Why is there 1 carb per cylinder when a car has one carb for multiple cylinders?

Discussion in 'Motorbike Technical Discussion' started by markc, Jun 11, 2006.

  1. Nah, I agree with you just as much. See other thread. But, as an
    illustration, that huge tome addressed to the guy with the dicky
    The Older Gentleman, Jun 13, 2006
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  2. Oh yes indeed :))
    The Older Gentleman, Jun 13, 2006
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  3. markc

    FB Guest

    Like I said before, I work from the presumption that anybody asking a
    question to rec.motorcycles.tech knows very little about motorcycles,
    but may have some misconceptions caused by his/her experience with
    cars. So I take extra effort to start with the basics.

    Too bad if that bothers you.
    FB, Jun 13, 2006
  4. markc

    FB Guest

    What kind of motorcycle is it? Is it an inline-4? Does it have a
    4-into-1 pipe? Or does it have a stock exhaust system with balance
    tubes between the pairs of exhaust pipes? Do you only ride in straight
    lines, with no slow corners?
    FB, Jun 13, 2006
  5. markc

    Sean J Kelly Guest

    Not to argue with you, but i also have a bike with pod filters and no
    (noticeable) flat spots in the midrange. it's a honda cb350 twin
    with stock
    (CV?) keihin carbs and k&n pods.
    of course, the stock air filters are just paper elements with a metal
    casing. and the exhaust consists of two seperate headers leading to two
    seperate mufflers. no cross-over of exhaust gases at all.

    But I've never had it on a dyno, so perhaps my perception is off.

    Sean J Kelly, Jun 13, 2006
  6. No, it's fine, but if you were to start with the basics on a CG125 (and
    bikes don't come any more basic), it would be a good idea to refer to
    slide carbs rather than CVs, which it doesn't use (and never has).
    Otherwise you wind up with something totally irrelevant and which has
    the potential to confuse a newbie to the point where he starts acting
    on incorrect advice.

    It also has the side-effect of showing that you have zero familiarity
    with the bike under discussion....
    chateau.murray, Jun 13, 2006
  7. markc

    FB Guest

    OK, so you have a twin cylinder machine with separate pipes and that is
    a major clue
    as to why you don't have the dreaded inline four flat spot.

    The original poster has a motorcycle with an inline four cylinder

    New riders who own inline fours often wonder why they have all those
    carburetors, and, wouldn't it be simpler to just have ONE carburetor on
    a simple log type manifold,
    like a car has. That's why he asked about motorcycles that had single

    As I explained previously, George Kerker and other motorcycle
    hotrodders decided to try 4-into-1 exhaust headers on the most popular
    inline fours to take advantage of the high RPM cylinder over-filling
    effect caused by another cylinder which is exhausting burnt mixture.

    This effect can only work if the cylinder that is filling has valve
    overlap. The intake valve opens early and the exhaust valve closes late
    and a cylinder that is on the exhaust stroke helps suck fresh mixture
    into the charging cylinder.

    Problem is that the beneficial pressure waves in the exhaust system can
    be out of phase with what you hope to accomplish. The speed of sound in
    a gas is based upon its temperature and the exhaust system has a
    certain tuned length and the pressure waves return to the cylinder at
    regular intervals that can be determined by mathematical formulae.

    The waves have to be "in synch" with the motions of the pistons. A
    4-into-1 pipe returns a beneficial sucking wave at high RPM but it also
    sends an adverse positive pressure wave back through the cylinder and
    out the open intake valve and through the carburetor at some RPM.

    The carb is a dumb device, it has no idea of which way the air is going
    through it. So it picks up a shot of gasoline out of the float bowl and
    throws it into the open air behind the carb, or into the air filter or
    into the air box, depending on what you have.

    Then the airflow reverses direction, the carburetor picks up a second
    shot of gasoline.

    The amateur tuner feels this problem as a flat spot between 5000 and
    7000 to 8000 RPM, and imagine he might be able to do *something* with
    jets, but it's a mystery. Nothing he does with jets works.

    The various aftermarket exhaust system manufacturers offered pipes with
    extra chambers to add volume, they offered 4-into-2-into-1 pipes and
    claimed that they were the solution to the adverse pressure
    wave-induced flat spot.

    Some inventors even suggested variable length 4-into-1 pipes with
    computer-controlled servo motors to vary the lengthof the system in
    order to get rid of the flat spot.

