Juice

On one of my 4-stroke setups I am not going to use a muffler, so there will be no back-pressure to pressurize the fuel tank. Someone gave me the idea to use a funnel or scoop on the pressure line. Basically the funnel (or scoop) faces forward and collects air from the prop.

Has anyone done this? How big does the funnel opening have to be?

The tank pressure depends on how much air is being pushed into the funnel. If I am tuning the engine on the ground, the air entering the funnel comes from the propwash only. Now when I fly, the air entering the funnel comes from the propwash and the oncoming rush of air the airplane is flying on. That would mean more air is stuffed into the funnel (when flying), therefore higher tank pressure and a rich running engine. Is my logic correct, and this funnel idea is a bad idea? Or am I paranoid?

Thanks,
Juice

w8ye

My Feedback: (16)

Back before the days of mufflers, I ran funnels at one time on my two strokes.

I never experienced any of the problems you are fearing. It worked fine. I had two 1/2" dia flares on the fill line and the overflow line.

I say it worked fine, I didn't seem to have any fuel draw problems. But I sorta wonder if the extra drag overcame any benefit?

The OS four strokes will run fine without muffler pressure anyway. At least mine do.

Enjoy,

Jim

Flyboy Dave

My Feedback: (21)

I tried that once on a Saito .80, one of the original ones....the muffler fell off
and I wanted to try the "air venturi" to the tank thing....it didn't work. []

Some 4-strokes work just fine without tank pressure. The carb is usually
mounted right behind the engine, and the fuel flow is short and direct. The
four strokes also have much better fuel draw than 2 cycles, due to long
duration of the intake stroke.

If the tank is mounted directly behind the engine, chances are it will run
fine without exhaust pressure to the tank. If all else fails, you could employ
a Perry pump.

FBD.

Ed

In two words - Drag & Dirt. You are gonna get both.

Muffler pressure is good, crankcase pressure is best !

> Jim

DarZeelon

Juice,


You are not going to get any pressure for your fuel tank, before reaching trans-sonic speeds; roughly 0.7 Mach.
This is the lowest part of the compressibility range and for some reason, I don't think your model will fly that fast...

Jimmbbo

Not to hijack the thread, but the forward facing "scoops" on the '60s - '70s muscle cars were totally useless, and may have hurt instead of improving performance... the max static pressure is at the junction of the hood and windshield, which would require an AFT facing scoop.... But THAT wouldn't sell, now would it??!! [sm=confused.gif][sm=confused.gif]

Cheers!

Jjim

FenceMagnet

My Feedback: (2)

as previously mentioned, IF you have any problems with fuel feed due to the lack of muffler pressure, your safest and easiest solution is a Perry (now Connely) Micro-Oscillating Pump... trivial to set up and very reliable....

[:-]

BillS

I am sure there is a good reason but wondered why the muffler would be omitted?

Bill

ronaldh

Juice,

Here in the UK, manufacturers recommendations for the Laser four stroke engines are to run without muffler pressure and as long as the vent faces forward it's fine. Facing to one side or backwards will create a slight vacuum. One or two fliers at the local field run a funnel set up but others don't....there doesn't appear to be any difference. In cowled situations with the very hot exhaust on four strokes the pressure line will eventually burn off. First sign is the idle becomes unreliable and cuts out.

downunder

You'd better tell that to an airspeed indicator connected to a pitot probe. Or the sensors in a gas turbine FCU that detect both static and ram air pressures. Or to guys who have the vent tube inside a cowl so it's in relatively static air instead of open to variable speed air flows outside.

Instead of a funnel, how about a reverse venturi to slow down the airflow inside it and build up pressure?

outssider

My Feedback: (23)

you could make something like this with silver/brazing solder. cup the end a bit to create some back-pressure

w8ye

My Feedback: (16)

Outsider,

We are talking about not running any connection from the muffler/exhaust to the tank. We are talking about running the tank vents into the slip stream to put a small pressure inside the fuel tank. We have been talking about the pros and cons.

You appear to be wanting to generate more pressure to your tank by altering your exhaust pipe? If that is so, you will have to put a slight restriction at the outlet of your pipe. But most often, the systems like you have perform satisfactory the way they are.

Enjoy,

Jim

Juice

Thanks for all your replies. They were very helpful. I am going to mount a 1/2" funnel between the cylinders of the dummy radial.

To keep my exhaust inside the cowl I have to use a 90-degree header and a flex pipe to route the exhaust out a scale location. No nipple and no back pressure available.

Juice

DarZeelon

Juice,


Installing a pressure nipple can be done on one of the headers.
It will likely provide sufficient pressure for your fuel system.

If the header cannot be drilled and tapped for a screw-in pressure nipple, perhaps you could braze a short piece of brass tubing into a drilled hole.

I cannot see what problem has to be solved.