2 to 1 exhaust

Discussion in 'Airheads' started by Sabre170, Feb 18, 2013.

  1. RGregor

    RGregor Been here awhile

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    The 2-1 exhausts from SR-racing are quite popular over here in Germany.

    http://www.sr-racing.de/anlagen/BMW/R100GS.jpg

    No one has ever complained about performance, in fact they give a rather significant boost over the complete rpm range.
    However, I would not use them if classic looks were important.

    Regarding the second crossover tube of the post '81 models: I have a technical article where the authors (engineers from zeuna staerker, the company that developed and manufactured the BMW exhausts then) describe the effect of the crossovers as follows:
    - The first crossover improves low-mid rpm torque
    - The second crossover improves mid-top rpm torque.

    My dyno runs have shown this exactly.
    #21
  2. Airhead Wrangler

    Airhead Wrangler Long timer

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    #22
  3. Prutser

    Prutser Long timer

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    You are right AW its the same as the siebenrock system.

    The system is not a big improvement on a stock 800 engine. The bike will loose bottom torque.
    But with some engine tuning the system starts to work.
    #23
  4. DiabloADV

    DiabloADV Semi-Occasional

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    I'm in the process of copying that setup. Keep getting sidetracked on the honey-do list...
    #24
  5. RGregor

    RGregor Been here awhile

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    I admit I don't know anyone with a stock R80.
    I even think I don't know anyone with any stock engine ...

    The Siebenrock exhausts are built from SR. Which does not necessarily mean they are the same.
    From SR-racing you normally get full service, which means they "build" the exhaust on your bike and tune the bike on the dyno afterwards. And they have a variety of different mufflers to fit.
    A couple of friends have these exhausts on their bikes with positive reports.
    #25
  6. supershaft

    supershaft because I can

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    It seems everyone is comparing the SR pipes to the stock setup. That I am not interseted in. Almost any setup is going to cost you some bottom end power over the stock setup with a stock engine. What would be intersting is to hear how they compare to other types of aftermarket setups. As far as welding up my own two into one setup for my bike? There are just too many variables and I don't have the dyno and the time to figure out collector shape and length. How far down the pipes does the collector work best, etc.. I could surely copy some that I hear work real well over other aftermarket setups. Personally, I think I am going to stick to two into two's just for tuning simplicity. Plus I have decided that I am not going to go looking for TOO much more through the exhaust because I know from experience that the big gains are going to be LOUD and I am now too old for LOUD. That and now there are a million Harely's out there and almost all of them are LOUD. I don't want to be a part of it.
    #26
  7. RGregor

    RGregor Been here awhile

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    No, not necessarily. The engines I know (with the SR exhausts) have more low end torque than stock airheads have at their peak.
    But this of course may be due to the other modifications. Including cam.

    Now, these guys did some cross comparison (e.g. to the exhausts from this guy here
    http://www.rs-haslauer.de/tuningteile_bmw.php ).

    And the answer is that there is no definite answer.
    Although the engine specs were similar the engines responded differently to different headers (crossover/no crossover) and mufflers.
    #27
  8. Boxer Metal

    Boxer Metal Mad Scientist

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    Why are you commenting on this thread? No disrespect just curious.
    #28
  9. chasbmw

    chasbmw Long timer

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    Jim Cray had Keihans make up some absorption type straight through silencers to his design.

    They look almost stock, but sound like a BSA twin on Goldie silencers. Jim did a back to back dyno run using a dual plugged but otherwise stock R100/7 and the results show a useful increase in power throughout the rev range.

    Here is a copy of the dyno chart, it's not that clear but I'm sure you will get the message. Keihans still make batches of these from time to time, or Jim has some in stock. 38 and 40mm.

    I like the idea of a stealth bike......a BMW that looks almost stock, but goes much better than people expect:clap

    [​IMG]

    [​IMG]
    #29
  10. chasbmw

    chasbmw Long timer

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    Rudi, how easy is it to get away with a non stock exhaust in Germany? Do people run what they like and then go back to stock when the bike needs its inspection?
    #30
  11. RGregor

    RGregor Been here awhile

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    What is legal or can be legalized depends on several aspects.
    The law says that your bike must fulfill the restrictions being valid at the time when it was first brought onto the road.

    Having a pre '81 bike you almost have no trouble at all.
    I run my bike with open bellmouths and Norton style mufflers, so it is rather loud when I push it.
    But I had it inspected and it was within the then valid limits. So it's all legal and registered in the vehicle documents. You have to register all changes made to the bike. As literally no part of my bike is stock any more I needed some additional sheets for the documents ..

    In '81 limits were lowered.

    From 1989 on things are different, according to the law you need to be within again lowered noise and newly introduced emission limits.
    So 1988 GS frames are rather rare over here :-))

    SR-racing however have certificates for their exhausts proving that they conform to the restrictions. Quite a few people wonder how they got them as the exhausts are not really silent ...

    Changing back for inspection is a way to cope with the limits, but it won't help you when the police stops you. In the Munich area where I live they were rather rigid against the Harley guys with their loud and slow bikes.

