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  1. #1
    James c harrell's Avatar
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    O.S. 46 FX on a pipe

    I recently got a Mach racer wit a O.S. 46FX with a tuned pipe. Fixed up the plane a little and did away with the gear. Looks like it shoul;d be pretty quick but I am not sure what prop to start with. I also don't know the first thing about tuning a pipe. I hope it is pretty much set byu the person I got it from so I have left it like it was set.It came with a 10-6 APC prop. The only thing I have changed is the prop. I went to a 10-6 Master Airscrew narrow blade. What size prop would you guys go with? And a short how to on tuned pipes would be great.

  2. #2
    combatpigg's Avatar
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    RE: O.S. 46 FX on a pipe

    First of all..a 6 pitch prop won't get you much more than 100 mph in most applications. If your plane is kind of heavy, draggy and you fly off a short field then a 10x6 might be the best all around choice.
    If you want speed from a .45 sport engine, then a 9x7 or 9x8 prop is more like it. Plan on dead stick landings if you have a short field.
    If that engine is broke in [no metal grit in the exhaust spray], then set it on the bench with open exhaust, wide open and get a tach reading with a 9x7 prop. This will be your base line target rpm.
    Now run it with the pipe and get a quick reading before the engine gets too hot, but make sure you get a good reading.
    Use a 32 tooth or finer hacksaw to remove a 1/4" slice off the header and get another reading.
    hopefully there will be a rpm gain. This means you are moving in the right direction. If not, then put the slice back into the silicone coupling and leave a small gap between the sections of tubing. Run it again to verify whether or not the system needs to be shortened or if it is either already short enough [or possibly too short] for the prop that you have decided to use.
    No matter what, every sport engine I've ever converted to full wave pipe needs to run at a higher rpm than it can with open exhaust or it is a waste of time in a speed application. I have spent more time over the years typing out how to tune pipes than I have spent actually doing the work, so don't be afraid to get your hands dirty and get after it. It's not brain surgery, it just takes a willingness to put in the time to do it.
    A spray can of brake cleaner is handy to clean out the aluminum saw dust after each trial cut on the header. I just patiently go 1/4" at a time until I see no improvement with the target prop.
    WHO GUNNA FEED MAW KEEEIDS..???

  3. #3
    James c harrell's Avatar
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    RE: O.S. 46 FX on a pipe

    I didn't think the 10-6 had enough pitch either but that is what was up there when I got her. I did away with the landing gear and took off all the repair work to the monokote and re-covered it, after I re-repaired structure. Alot of weight! It will belly land now so I put some skids on it. Think I will take your advice and start with a 9-7. By open exhaust you mean open header right? Then take another reading with the pipe on. If I get a good RPM increase it might just be already tuned but I doubt it. It did have a 10-6 on it.

  4. #4
    combatpigg's Avatar
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    RE: O.S. 46 FX on a pipe

    Yeah..open header will give you a rough idea of what to expect. The important thing is to have a prop that is a good match for the plane, engine and exhaust. The prop is like the wedding ring for the other 3 components.
    I think you will find the narrow blade MAS props in that size range to work pretty good and of course the APC 9x8 should give that combo impressive speed too. The 9x7 is good to start with since it will be easier for the engine to turn.
    Save 1 or 2 of the little hunks of exhaust tuning should you ever want to lengthen the system a little..to accomodate a different prop.
    I never have seen the Mach Racer in person, but it looks like a pretty good model if the factory glued it together well.
    WHO GUNNA FEED MAW KEEEIDS..???

  5. #5
    James c harrell's Avatar
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    RE: O.S. 46 FX on a pipe

    I noticed when i got it that there was a pretty big gap where the coupler is. If I understand you right I need to fill the gap with the pieces I cut off, right? This coupler feels a little soft to me as well so I think I will get a new one when i go get some new props in the morning. I snapped a couple of pics to let you see where I am starting at.
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  6. #6
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    RE: O.S. 46 FX on a pipe

    You will want to ditch that plastic spinner as well. Get a two piece or at least get the solid Dubro nut.
    A guy who used to live on this site and now is rarely here due to the "upgrade".
    Most likely fading away till the My Forums is fixed. Too bad.

  7. #7
    James c harrell's Avatar
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    RE: O.S. 46 FX on a pipe

    Just put that up there! Well, it looked faster!!

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    RE: O.S. 46 FX on a pipe

    Hey James,

    Another tip I've read about is that if you get the pipe too short, the motor will become difficult to tune meaning that the motor will go from too rich to too lean with just a couple of clicks of the high speed needle on the carburator. I have never had a plane with a tuned pipe either, so maybe one of the guys here can elaborate for us.

