Any exhaust experts out there?

Discussion in 'Airheads' started by griffg, Apr 23, 2012.

  1. griffg

    griffg n00b

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    I'm going to build a stainless steel system for my R80/7 cafe racer and need some advise. The standard chrome system on the bike has a single balance pipe on the downtubes just after the first bend from the cylinders. I've done a bit of research and info that I found says that the balance pipe improves torque and also reduces noise. When I build the system, I'd like to leave out the blance pipe, this is mainly for looks. My questions are 1. would not having the balance pipe make much noticable difference to performance and 2. if it does make a difference, does it matter where the balance pipe is, ie could i position the balance pipe under the sump?
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  2. Airhead Wrangler

    Airhead Wrangler Long timer

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  3. MODNROD

    MODNROD Wheat and Sheep

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    The balance pipe will increase torque IF it is placed at the right tuning length to suit that rev range, if the balance pipe is placed elsewhere it can reduce torque at it's peak, but MAY increase it elsewhere, depending on the tuning length.

    There are a few web-based quick and easy pipe length calculators around that can give you an idea of where best to place the balance tube. As a rough guide, if you want to maximise the torque curve between 4000 and 6500 RPM, a tuning point between 24" and 27" off the head will work fine, so will 13".
    You'll probably find the BMW balance tube around 13" anyway, another "cheats" way to do it is you can double the length from the head the balance pipe sits without affecting the torque curve much, it SHOULD be slightly less at peak, but have a slightly broader curve. This will probably get your balance pipe under the motor where you want it.

    Balance pipes may not reduce your total noise output much, but they do reduce the "blatty" sound out the back, especially on the over-run.
    #3
  4. simonboxeproject

    simonboxeproject Adventurer

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  5. griffg

    griffg n00b

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    Thanks for the replies. The link from Airhead wrangler was interesting reading with no definitive answer.
    MODNROD seems the most straight forward and the one i'll probably go with. 26" from the head will work nicely.
    #5
  6. ericrat

    ericrat Long timer

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    I have used a program called Pipemax with some sucess in the past, you might want to take a look at it. Even the demo pages with calculation examples are helpful in getting the idea. No afflilation, blah, blah, just something that has worked for me.

    Eric
    #6
  7. lkchris

    lkchris Albuquerque

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    Chances are at least 80% and probably closer to 99% that you'll come up with something worse than did BMW. Not personal, this applies to anybody else.

    Yeah, BMW has "programs," too, plus years and years of experience and lots of expensive resourses to use in testing and verifying, i.e. R&D. BMW used crossovers way before there were emissions requirements, so don't bother with that old aftermarket hype as it's just myth.
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  8. Houseoffubar

    Houseoffubar HoFmetalworks.com

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    I would agree, if you had to accomplish all the same things BMW did when building their exhaust. However, if a few more decibels are acceptable, it would be hard to not make more power than the stock exhaust! I am not talking about loud exhaust, but a much freer flowing exhaust can be had, with a slight increase in sound.
    #8
  9. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Back in the 80s (I think it was), a guy here in town by the name of Tom Webb was making stainless mufflers for these bikes. Nice ones, too, I might add.

    One of his friends understood exhaust science and figured the flow through the muffler was all wrong - entirely backwards. So he had Tom build a set to his specs and what do you know - they netted something like 7 more HP (per dyno testing) on a stock R90S.

    The interesting thing is they're not particularly louder, either - but they've got a different sound. It's hard to describe.

    So if BMW is so smart and extracted the most HP from these engines - why was it that an unknown out here could extract ten percent more with different mufflers? And don't tell me my data is BS or I'm dreaming! I wasn't there during the dyno runs, but if you know Tom, he wasn't making this up, and he's the one who told me about it. For quite a while he produced and sold these mufflers as HiPros, and I'm sure there are quite a few sets still out there in use.
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  10. robtg

    robtg Been here awhile

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    Some people are believers and think the BMW engineers can do no wrong.
    Some people are thinkers with imagination and go beyond the engineers limits imposed by the factory to customize their own machines. Sometimes it turns out better, sometimes not, when it does it gives the satisfaction that you were at least thinking and doing something about it and not just wishing it could be better.
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  11. Beemerboff

    Beemerboff Long timer

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    Doug Hele, one of the greatest motorcycle development engineers ever, was one of the first to use balance pipes, in his case to reduce the noise on the Triumph Bonneville road bikes.

    The production racers with their , err, freer flowing exhausts had 7 to 8 HP more than the more strictly silenced road bikes, and Doug was given the task of eliminating the power difference while still complying with the road noise regulations.

    He spent a bit of time developing the longer "Cigar" muffler, which gave him 4/5 HP, and the balance pipe took him the rest of the way.

    When interviewed, he was quite candid in stating that he didn't know why they made a difference, and also that, to his surprise and contrary to his expectations, the location made absolutely no difference to the power output at any revs.

    On the Bonnie they were located close up to the head as this was the easiest (cheapest) place to put them.

    So, if you are tuning a Bonnie, don't bother with the fancy computer exhaust software.

    And for anything else, the formula for four stroke pipe lengths is in Phil Irvings book - Tuning for Speed, and it isn't a complicated calculation.

    So it would seem that if you plan on fitting a free flowing exhaust with race levels of noise output, there is not a lot, if any, benefit in the balance pipe, and the benefit will increase with the amount of restriction in your muffler, and stock BMW mufflers are notoriously restrictive.
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  12. Voltaire

    Voltaire Titanium and Ceramic Hipster

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    My racebike has a exhaust that was fitted as it was available, its a 2 into 1 unequal lenght with original headers. the 'muffler' is one of those taiwanese shorty Norton copies, it has no innards apart from a plate welded in with some holes in it. The track is tested to 95 DB and its very quiet.:huh
    It was suggested today by another airhead rider after he rode mine I should put a BMW 2 into 2 on it as they are better and all the racers in the old pics have them.....
    The one called the 'Black Hole" look interesting...http://www.rbracing-rsr.com/bmwexhausts.html
    I think I'll stick with my 2 into one until after I do a dyno run and see what they say.
    #12
  13. Wirespokes

    Wirespokes Beemerholics Anonymous

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    Exactly right!!! :clap

    Very well said!!

    If these bikes were perfect in every way we wouldn't have to deal with side stands that fail or pop up in a side wind leaving the bike unsupported. Or center stands that undo their fixings, trashing their mounts. Or the hernia causing center stands. And how about that flimsy ground connection at the transmission? And the wobbles and tank slappers they couldn't fix other than lengthening the wheel base? The valve seats they changed that caused valve recession problems, and denied it had anything to do with them? There's the plastic speedometer parts that snap off, like the GS speedo cable mount. And how about the odometer problems that were never even addressed, much less resolved, that continued for the complete line from 74 to 95?

    It's ludicrous for anyone to hold the opinion that BMW could do no wrong and we're idiots attempting to change anything! :lol3
    #13