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Boring a Tee Dee for pressureized fuel feed Empty Boring a Tee Dee for pressureized fuel feed

Post  RknRusty on Tue Dec 13, 2011 11:36 pm

What's the best venturi bore size for a pressurized fuel feed for my fun-flying? I know the one I have on it is larger than stock but not sure exactly how much it's bored. I tried to see what size drill bit will fit through it, but it's in between bit sizes.
I do have a screen on it, so that might change the bore requirement, but it's better for hunting gophers.

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Post  Jaspur_x on Wed Dec 14, 2011 12:21 am

I`m waiting to hear too, I never went overbore with mine, stayed stock until I carbed it.
I never presurized my fuel tanks either;but i liked using the cox tank/mount on mine which may have been why I never needed pressure;even vertical flying.
Maybe your overbore is/was the source of your fuel draw dillema??
I don`t know, but, when you open up the bore diameter you increase flow volume;decrease vacuum fuel draw.......in theory.............. and depending on how far the bore goes will have effect too.

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Boring a Tee Dee for pressureized fuel feed Empty Re: Boring a Tee Dee for pressureized fuel feed

Post  RknRusty on Wed Dec 14, 2011 10:39 am

Jaspur_x wrote:I`m waiting to hear too, I never went overbore with mine, stayed stock until I carbed it.
I never presurized my fuel tanks either;but i liked using the cox tank/mount on mine which may have been why I never needed pressure;even vertical flying.
Maybe your overbore is/was the source of your fuel draw dillema??
I don`t know, but, when you open up the bore diameter you increase flow volume;decrease vacuum fuel draw.......in theory.............. and depending on how far the bore goes will have effect too.

When I was using vacuum feed I had the stock venturi on it. I think with pressure feed, the venturi has to be wide or else the engine can't get enough air to mix with the magnum flow from the bladder. I posted the question at RCG but I stuck it at the bottom of another thread. If no one answers soon I'll start a new one. I expect Bare or Earlwb will see it today. Both of them will know the answer.
I forgot to mention it's an .049.

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Post  fit90 on Wed Dec 14, 2011 10:50 am

Rusty,

It is not necessary to bore the venturi just because you are using a pressurized fuel system. When I get home tonight I will measure a bored venturi and get back to you.

Bob

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Post  nitroairplane on Wed Dec 14, 2011 10:58 am

You don't bore it out so you can pressurise it you pressurise it so you can bore it out.
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Post  fit90 on Wed Dec 14, 2011 4:11 pm

Just measured it. .153" with uncertified calipers. A #22 bit was the largest I could get in the venturi without working at it.
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Post  RknRusty on Wed Dec 14, 2011 4:39 pm

fit90 wrote:Just measured it. .153" with uncertified calipers. A #22 bit was the largest I could get in the venturi without working at it.
That's probably the size on mine, seeing as how you're probably the one that bored it. It's just a point of curiosity more than anything. Mine runs good with the current setup. In fact it runs so good, I'm fixing up the plane. I even made a new pilot head for it. The old one must've gotten scared and bailed out while I was test flying.Boring a Tee Dee for pressureized fuel feed Icon10

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Boring a Tee Dee for pressureized fuel feed Empty Re: Boring a Tee Dee for pressureized fuel feed

Post  fit90 on Wed Dec 14, 2011 5:13 pm

Rusty,

I just checked the venturi with fractional bits. The venturi was bored to 5/32". If your venturi has a screen on it then I did not bore it out. I always remove the screen first.

You could start out with a 1/8" bit and run the engine and keep working up to bigger bits as long as the venturi does not get too weak and you like how the engine runs with it.

Good Luck,

Bob
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Boring a Tee Dee for pressureized fuel feed Empty Boring out a TD venturi

Post  Paulgibeault on Mon Dec 26, 2011 3:29 pm

Gentlemen,

First off, boring out the venturi only really works well when the engine rpm is expected to go over 20,000.
Assuming your prop/ fuel/ engine combo will do that then you with definately see then benefit of using a pressurized fuel system. Going up to .140" I.D. is the standard number. The late Joe Klauses K/K pressurised venturi's went as high as .150". They work well in the 22-24,000 rpm band. Of course I wanted to know the limit & went up to .165" ID. That venturi lost noticeable rpm compared to the .150 venturi. So I learned that bigger is not always better. IF one cuts the prop down to a silly small diameter & raise the engine rpm up to 26,000, then the .165 can show a small gain. BUT such a prop is useless on a plane & also causes the engine to die a very early death w/ piston failure etc.
And another thing. Joe Klause decribed enlarging the venturi using a tapered reamer, keeping things very nice & smooth. Just drilling out a bigger hole with a common drill CAN in some cases cause a loss of rpm.
If anybody is interested, I have a supply of TD pressure venturi's for sale ($10 each + post) or can ream a new one to your 'magic number' if you wish...

Merry Christmas Everybody!
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Boring a Tee Dee for pressureized fuel feed Empty Re: Boring a Tee Dee for pressureized fuel feed

Post  RknRusty on Mon Dec 26, 2011 10:51 pm

Paulgibeault wrote:Gentlemen,

First off, boring out the venturi only really works well when the engine rpm is expected to go over 20,000.
Assuming your prop/ fuel/ engine combo will do that then you with definately see then benefit of using a pressurized fuel system. Going up to .140" I.D. is the standard number. The late Joe Klauses K/K pressurised venturi's went as high as .150". They work well in the 22-24,000 rpm band. Of course I wanted to know the limit & went up to .165" ID. That venturi lost noticeable rpm compared to the .150 venturi. So I learned that bigger is not always better. IF one cuts the prop down to a silly small diameter & raise the engine rpm up to 26,000, then the .165 can show a small gain. BUT such a prop is useless on a plane & also causes the engine to die a very early death w/ piston failure etc.
And another thing. Joe Klause decribed enlarging the venturi using a tapered reamer, keeping things very nice & smooth. Just drilling out a bigger hole with a common drill CAN in some cases cause a loss of rpm.
If anybody is interested, I have a supply of TD pressure venturi's for sale ($10 each + post) or can ream a new one to your 'magic number' if you wish...

Merry Christmas Everybody!

Thanks Paul
I have one that's apparently stock and one that's bored. I'm not competing or anything like that, I just play at stunt flying. So whenever I can get some flying time I'll switch between the two venturis I have and just decide whichever one runs best. I'm more concerned with consistent speed than highest speed.

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