Engine basics > Engine set up tips
Venturi inside diameters
Brett Buck:
--- Quote from: Dan McEntee on March 22, 2022, 06:35:10 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.
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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
BillLee:
For a calculation of the effective venturi area/spray bar combination, see http://www.nclra.org/Programs/Venturi.php
Paul Smith:
--- 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?
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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.
George Fruhling:
--- Quote from: Dan McEntee on March 22, 2022, 06:35:10 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.
Type at you later,
Dan McEntee
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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.
Motorman:
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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