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Engine basics => Engine set up tips => Topic started by: Andrew Tinsley on July 10, 2011, 02:21:04 PM
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Hello,
I have been following the OS 46 LA setup topic with some interest. I run mine with the stock large OS venturi and NVA, in a wet two stroke with an APC 12.25 x 3.75, when the grass is short and an APC 12x4 when it isn't! Oh and maybe an extra head gasket if required (some of my LA 46s need it and one doesn't)
My query is why do so many people use the smaller OS venturi used on the FP 20 /25 and I believe the LA25? I appreciate that the smaller venturi would help to keep wind up to a minimum. However, my setup with the larger venturi doesn't suffer from such problems. So what is the advantage of using the smaller venturi? (I know that the larger OS venturis have varied in size over the years, but I don't think that is significant here). I am not trying to be argumentaive, just curious, obviously a lot of people get good results with the smaller venturi. It makes me wonder if venturi size is as important as some people claim. Anyone got any comments?
Regards,
Andrew.
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Andrew, i started using that setup a while back and the advantages i have found are no loss in power and fuel savings up to an ounce per pattern. Most of my planes using the la46 were originally designed for 35 to 40 engines anyway so any slight loss of power is not noticed. One other advantage is the venturi will still work on a 20 or 25 size engine if you need it. The reason i originally tried it was i needed a venturi for a 25 la for speed limit combat and used a .46 venturi, that resulted in an easier needle setting and still did not exceed the speed limit, so i have switched the 46 venturi's to my speed limit 25's and use the 25 venturi's on my 46's. Try it, you'll like it!
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The thing I found with the smaller venturi,, talking .272 to .275, with a ST needle ( or randy smith needle) is that the engine is MUCH more needle friendly, uses less fuel, and runs just as strong. I noted with the larger venturi that it was spitting fuel all over the front of the airplane as well, possibly part of the fuel consumption issue. I also get better more consistant starts with the smaller venturi.
These are my observations, and could be based upon misguided observation but Pat Johnston and I both came to the same conclusion so take it for what its worth. He uses 5% 22% fuel, I use 10 22 fuel. I burn right at or less than 4 oz to 4.25, he is now using about 3.25 to 3.5 oz for the pattern.
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Thanks guys,
Why didn't I think of fuel consumption! I am very happy with the needling using the larger venturi, but if it gets better with the 20 FP venturi then I think I should give it a try.
I am not mean or tight fisted, but I am averse to spending money unnecessarily! The saving of an ounce per pattern sounds very attractive. I have quite a few BBTU FP20s, so I shall steal some venturis from them and give it a try.
Thank you all for your comments,
Andrew.
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Remember that the less fuel used, the more variation there will be in run-time accuracy. If you run 5 oz, then 1/4 oz error won't be critical, but if you run 3 oz, 1/4 oz is a bigger percentage. Similarly, a change in NV setting or slight bit of chit in the spraybar will have a bigger effect. And then, there's the problem of adding nitro to adjust for density altitude changes, and your 7 minute flight time vs. our 8 minute limit. Many things to consider. :o Steve
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Steve,
on the surface you are correct, however one of the anomolies we found with the "old" setup, ie larger venturi, 11.5x4 prop, is that it was actually MORE critical on fuel burn with relation to needle setting. a small variation in needle setting made a HUGE impact on flight times with the old setup. with the smaller venturi, even considering the smaller fuel load, it was much easier to regulate the fuel needed since the setup was more stable in different conditions. Keep in mind I go from 2500 foot altitude( and much higher density altitude during the summer) to virtually sea level when I attend contests. I found that I had much less trouble gauging fuel load with the smaller setup .
HOwever you are correct in your observation that a 1/4 ounce is a much higher percentage on 3.5 ounces than on 5 ounces, its just this particular setup makes it less critical in my experience
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Hi Steve,
I alredy have big problems with running an MVVS 49 diesel in F2B that causes a huge variation in run time depending on conditions! The variation in conditions are not obvious either, I finished up by overfueling and run a very small electric fuel cutoff! So I have added weight from both causes. Still I like diesels because the smell is beautiful!
I have all the bits to hand to try the smaller venturi and give it a whirl Maybe a fuel guzzler setup has its advantages, thanks for pointing that out.
Regards,
Andrew.
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I have not tried this, or had this experience, but some have reported that a small venturi made their engine run as though it had low oil content in the fuel. Theory is that increasing the venturi size increased the amount of fuel and oil going through the engine, and was supposed to fix this. I'm not so sure that just adding oil to the fuel wouldn't do the same thing. I can see that if the engine is overheating (due to too much prop load), adding nitro might make it run cooler, since you would have to run more fuel and thus more oil through the engine during the flight. And of course, the engine would produce more power, so swing that prop easier...and a richer setting for the same rpm at launch. H^^ Steve
Edit: I did have to reduce the venturi bore to .272" (.156" spraybar) on the .46LA in my ex-McClave Skylark, just to get enough run time, even at the altitude in Tucson. I'd thought it would be ok with the .281", but wasn't. I don't really know how big the tank is, but I think Don told me it was 4.5 oz. I was running the 11.5 x 4 APC. If I'd run the 12.25 x 3.75, I would have needed a smaller venturi, because the fuel consumption would probably be higher.
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Hello,
Has anyone tried using the .25 LA venturi in the .40 LA or .46 LA?
Thanks,
H^^
-Danny
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@ Steve,
this is a new one on me, I have not experienced this
@ daniel
thats what we are talking about, the FP/LA venturi
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OK, I wasn't aware that the LA and FP venturi were identical for the .25.
Has anyone tried this venturi in the .40LA?
Thanks,
-d
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I would venture to suggest that the runs would be similar to the LA46, although I have not used an LA40 myself. All the above advantages with maybe even more fuel economy?
Regards,
Andrew.
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From what I have seen, the LA 40 is no where near as friendly for stunt as the LA 46 is. Its fairly tempermental in comparison, though some reports say otherwise, my experience shows otherwise
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Usual problem with the .40LA is too much prop load. The TT Cyclone 11 x 4.5 works well here, on 10-22. With less nitro, maybe too much prop. At higher altitudes, a 10.5 x 4.5 APC would be a good bet, and the 11-4 APC might work. Adding a head shim might be in order. I'd stick with the stock venturi and put a .156" Randy Aero NV Assy. in it, but would eventually try the .25LA venturi.
FWIW, I have my .25LA running on a venturi drilled to .272" with a .156" spraybar. I can restrict it down with a few layers of pantyhose stock, or a Bruline air filter, but it runs alright, and without muffler pressure. y1 Steve
Edit: This reminds me...why not use the stock .40LA-S or .46LA-S venturi with any spraybar you like, then add some air filtration to make it think it's smaller than it really is? It does work, and you don't have to remove the venturi to test changes. I will say that I see no way of getting any consistent adjustments out of pantyhose material, so I do prefer to use either the coarse or fine Bruline filters.