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Old Apr 25, 2012, 01:29 PM
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Cox TD .09 Venturi Bore

I have a new TD .09 and am wondering how far to bore the venturi to run on bladder presure for free flight. The bore is about 1/8" stock, barely more than the .049, which can be bored to 5/32" or more. There appears to be more than enough meat to allow for enlargement within reason. I'm thinking at least 3/16". Anyone else done this?
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Old Apr 25, 2012, 02:57 PM
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I haven't done this first of all, but the undercut is .270" so to allow any meat left, I wouldn't go over 210" myself. Normally any bigger than the crank bore is the max. for the venturi. The needle valve may be hard to set at .210", and there are no fine thread needles available, so 3/16" sounds good. The rc .10 sizes are normally 4mm. or .160" and they run on suction fairly well.
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Old Apr 25, 2012, 04:22 PM
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Been there, done that, broke a crank. The TD 09 stock needle will work OK on crankcase pressure. Kustom Kraftsmanship made a fine thread needle assembly for the TD 09, but I've only seen a single one.
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Old Apr 25, 2012, 04:22 PM
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The crank bore is slightly over 1/4". I don't intend to go that large. I know that this engine has a reputation for being one of the best of the TeeDee line, but with the venturi so small I think there is a lot of room for improvement, especially since this engine will be run on pressure.
Jim, how hard were you leaning on the engine when the crank broke, nitro -wise?
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Old Apr 25, 2012, 05:54 PM
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Originally Posted by Gnu4FF View Post
Istock, barely more than the .049, which can be bored to 5/32" or more.
There's NO 'or more' onna TD .049/051.
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Old Apr 25, 2012, 08:47 PM
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I probably was running 30% nitro. In recent years, flying control line stunt models, I have been using 10% with stock venturi and crankcase pressure. It seems easier to get to run consistent than the higher nitro I used to use. I have broken a crank under those circumstances as well. Fortunately I got three cranks cheap off eBay some years ago.
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Old Apr 25, 2012, 09:48 PM
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I run my Tee Dee's on Sig 35% fuel, with no added castor. The "Cox" recomended bore for a .09 venturi on pressure is 7/32" however, I'd stay at about .200" , that last little bit of bore will not give you much advantage and the slightly smaller bore will be easier to dial in.

George
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Old Apr 25, 2012, 10:47 PM
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Originally Posted by Bare View Post
There's NO 'or more' onna TD .049/051.
Just out of curiosity I plan to run a comparison of 4 venturis on the same TD .049 engine/gas/prop/plug to determine what's what with bore diameter. I'll let ya'll know how it comes out.
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Old Apr 26, 2012, 09:18 PM
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Fill yer boots .... just be aware that It's all 30+ year old.. Proven.. history tho :-).
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Old Apr 26, 2012, 09:23 PM
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Originally Posted by Jim Thomerson View Post
I probably was running 30% nitro. In recent years, flying control line stunt models, I have been using 10% with stock venturi and crankcase pressure. It seems easier to get to run consistent than the higher nitro I used to use. I have broken a crank under those circumstances as well. Fortunately I got three cranks cheap off eBay some years ago.
Try ? a bit higher Nitro and a Baby Bee (lo comp) Glo head
Wayyy easier on the crank (which IMO breaks very easily) and you get a larger Needling range at the loss of only a very few hundred rpms.
Never could get the TD crankcase pressure to work worth a darn..a balloon tank simply works very well ..without it.
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Old Apr 26, 2012, 11:02 PM
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Everybody needs to do their own thing. And must admit I've never dealt with TD.09. However, I seem to recall that the TD.09 was rated the most powerfull engine of its class for MANY years. Having flown a number of TD.049/.051;s in FF and CL, I can't imagine how the follow on Russian and Chinese engines had more power.?? Not that I can't imagine more power, it's just what does one do with more power than the TD's?? I'm kinda glad that I'm not competing anymore. The last time I flew a TD in a controline combat model, I couldn't keep up with it. Eventually planted it on the far side of the circle, as I attempted to go inverted. It happened sooooo fast...

I tried to concentrate on reliable engine runs without resorting to drastic "go fast mods". From what i saw with guys modifying for crankcase preasure, didn't result in reliable runs. Guys were very competitive with pacifier preasure , but I always used stock TD's on normal suction and gave them a run for the money. I was always looking to build a better lighter model (usually a Starduster) and hot prop combo.(I liked to modify a Top Flight 6-3 or 6-4 wooden prop, by cleaning up the hubs, thinning the blades, rounding machined square edges, refinish and careful balnce etc.).

I can't imagine(and i'm ok with that) making a TD. 09 into more of a screamer than it already is/was. Always wanted one, but never did.
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Old Apr 27, 2012, 10:27 AM
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With the TD's, all sizes above 020, I run a one way valve with crankcase pressure and get reliable runs for control line stunt. 1/2A stunt used to be a popular event. According to Aeromodeller review, May 1963, stock TD 09 on 30% nitro fuels peaks at .235 BHP at 19,000 RPM. I turn mine loose at @15,000 on the 10% nitro and a Cox 7 x 3.5. I have tached one at 17,000 on the same fuel and prop on the test stand.
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Old Apr 27, 2012, 11:41 AM
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Thanks all. I know this is old stuff, but so am I. I've had these Cox engines a looong time and I refuse to pay $200+ for a competitive modern FF engine. So I'll doodle along with what I have, for now.
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Old Apr 27, 2012, 04:43 PM
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One of the reasos I stayed with stock TDs was the relative high cost (for me as a kid in the 60's). Even in the 80's, the thought of losing an engine OOS was not pleasant. I could never justify the mods that, if done incorrectly, could easily turn a fine piece of machinerey into junk. I guess I've always been a bit cheap when it comes to engines

By all means do what you like, as I got no say in the matter . I am , however, very interested in your doodling results. thanks for sharing.

BTW-Where is the best place to find replacement parts for TD's? I've got an original .010 with red carb body and .049 ,also fitted with red .051 carb body(parts were very hard to find back th late 70', as I recall) that has deteriorated and cracked. Also a medalion 15 thats failing in that regard, too.
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Old Apr 27, 2012, 08:45 PM
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You could try Xenalook for plastic parts. he has a few things to choose from. Not much for .15's. Texas Timers has some stuff too for the more popular size motors. I kind of like the AP .09 and .15 for throwaway motors. They are only $30 and worth it considering no one else makes an .09 anymore in glow at least. The Cox stuff is so old and hard to find parts for I don't like to bother with them much other than using up what I have. The reedies I hardly look at any more.
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Last edited by aspeed; Apr 27, 2012 at 08:52 PM.
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