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Fuel pressure question

Fuel pressure question

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Old 09-18-2009, 07:18 AM
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Default Fuel pressure question

Does exhaust from the pipe soley provide fuel pressure, or is there any vacuum generated by a running engine that also sucks in the fuel? I'd figure that if I open the tank to refuel, the engine would stall because loss of pressure but it doesnt?
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Old 09-18-2009, 07:27 AM
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Yup, it has vacuum.
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Old 09-18-2009, 08:28 AM
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its a little of both. more of the pressure though
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Old 09-18-2009, 09:36 AM
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A good engine needs very little fuel pressure at all.

And unless you're already running excessive pressure or have a worn sleeve; the engine won't stall just because you leave the lid up.
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Old 09-18-2009, 09:55 AM
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And engine NEEDS the pressure lie from the pipe, If not it will not run.
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Old 09-18-2009, 10:10 AM
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You can tune them to run without the pressure line. Its more of a tank breather than a psi line. Did it all the time with airplane engines. There really isn't much pressure in that pipe. Allthough the engine is desighned to have a pressure line, my cars run decent with the tank open for a little while.

Quick test for fun, remove your pipe. put a 2' fuel line on the pressure tap hold it like a U, kinda like a sinks trap with the open end if the line going vertical. Fill pressure line patially with water so that both sides of the U have plenty of fluid in them.

Blow into the pipes inlet, notice how much that water moves. If the water moves up your fuel line about 1.5-2" thats close to 1 or 2 once of pressure.
Google "manometer" to see how this works.

You can try this on your car as well to see exactly what pressures you have when the engine is running. It ain't much.
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Old 09-18-2009, 10:20 AM
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I view it like this. The pressure from the pipe is like the accelerator pump in a real vehicle carb. At idle your engine is tuned to run on very little pressure since the pipe pressure is relatively low at idle. As soon as you start opening the throttle and rpm rise the pressure in the pipe also rises increasing fuel flow to the carb. I think this also has a lot to do with why different pipes run so differently on the same engine.

If your engine stalls when the lid is open you are way too lean on the bottom end and if you have a problem stalling after a fuel stop you are too rich on the bottom end.
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Old 09-18-2009, 11:53 AM
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Originally Posted by token View Post
And engine NEEDS the pressure lie from the pipe, If not it will not run.

This is not 100% true.
I ran my GT2 w/ DPS.12 with no pressure line
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Old 09-18-2009, 12:02 PM
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Originally Posted by scwrod View Post
This is not 100% true.
I ran my GT2 w/ DPS.12 with no pressure line
ok
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Old 09-18-2009, 12:34 PM
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Even though two strokes don't have much vacuum, the venturi effect of the carb also has the ability to draw fuel from the tank.
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Old 09-18-2009, 12:46 PM
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Our r/c 2-stroke engines don't need the pressure line to run. I use to run my 1/10 gas truck without a pressure line all of the time. It made the throttle super smooth and a lot more linear/easier to drive. You need to run a pressure line from the tank's lid and secure the other end of the line (that normally goes onto the pipe) but position so when the car is upside down the end of this lins is above the bottom of the tank so fuel doesn't spil out when you're upside down. The take a 1" or so long piece of tubing, put it on tank with a screw or some plug on the end of it. Richen motor top & bottom between 1/4-1/2 turn and good to go. You do lose the ability to prime the engine/fuel line which can cause problems for re-starting after flameouts. If engine is super hot/vapor lock, it might not start/draw the fuel.
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Old 09-18-2009, 12:52 PM
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yep no pressure line was a hot setup on the gas trucks "back in the day"lol
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Old 09-18-2009, 12:54 PM
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Originally Posted by token View Post
And engine NEEDS the pressure lie from the pipe, If not it will not run.
wronge
you lose
you get nothing
good day sir!
lolololololol
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Old 09-18-2009, 12:54 PM
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Jesse thanks for posting, empirical data from pros is priceless on an ordinarily not-so-certain forum.


Kinwald also used to run his Losi's without a pressure line to the engine.
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Old 09-18-2009, 01:27 PM
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Originally Posted by allan42r View Post
wronge
you lose
you get nothing
good day sir!
lolololololol
ok - if you are a racer are care about performance - YOU NEED THE PRESSURE LINE -

If its not needed why is it there, why does it make such a huge difference in tuning. I have worked on way to many engines solving problems and it turns out it would not stay running becuase the line had a whole or the nipple on the pipe was broken.

Go head take your 1/8 to the track a TRY to run with out that line. better yet try to start with out the line. I will be here Monday to hear the results. - until then ladies
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