    But the system that works is Yamaha's ExUP, the computer-controlled
    servo-operated exhaust throttling system. All of the Japan Inc.
    manufacturers have adopted the ExUP system, with various acronyms that
    seem to indicate that they invented it themselves.
    FB, Jun 13, 2006
  8. Um, just a question:

    How come the manufacturers manage to produce in-line fours with
    four-into-one pipes, without flat spots? I remember a very respected
    tuner saying that the one huge mistake to make was that the factory
    designer knew nothing about exhaust systems...

    That said, a lot of aftermarket four-into-ones have been utterly

    Actually, no, they haven't, because Yamaha's patent lawyers would have
    their arses if they did.

    They have variations on a theme, agreed, but Yamaha's EXUP is indeed the
    original and best. As was its YPVS system on its two-strokes. Yes, other
    makers built power valves, but none was as elegant and effective as
    Yamaha's, and nobody copied it for exactly the same reason they haven't
    copied the EXUP. Patents.

    "Yamaha's EXUP system that functions to eliminate torque valleys and
    provide excellent power development characteristics has been made more
    compact on this model while retaining the same basic operating
    principle. Unlike the former system that included a separate valve for
    each of the four exhaust pipes, the system on the new YZF-R1 places just
    two EXUP valves at the points where the four exhaust pipes merge into
    two, thus creating a more compact system that controls the pulse based
    on calculations of the 1st and 2nd cylinders' exhaust and the 3rd and
    4th cylinders' exhaust."

    (From Yamaha's website)

    And it's not just fitted to four-cylinder engines, either. *Please* stop
    relying on your own perceived wisdom.
    The Older Gentleman, Jun 13, 2006
  9. markc

    FB Guest

    You can use zero overlap valve timing.
    Snip a bunch of cut and paste...
    Why? In a desert of one-liners and other egotistical post, my own
    perceived wisdom is what I have to offer people who ask honest

    If you will pardon me now, I shall go and assist the happy campers at
    the lake...
    FB, Jun 13, 2006
  10. I've posted this before in this forum, but for the record it's a 1980 CB750K
    stock, 4 into 4.
    Only when I'm on the freeway (every day)

    Seriously, I think your putting way too much reliance on any Hemholtz
    resonance in the airbox fixing a flat spot. I have no doubt that
    the airbox changes things, but I don't think it's as important as jetting
    and general maintainence of the motorcycle - at least, not on a larger
    displacement bike. Perhaps a 50-100 cc engine it would matter,
    or perhaps if your racing at high speed (not some offroad dirt bike
    thing) you would see a difference.

    On my bike, the pod filters were on it when I bought the bike. I did
    get the airbox from the prior owner, unfortunately it's missing one of
    the manifold rubber connectors so it's just sitting until I can find a
    replacement that won't break the bank.

    Incidentally, there WAS a flat spot at 6000 RPM when I bought my
    bike, in fact the seller commented on it. I thought it was carb settings
    but I haven't yet got around to doing anything about the carbs. Do you
    know what fixed it? I'll tell you. Replacing the chain.

    When I bought my bike the chain was an old o-ring chain stretched to
    the limit, most rings had fallen out of it, it was rusted, the front and
    sprockets were both pretty worn. I ran about 400 miles on that mess then
    replaced everything, a new steel front sprocket, new aluminum rear sprocket,
    and new O-ring chain. I did NOT change number of teeth on either sprocket,
    and took pains to make sure that the sprockets had the same number of
    teeth as the factory stock sprockets (the existing ones did, and the new
    do) In fact, seat-of-the-pants performance of the bike increased after
    doing this.

    My assumption is that the old chain and sprockets were so incredibly
    worn and also adjusted too tight, that they somehow introduced enough
    drag at speed as to create the flat spot. Beyond that I have no other
    explanation, and I really don't like that explanation anyway, because
    if the chain could have actually taken that much horsepower, why didn't it
    break, or melt into slag.

