Help with venturi diameter

[Andrew Borgogna]

Guys, I have an O/S LS.65 R/C engine that I want to use for control line.  Ignoring the weight of the engine, it's heavy, can any of you suggest a good starting point for the internal throat diameter for a venturi.  I want to remove the R/C carburetor and replace it with a standard U/C carburetor.  The R/C carburetor is .323" diameter at the base and .468" at the intake.  These dimensions assume the throttle barrel is locked full open and does not account for any leakage past the barrel, if any.  The engine has a remote needle and I plan to use it with the new carburetor, I may later change to an NVA in the carburetor but not to start with. 

Any help would be greatly appreciated.
Andy Borgogna

[Paul Smith]

I've never messed with venturis on 60's, but based on smaller engines, I'd start at .275" and put it into the air.  See what happens.

Venturi size isn't all that critical, unless you're trying to get every last RPM before it sags off.   You can drill or ream a small venturi, but if you start off oversize, you need another venturi.

[Andrew Borgogna]

Thanks Paul for your input.  Actually I am trying to get better gas mileage from the engine.  I have it on a Score and with a 8oz. tank I can't get the entire pattern in.  I am hoping with a smaller venturi I can extend the mileage to complete the pattern. 
Andy

[Paul Smith]

One thing that REALLY helps is a muffler with more back pressure. 

Assuming you can spare a few RPM, you'll gain a lot more run time from the muffler than you will from the venturi.  If your muffler outlet is way oversize to begin with, you won't lose any power at all. 

The theory of believers of this concept is that the back pressure reduces the unburned fuel that's thrown out and wasted.  I've gained enough time to do the final two stunts of the pattern just by plugging two holes of a tongue or putting a 4/40 bolt across the outlet of a tube.

[Andrew Borgogna]

Good point Paul.  The IC engine is an air pump, you can block air coming in at the venturi or restrict it's exit at the muffler.  Either case reduces the amount of air moving through the engine, when you reduce the volume of air you must also reduce the volume of fuel therefore increasing the mileage.  I will investigate your approach along with the new venturi.
Andy

[Paul Smith]

One further point:

An oversize venturi will tell you right away, in the form of not getting off the ground or sagging in turns.

But an oversize exhaust won't show itself other than by excessive fuel consumption.  One technical manual I happened to read pointed out that the exhaust port is open less than 25% of the time, but the majority of the noise leaves the engine in the first 5% of the exhaust cycle.  But the ultimate muffler opening is open 100% of the time.  So the ultimate muffler opening (assuming the muffer has some volume) only needs to be 20% as big as the engine's exhaust port, at most!

A 60 is only 50% bigger than 40.  An OS 40 can easily do a stunt pattern on 4 ounces of fuel.  8 ounces in 100% more than 4 ounces, so fuel consumption is definately excessive. 

[Andrew Borgogna]

Paul
Here is my first step.  I had the extra venturies that come with the Evo .36.  The large one is very close to your starting point, and the size of the base is a couple of thousands under the O/S carb.  I shimmed up the base with high temp duct tape.  It is very thin so a few layers brought the diameter up to the correct size.  I then drilled and tapped two holes to lock it in and cut to 4-40 screws to secure it in the engine.  I ran the engine at 10K on a 12x5 zinger wood prop and got 3 minutes and 45 seconds out of a measured 4oz. of 10% fuel.  That equates to 7 minutes 30 seconds on the 8oz. tank I was using.  Should be plenty of time to get the pattern in, but as you said the only way to tell is fly the plane.  One last thing, I weighed the O/S R/C carb  on my letter scale it weighed 1.3oz. the replacement U/C carb weighed .4oz so I saved almost a full ounce off the nose of the plane.  I will let you know how things work out after I fly it.  If I still need to save fuel I will attack it from the muffler end.  Thanks for you help.
Andy

[dale gleason]

Hi Andrew,
Locally, Bill Wilson has had great success with his LA65 as a stunt motor. The weight isn't a problem on his plane as it has a very short nose. And it develops the power of a 90. He uses numerous 1/32 plywood wedges jammed in the venturi vertically to reduce area and thereby reduce fuel consumption. You can do the same thing by stretching layers of panty hose material over the venturi, holding them in place with a couple of "O" rings. I like the latter because it works as a filter, too. Hope this helps.
Hope to see you at the VSC,
dale g