Tuned pipe pressure

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Roger J. Newton

Well-Known Member
Joined
Nov 29, 2003
Messages
76
I should know the answer to this question but I wanted to verify this with the group. At this last RCU race my son was having a fuel problem with his 1/8 scale. It would run fine for a couple of laps and either die or slow down. He changed or checked everything he possibly could. Just prior to the first heat he removed the sump tank. After that the boat ran fine. My theory is that with 2 8oz. side tanks and the sump tank the engine was out drawing the pressure. CMB.67/ Muck pipe/ pressure fitting is about 1/2 to 3/4" behind the weld. Anybody ever run into this problem?
 
Roger J. Newton said:
I should know the answer to this question but I wanted to verify this with the group. At this last RCU race my son was having a fuel problem with his 1/8 scale. It would run fine for a couple of laps and either die or slow down. He changed or checked everything he possibly could. Just prior to the first heat he removed the sump tank. After that the boat ran fine. My theory is that with 2 8oz. side tanks and the sump tank the engine was out drawing the pressure. CMB.67/ Muck pipe/ pressure fitting is about 1/2 to 3/4" behind the weld. Anybody ever run into this problem?
Click to expand...

make sure the pressure fitting is open and not blocked, which can happen if the fitting is screwed on to far and not drilled out. ask me how i know.
 
steve wood said:
Roger J. Newton said:
I should know the answer to this question but I wanted to verify this with the group. At this last RCU race my son was having a fuel problem with his 1/8 scale. It would run fine for a couple of laps and either die or slow down. He changed or checked everything he possibly could. Just prior to the first heat he removed the sump tank. After that the boat ran fine. My theory is that with 2 8oz. side tanks and the sump tank the engine was out drawing the pressure. CMB.67/ Muck pipe/ pressure fitting is about 1/2 to 3/4" behind the weld. Anybody ever run into this problem?
Click to expand...

make sure the pressure fitting is open and not blocked, which can happen if the fitting is screwed on to far and not drilled out. ask me how i know.
Click to expand...
Thanks Steve. We did check that. A few times. There is another fellow racing with us that did find that problem. But it still did what my sons boat was doing. He had a longer than need be line from the tank to carb. We shortened the line and it fixed his problem.

Roger
 
Roger what size lines are you using between 1st & 2nd main tank? We used at least 5/32" on the outlet of tank one & inlet of tank two & 3/16" is even better if you can do it. The problem we ran into with 3 tanks (2 mains & hopper) was the motor would go rich when it switched to the second tank due to the pressure change when the first tank went dry. We also used to drill out the pipe pressure fitting a few sizes larger (BTW- check that the pressure fitting isn't partially restricted). All the scales I now build use a single custom metal tank with internal hopper set in an under deck tank pocket on CG. That 3 tank sxxt got to be too much hassle. :blink:
 
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I don't know Roger, that pressure fitting placement sounds strange to me?

1/2" to 3/4" behind the weld (I'm assuming closer to stinger) yes?

I have always been told as close to the weld/fatest point on the inlet (closest to engine) side

Something about positive and negative pressure waves, and you can actually draw a vacuum on the tanks.

Just something to consider?

Oh yeh, make sure the exhaust coupler is not coming apart, when running, pieces can cover the pressure fitting form the inside, and on the way back to you, in the retrieve boat, they fall into the pipe, and you will drive yourself nuts trying to find that problem.

Rick Bellinger
 
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I go with Don on this one. You are losing fuel. Try bigger lines or extra lines. I had a boat with two tanks and when the first tank drained it would do silly stuff. I added another crossover line and the problem went away. As long as the fitting is in the fat part of the pipe you are fine, that is where the highest pressure is. Forward or aft of the weld does not matter either.
 
I had a fuel hungry engine was trying to run a offshore race so i had three tanks i used the bigger fuel lines then checked the draw by sucking out all the fuel with my six shooter.It drained out fine threw it in ran just like before.I had a 16 oz and two 4 oz tanks.Sounds like a restriction for sure.
 
