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14114Re: [Small4-strokeEngines] single carb

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  • factoryfit
    Mar 4, 2014
      Exhaust headers were a compromise (they always are)
      Nosing around the racing sites showed that the boys with best power had got long headers, and for some reason always siamesed. Using this online calculator with a target rpm of 4,500 and the camshaft timings gave 35" - 38". 
      You can draw a degree wheel and Bluetak it to the crank pulley with a wire pointer, but recall the numbers were at 0.50" lift, at this stage I didn't even know if it would hope to fly, as the aircraft needs everything a Rotax 447 can give, and that is 39hp. Plus it was heavier.
      The head ports sloped upwards, so that plus the long lengths given by the calculator meant the first 90 degree bends were set upwards, then both angled inwards at 45 degrees off the flange plates, and to complete added two 90 bends then 45 degrees to run down the sides of the reduction belt. Easy decisions, based on the above factors with the bonus that all the parts were stock Walker 1.25" external, which just happens to be 29mm bore, the number the calculator gave as perfect for that revs.
      So cheap, long, easy to get and I could tack weld it all up on the motor protected with rags and aluminium foil, then unbolt and take it to the shop for TiG welding up.
      Design was unscientific, in that if Mudbuddy thought it a good idea to have coiled up headers, and my solution had gentler bends, and it might take a few tries to get it right so low price and easy to get parts would be nice, so that was that.
      Other considerations were that the crank really should have been balanced, so siamesing the lower pipes made a strong triangle to bolt firmly to the casing, long curved headers could then absorb any flexing and also cope with thermal expansion.
      Four tries to get silencing right; first to get off the ground was squash the end of two pipes then drill flute holes, noisy but it flew. Right, next check was a motorcycle can, block one end and fire the noise into the 2" tailpipe, so the gas ran down the can, bounced back and got away via the annular ring formed by a small pipe wiggling around inside a bigger one, this was finefine, as the can could be mounted rigidly and isolated from engine movement by the clearance, this was reliable enough to fly down country and across to France and back, but left smoky deposits over the engine. Third try was a cheap Chinese pitbike can in blue alloy with upgraded guts, really noisy and although straight through, loss of performance. Finally made the can from 22gauge flue liner caps with a straight section and pierced tube inside connected to the outlet, this enabled the exhaust pulse to expand into the flask, but then find its way out via the pierced tube more smoothly, Cessna or Continental exhausts are like this.
      So no clever stuff, just the best mix of price and near as possible to ideal length, plus enough shove to get me flying.


      PS please DON'T fit one big single carb, it screws up the mixture distribution and shortens the life of the engine, keep to two separate inlet tracts, Briggs etc achieve this with one carb and two chokes, each running to its, own cylinder. The big carb off the 61" or 990cc engine is the answer.

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

      I see that I made a typo, and it was a serious one:


      " If the 4 ft length happens to resonate at 6000 rpms, the suction returning from the 6 ft point might be 4000 rpms. (These are not real numbers - I am just trying to keep it simple).  You will NOT have a reasonable suction at two points in the powerband"

      The last sentence in this para should have read: "You will NOW have a reasonable suction at two points in the powerband."



      Gary Wolf


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