Best Venturi size for LA .46

All,

In your experience what is the best venturi size to use on an LA .46. The only caveat is that I definitely want it to use less than 5 ounces of fuel for the pattern. Right now I plan to drill a stock Tower .40 unit out to .281, 9/32. Will this get the job done with less than the 5 ounces for the pattern? The stock unit is now coming with a .303 according the Len. What is the take on fuel draw with that larger of a venturis? Seems to me that at .303 is getting in the range that Super Tigre .60's use? All comment welcome. Of course I am planning on using mine in a Palmer T-Bird and later a Warden Continental. Both planes should be under 50 ounces.
Thanks again guys!

Jim, do a search for LA 46 venturi and you will see what works for some doesn't work for others. I started with a 293 venturi, Super Tigre needle and had fuel delivery issues during the pattern, a Bru-Line air filter fixed it.

I was running a 4.5 ounce tank which wasn't quite big enough. If I got off just a little rich the plane would fly great but would run out of fuel in the clover.

How much would the type of of run preferred (4-2-4 VS 2-2-2) affect the venturi size needed, if at all?

Jim,

I'm using a 4.5 oz tank with mine in the Time Machine 50. .285 venturi with ST needle. runs about 7 minutes.

LAST EDITED ON Jan-12-04 AT 06:24 PM (CDT)
 
Jim,
I used a .272 with a .135 spraybar and used 5.5 oz of 10% Powermaster. 5 oz is cutting it thin IMHO.

Crist,

I will run it on a bench for a while and then check the fuel consumption after about 30-45 minutes of run time.
5.5 OZ is a lot of fuel consumption for a .272. Did you try a head
gasket or anything like that?? Just wondering!

LAST EDITED ON Jan-13-04 AT 06:32 PM (CDT)
 
>Crist,
>
>I will run it on a bench for a while and then check the fuel
>consumption after about 30-45 minutes of run time.
>5.5 OZ is a lot of fuel consumption for a .272. Did you try
>a head
>gasket or anything like that?? Just wondering!
>
>Jim Pollock

Jim,
No to the head gasket. Just a stock R/C engine with the carb removed. I ran an APC 12.25 x 3.75 cut down to 11.75. Launched @10,200 rpm. Thunderbolt R/C long with idle bar. Keep us posted as to how yours turns out.

Here is my setup:

OK, here goes. I bought a R/C version and installed a .272 venturi and a .135 spraybar both from Russ Gifford. You will not be disappointed with his stuff. I then replaced the plastic backplate with a Tower 40 metal one. I broke it in on the run up stand and ran about 3/4 gallon of SIG 5% Champion fuel through it. I did the 2 minute run and cool off routine for the first 24 ounces, then increased the run time to 4 minutes for the next 24 ounces. Then bacisally ran the engine for 7 mins runs after that.

I'm using an APC 12.25 x 3.75 prop cut down to 11.75 dia. I just rounded off the tips, and balanced the prop. I'm using a Thunderbolt R/C long glowplug with an idle bar. The fuel is Powermaster 10/22 with 5CC of Grape fuel fragrance added to it.
I'm using a Sullivan 6 oz RST Flex tank set up for uniflow and muffler pressure. I run the uniflow vent to the middle of the tank (fore and aft) positioned it about 1/2 inch below the TOP of the tank near the outboard wall. I adjust this up or down to get the type of engine run I want during upright and inverted flight. I like to have the engine speed up just a touch during inverted. It helps a bit with the vertical stunts. BTW, my engine bearers are under cut 1/4 inch and the engine sits on top of 1/8 phenolic pads. I'm using an RO-Jett muffler with the stinger drilled out some. I drilled out the stinger for another perceived problem that really wasn't a real problem. My guess a stock muffler would work just fine. I have a Dubro exhaust deflector on the outlet and it is cut down, at an angle, to 1 inch after the bend. I launch at about 10,200. This speed will vary depending on weather conditions. I like to "lean" it out a bit more when the weather is hot and/or windy, like we had at Rockford. I also use a Sullivan "Crap Trap" filter and Prather "pink" medium fuel line. I feel that the Prather fuel line is a lot stronger and it holds up to the nitro better. I use a SIG 2" spinner and yes, I have to enlarge the cutouts some for the APC prop.

I fly, er make that flew, the 47 oz Actroma on 62 foot (eyelet to eyelet) 7strand lines using Ted Fancher's Precision Pro handle. Lap times were about 5 to 5.1 seconds. It's been a while since I timed my lap speeds.

I need to make sure that you realize that there were NO mods done to the engine other that adding the venturi, spraybar and a metal backplate. NO shims, NO grinding, NO nothing. Just run it and enjoy it. The engine cost me $65 and the backplate $3 from Tower. The venturi was about $12 and the sparybar was $15. I really can't remember what Russ gets for these, but the costs listed are close.

>How much would the type of of run preferred (4-2-4 VS 2-2-2)
>affect the venturi size needed, if at all?

The type of run, the prop you are going to be using, the muffler you use, and definitely the venturi and needle (whether .135 OS 0r .155 ST type) will affect the fuel that you use. So there is more to the question than simply asking for the size of venturi or size of tank.

Len,

I would like to run in mostly a 4 stroke with a .135 OS needle valve.
However, depending on fuel use issues I could always chage the style of run. Looking to use an 11.5 X 4.5 or in that range at about 10,300RPM at release. I probably will just use plain old Sig blended to be 7% nitro with 20% 1/2 and 1/2 oil.

Hi Jim, As one of the pioneers in the use of this engine in Stunt(5 full seasons) I have learned at least for me the ideal starting point in venturi seize is 13 square millimeters. This is a constant and may be adjusted acording to plane, weather, and flying style requirements. I prefer the 4:2:4 routine and have found that 11X5 props run quite well. Again change with conditions. My number 1 engine and I now have well over 400 patterns and that's a lot of togetherness! I prefer the OS NVA and have changed the id from time....rarely, via a tapered reamer. NEVER NEVER EVER use drill bits as the impact is way more than needed even if you only move one drill bit number. My plane uses just under 5 ounces for the pattern. My fuel is 7% Nitro, 22% castor oil and 3% synthetic oil. My deck height starts at .043 and is adjusted by shims. The St .45, .46 and .51 head shims all fit and are thinner by about 50% and thus give a much finer adjustment. On occasion 5 and 6 percent nitro may be used for gains in economy and cost! After all the running my engine has had it has shown no sine of getting tired! Incidentally I am using the stock, RC type muffler on some of my set ups but on my favorite I am using a Randy Holcroft muffler. Sadly he isn't making them any longer.Big Art tube mufflers work equally well too. The back plate is stock with stock screws. Works just fine and gives a bit better stuffing than the Tower .40 BP. You may replace the phillipps head screws in the head with cap screws if you wish,but leave the phillipps in the bp so as not to distort the BP.

Pancho,

After figuring out how to do calculations from millimeters to inches. I calculated that 13MM would require a venturi bore of about .312. That seems rather large to me. You must be using some head shims to get the fuel used down to the under the 5 ounce range. I guess the moral of the story is that there are plenty of ways to skin the Cat, Koshka, or whatever you want to call it!!