Roger J. Newton said:
I should know the answer to this question but I wanted to verify this with the group. At this last RCU race my son was having a fuel problem with his 1/8 scale. It would run fine for a couple of laps and either die or slow down. He changed or checked everything he possibly could. Just prior to the first heat he removed the sump tank. After that the boat ran fine. My theory is that with 2 8oz. side tanks and the sump tank the engine was out drawing the pressure. CMB.67/ Muck pipe/ pressure fitting is about 1/2 to 3/4" behind the weld. Anybody ever run into this problem?
Click to expand...
I think the key thing here is ; you said behind the weld 1/2" to 3/4". Pressure fitting should always be in

front of convergent cone. or as close as possible to it. Never behind . Pressure at widest point of pipe,

just ahead of that.
 
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I have a PT SP40 II, that has two 8 oz side tanks and a sump, have replaced the lines from tank 1 (8 oz) to tank 2 (8 oz) with two 5/32 tubes and large fuel lines and replaced line from tank 2 to sump (2 oz) with 1 5/32 tubing and large fuel line, still goes a little rich after draining tank 1. This engine uses most of both tanks during a race. Took half a season to straighten the fuel feed out. Used to run fine in a PT SP40II with front fuel tank normal plumbing. Short answer: I agree with Don! Rudy
 
I am using Large fuel tubing and 3/16th stainless in the fuel tanks. I tore the 4 oz tank apart last night, and I could not find a single thing wrong with it, the tubes were all together had no cracks or holes and did not leak any pressure. I have ran this pipe for several years now with the pressure fitting behind the weld past the flat part. Maybe I will have to try putting one in front of the weld like you guys are suggesting. Another question, I am wondering how big of a pressure fitting do you guys have on the pipe? I have heard many theories on this, but I am not sure if you want a bigger fitting or smaller fitting for the pressure line on the pipe.
 
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Question- how old are the bearings in the motor? Do you have or already tried swapping out the motor either for a known good one or try that one in a different boat? :blink:
 
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miniczar said:
I am using Large fuel tubing and 3/16th stainless in the fuel tanks. I tore the 4 oz tank apart last night, and I could not find a single thing wrong with it, the tubes were all together had no cracks or holes and did not leak any pressure. I have ran this pipe for several years now with the pressure fitting behind the weld past the flat part. Maybe I will have to try putting one in front of the weld like you guys are suggesting. Another question, I am wondering how big of a pressure fitting do you guys have on the pipe? I have heard many theories on this, but I am not sure if you want a bigger fitting or smaller fitting for the pressure line on the pipe.
Click to expand...

I wouldn't worry about the pressure fitting location, although most seem to put it on the divergent cone side near the weld I know a pretty experienced boater (Gary Preusse) that puts his on the convergent side.

Far as I know a standard sized pressure fitting works well, pipe pressure is supposed to be around 3psi and making the fitting bigger or smaller isn't gonna change what the tank sees.

Is the sump tank lower in the hull than the 2 main tanks? :rolleyes:
 
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Obviously the problem is pressure or the header tank. It can't be due to fitting location or it still would not run.

Just athought but why not lose the header tank and only run the other two. The first will act as a header and you probably will get some air in the second but I seriously doubt it will affect the performance. After the first is emptied you have air in the second anyway.
 
Hey Dave, I had trouble all year last summer. I tried everything, new tanks, sump tanks, one tank, 3 tanks, big lines, etc. Over the winter I tried a diferent pipe and the problem magically went away. I was running a AB67 now I have a CMB 67 fat pipe. Now I'm back to my original configuration with two side tanks.

Brent
 
Well I would look at it this way,

get rid of the 2 - 8oz saddle tanks,

put one large 16 oz tank in the right side (Sullivan 16oz flat tank)

5/32 tubing from the 16 oz to the 2 oz hopper

CMB 67 motor, run a Bob Violet needle (high volume needle) vs others.....

The shortest fuel line as possible from the needle to the carb.

go like hell..........
 
Thanks for all the input. I have only run 3 races on these bearings, and they are still in good shape. I did try running a different engine in the boat, and did not change performance at all. I will try everyones suggestions, and see which one will work best, nothing like trial and error, right? I definetly will try to go like hell! lol
 
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