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14098Re: [Small4-strokeEngines] single carb

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  • mirco@jps.net
    Mar 2, 2014
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      Hi, exhaust tuning is very important to high performance two stokes, important to four stokes with a lot of cam timing overlap and of much less use on mildly tuned low rpm engines.  If you do not add too much weight and drag, it can be worth doing even on these little engines but practical considerations should come first.  Prop rpm is constant for tuning purposes compared to automotive applications.  The formulas will get you close and then a bit of cut and weld tuning will get that last 10 or 20 rpm.
      Murry I. Rozansky
      ----- Original Message -----
      Sent: 3/1/2014 3:22:15 PM
      Subject: Re: [Small4-strokeEngines] single carb


      Hi Darrell

      You may not be an engineer but you are the designer of your particular craft.  Without knowing the reasons why things are done the way they are, or by not asking the right questions, do you ever wonder whether you are copying someone else's mistakes into your design?  I am not intending to imply that Kevin has made any mistakes in his design, but without understanding the engineering part, it seems to me this should warrant genuine concern on your part if you are attempting to copy it.  How will you ever know that you've not introduced your own mistakes by, for example, changing a pipe diameter?  You can always cross the 'ol fingers and hope for the best.  Your choice I suppose. 

      You bring up another facet of exhaust design I hadn't touched on; the two into one collection.  Does the 2 into 1 collector ALWAYS scavenge?  Will there be a physical arrangement and operating mode where the placement of the merge coincides with a particular RPM to actually increase back pressure on the cylinder you are attempting to scavenge from, totally defeating the purpose?  How does one design the physical arrangement- the placement and angle of the merge?  Without some insight into the design, it seems to me one is shooting in the dark and claiming a hit. 

      You go to some length to expound the use case differences of cruise vs WOT, but you never really answered the question I posed.  Please tell what target RPM will you be designing your particular exhaust.   What made you choose that operating point vs somewhere else on the curve?  How will you know you have succeeded?

      I posit that if there are truly 10% power output gains to be had, on top of increased fuel efficiency,  the weight and complexity of an in-flight variable length telescoping exhaust would be warranted.  Such an arrangement would also allow precise tuning for any RPM.   But I've never seen or heard of such a critter.  Has anyone else?  I am sure there are gains to be had, but I am quite dubious of the 10% number.

      I concede that indeed, the whole thing it is quite complicated.   There are numerous simultaneous interactions occurring.  But this is 100% science. There is no magic and the true extent of the art is purely aesthetic.  I don't want to be too cynical, and I am certain blind-assed luck plays a role too, but without a measurement of some sort, in my eyes all claims of benefit are suspect. 

      If you don't know the answers to any of the questions that's OK, neither do I.  But the whole thing does give me pause for consideration and questioning.    And yes, kevin's wrap-around exhaust looks very nice and quite professional.   

      Mike Phillips

      ---In Small4-strokeEngines@yahoogroups.com, <dagwodzz@...> wrote:

      Hi guys, If I may just interject a bit. While I���m no Engineer on this subject. I can tell you there are tons of science at work here. As with any subject one can discuses the minutia to infinity. The basics are. To find the target area (RPM) of most importance to you. There is no one size fits all here. One can tune the exhaust for low end torque , or for WFO. Pick one. If we were building a pulling mower for example. We would tune the exhaust for WFO because that is what we need for the WFO full pull. Get up in Rs and in HP and go. 
         For us we need to ask our selfs a few questions. Where do I spend most of my flying time in terms of throttle position and RPM? For me cruise is 3K for level flight. Now I still want and need to let the engine make the best HP it can at WFO. In general for MAX WFO . larger tube selection would be the best at around 36 inches ���or so��� with little to zero back pressure.  I used 1.25 OD tubing. The ID of the exhaust port is 1.25. What this does is keep the exhaust moving quicker in the mid RPM range. Better mid range torque and fuel burn and (throttle response. Very important in a PPC) 
          Now the collector. The purpose of the collector of course is use the other cylinders exiting gasses to draw (scavenge) the intake from the sister cylinder. Doing so helps draw fresh intake charge in and drain the remaining unburned gasses out. One thing to note. The exit pipe must be bigger than the two incoming pipes flowing into the collector so as  not to create back pressure. Let it flow.
         Now, having said all that. Even if the system is not perfect in the science end. A 2 into one is better than not because of the scavenging. In a perfect world. We would want the pipes straight for a smooth un restricted flow. But its not a perfect world. In my case. In my quest to keep the pipes straight. The simple design caused to much(long) pipe to be bull whipped by the engine vibrations causing it to crack and brake off, and you all know where those parts went?! 

         So I have look over Kev���s set up. I like how he has taken up the needed length by snaking it up and over the upper pulley. Doing this now has very little hanging under the craft. If you notice. He also built it in section with slip joints. This will also help keep the vibs from traveling down the pipe. Does the curved pipes around the pulley restrict the flow? No doubt. But you have to do what you have to do. I like his design and I���m in the process of duplicating it. The other challenge of course is the weight.  

       Whats a fella ta do?!


      On Mar 1, 2014, at 8:12 AM, mostsigbit@... wrote:

      Hi Thomas

      BTW my name is Mike, not Philip.  
      What I was attempting to ask, knowing exhaust design is both art and science, is whether Kevin had done any of the science .  I am asking whether he made any quantitative measurements of the total power output, or is it all just art and blind faith and 'seat of the pants' feel.  
      You say the first harmonic gain is a possible 10%.  What happens out of the target rpm band?  Are there any possible losses, or is it all possible gains.       

      I was also genuinely asking whether he considered the numerous exhaust bends in his calculations, or whether they can or should be considered at all, or does one simply have to 'live with them'.  I was also attempting to ask whether he 'fine tuned' his tuned exhaust, or simply went with the calculations.  

      Is the effective tuned band as narrow as you indicate, 100 rpm? Or is that just a number to illustrate the proportionality of the power band to the design rpm?  Without a variable pitch prop, it seems it would be difficult to realize any true advantage.  Altitude and meteorological changes would ensure you would only be in the tuned range almost by chance.  Since you can't have both, do you design for full power or cruise rpm?   Can anyone provide reference to the theoretical power curves for a representative tuned system?   In general, is it correct to assume as you increase the harmonics, the width of the power band increases, but the gains decrease?           

      I'm not trying to stir the pot, I'm just being inquisitive.  Ultimately, I am trying to determine whether a long tuned exhaust is 'worth it' on a small v-twin 4 stroke.     
      I realize every application will be different.   In your example, you have a tractor configuration, thus a straighter exhaust is possible.  Still, you say the ideal length is 1.5 to 1.7m.  That is a difference of nearly 8 inches.  Do you have any intentions of tuning to 'best' length, or will you just go with what works out the best from an aesthetic or installation ease length?

      Mike Phillips

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