I run some of my boats with a gravity feed fuel system. The Mean Green and Tylers boat are both gravity fed,along with the 108 boat, and another one of my brothers.

Gravity fed meaning that the fuel tank sits at the carb level or slightly above it about a 1/4", I also run the shortest fuel line possible to the carb. This allows the fuel to get to the carb without any pressure from the motor. Which allows you to take out the baffle or do like I do and remove the entire exhaust tip.
This give you an open header!!!!! Now the engine has no back pressure, which will allow it to turn up the rpm's quicker and doesn't take as much hp to get its fuel.....she becomes pretty "peppy"!!!! I set the needle valve like you would any other set up.
And from my experiments I get better engine performance and fuel economy from this set up. The only down side to this set up is you give up a little bit of initial response from an idle to acceleration (not enough to make a difference),and its a little louder. But then again I don't idle my boats ever...they were built to run and that's what I do with them! I run 'em like I stole 'em!!!!!!! (Please don't take me for a thief....its just a metaphoric phrase.)

I have experimented with the set up quit a bit. I run a conventional set up on 3 boats a 46FX, 46LA, and a Magnum 108. Then I converted them to gravity feed and the response was better with the gravity. I haven't used a tach on the two set ups but just by running them I can see a difference. And another thing....it costs you exactly $0.00!!!!!!

Check out the video footage on my web page and you can see how the gravity fed motors run.

It's late and I gotta work tomorrow. Any questions or comments?...I'll be more specific if anyone has any questions.

In about 1 week, I will have two boats with a Pro.46 engine on them. I plan to tach both of them. Onw will be pressure, one will be gravity fed. I can say though....my OS .25 with gravity feed runs great.[8D]

Let me know what the results are. Could you see a difference when you gravity fed the .25?

That is all the .25 ever had....

Is no one interested or you guys just think I'm [sm=spinnyeyes.gif][sm=spinnyeyes.gif][sm=spinnyeyes.gif][sm=spinnyeyes.gif]???????

Here is a couple of pics showing how the tank level is.

A little engine history and faq.
Any engine run without a muffler WILL have more rpm and power than run with a standard muffler. This has been a proven fact for several decades.
Mufflers came into wide use due to areas for model activity were being lost due to noise. Mufflers made things quieter. Non-modeling folk were happier. RC folk lost less places to do their activities.
Due to loss of power with standard mufflers, tuned pipes became more popular for everyday craft of all sorts and no longer termed for competition uses only. Not only did pipes increase rpm's, but it did it quietly.

Pressurized fuel systems was the only thing to use to be consistant in CL and FF aircraft competition since the 1950's. This was always crankcase pressure ( much higher pressure than exhaust pressure) and in a sense, was a poor mans supercharger, as the fuel was forced into the engine and easily consumed due to the use of VERY large VENTURIES (intakes) and humongous porting of the engines.

RC folk tried this route to increase performance but quickly determined that the pressure was too great for CARBS to handle (excluding PDP modified engines, but that's a story in itself). It was also determined that exhaust pressure was more than suitable, as it is continuously variable( the higher the rpm the engine is run at, the higher the pressure created). Though it did not increase top rpm as much as they had hoped, it did help greatly in the throttle in terms of transition smoothness and overall running consistancy. Exhaust pressure also allowed the fuel tank to be positioned lower than what is normally recommended due to the slight force feeding characteristics.

Ok , here's my pennies worth ...
First , on a 4 stroker , a wider , larger exhaust opening ( no muffler , no pipe ) , is effective in overall top end performance , allowing both higher rpm range and hp .

On a 2 stroker , a tuned pipe , utilizes the pressure waves created by the exhaust gases to aid in the scavenging of those gases to create a "supercharger" , if you will . A tuned pipe will pull the fresh fuel into the cylinder better while also making sure that there is a minimal amount being kicked back out the exhaust before the fuel ignites . Now , with this , you will get a larger than normal ( or should I say , more than an open exhaust ) rate of fuel and compression , and hence , more power ..[8D] Now , how this "extra" power is used , is decided by which tue pipe and header length you use . It could be user for a higher top end , wider RPM power band , ability to swing a larger prop . Some other noticable attributes are : less wasted fuel ( meaning that less is kicked out the exhaust and basically just wasted ) , a cleaner burning/running engine , better throttle response and last but not least , one that has been bugging folks a LONG time which has so had many turned away from our local ponds and ponds closed due to this factor - LESS noise .
Now , I am not condoning, or condemming the use of a wide open exhaust , just stating that there have been MANY , MANY complaints over the years that has caused MANY ponds and other areas for RC'ers to be able to go and enjoy their hobby .
As a matter of fact , as much as I loath the "clubs" for boating ( NAMBA , IMBPA and the such ) because of their "rules" , I think everyone should also know that they have incorporated a "rule" which not only makes sense , but is " neighborhood" friendly - I believe they have lowered the DB for the 50 ft distance to 85 db's .
FTR - I own a BIG cat , with twin Zen's , both have QD WTC pipes . Now , what is my reason for telling you this ? Well , you guessed it ... I'll tell ya .
When I fire up those twin Zens , the sound is sooooooooooo sweet .... but ( yeah , you knew that was coming )
Talk about LOUD ???? The lake is off a kinda major highway ( for the area anyways .. LOL ) and I actually had a passer by , stop, and was watching and after I brought the boat in , the fellow ( who also has his wife and kids with him ) said to me ... shoot , we heard the " sounds" ( he was being nice and not saying NOISE ) from the road and thought that they were having some kinda boat races here ....
Then he went on to say that he was REALLY surprised to see that it was only from one "little" boat ( little meaning that he was initially referring to a FULL size , REAL boat [&o] )
So , to any who plans on going NO exhaust , please be considerate of the "neighbors" .
( man , I hate preaching ... )
But it is a fact ... most ALL areas have " noise" level laws , and besides, you also don't want , or need the local PD hassling you , OR worse , forbidding you , from running on your local pond, or where-ever .
So , in cutting to the chase .... make your decision wisely , otherwise , you might not have a decision to make for future use . ( meaning , you WON'T be allowed to run your RC's because of the noise levels )

Eloquently put.................

If I did not use a muffled engine at the two places I run, I would not have the privalege to do so anymore. One of the areas is a pretty remote lake next to a major highway so there is already a lot of noise. But the frequency of the noise our engines make is distinct.

All it takes is one person to complain and you could be shut down in a heart beat.

I don't have a problem with people complaining about noise. There is no noise orinance in the county , only in the city. I don't run this set up all the time...just occassionally. Was gonna put the tip back on for the black and silver boat but I have missed placed my tip and baffle!!

Is ther eany benefit running with the tip and no baffle? Just curious, I am running the .25 like this....

No difference......that I'm aware of.

It is the internal baffles that cause the quieting effect. These baffles usually have slots or holes that the exhaust must pass through on their way out of the muffler. By removing the baffle and still keeping the end piece in place has the same effect as removing the end piece, but gives the engine a different sound, with a slightly mutted sound.