Pressure to for fuel tank???

[Don Templeton]

Hey guys,

Just how important is pressure to a properly built fuel tank?

Just asking because I raced 67 riggers for a number of years without pressure and never, never had a problem with fuel feed. In fact, most of the really fast boats at that time didn't use it either. I don't remember anyone one having a problem.

Isn't the pressure just to make sure you get fuel to the engine but has nothing to do with speed or RPM?

Just need some qualified input. :-

Thanks,

Don

 

[Atious]

Without some type of pressure or pump, other than gravity feed, how would you otherwise get fuel to the carb?

 

[Don Templeton]

Engines "Suck" ;D

 

[Propjockey]

The venturi effect of a carb produces a certain amount of suction -- without tank pressure, this suction has to overcome a bunch of variables to get fuel into the engine. Things like - fuel level, carb height, atmospheric pressure, length and diameter of fuel tubing, etc, etc.

While it's true that some engines don't require tank pressure to run efficiently, I've found that I get the most consistant needle settings (at any RPM) with tank pressure and a one-way check valve in the pressure line.

 

[Don Templeton]

Atious,

I don't care where the tank is. If you try to start that engine without a plug you'll get a face full of fuel!

 

[Slideblues]

Been there, done that, and it dont taste like chicken!!!!!

Gene ;D

 

[Don Templeton]

Well, it's true the top of the tank was higher than the venturi only that I used but the needle never seemed to change until the tank ran dry. I also noticed the same with most of the other boats of that time.

 

[Slideblues]

Back a ways I rigged one tank level with carb and it ran ok, seems like I did my old shovelnose that way,

Gene ;D

 

[waterbug]

As long as you can get the top of the tank 1/2" above the carb centerline it will draw fuel without getting lean at the end of run when fuel is low.

Now that applies mostly to planes because of the designs. The engine can not be lowered to compensate [ie.] Tank pressure !! must be applyed.

The reason you can do it with the 67 is because the carb is low 1/4" -1/2"or a little more off the boat or tub floor !! Most people put foam around the tank to stop foaming of the fuel . [ RAISING THE TANK MORE} + dead rise in the hull . Most boats & Hydros have the tank in front of the engine . [ie.] raising the tank.

Tunnels and canard outboards and Hydro outboards are the exception . They need tank pressure and occasionly a hopper tank .

Could you imagine a tunnel with the tank raised up to were it's tank top is 1/2" above the carb centerline ?? It would roll on to the cowl top so fast, it would take a week of mumbling to yourself to figure out what happend !!! You would have the world record for barrel rolls in a lap !!!

 

[Don Templeton]

Very interesting! Sounds like with a .12 you could have some problems without the pressure.

Thanks for the input guys,

Don

P.S. Gene you'r right about the chicken. I've raced a heat or two almost blind after cleaning out the water. ;D

 

[Ian Inverarity]

Don,

Years ago I read about positioning the tank centreline at the level of the carb, but that was for planes. I have since built and run many boats with the carb higher than the tank. I currently use a setup with the tank higher than the carb, which is a pain due to fuel draining into the engine all the time, but when it is running the system works fine. I dont think tank height is very important at all.

I have run boats without pressure, and they have worked okay. I do it when I am trying to find out what is causing an engine to stop.

The carbs draw fuel due to the venturi effect, so can run without pressure. But, to the best of my knowledge the pressure system is designed to give a good enrichment of fuel at full speed. This is useful for simple carbs which have no low speed mixture. At lower speeds the exhaust pressure is low and the demand for fuel is also low, so it works well. Having a pressure system to assist with fuel flow would also tend to make effects like the fuel level difference less significant.

With a really large bore carb you may struggle to run the engine without pipe pressure. I personally use pipe pressure for all my boats.

Ian.

 

[Don Templeton]

Well this is interesting.

I just measured my old Crapshooter .67 and the metal tank at it's highest point is 1/2" (Below) the fuel inlet on top of the venturi. I also always cleaned out my engine of fuel and water between heats by first running fuel through the engine with the starter and then pinching off the fuel line totally before stopping the starter. It always started very quick. Go figure! :

Don

 

[waterbug]

The bigger the engine the more" SUCTION" u know like that _ _ _ _ down the street !!!

 

[Don Templeton]

"10-4"

 

[waterbug]

No realy I've noticed this on my planes. I have a mod. K&B 61 and it will pull the fuel from the tank until it gets kind'a low. But i have to run large fuel line to get enough fuel, pressured or not. I've flown with the pressure line off at the tank inlet with the med. tubing and it got lean at 1/4-1/3 tank left. When i pull the tank I found the pressure line off, and replaced both lines with LG. tubing. Then when i flew i took the pressure line loss just to see what happened !! But I like pressure when I'm inverted 3 feet off the deck !! he he he !!! I'm not to old yet !! if you dont get scared at least once in a run you will lose intrest !!

IF YOU DONT GET A RUSH AT LEAST ONCE IN EVERY RUN !! YOU NEED A NEW BOAT OR PLANE !!!!

 

[Don Templeton]

You've got a lot of guts to try those things with an airplane but I guess it's the only way to learn what's really going on.

I once won a very importand heat at the IMPBA Worlds crossing the line 3 feet off the deck and inverted but it was definitely not intentional! ;D

Best thing about boats is you can try a lot of things while thrashing at the local pond without going home carrying a basket.

Thanks,

Don

 

[Propjockey]

Yeah, but if things go really wrong with a plane, you go out with a trashbag and pick up the pieces...with a boat, the really important stuff sinks! :-

 

[waterbug]

Guys,

This is true!!! Just about 2 months back my wifes pride and joy covering job, purple & pink diamond clad 4 star 40. I put it into a inverted flat spin, seeing how low i could get before pulling out!!!

4 times at around 30-40 feet OOYEAH!!! I got this!!!!

The 5 th time I'll just leave the power on high and snap it out !!! didn't pay attention to were i was and spun down went to snap out and made it to 3 quarters of the way to level flite HEDGE ROW !!!!!!!!!!!!

It was over before I new what happend !!!! :'( :'(

All that was left was 1/2 of the fuselage in 1 1/2 feet of water !!!! Goodthing I have 3 planes of my own designs ready to cover !!!!

 

[Hurricane]

One thing is being missed here, maybe! Todays new motor require so much more fuel that a few years ago you cann't get enough to them with the large carbs with out the pressure on the tanks. My MAC 67 uses 20-25% more fuel than my Blue Head Picco 67.

 

[Don Templeton]

Hurricane:

That's a good point. I'm not really familiar with the new carbs and engines.

Propjockey:

In eight or more years of racing riggers, I never saw one sink no matter what kind of crash it was involved in. If they sink today, it's because of the very light building techniques that every one is using. I guess that's the game if you want to go over 80!