    Police stopped me once and asked, if exhaust was stock. I showed them the entry in the documents.
    Next question was: how much does a bike like that cost?
    We had a nice chat then for a quarter of an hour.
    #31
  12. RGregor

    RGregor Been here awhile

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    Some guys here run these mufflers.
    I don't think anyone has dynoed them. Thanks for the charts.
    I remember that Dirk Scheffer used and recommended them for a while.
    I knew this chart here (which was taken with a bike modified by Dirk Scheffer):
    http://www.boxup.de/images/DiagrammWEB.jpg
    But here it's not clear what modifications the engine had.
    #32
  13. supershaft

    supershaft because I can

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    Like I said, almost any setup will cost you low rpm torque with a STOCK engine. Don't forget that most wheel dyno charts START right around half way through our stock rev range.

    I didn't see any airheads on that link?

    I can believe that about modified engines having more low rpm torque than a stock engine's torque peak. My engine with a 336 cam had almost a flat curve all the way across to over 6500rpm. Now it has way more midrange and low end than it did then but I haven't run it on a dyno yet. I am going to break it in a bit more.
    #33
  14. RGregor

    RGregor Been here awhile

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    OK, I didn't get this right.
    The dyno charts I saw started at around 2500 rpm.
    But more important is I rode the bikes and listened to my butt dyno.

    Probably they make more money with the modern bikes and advertise these.
    They make 2-1 systems for airheads, but contrary to SR-racing their headers do have one crossover in front of the engine.
    #34
  15. supershaft

    supershaft because I can

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    The dyno chart posted in this thread is a lot more closer to what I usually see.
    #35
  16. Beemerboff

    Beemerboff Long timer

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    Overland in Melbourne make a 2 in 1 at a sensible price.

    I found one stock muffler didnt flow enough for a 2 in 1 and 900cc s, but a low cost Dunstall Decibel does, and looks and sounds a lot better too.

    I wouldnt get to excited about equal length headers , while they might show up better at a few spots on a dyno you probably wouldn't notice the difference from unequal in normal road use.
    #36
  17. motu

    motu Loose Pre Unit

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    My first 2 into 1 was a nasty thing I cobbled together, and was unequal length, this was just a test set that was on for over a year. Then I made the equal length, and no difference in performance....I just think equal length makes more sense.
    #37
  18. RGregor

    RGregor Been here awhile

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    Yes, dyno charts are a story of their own. I think you said that once or twice.
    And you're right.
    Looking at the charts shown in this article you see the hole in the torque curve at around 3000 rpm (version 3).
    http://www.advrider.com/forums/showpost.php?p=19636511&postcount=41

    This seems to be characteristic for the SR-racing exhausts as I've seen it quite often.
    On the dyno charts taken at SR racing you won't see it as they usually start at 3000 rpm or above.

    Never believe the charts of the man who wants to sell you something. Or prove the result of his work.

    The charts I was talking about were taken on an independant dyno. And the level in the hole was still in the 80Nm range.
    #38
  19. Edelweiss-Motorsport

    Edelweiss-Motorsport Been here awhile

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    Hi all,

    found this thread by accident - very nice forum with a lot of good technical discussion !

    Beemerboff - yes you are right when it comes to a BMW 2V street / sport engine in the range below 100 HP an unequal designed header can even have some pros ! It makes a much more flat power and torque curve and cleans out the peak spots. The max levels are more or less the same if the package is sorted out right. The most important coherence is to find between cam and exhaust.

    If you start using cams with more and more lift at TDC your system starts to get more and more reactive to what happens in the direct periphery. In other words using the exhaust gas energy to motivate the fresh gas side the correct header design as well as the length on the carb side gets essential.

    Also a right placed and dimensioned interference can be of great help when it comes to the search of low end torque.

    If you know how....80HP and 90NM are easy to find in a std. R100GS engine.

    You will not be in need of very much or fancy material just a cam and a header -
    the std. silencer works fine up to 80 - 85 HP !

    Very much of importance is the right modification of the squish band - at this point please know that you do not need to buy the 70 HP hc pistons, the 60 HP ones to be found in all R 100 GS / R will do the job perfect.

    Air cooled for ever !

    Dirk
    #39
  20. Beemerboff

    Beemerboff Long timer

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    Dirk , have you any details of your squish band modifications - good information is hard to find.

    There are people claiming they run 26 thou clearance with no problems, and others who claim that anything less than 65 thou and the piston hits the head, and everything in between.

    And no one has any detail of what they do at the back of in particular the inlet valve, maintaining the squish band here would appear to shroud the valve.

    The 2 in 1 on my stock cammed 900cc R75/7 is just about as unequal as you can get , with the LHS twice the length of the RHS, and the Dunstall Decibell works better with this set up, but I suppose I don't really know if it better flow of better harmonics, I had just assumed that because it flows better that was where the improvement came from.
    Big mistake on this forum- you dont get away with much here!
    #40