you with calculation to show speed/rpm at various gearing if you would like. Also, the 11/33 final drives are very desirable if you want to sell it later.

    Oh, you said should dislocating, well hunt around for the highest numeric ratio you can find, or buy a faster bike.

    Good luck,

    Eric

    Eric
    #6
  7. Hawk Medicine

    Hawk Medicine Coyote's Brother

    Joined:
    Jun 8, 2008
    Oddometer:
    3,304
    Location:
    NOR CAL
    The Lufty turbo really didn't work that well (But could probably be made to work if you have an engineering degree.). Keauser heads were selling for about $3000 per set five years ago, they're delicate, can tricky to setup and are currently unobtanium. Haven't seen a set for sale in ages. The Sam Jose kit has issues with extended use....

    Probably the best bets would be to either spring for one of the Siebenrock 1050 or 1070 kits or stay at 1000cc and build a high compression engine. Any of these methods will deliver higher performance but since youre after torque,
    dollar-for-dollar, I'd go with the Siebenrock 1070 with some nice big carbs.

    Don't forget that somewheres along the line youre going to need better suspension and a stiffer frame too.
    #7
  8. nella

    nella Been here awhile

    Joined:
    Apr 16, 2010
    Oddometer:
    403
    Location:
    Raleigh, NC
    I was not really serious about the turbo kit or the Krauser heads, but just trying to make the point that any significant performance changes in these bikes is generally an expensive endeavor and that the original poster's goal of "shoulder dislocating mid range" is not that realistic.

    Mikunis were mentioned above. I have TM's on my R100 and think they improve throttle response but didnt seem to impact mid-range torque in any noticeable way.
    #8
  9. Plaka

    Plaka Brevis illi vita est

    Joined:
    Jun 19, 2010
    Oddometer:
    4,623
    The simplest things are to up the compression (and buy more expensive gas) and/or drop the rear end ratio.

    The Mikuni's will give a snappier response--but you always end up wanting more.

    For the cost of the big displacement kits you can pick up a 75,000 mile K100 and get it all out of your system. The thing has a gear indicator on the dash because very often you can't tell where you are. Serious torque. Or there are ton of other inexpensive jap bikes that will do the same or better.

    Straining shoulder ligaments isn't what airheads are about. As soon as you start trying you spend a lot and don't get what a real performance bike will do while losing the best virtues of the Airhead. Tune the Airhead for smoothness, reliability and very long legs. Then get an RD350 or 400 for those midlife moments.
    #9
  10. SprintSix

    SprintSix Been here awhile

    Joined:
    Dec 24, 2009
    Oddometer:
    152
    Location:
    Ohio
    1981 r100rt. I got very noticable mid range improvement from 34mm mikuni round slides. Stock for me is 40mm bings. More power across the band as well. At a guess I'd say a pumper would be even better. Compared to stock it had the arm yanking power you describe. No other mods. Long winded +1 to smaller carbs.
    #10
  11. Cogswell

    Cogswell Spudly Adventurer

    Joined:
    Jan 3, 2005
    Oddometer:
    5,961
    Location:
    Riding with my pal Richard Cranium
    I'll have to agree.

    I bought my airhead knowing what it was, it's limitations, and capabilities. It is fun to get on periodically and just plain enjoy for what it is. It does not do any one thing spectacularly, but it does a little bit of everything well.
    I have other bikes to suit the other riding needs I have when that time arises.

    Mike
    #11
  12. ericrat

    ericrat Long timer

    Joined:
    May 18, 2007
    Oddometer:
    1,314
    Location:
    Cincinnati, OH
    Check your PM.
    #12
  13. beemercafe

    beemercafe Cafe-racer

    Joined:
    Nov 1, 2010
    Oddometer:
    4
    Location:
    Australia
    I have just fitted 34mm Mikuni round slide carbs to my R65LS cafe racer and am looking for a base in the jet sizes for tuning

    Any information would be gratefully accepted
    #13
  14. supershaft

    supershaft because I can

    Joined:
    Sep 9, 2010
    Oddometer:
    9,116
    Location:
    San Francisco Bay area
    336's have no midrange? I guess that false rumor got started back before they advanced the timing? When I installed my 336 three different mechanics and the owner of the BMW dealership where I worked at were all telling me that my bike wouldn't idle below 2000rpm and that it would have NO bottom end and midrange. All those warnings were all 110% BS! Right after I installed the cam I rode over the mountains to a BMW rally. A fairly local guru with a very popular BMW website that pops up by the time you type BM into your computer listened to my bike and declared to all that were listening that in fact it was not a 336 cam I had just installed. He said it idled too smoothly! My girlfriend and I got a good laugh out of that! We just got through hauling ass over Donner Pass and we were both stoked about all the extra pull with almost NO down shifting where before the same highway with exactly the same two up for camping load would take rowing the gear box from third to fifth back and forth constantly and we still weren't going as fast! (I usually go over that pass in the mid 80's mph and it is usually through a lot of traffic.) If you are a below 2500rpm engine chugger, you won't like a 336. Other than that, they get you WAY more midrange and top end.

    IMO, 980cc's IS a big bore kit. Basically, your 1000cc beemer is a hogged out 750! BMW stopped hogging her out at 980cc for good reasons. I don't recommend big bore kits. They do run great for a short while!

    I would and have started more than a couple of times with a 336. Clean up the ports, raise the compression, put some Dellorto's on and JET them and I have had some very happy customers. I got a lot more midrange out of my setup and I didn't raise my stock 8.7:1 CR. SO many airheads are modified ass backwards with marginal results. No one changes the cam and that is where the big diff comes from! Big bore kits often turn into oil pumps. I can't tell if they pump more oil out the exhaust or through the gaskets! Big bore kit alone? Ride a 750 and then ride a 800. It's a tiny bit bigger diff than that! That is before the power falls off due to corrugated cylinder walls. That's my $.02.

