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Author Topic: Venturi size and effects.  (Read 1018 times)

Offline Bob Reeves

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Venturi size and effects.
« on: April 01, 2008, 03:22:02 AM »
Lets say for argument sakes one puts a venturi in a ST 51 that is sized for a ST 46 (say a 0.177 sprinkler). Are you really gaining anything over a ST 46 with what would seem like a choked down ST 51?
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Offline Dick Fowler

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Re: Venturi size and effects.
« Reply #1 on: April 01, 2008, 04:52:27 AM »
Sure. Within reason, the charge size is still larger in the larger displacement engine thus the resultant power produced is greater. Less than the output with the larger venturi but still more than the smaller engine. There is a point where the inlet restriction becomes so great that the large engine cannot take in as much fuel/air volume as the smaller unrestricted engine. At that point the power produced is less for the larger displacement when compared to the smaller engine.
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Offline gary tultz

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Re: Venturi size and effects.
« Reply #2 on: April 01, 2008, 08:51:27 AM »
Makes sense to me, besides I've always heard that men with big feet have-----Big shoes.  ;D H^^
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Offline phil c

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Re: Venturi size and effects.
« Reply #3 on: April 01, 2008, 12:21:30 PM »
The venturi controls how much air the engine can pull in.  Changing the venturi size is akin to adjusting the throttle on an RC engine.  If the engine is already loaded down to a fairly low rpm(9-10K) the venturi(throttle) can still pass enough air for it to run at its full speed.  The restricted opening will keep you from getting the max HP the engine can produce by running a smaller prop at a higher rpm.  So something like the LA 46, which has a peak HP of around 1.2 at 14,000 rpm will only get to 14,000 on a smaller than usual prop, and produce less than 1.2 hp as a result.

In other words, to a point, restricting the venturi mostly affects the high rpm peak power of the engine, and not the lower rpm range.

More displacement will let you run a larger prop at the same rpm.  It has more torque across the board.  This is really apparent with the LA 40/LA 46 engines, since they are so close in design.  On the exact same venturi and fuel the 46 can turn about a half inch larger prop than the 40.  You can adjust the 40 to turn the larger prop by opening the venturi/more nitro and other tricks, but when you do it is working much harder than the 46 and is more likely to overheat and runaway.
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phil Cartier

Offline L0U CRANE

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Re: Venturi size and effects.
« Reply #4 on: April 01, 2008, 03:40:11 PM »
Phil has it just about nailed down.

Another way to see the same thing is that we don't run our engines to the RPM they could ultimately reach on whatever venturii ID we have. We restrict RPM by prop, fuel, plug, needle, to get the airspeed and running characteristics we want.

The prop is the load... If we went to the ultimate RPM possible on a given prop, that would be more than the RPM that "behaves" dependably for CLPA.

Fuel influences how the combustion proceeds: Alcohol is good at cooling when it vaporizes, so it keeps the engine from overheating if we do it right. An excess of alcohol somewhat damps the combustion by chilling it, as does the oil. We don't fly stunt with the engine totally leaned out to were we couldn't dial in a few RPM. So, there's always a bit of excess fuel going through the engine...

Plugs operate in some magical, mystical way, and we need to start by trying what others have found works in our particular engine. If that doesn't work, try other well-recommended plugs until you find what does.

 

What the needle does is pretty well suggested above. It is nice to have a progressive needle - which means that the engine richens or leans consistently and steadily as we adjust.
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Offline RandySmith

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Re: Venturi size and effects.
« Reply #5 on: April 03, 2008, 03:23:52 PM »
Quote from: Bob Reeves on April 01, 2008, 03:22:02 AM
Lets say for argument sakes one puts a venturi in a ST 51 that is sized for a ST 46 (say a 0.177 sprinkler). Are you really gaining anything over a ST 46 with what would seem like a choked down ST 51?


Hi Bob

Yes  Dick has it correct, plus the fact that it not only takes in a larger charge thru the same size venturie, the 2 motors are differant ,the old 46 is an older generation, single bypass  ,cross flow engine ,
and the newer  G51 is a more efficient Loop charged motor...IE  more of its charge will typically get burned, instead of flowing out the exhaust...how was that for a  run on statement? ;D

Regards
Randy
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Offline Bob Reeves

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Re: Venturi size and effects.
« Reply #6 on: April 07, 2008, 03:51:34 AM »
As usual you guys were right on...

Made a spigot venturi for my 51 that calculates out to the same as a stock ST 46 sprinkler and it worked great. I fought engine run issues with the 51 in my Skylark all last summer without a thought that the spigot I made with the choke area slightly larger than the stock venturi just might be too big. Should have suspected, it was telling me what the problem was (5 minute runs on 5 1/2 ounces of 10-22 and slowing down in the squares) but I wasn't listening.

Next step is to make a larger venturi with a nylon choke adjust screw like I do on my 4 strokes. Might be a good subject for another thread...
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