When installing a 6-32 pressure tap for tank pressure does it go in the muffler or the cover plate on the back of the crankcase?

Generally, tank pressure is taken off from the muffler...

You can also use case pressure...it will be greater, but muffler press. is usually sufficient.

Most mufflers have a press. fitting...what type engine are you talking about?

edit: one thing better about muff. press. is that if there is any back flow, it will have less chance of flooding the crankcase w/ raw fuel if the line is connected to the muffler.

I am refering mainly to marine glow motors in race boats that flip. I am concerned about water getting into the fuel tank and think this is a safer way to do it.

David,


The average crankcase pressure varies from a negative value at low throttle openings, to a positive value of about 5 psi at maximum power.

An R/C carburettor, even in a boat, will not properly cope with such changes.

An engine running at constant full throttle, like F2D C/L speed, or F3D pylon racing (I don't know the boat racing class...) will do much better.


For an R/C engine, just use muffler pressure.

jetpack,

It depends on wether you are using a pump or regulator or not. Muffler pressure will work fine with no pump or regulator on most 2 cycle and 4 cycle engines. If using a pump (VP-30) or a cline type regulator with a 2 cycle engine you use crankcase pressure with a one-way valve to keep a pressure head in the tank and prevent back-flow. A 4 cycle uses muffler pressure with the valve if using a regulator,( I am not sure about a VP-30 using muffler pressure as I only use a cline regulator on my engines).

Phil

jetpack, I have used crank case pressure successfuly in C/L and freeflight applications, but agree with Dar that problems could arise in a throttled application.

jess

I guess the best way is to put the question in another light would be how would you approach keeping a water tight fuel system or one that is less likely to take in water if capsized?

Run a pressurized system, right?

How about a "bubbless" tank...or a Tetra tank? (bladder type? )
The pylon guys use them...you fill the tank with a big syringe, and I think there's only 1 line...somebody else who's into pylon might chime in here?

Edit: I edited out the V-30 pump/regulator part...thinking the vent would allow water to enter the system...but the snorkle sounds like an interesting idea...

There are a couple others here who also run boats...
Have you looked in the boats forum too? (probably a lot more "traffic" here though...)

I haven't counted out a perry system, and your point about the vent taking in water I considered also. The carbs do need pressure per instructions, I was just trying to get away with a more simple closed system which by others the webra's only been needing pipe pressure and a good tank location.

The vent I guess could be delt with just simply pointing it upside down in the boat, or a snorkel where when its capsized would clear the waterline.

So with the vent problem solved Im going back to the ideas again.

Perhaps a check valve in the pressure line from the muffler to the tank. Keeps positive pressure on the tank and keeps water out as long as the tank was pressurized just prior to a boat "flip." Or a complete Cline or Iron Bay system that will hold pressure until relieved -- total closed pressure system.

Cheers,

Chip

Chip,


With a muffler pressure system, that would be wrong!

As the engine is throttled down, excess pressure in the tank is vented back into the muffler.
If you put a one-way valve there, there would be too much fuel pressure at part throttle/idle and the engine could be dowsed and die...

You need maximum pressure only at maximum throttle.

A small engine with a large tank is more likely to die, as the throttle is snapped shut, since it takes too long for the tank pressure to be vented; even if there isn't a one-way valve on the exhaust pressure line. A one-way valve will only make matters worse.

jetpack, piper chuck knows a great deal about boats. You might e-mail him?

jess

After reading all the aspects sounds like the only way around getting any kind of water in the fuel would be going with a fully closed system. I cant picture a way other than what Chip touched on with the check valve arrangement, but hard to figure out the schematic for me if Im using a Perry system. Now its turning to a fuel system problem for me...lol..duh. I'll just go ahead and state what I would like to see for a system and go from there.

Here is what my initial intention was with the fuel system before I got sidetracked with my ideas: VP-30 Perry Pump, Check Valve, T Fitting, Third Channel mixure valve, Fuel Filter, Dubro quick-fill port, and a SLEC tank. I'm going to work on a sketch of the plumbing and post it. Its the vented tank thats my hangup due to water spoiling a tank of fuel or others to follow if it doesnt get sucked out.

David,


According to Perry, when using a VP-30, the tank must be vented and not pressurized.

I don't know how that will be done, risking water ingestion on a boat...

Chip has the perfect idea, using the Cline or IronBay regulator not only will you have completely closed sytem but you'll have uniform engine performance from full tank to empty. You can also run the engine a little leaner because you don't have to run it rich at the beginning of the tank to prevent it from being lean when fuel level gets low.

The Perry pump will work with muffler pressure. But when I did that (I missed that part in the instructions) the pump only lasted a little over a year. However I don't see how using muffler pressure is going to prevent water to enter the tank, most mufflers have about a 3/8" hole for water to enter them and thus the tank.

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When running crankcase pressure, a one way valve must be used, otherwise the end result is no pressure at all, since the piston goes up and down, causing the pressure to swing from positive to negative to positive over and over again.

Crankcase pressure should not be used unless a pressure regulator is employed at the carburetor.

Timed pressure can be obtained by tapping the crankcase under the rotary valve window that is cut into the crankshaft. Again, this should not be used without a regulator of some time to prevent flooding the engine.

Ed,


I am afraid you are not completely right.

What the tank 'feels' is not the pulsating pressures, but the average pressure.


This average has been found to be negative (sub-atmospheric) at low throttle openings, which may cause the engine to draw fuel through the 'pressure' line...

At high and at full throttle, the average pressure can be as high as 5 psi.

If you connect several inches of fuel line to that crankcase nipple, the volume of the fuel-line will be enough to completely dampen the pulses. This is why a pressure pulse operated pump needs a short line, with a relatively large diameter.

nope, case pressure doesn't need a check valve to work. in fact, at idle it will still make enough pressure to flood an engine.

OLD and CHEAP K&B outboard mod for making power: carve away the muffler portion of the outboard under the engine and install an exhaust throttle, chuck the carb and put a neele valve through the carb screw holes and finally, put a pressure tap in the case to feed the tank....works with plane engines too!


dave

you left out the part about the intake valve being open for most of the time that the piston is going up....


dave