Hello Frank I think the accepted wisdom is not to do it (unless it helps!) . I think why not try something especially if it is not too critical , you have little to loose and you can change it back if it mucks up your run. Regards Gerald

Frank, you're correct about pressure to the overflow, and plugging the uiniflow. Might as well use an old style suction tank with muffler pressure.

I've heard reports of guys having issues with pressure on uniflow tanks. I believe a lot of problems arise from  what style of uniflow you use.

A standard metal uniflow tank has the uniflow fuel tank end soldered to the pickup line. This works fine, but are susceptible to several possible issues.

First, you will have to physically raise or lower the tank to adjust the run, upright, and inverted. If you don't have the room to move the tank, you can remove the back, and re-locate the uniflow end.

Second, Pointing the outside uniflow end into the airstream to take advantage of ram air pressure has it's own problems. Going into the wind, speed slows due to the force of the wind. For instance, 55 mph stunt airspeed + 15 mph head wind = 40 mph actual speed relative to the ground. Pressure goes up, and engine richens up, slowing your plane further. The opposite occurs on the downwind leg. This can cause your plane to speed up downwind.

Third, Attaching muffler pressure to the uniflow may be adversely affected by the location of the inside end of the uniflow. It's generally believed that the inside uniflow end should be 1/4" to 1/2" forward of the pickup end. Too close, and air bubbles can be sucked into the pickup. It's not well understood that the inside uniflow end can be much further forward, up to about the middle of the tank, and work as it should, without the chance of introducing air bubbles into the pick up.

Fourth, It mightr be best when using an unpressurized uiniflow tank to place the uniflow opening inside a protected, from varying ram air pressure, area inside the cowl, high enough so it won't siphon out the tank.

My preferences, and personal choice for uniflow tanks is the humble RC style clunk tank. I've used them on every plane, except one, (An Excalibur !, a Dick Mathis design) for the last 25 years. I'll try to locate my tank illustration, and post it here so you can get a better idea of what I do. Thanks to My late friend Allen Brickhaus who clued me into this style of tank.

Over the years, I've streamlined the concept to where it seems to work better. There are some real advantages. Let me list a few.

First, These are easily available, and come in usable sizes.

Second, They are lighter in weight, size for size, than metal tanks.

Third, Set up like mine, they are true clunk tanks. The uniflow is not affected by the movement of the clunk fuel pickup. If the uniflow is tied to the clunk, so it moves with it, The apparent height of the tank is constantly moving,up and down as the clunk moves up and down. Not a good thing and defiantly not running like a true uniflow. It also negates the next advantage.

Fourth, The uniflow inside end is moved to about the middle of the tank. It's designed to allow the apparent tank height to be changed by rotating the uniflow tube. This makes it easy to set the tank height without having to move the entire tank. With the uniflow end away from the pickup, muffler pressure works very well. Engine rpm remains constant up, and downwind.

Carefully study the illustration. This really works well.

John Miller

While it still functions somewhat like a uniflo tank it is NOT the same. Uniflo venting (Like a sparklets water dispenser) is designed to equalize the atmospheric pressure at the fuel area surface and the outlet (needle valve on engine, spigot on water cooler. With muffler pressure you are increasing the pressure at the fuel surface in the tank to above ambient atmospheric (.5-1.5 PSI depending on muffler) but the atmospheric pressure at the outlet or needle valve is still at 1 atmosphere (ambient). This will effect the "break" or amount power changes with altitude and/or G load. In affect it will act like a slightly smaller venturi

No, uniflow is still a uniflow, regardless of pressure.
Unless you want more power (bigger venturi), the flow rate in needle valve is still the same, and same fuel head pressure and replacement air pressure exist and the self regulating effect works.
Perhaps you see an interaction in wrong place? It’s typical for uniflow (without pressure) to go slightly richer towards end of flight when a non-uniflow does the opposite. It gets even more rich when you feed hot exhaust gases to tank. So with some luck you may see some sort of a better compensation with a pressurized standard tank.🦄
L

The reason a normal tank gets leaner gradually throughout the flight is the change in pressure differential is constantly changing as the fuel level (weight of fuel at centrifugal force) drops and the fuel is replaced by air at ambient pressure. A uniflo in flight has the same atmospheric pressure at the feed line and the vent line throughout the entire tank until the uniflo uncovers
You're arguing against very sound engineering principle, not with me. Your personal experience using muffler pressure may show little effect depending on venturi size, ambient barometric pressure and load on engine but it is NOT the same as no pressure. Imagine if you put 10-12 PSI from an air filled bladder into the uniflo vent. Will it run the same? No You will have to close the needle way down and there will be NO Break at all

This should give the idea

While it still functions somewhat like a uniflo tank it is NOT the same. Uniflo venting (Like a sparklets water dispenser) is designed to equalize the atmospheric pressure at the fuel area surface and the outlet (needle valve on engine, spigot on water cooler. With muffler pressure you are increasing the pressure at the fuel surface in the tank to above ambient atmospheric (.5-1.5 PSI depending on muffler) but the atmospheric pressure at the outlet or needle valve is still at 1 atmosphere (ambient). This will effect the "break" or amount power changes with altitude and/or G load. In affect it will act like a slightly smaller venturi

  If you assume that the muffler pressure is a constant pressure source, it works exactly the same as a uniflow tank vented to the atmosphere, in that the outlet pressure stays constant regardless of fuel depth.  The pressure in the ullage is still reduced compared to the input by the hydrostatic pressure, even though the absolute pressure is above atmospheric, and the pressure at the outlet is higher than it would be vented to the atmosphere.

