Hello All:

        After researching this question, I am still slightly confused.  This question applies to a venturi with a through spray bar and a venturi that is called a spigot type where the spray bar only goes about one half of the way through the venturi.

                                                                                                                                                                   Tia

                                                                 

       

     I have deceided that about .280 may be a valid starting point.  MECOA sells a spigot Venturi with a 5 mm bore.

   I was avoiding this thread (since as far as I know there is no such thing as a HP 40 Stunt...) but .280 with which spraybar? 5 mm with what size spigot?  I don't know what size the HP spraybar might be. .280 is probably too big for a Silver Star with an ST spraybar, and way too big for a Gold Cup. With an OS or OS-copy spraybar, its vastly too large for either engine. .197/5mm is way too big for either one with flush outlet, but a large spigot with a long projection could be in the ballpark, with the typical 1/16 diameter spigot with .030 projection, still far too large.

   It also depends on what sort of prop you are planning on using, large high-pitch props will tolerate less venturi than small low-pitch props.

  I would always start with the very low end of venturi size and work up, not start out too big and work down. That means a starting choke area of .0170-.0175 square inches or so, I think.

     Brett

p.s. by the way, what did you "research" to come up with these sizes? I am not aware of any serious efforts to use these engines, unmodified.

There difinatively wuz , you saw adds forem in the late 70s , with the wierd side feed tall funny venturie . The Factory fit .

  The second picture, at least, looks to be a Silver Star 40S. "S" doesn't stand for "stunt" but for "standard" (i.e. not RC). That does look like the factory venturi/spraybar that I recall. The first picture looks like a Silver Star with a Gold Cup anodized head, but of course we can't see inside.

  I always figure the Silver Star 40 was a potentially usable stunt engine, back in the days of K&B 40s, but I never saw anyone spend any real time trying to develop it. Bob Hazle spent several seasons with a 40 Gold Cup with mixed results. Bob McDonald ran one for a while, too, but with internal modifications.  It wasn't nearly as good as an ST46 as far as I could tell.

   Randy may know what you need for a Silver Star, and he definitely has sold modified Gold Cups over the years.

    Brett