    Are you bringing the plane to the field on Wewdnesday?

    See ya,

    David
    Fly Safe and Have Fun ! ! !
    Ultra Sport Brotherhood #22

  9. #9
    James c harrell's Avatar
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    RE: O.S. 46 FX on a pipe

    I plan on it. I should have three to test top Wednesday! Two Spitfires and this one. See what happens when they tell me to stay home and get some rest Huh!!!

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    RE: O.S. 46 FX on a pipe

    Oh Shoot, I forgot. Maybe you should stay home and take it easy, you don't want to over do it. With any luck, I may actually have some planes to fly too.

    Hope to see you Wednesday,

    David
    Fly Safe and Have Fun ! ! !
    Ultra Sport Brotherhood #22

  11. #11
    combatpigg's Avatar
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    RE: O.S. 46 FX on a pipe

    Buy a new coupler and when you make length adjustments just leave enough space between the parts to avoid chafing. Remember that the engine takes "bong hits" off the return pulses as part of the magical cycle, so you don't want to feed it ground up aluminum paste.
    Aluminum grit can coat the glow plug and foul it.
    WHO GUNNA FEED MAW KEEEIDS..???

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    James c harrell's Avatar
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    RE: O.S. 46 FX on a pipe

    No worries there, I keep brake cleaner and carb cleaner by the case! And Dave, I have been wanting to get out of this house and fly so bad that if I don't I think the wife is gonna kick me out! Combatpigg, you would definately start by going shorter?

  13. #13
    combatpigg's Avatar
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    RE: O.S. 46 FX on a pipe

    Yes, in general if you have been equipped properly with a new system it is made extra long and it is up to the end user to find the correct length for his chosen prop by removing length in 1/4" increments until no further gains are seen. If the guy before has already shortened the system you might see evidence of a ragged cut.
    Also...I don't think you want to use a tubing cutter and take a chance on flaring the tubing inward.
    WHO GUNNA FEED MAW KEEEIDS..???

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    1320Fastback's Avatar
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    RE: O.S. 46 FX on a pipe

    The gap in the coupler looks too wide.
    This will affect the sound/pressure wave a little.
    Try to get the gap to about a 1/8" which will keep the silicone from collapsing in the middle.

    The length is critical for the life of the engine.

  15. #15
    combatpigg's Avatar
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    RE: O.S. 46 FX on a pipe

    I notice the pressure tap in the header. The factory tuned pipes I've seen place the tap at the fattest part of the pipe.
    After taking a good look at your photos it looks like the header is already shorter than a new one from Macs..but that's my guess from memory. You might find that tach readings go down slightly if you slide the pipe closer to the header with the 10x6 prop..?
    Regardless, I don't think the 10x6 has any future on that plane if you are trying to make it run as fast as possible, so your relevant tach readings during your tuning session are made with the 9x7 or 9x8 prop.
    WHO GUNNA FEED MAW KEEEIDS..???

  16. #16
    James c harrell's Avatar
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    RE: O.S. 46 FX on a pipe

    44 years old and still learning stuff!! I can't thank you guys enough. I am on the way to the LHS to get some props and a new coupler and some clamps too! Think I better get a couple of props of both sizes, I tend to be tough on props. I will definately let you know the progress and results and THANKS AGAIN!!

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    RE: O.S. 46 FX on a pipe

    It looks like you're new to the world of trying to go faster.

    First off a Master Airscrew prop has nowhere near the pitch of an APC with the MFG stated pitch on the prop. Take an APC and MA prop put them on a flat table near the edge. Now look at them with your eyes even to the table then you will see MA's have less pitch. It’s not they may not have measured at 6" pitch but there are different ways to measure pitch of a prop. MA props also flex more and may de-pitch in flight as rpm goes up.

    You can tune your Tuned pipe on the ground but there is more to it than just getting the max rpm on the ground. Some engines will like higher rpm and other will like less rpm but with more torque. Once you find where your engine likes to operate best then you will know what prop to run on it assuming you can fly the plane at those increased speeds. So if your engine performs best at 17,000 rpm then you don’t want to set it at that rpm on the ground. You want it to reach that rpm in the air. So setting it for 15,500-16,000 on the ground is a good start. I would fly the plane with a 10x6 APC prop just the way it is. You will be hand launching the plane. This will give you a bunch of thrust. Once you have a couple of flights switch to an APC 9x8 as they are close to the same load on the engine.