     That's the reason to use it  -which you largely have correct, it reduces the dependence on fuel draw, so you can increase the venturi size for the same run quality, or run the same venturi and get less pressure variation. The higher absolute pressure does have some other side effects - like, the needle opening has to be smaller - which can have some side effects. And also, you are heating the fuel, which *sometimes* makes it foam where it did not before, and "run away" lean at some point in the run. I have had that a few time, but have also run many, many successful (and constant-speed) flights with it, too.

      It's one of the 4 variants to try -

 straight uniflow (vent open and overflow plugged) - constant atmospheric pressure at outlet
 uniflow with pressure (pipe tap to vent and overflow plugged) - constant muffler pressure at outlet
suction with pressure (plug the uniflow and hook pressure to the overflow) - pressure varies with fuel depth, starts at hydrostatic + muffler pressure diminishing to muffler pressure at end
suction with open overflow (plug the uniflow and leave overflow open) - pressure various with fuel depth from hydrostatic pressure+atmospheric pressure, diminishing to atmospheric

   Note that the delta in the pressure from beginning to end with the uniflow is always the hydrostatic pressure, but that the absolute pressure is higher if you have muffler pressure, so your percentage change is smaller, which reduces the speed change from beginning to end.

     Brett

(Yeah you are in effect running burned exhaust residue including oil thru your engine)

Never thought of that before.  Somehow I think it is very minimal but I am only guessing.  Another point of curiosity is why we do not use crankcase pressure.  I used it in the 70's on my McCoys and it really helped.  I am not advocating it, just curious.  I am sure there is a good reason.

Ken

Technically in a standard uniflo set up there is a slight Vacuum at the outlet of the tank due to the suction of the venturi when the engine is running. That's what makes the fuel flow when the vent is at atmospheric pressure and the fuel is replaced by air as the fuel is consumed. When you pressurize the tank (By any means) it is no longer a "suction" fuel system; it is a pressure fuel system and it will affect the run regardless of where you inject the pressure.

 Sort of. Technically, a uniflow tank runs a constant feed pressure regardless of fuel depth. That can be at atmospheric pressure or muffler pressure, but it is unaffected by fuel depth. If the only thing going on was the outlet pressure, then, it would run the same setting throughout the flight, as opposed to suction tanks which go leaner as the fuel depth decreases.

   What is required to make the constant outlet pressure happen is a pressure differential between the pickup and the tank ullage equal to the hydrostatic pressure. If the vent pressure is atmospheric pressure, there is a slight vacuum in the ullage that increases with fuel depth. If the vent pressure is (contant) muffler pressure, it is a positive gauge pressure, but is still lower than the muffler pressure that changes with the fuel depth, since the hydrostatic pressure still opposes the pressure at the vent.

   The effect in either case is to run at a constant outlet pressure, which if nothing else happens, should keep the engine running at the same mixture and a constant RPM.

    Brett

   It has been discussed over the years of using inserts of different inside diameters in muffler pressure lines to help avoid over pressurizing a fuel tank, not out of worry of bursting it, but trying for a more even run. When making fuel tank and fuel line plugs I always use something larger than the ID of the fuel line. When Windy mentioned using the pegs from the Lite Bright toys for that purpose, I rounded me up some and use those a lot.  They tend to glow a bit in the sun and makes them easy to find if you drop it. Ordinary BB's make a good plug for a fuel line, just stuff a couple up the short length of fuel line and it gives you something substantial to grab when pulling it off.
   Type at you later,
    Dan McEntee

I once read, or was told, that the Asian engines seem to like muffler pressure, but others didn't. I have not tried it on a Euro engine or even Yankee engine. Randy Smith is against it, and I value his opinion, but I have always used muffler pressure on Magnum XLS .36 and OS .46LA with no problems, and like some things about it.

Keeps the bugs and little seeds out of the fuel filter, at the very least. I like that! I also pinch off the pressure hose a few times to lean the engine and help heat it up to running temp before setting the NV. Some don't heat the engine up, but just leave it set as per the last flight. I don't find that to work for me. I find a need to set the NV with a tach, and that setting changes from AM cool to PM heat, muffler pressure or not. 

As Brett pointed out already the flow into the tank is relatively tiny...air or exhaust gas inflow is limited by the rate of fuel being burned off. It will NOT crap up your tank with exhaust residue.

The KEY THING, if you decide to try it, is to be VERY certain that your muffler will not come loose, the pressure tap is NOT going to come out, and the pressure tube and tank is NOT going to spring a leak. The result of any of these is a very lean engine, and that's NOT good. By the way, DO NOT use a muffler gasket. If you do and it blows out, it will be BAD for your engine.  Basically, if you struggle with mechanical issues like the above, don't do it. Not for everybody.  I tried pipe pressure on a .46VF, simply because the pipe was already setup for it (bought used), but I didn't like it at all. Not because it didn't run ok, but because fuel siphoned into the pipe and left me short of fuel a few times too many.  Steve