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  • Re: 912UL Carb sync

    by » 9 months ago


    Hey Wizard! What old hot rods and engines? Thanks for the Helmholtz resonance mention, I’d never heard of it. I looked it up on the Google machine and found it very interesting. But isn’t an air box required to employ it? What if one is just running their 912 with the K&N filters?


  • Re: 912UL Carb sync

    by » 9 months ago


    The tuned runner/intake resonance theory has a problem: The 1/2 runners are longer than the 3/4 ones. No self-respecting car hot rodder would be caught in public with such a setup. With an airplane engine, induction & exhaust are usually quite compromised in order to get a compact package. Usually, we picture the function of carbs happening with a nice smooth flow of air going through it. This is far from the case. A cylinder sucks only once every other crank revolution. Spin it fast and while it may appear flow is constant, in reality it's coming through in a series of pulses. Worse, as the intake valve closes, the onrushing air gets stopped and sets off a reverse pressure wave. How is the poor carb supposed to get the mixture right? It can't, other than some average right. Play with resonance tuning of the exhaust and the pressure waves both in and out get amplified. Tuned carbed engines of the past ran great at the tuned rpm, but lousy everywhere else - easily finding conditions with the air flowing backwards through the carb, getting fuel added to it, the shooting flame out the intake or blowing up the intake air box.

    How to fix? Even out pulses by adding more cylinders serviced by each carb, for one. And/or add a balancing tube. With dual synched carbs, the average vacuum between them isn't much. But the pressure difference between *pulses* can be considerable. The tube serves as a sort of filter capacitor to mitigate the effects, get better carburation and hence run smoother.


    Thank you said by: Sean Griffin

  • Re: 912UL Carb sync

    by » 9 months ago


    Hi Jeffry -

    "Tuned carbed engines of the past ran great at the tuned rpm, but lousy everywhere else" 

    Your words instantly evoked memories of  sports cars of the pre computer era - that wonderful discordant rough rolling idle, followed by the snarl of acceleration - great stuff, now  confined to history.

    My thanks. 


  • Re: 912UL Carb sync

    by » 9 months ago


    LOL all, sorry but tuning for resonance has everything to do with it.  First we are dealing with side draft carburetors and the complexities of a prop on one side.  The issue of making the manifolds all equal length would prevent a lot of problems with installation and I agree that would be the nice way to do it all however we need to consider it has to fit into an engine cowl and also fit within the design concept.  Naturally a equal length intake and the same for exhaust runners would be ideal.  

    Rotax had to make some compromises.  it is understood that engines are not flying normally at low power settings so that range for tuning was not the concern.  The most important areas are the 65% to 100% power ranges.  We can see this in a fully instrumented engine.  For use financially challenged people a good set of EGT will also give us a good window into what is happening.  In testing a number of the carb engine 914 types you can see this very well on an engine with 4 EGT sensors.  For the most part we need to understand that there is some imbalance for temperature due to airflow considerations, the prop has a big influence on this part in cooling the ½ side barrels and heads as a tractor and a smaller effect on the cooling of the 2/4 barrels as a pusher.  This is due to the fact that air is compressed on the thrust side of the prop as opposed to the suction side, the compressed or thrust side depending on your cowl or lack of cowl can have a big influence.  That said the obvious difference in the manifold length of the runners of the ¾ cylinders and the ½ cylinders will always show 2 cylinders run richer that the other.  This will always show an EGT split due to the slight fuel imbalance due to the tuning pulse.  

    Given we have a reference now we pick up the throttle and watch the EGT, as it advances the flow will change due to the wavelength of the intake valve pulses back into the manifold. (resonance) At about the 65% power area is reached we get to the designed tuning length and front that point the EGT now drift toward equal readings.  At the design operational speeds EGT is almost always completely balanced.  Give it a try, it never fails to amaze.  When you land and taxi back to the hanger a plug check will show the same old rich cylinders due to the run at lower speed as you taxi in and the fuel imbalance again shows up.  

    In short, it is designed that way.  The injected engines have greatly reduced any of those kinds of imbalance to the point that nobody complains about EGT differences anymore.  So Jeffry is right, we have to look at what the turned lengths were designed for and it was never for idle speeds.  

    I worked for many years with 2 stroke race engines and can tell you it is all about tuned waves and pulses.  Most 4 strokes in aircraft have huge differences with intake systems and for the most part exhausts are almost unexplored territory for a lot of engine companies.  

    Cheers

    Just a PS on the original 9 series design.  This engine was laid out on paper, not a computer, in the mid 1980s.  I had the pleasure to see some R&D runs before serial production and can tell you for a fact that all these concerns were major discussions.  There were many trials and many failures before the production series were made.  And yes they have great beer in Austria. 


    Thank you said by: David HEAL

  • Re: 912UL Carb sync

    by » 9 months ago


    Rotax Wizard, thanks for the explanation.

     I owned a Zenith701 with a 912UL for a year before selling it to a friend as per our original plan. Frankly, I have not understood everyone’s love of the Rotax, but that is because I am old and already accustomed to the Continental/Lycoming issues. That year in Powerplant school in 1968 didn’t cover electronic ignition or balancing carbs. There are also other local codgers with experience on those engines and the Rotax man moved 200 miles away. The Rotax learning curve has been rather flat through no fault of Rotax.

    I really like your closing paragraph about how the issues were major discussions during development of the 9 series. My fall back response to myself when I have not understood just why something was “different” has been the general idea that they are getting the most HP/LP that they can and still sell the engines, so follow the manuals.(which I have downloaded) Your comment makes me feel better about that.

     I am still helping my friend with the 912, but he doesn’t know anymore than I do. ?


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