    Your new plane, remove the fuel pressure nipple from the header. That will not pressurize the tank as the pulse move back and forth with the engine creating a negative and positive pulse. Move it back to the largest Diameter of the pipe if it is thick enough to install. You can have that large gap in the coupler but you will need a wide zip tie that covers 80% of the gap to keep it from expanding as the positive pulse heads back to the engine. Not the best set up but from my own testing it only cost around 100 rpm that way. Make sure all slop is removed from your control horns.
    AMA # 126183
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  18. #18
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    RE: O.S. 46 FX on a pipe

    The piped OS should be very close to the same power as my Rossi .45 with Ultra thrust. The best prop for my particular airplane turned out to be an APC 9.5X7 pylon prop reduced down to 9.25". I get 16,200 on the ground with that combo.

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    RE: O.S. 46 FX on a pipe

    I wouldn't move the pressure tap , you can count on rossi and novarossi etc and nelson engines to know where to place a tap for pressure and they often place the tap on the header much like yours is, if you own a nelson ultrathrust muffler take a look at their placement of their pressure tap..... you will find that you will have to take at least two or more inches off the header length to be in the right resonance.

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    RE: O.S. 46 FX on a pipe


    ORIGINAL: lfinney

    I wouldn't move the pressure tap , you can count on rossi and novarossi etc and nelson engines to know where to place a tap for pressure and they often place the tap on the header much like yours is, if you own a nelson ultrathrust muffler take a look at their placement of their pressure tap..... you will find that you will have to take at least two or more inches off the header length to be in the right resonance.

    ifinney I am supprised you dont know how a ultra thrust muffler works....... The UL Thrust muffler like the jet and the Orginal Super Tiger muffler is a 1/4 Wave mini pipe. These hook up just like a normal muffler but have a smaller inner header that extends back into the muffler. The Pressure tap is installed near the front of the engine BUT BUT in the large Diameter portion of the 1/4 wave mini pipe.

    So again to the OP you will need to move the pressure tap to get the most pressure to the tank. Now some people have attained good results directly to the header like that one but noway is it the best.
    AMA # 126183
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  21. #21
    combatpigg's Avatar
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    RE: O.S. 46 FX on a pipe

    It really doesn't take much pressure to keep up with the fuel demand of a sport .40 getting run like this one....especially with a throttle.
    The important thing is consistent pressure that gives the carburetor something to work with till the tank is empty.
    It doesn't seem wise to move the tap unless this setup proves that it needs more pressure.
    So of course, I side with lfinney......[8D]
    WHO GUNNA FEED MAW KEEEIDS..???

  22. #22
    speedracerntrixie's Avatar
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    RE: O.S. 46 FX on a pipe

    One of my Race airplanes is powered with a Rossi FIRE 60 with an enclosed pipe. I have fitted the engine with a 12mm Picco carb. Running the tank pressure from the large part of the pipe led to the engine going lean in the turns. It did not do this with the stock Rossi 10mm carb. My fix was to put an additional fitting on the header and then T them together. Worked great. I have a Ultra thrust on My Rossi .45 and have put the fitting right on the header flange maybe 1/2" from exhaust port. Running that one with a 10mm carb with no issues. IMO the pressure is the same along the whole of the exhaust system.

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    RE: O.S. 46 FX on a pipe

    shawn I almost bought me a 115 today. I need to get a couple for next years racing season. I need to get to working on those molds also.
    AMA # 126183
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    RE: O.S. 46 FX on a pipe

    actually you are wrong..., and I was a employee at macs making wizard type 1/4 wave pipes in the early 80's , and and have used pressure taps on header plates since then. if you will examine a nelson ultrathrust muffler you will see that to be a fact, I own 4 of them and 7 of the Jett variety which do tap in to the resonant chamber area... my point is that the pressure is the same but is varied by exhaust pulse frequency, its not a error either as rossi, picco OPS etc all have done the same of putting pressure taps in headers for the last 30 plus years.. the biggest error one can make is to have to small of hole in the pressure tap...this causes some lag and pressure delay, hence the offering of 8-32 pressure taps to adequately drive sufficient volume of fuel to carb or venturi.

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    RE: O.S. 46 FX on a pipe


    ORIGINAL: airraptor

    shawn I almost bought me a 115 today. I need to get a couple for next years racing season. I need to get to working on those molds also.

    Jeff, I thought I was at the point of going YS but then I found a Rossi 90 FIRE that is the same case dimentions as my 60. The engine will be here tomorrow. If I'm getting 13,500 RPM out of my 60 with an 11X11 then I'm thinking maybe close to 15K with the 90 and a 11X12 or maybe 12X12. That would be a good jump in speed with no weight gain.



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