    Happy Thanksgiving inmates!
    #14
  15. supershaft

    supershaft because I can

    Joined:
    Sep 9, 2010
    Oddometer:
    9,116
    Location:
    San Francisco Bay area
    Were you setting them up to pull one of those plows made for a two wheel drive Rokon?? I have never felt the need to tune one that way. I have used bigger carbs, bigger valves, and opened exhaust and got way more midrange down to just below 3000rpm. I don't chug my bikes with much of a load at all below that anyway. They don't like it no matter how they are tuned.

    Man, now you got me thinking about two wheel drive Rokons. I want an old one with a Continental I think two stroke. Two strokes won't dry sump when you are climbing that tree!! I figured that out mowing lawns! :norton
    #15
  16. 2xdisco

    2xdisco Been here awhile

    Joined:
    Dec 11, 2007
    Oddometer:
    279
    Location:
    WNC western NC
    i have experience with these models. dual-plug the bike while doing this eather shave the heads or add HC pistons(better).... have the bike timed and tuned proper!.. i have found that most shops do not know how to do this right.... they do it ok but miss out on the total potential of the motor.... with it timed right. the motor will pull so hard from 1500-5500 that u actually have to hang on. after 5k the bike will accelerate extremely fast. this way u can leave everything else stock , exhaust, 40 bings, ect.... this mod will also increase fuel millage.. i have this same set up on a r100rs/s i get 45-50mpg at 75+ loaded. HP with this done right, will put u in the 75HP range. and torque will be lowered due to the retardation of the timing and a widening of the advance range...
    #16
  17. supershaft

    supershaft because I can

    Joined:
    Sep 9, 2010
    Oddometer:
    9,116
    Location:
    San Francisco Bay area
    75hp at the rear wheel from dual plugging alone? Even at the crank would be a miracle! I wouldn't count on that happening twice here on planet Earth.

    Dual plugging? I am all for it. Remember that I didn't raise my compression? It's because I wouldn't raise my own compression without first dual plugging and I opted out. Besides, my boss and a bunch of mechanics I worked with the last time I had my heads off said that you HAD to raise the CR if you put in a 336. Wrong! For 1000cc, my 8.7 is about as high a CR as I would run single plugged. It runs on regular just fine when it isn't too hot and when it is hot it needs premium to keep from pinging. That's about as close as you can get to the highest CR doable IMO.

    I have seen F'ed up machining on a lot of dual plugged heads. Usually the plug is way off center and if it is off center towards the exhaust seat too much, the head will crack between the spark plug hole and the exhaust seat. I have seen a lot with Fed' up threads right out of the shop too. I have seen some that have had coil inserts installed right out of the shop because the first attempt at cutting the threads was a fiasco!

    I have seen even more dual plugged setups timed wrong. There are two schools of thought on this subject. One works WAY better than the other. Oak and Snowbum on on the wrong side of THAT fence.
    #17
  18. 2xdisco

    2xdisco Been here awhile

    Joined:
    Dec 11, 2007
    Oddometer:
    279
    Location:
    WNC western NC
  19. supershaft

    supershaft because I can

    Joined:
    Sep 9, 2010
    Oddometer:
    9,116
    Location:
    San Francisco Bay area
    That article has got some good, some bad, and some ugly. First off, Chris is a real good guy and has set up the best running airheads I have ever been around but he doesn't have an aeronautical engineering degree. He said himself just a while back on this forum what kind of degree he has. It's an aviation management degree or something like that. It's really no secret.

    I wonder how this myth of /6's and /7's getting 55mpg got started. I worked at a BMW dealership when those bikes were being sold brand new. Back then as now, it would take getting on a thumper to get that kind of mileage. At least at decent speeds. I have ridden with so many /6's is isn't funny. They have all needed to stop for gas about the same time I do on my non /6 and /7 airheads. I am not saying I couldn't maybe get that kind of mileage out of one or any other airhead but I would want a safety car behind me with flashing yellow lights!

    60+ rear wheel hp from a stock R100GS? Sorry but that just ain't going to happen. BMW "claims" 58(?) at the crank! By the time I worked with Chris, he knew his dyno that a local guy had made read high. If I remember right, he had the program re-wrote to get the numbers down and we still recalculated the figures to get them down into reality. But then most others make out of this world claims so what are you going to do? You tell your customer that we got 20% more power out of his bike and the power is now reading 58hp and the customer will scream that it made 60hp stock! SURE it did! Not YOUR engine but a FINELY tuned and setup prototype somewhere deep in the bowls of BMWAG on THEIR dyno and AT THE CRANK! Why do people even bring up claimed hp or weight? It's la la land! Sure, BMW stretches the truth a LOT less than most other companies but still!

    That article reads like he didn't yet have a gas analyzer but I don't know. It reads pretty out there in spots. I am sure it would read very differently if Chris himself wrote it. His recent posts were GREAT!

    I hope I am not going too far sharing my experience with Chris. For instance, I have never revealed what I saw some of his race bikes make on the dyno and other things I would consider proprietary. It's just that I have to deal with people wondering why their RS doesn't make 70hp or why their bike isn't getting 55mpg ALL the time.
    #19
  20. bmwrench

    bmwrench Long timer

    Joined:
    Nov 10, 2006
    Oddometer:
    3,460
    My more-or-less stock R100/7 (modified airbox, RS Fairing) routinely got 50 MPG. A friend's R90/6 with an R60 rear drive(!) also got 50+ MPG. We weren't in any danger of being run over, either. This was back in the late 70s, when gasoline was far better than what we can buy today. The best mileage I've got in recent years was on my K1!
    #20