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Author Topic: Venturi inside diameters  (Read 6802 times)

Offline Robert Zambelli

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Venturi inside diameters
« on: January 24, 2021, 08:34:26 PM »
Assuming a Super Tiger size spray bar (around .157 diameter?), I'd like to hear from you regarding you best choice(s) for a venturi ID for some of the more popular engines:

Max S 35 -
Max S 25 -
FP 40 -
FP 35 -
FP 25 -
ST 35 -
ST 46 -
ST 56 -
ST 60 -
(STs are all baffle piston types)
Max H 40
LA 25 -
LA 40 -
LA 46 -
And any others you might think of!





Offline RandySmith

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Re: Venturi inside diameters
« Reply #1 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
« Last Edit: January 29, 2021, 07:03:46 PM by RandySmith »

Offline Robert Zambelli

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Re: Venturi inside diameters
« Reply #2 on: January 29, 2021, 04:27:34 PM »
THANKS, Randy - just what I needed.
Bob Z.

Offline Dan McEntee

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Re: Venturi inside diameters
« Reply #3 on: January 29, 2021, 04:50:32 PM »
Max S 35 -   .265 to 300 restrictor  depends on muffler
Max S 25 -    260 to 280  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
ST 46 - .165 to .170 venturi
ST 56 - 175  to .183 venturi
ST 60 -.181  to .190 venturi
G51  loop charged   .175 to .175 venturi
(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

     Maybe this needs to be pinned up at the top wit6h the other stuff.  Between this info and a good search f the section you should be able to set up just about anything.  Only thing missing is ID for a restrictor set up for a ST.60  and G.51.

  Type at you later,
  Dan McEntee
AMA 28784
EAA  1038824
AMA 480405 (American Motorcyclist Association)

Offline frank mccune

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Re: Venturi inside diameters
« Reply #4 on: April 15, 2021, 10:32:07 AM »
           Hi: 

           I too think that Rand’s list should be listed at the top.

           

Offline Allen Eshleman

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Re: Venturi inside diameters
« Reply #5 on: May 05, 2021, 06:54:27 PM »
Would it be possible to explain the part where it depends on the muffler?

Offline Chris Wilson

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Re: Venturi inside diameters
« Reply #6 on: May 06, 2021, 03:21:07 PM »
Would it be possible to explain the part where it depends on the muffler?
To further that, compared to open exhaust please.
Chris. Is
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Offline RandySmith

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Re: Venturi inside diameters
« Reply #7 on: May 06, 2021, 06:33:13 PM »
Would it be possible to explain the part where it depends on the muffler?

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

Offline George Fruhling

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Re: Venturi inside diameters
« Reply #8 on: March 22, 2022, 12:24:16 AM »
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

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. 

Offline Dan McEntee

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Re: Venturi inside diameters
« Reply #9 on: March 22, 2022, 06:35:10 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.

    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
AMA 28784
EAA  1038824
AMA 480405 (American Motorcyclist Association)

Online Brett Buck

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Re: Venturi inside diameters
« Reply #10 on: March 27, 2022, 01:21:59 AM »
    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.

  The critical matter in all cases is the "choke area", that is, the cross-sectional area of the smallest point or restriction,  not necessarily the diameter. While you *can* use larger venturis on larger engines, almost all successful stunt engines run in the range of choke areas from about 0.018 to 0.023 square inches. It's not a straight function of the displacement, it's closer to a function of the "very narrow" range of required power levels, anywhere from around .35 to about .6 Hp.

    While they don't respond the same, a "straight hole" with a flush inlet (which Randy refers to as a "venturi" or a "true venturi"), a "fuel post" or "spigot" venturi,  or a "through the middle" spraybar (which Randy refers to as a "restrictor") give about the same overall power assuming they are of the same choke area.

    Larger engines can develop more power from a given venturi size than small ones, so you might run a smaller venturi on PA75 than you would on an 20FP - which is exactly what happened when David Fitzgerald won the world championship, the venturi choke area was substantially smaller than the venturi choke area used on a stock 20FP straight out of the box.

   It also depends heavily on the RPM you want to run - if you want to run it slow like in ancient times, you *have to* run a smaller venturi to get adequate suction, and if you run a large venturi you have to run it faster to get adequate suction. This is just another way of saying you are adjusting it based on the required power instead of the displacement, slow = lower power, fast = higher power.

    An extremely common problem (as illustrated by75% of the "engine setup forum" posts) is that people want to run a larger venturi to "Get More Power" and then it doesn't respond properly in the maneuvers, or they cannot needle it properly. If you make a mistake, or if you don't know for sure, you are *MUCH BETTER OFF* guessing on the small side, because at least you can still adjust it properly. Far too often (almost all the time, in fact) people try to "Get More Power" with a bigger venturi, it won't suck fuel properly, and they try to fix it by sticking in head gaskets. That doesn't improve the fuel draw, it just makes the engine weaker and even more prone to the dreaded "runaway", which means, it it trying to run the way you set it up - faster for the larger venturi.

     Brett

Offline BillLee

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Re: Venturi inside diameters
« Reply #11 on: March 27, 2022, 06:05:12 AM »
For a calculation of the effective venturi area/spray bar combination, see http://www.nclra.org/Programs/Venturi.php
Bill Lee
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Online Paul Smith

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Re: Venturi inside diameters
« Reply #12 on: March 28, 2022, 08:40:10 AM »
Would it be possible to explain the part where it depends on the muffler?

The key element is total air flow. 

.25 cubic inches times 14,000 RPM equals 3,500 cubic inches per minute.  This would be the maximum possible air flow. 
But it's not a perfect world so we never reach the theoretical maximum.
Port timing limits the air flow, along with the venturi size and exhaust restriction.

The mistake people make is the assumption that bigger venturi will always increase power & speed.  What big venturis really do is reduce intake velocity and reduce suction.  Of course if the venturi was way undersize, increasing it to the proper size would help.

In conclusion, the answer your question, an increase in muffler restriction might trigger the need to rethink venturi size.
Paul Smith

Offline George Fruhling

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Re: Venturi inside diameters
« Reply #13 on: April 04, 2022, 01:35:12 PM »
    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

Looks like a venturi  set up, not smooth but like it was step drilled. Anyway, a 5/32 bit is a very good fit. According to my math, that's 0.15625" which is kinda small considering   0.165" to 0.170" range. I'm having trouble starting it and keeping it running right. When it starts, it will run maybe 30 seconds  and then go rich or lean. Hard to tell. Running a stack muffler with pressure tap for muffler pressure. Is that the problem? Am running a 10 6 prop for now. Probably on the small side.

Offline Motorman

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Re: Venturi inside diameters
« Reply #14 on: April 04, 2022, 10:31:02 PM »
Sounds like you have a factory stock venturi. Yes, they are stepped. Couple thing to check. Venturi should have an O-ring under it. Make sure the small peripheral jets are clean and make sure the spray bar discharge hole is in the right position and not blocked by the crankcase or anything.

An easy way to check is to screw the needle out 3 turns and apply the squeezy bottle to the fuel inlet. The venturi should easily flood, if not take it apart and see what's wrong.

Motorman 8) 


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