    Ted Mittelstaedt, Jun 14, 2006
  11. Fine. As long as they get accurate answers....
    The Older Gentleman, Jun 14, 2006
  12. markc

    FB Guest

    Well, I don't try to remember what every user in this NG rides or owns
    or claims to have ridden or owned. The main idea of
    rec.motorcycles.tech is that a newbie asks a question and we respond to
    *that* question, avoiding turning the question into a debate.
    OK, that's what it is. You don't notice the flat spot when riding
    straight up in a straight line because you can accelerate rapidly
    through the RPM zone where reverse flow occurs.
    Racers routinely removed airboxes from their motorcycles in the early
    1980's and replaced them with K&N's or foam Pods. The early 80's was an
    era when drag racing
    fascinated grass roots riders who weren't all that keen on going around

    But, when the production racers like GSXR's became available the focus
    changed to going around tight corners at part throttle and accelerating
    out of the corners.

    The racers began putting their airboxes back onto their bikes and
    pretty soon they were also ducting cold air into the boxes and putting
    ram air scoops on the fairings.

    The ram air system may only give about 1/3rd of a pound of pressure,
    but the extra density of the cooler air, combined with the resonant
    frequency of the airbox helps to flatten out the midrange torque
    FB, Jun 14, 2006
  13. I had to X-post this to ukrm, where there are plenty of People WK.
    The Older Gentleman, Jun 14, 2006
  14. markc

    Pete Fisher Guest

    In message
    People Who Kayak ?

    I will thumb through my ancient dog eared 'Tuning for Speed' to see if
    Saint Phil has anything to say on the subject, but as it was written in
    the days of 7Rs (that's PROPER ONES CHAMP) and Vincents he may be silent
    on the matter.


    | Pete Fisher at Home: |
    | Voxan Roadster Moto Guzzi Mille GT/Squire RS3 Gilera Nordwest |
    | Gilera GFR Moto Morini 2C/375 |
    Pete Fisher, Jun 14, 2006
  15. Individual foam filters are only of any use in a custom mode - where you
    don't or can't get a suitable airbox into the space available. In that
    case they provide a cheap and easy fix with suitable jetting. Some
    numbnuts run with only trumpet intakes and stone guards, but they're,
    quite frankly, off their heads.

    The manufacturers' airbox is a thing of beauty, designed to flow
    sufficient air at all parts of the rev range and avoid such problems as
    mentioned above, such as flat spots, etc. If you had to pay for an
    airbox solution and somebody was selling the same, you could bet your
    arse that airboxes would be the next cool thing.

    You get them for free with most bikes and they do the job they're
    supposed to do. People who throw airboxes away are idiots.
    Grimly Curmudgeon, Jun 14, 2006
  16. markc

    ChrisDC Guest

    Maybe, but I used to have all the dyno graphs from the work on my
    XJ900F, done at Dynojet's place in Warrington. They borrowed my bike
    to develop their stage 1 and stage 3 kits for the XJ, and did 30 - 40
    dyno runs in all.

    At stage 3 it had individual K&Ns and a Laser race 4-2 exhaust plus
    the appropriate Dynojet kit.

    The torque curves were beautifully flat, with no holes at all, and the
    power was up by 12% on stock over most of the range and rather more
    where it had filled in the mid-range dip.
    ChrisDC, Jun 14, 2006
  17. markc

    mb Guest

    Yebbut, midrange dips were designed into engines to get through noise
    regs... weren't they?
    mb, Jun 14, 2006
  18. markc

    ChrisDC Guest

    So I believe. The race cans made that irrelevant.
    ChrisDC, Jun 14, 2006
  19. markc

    Pete Fisher Guest

    <suitably chastised>
    Oops. Tired and emotional at time of posting. All misty eyed after
    blowing the dust of TFS and opening it to find all my old gear ratio
    calcs from the two stroke single class road racing days.


    | Pete Fisher at Home: |
    | Voxan Roadster Moto Guzzi Mille GT/Squire RS3 Gilera Nordwest |
    | Gilera GFR Moto Morini 2C/375 |
    Pete Fisher, Jun 14, 2006
  20. Using the patented Mavis Beacon "Hunt&Peck" Technique, Champ
    And this negates the "off their heads" assertion how, exactly?

    Wicked Uncle Nigel - Podium Placed Ducati Race Engineer

    SBS#39 OMF#6 Enfield 500 Curry House Racer "The Basmati Rice Burner",
    Honda GL1000K2 (On its hols) Kawasaki ZN1300 Voyager "Oh, Oh, It's so big"
    Suzuki TS250 "The Africa Single" Yamaha GTS1000 Honda ST1100 wiv trailer
    Wicked Uncle Nigel, Jun 14, 2006
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