Engine basics > Engine set up tips

Venturi inside diameters

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Allen Eshleman:
Would it be possible to explain the part where it depends on the muffler?

Chris Wilson:

--- Quote from: Allen Eshleman on May 05, 2021, 06:54:27 PM ---Would it be possible to explain the part where it depends on the muffler?

--- End quote ---
To further that, compared to open exhaust please.
Chris. Is

RandySmith:

--- Quote from: Allen Eshleman on May 05, 2021, 06:54:27 PM ---Would it be possible to explain the part where it depends on the muffler?

--- End quote ---

Yes..  2  end  of the hose, closed down  mufflers  make the  venturi  "act"  smaller, so  open mufflers  make the  venturi  "act"  larger,   example is a  stock  os FP 35 40  muffler, it has a  tiny  outlet hole,  so did the  stock  OS 35S  muffler, , they both  worked bettter  with a  huge  300  venturi, but the  real  FIX   was  to open them up and  run a more  normal size  venturi, like a  .275  to  .285.

Randy

George Fruhling:

--- Quote from: RandySmith on January 26, 2021, 06:31:25 PM ---Max S 35 -   .265 to 300 restrictor  depends on muffler
Max S 25 -    260 to 281  depends on muffler
FP 40 - 265 to 281  restrictor  depends on muffler
FP 35 - .265 to 275 restrictor
FP 25 - 260 to 270  restrictor
ST 35 -  .157 to .165 venturi       or  265 restrictor
ST 46 - .165 to .170 venturi        or  265 to 281  restrictor
ST 56 - 175  to .183 venturi     or  275 to 285 restrictor
ST 60 -.181  to .190 venturi       or  275 to  295 restrictor
G51  loop charged   .175 to .181 venturi    or  270 to 281  restrictor
(STs are all baffle piston types)
Max H 40  265 to 290 venturi depends on muffler
LA 25 -  260 to 280  depends on  muffler
LA 40 -  265  to 281  restrictor
LA 46 - 270 to  285 restrictor  depends on muffler

--- End quote ---

I assume the dimensions are in inch size. I have a Super Tigre   G 21 46 I purchased new circa 1980. It was originally an RC engine. The owner of the hobby shop called Bluff City Hobbies in Memphis TN removed the carb, deducted the price, and installed a venturi, adding that cost.  But I have no idea what size it is or that they made different sized.  Do these numbers work for my motor?  I was thinking of using drill bits to measure mine. 

Dan McEntee:

--- Quote from: George Fruhling on March 22, 2022, 12:24:16 AM ---I assume the dimensions are in inch size. I have a Super Tigre   G 21 46 I purchased new circa 1980. It was originally an RC engine. The owner of the hobby shop called Bluff City Hobbies in Memphis TN removed the carb, deducted the price, and installed a venturi, adding that cost.  But I have no idea what size it is or that they made different sized.  Do these numbers work for my motor?  I was thinking of using drill bits to measure mine.

--- End quote ---

    Yes, the measurements are in inches. If the venturi is a stock unit, which were aluminum, a true venturi, and they have a little step at the bottom of the bore, you might want to try it like that. The listing shows the dimensions for that type of venturi first. If it has the spray bar going through the venturi,  that is a restrictor type and use the second set of dimension for the ST.46. Use the shank of drill bit to gauge the size of the hole and then measure the shank. The edges of the flutes on a drill bit are what sizes the drill and the shanks are a bit smaller. I just don't like to stuff the sharp edges down the bore.

  Type at you later,
   Dan McEntee

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