Pros and cons of using the 7th injector with GTSIO-520-N

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  • #83772
    sworley
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      Does anybody use the POH procedure and have the mixtures in the ground start position when they fire up the N engines? What are the pros and cons of this procedure vs. the typical Continental start? How do you transition from the primer to the mixture control sending the fuel?

      I gather few people take advantage of this on the Ns. My theory is instructors and ground schools like to simplify things into something that will work for all 421s and don’t bother teaching a different method for the later engines. On the surface it seems like a great way to do hot starts, if not all starts, but I want to make sure there isn’t something I’m missing before trying a starting technique other than the conventionally taught way.

      thanks,
      Spencer

      #96640

      Hello Spenser,

      I am not an expert on the N model but I do know that it allows fuel to be injected into the spider manifold above all cylinders instead of the intake of each cylinder. The theory is to put fuel in the manifold and then the vapors are easier ignited in an already warm engine, instead of trying to shoot fuel through very hot little tubes and expect a steady stream. That is good but the supply lines are small stainless steel lines routed in an area that is overcrowded to begin with, raw fuel sitting in proximity to chaffe points. Being able to hot start easier is not my engineering, but think of the marketing idea of Cessna in the 80’s, but if that’s what you have make sure all lines are secured and not rubbing anywhere. A properly installed 7th injector is completely safe. I have had to make custom brackets to give proper clearance from the fuel lines and reference lines in the area,1/4″ is minimum 1/2″ is preferred.

      Hot starting a C model is a piece of cake with the L or N engines. I use mixture idle cutoff 30-60 sec with fuel pump on high, pump off, mixture rich, .5 sec of prime and voila.. If it dies after start a tickle of the prime switch with keep it going but do not hold it on prime it will flood and go too rich quickly. Warm the engine needs little fuel to run and start, once started if power is low you may need low pump or keep power above 800 RPM’s for good flow through injector lines till they are cool again and you are set. Hot starting a B model on the left side is a tough one, fuel lines are too close to slip joint!

      The N engine is a unique design with a 2 stage engine driven pump either lear siegler or Bendix I forget coupled with a TCM fuel control attached to the throttle body. Setting these fuel flows up are VERY different that the TCM full system. I have found that at the same power the N needs .5-1.0 gph more to be happy, why I do not know, power is power, may be efficiency of atomization given the fuel pressures which are different.

      There is a specific fuel setup that is not in the Cessna book that I got from Western Skyways specifically for the N model, let me know if you need a copy.

      Peter Danto
      Premiere-Aviation Inc. 8)

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