pressure line causing tuning issues??

[bigboystoysguy]

hey guys,
ok the fuel line going from the pipe to the cap on the fuel tank(aka the pressure line, right??) well my mamoth, when i took it out of the box had 8-10 inches of pressure line. i took this off and put maybe 3-4 in. my question is can i have to much back pressure?? would some thing like this cause tuning issues??

 

[FastEddy]

To short of a line may cause pulsing inside the tank. Pulsing will make tuning difficult at best.

If it came with 8-10 inches put it back and see what happens.

 

[bigboystoysguy]

ok ill try that. the only reason i took it off to begin with, was cause i wanted to make the chassis look cleaner. what do you think would happen if i put like 3 ft on there?? I'm not going to but would that also have adverse afects??:buggy1:

 

[El Pirata]

You'd have major tuning issues if you put a 3 feet length on there. It's likely you'd never be able to get enough fuel to the carb.

 

[SkyMaxx]

8 to 10 inches of fuel line between the tank and the carb (fuel feed) and between the pipe and the tank (pressure feed) is the correct amount.

As Eddy pointed out, too short and you will get pulsing in the tank. More importantly, too short will over-pressure the tank and lead to an over-rich condition at the carb. This may push you to a position where you are unable to tune the over-rich condition out.

Too long and you underpressure the system leading to an over-lean condition at the carb.

If you are trying to clean up the appearance of the chassis, look into some other options:
1. Fuel line covers (CLICK HERE to see my recently installed covers)
2. Fuel line clips that will allow you to coil some of the excess line.
3. Alternate routing of the pressure line to give a better appearance.

Remember, looking good at the expense of performance is just plain bad.

 

[lykan]

I noticed on my XTM .18 they haad the fuel line routed a certain way and a certain length, mine got frayed. I replaced it with what fit, not too long not too short..

My tuning went to hell. I dug the old lines out of the trash, used em as a guide, cut new ones to length, and my tunning issues went away.

I don't know if it's just XTM, or ay other companies, but they've got their lengths down lol.

 

[Nitro_Keebler]

Thats interesting Lykan... I'm wondering if I need to do that to my buggy engine.

Wish I could find a spec on fuel pressure line lengths in relation to what engine is being used...

 

[lykan]

I think a lot is dependant on yout fuel tank. Too much pressure in a little tank will cause a lot of air bubbles, and like Skymaxx says pulsing and a lot or turbulence in the tank. That means possible air bubbles, and a lot of turbulence.Too long and it might not have adequate pressure. I do know the smaller the tank, the more even and strong the flow from beginning to end.

I would rather have a tank 1/2 the size of the maxx tank. When full its got way too much pressure=rich. At the bottom of the tank it has to compress all the air in the tank before it can move any fuel.

If you want to experiment with lengths, wath the reaction in your tank. Test it about 1/8" from full so you can see what it does to the fuel.

For more pressure, above what you can get from a fuel tube, a good pipe is the best bet. Some provide a lot of fuel flow while others not nearly enough. I think the position of the pressure nipple on the pipe really makes a huge difference.

 

[bigboystoysguy]

i put it back on, 10 in exactly and tuning resumed its normal routine. it was like a magic trick or something. is 8-10 in standard or is it like lykan said about the size of your tank varys? my t-maxx runs great but could i change the fuel tubing and make it better?? i have about 7-8 in on my t-maxx.

 

[FastEddy]

I have always used the rule of thumb of .75-1' of return line per each 10cc of tank volume. Its a good place to start and its never let me down.
___________________
Edit:
Must have been a bit sleepy when I posted. I think my thumbs were swollen as well. It should have read. .5-.75 inch per 10cc.
Thanks for the wake-up Sky.

 

[bigboystoysguy]

I've never heard that before, see you learn somthin new every day!! Thanks for all your help guys!! :smoke:

 

[NooP]

I got my lines as short as possible on all my cars. THey all seem to hold a tune real well, the only thing I have noticed is that I have about a 40 degree difference from full tank to empty. I'm hoping longer lines will solve this problem.

--NooP

 

[SkyMaxx]

There is Eddy's rule of thumb and then the normal rule of thumb that says that anywhere between 6 and 12 inches is fine. I tend to keep it more centered on the 8-10 range due to the fact that 6 inches has proven to make things a bit rich for both a .15 and a .21 size engine. The 12 inches just seems to drop the things a bit on the lean side. But if you must run on the long side, do it with the pressure line, just make certain to shorten up the fuel feed line about the same length. Keep a balance, and things should work just fine.

 

[CJ-FOREVER]

Hey Sky is the common lenght for all rc or just MT. I have an onroad and was wondering if it applied to the 8-10 inch rule?

 

[SkyMaxx]

I have used this with MT and buggy alike. Onroad cars are set up a little different with fuel tank size, engine size, and things of that nature. I have seen some onroad cars with less than six inches total between fuel feed and pressure line. I wish I could give you a better answer, but I am mostly an off road basher and that is where most of my knowledge lies.

I will say this, if the car is an RTR then the length that the manufacturer puts on tends to be right in the middle of the range required. From there, you can deviate in either direction with the same results listed above (too short, too much pressure = rich and too long, too little pressure = lean).

Hope that gives you an idea.