Fuel line diameter- Pros and Cons

[namba969]

What if... you ran a small air pump to pressurize the fuel tank instead of pipe pressure? You could have variable speed control of the pump to regulate tank pressure. Tie it in with your throttle servo, more throttle more voltage to the pump thus more tank pressure.

My random thought of the day.

Andrew

 

[Jim Allen]

what pressures are our fuel tanks running at in psi or inches of water collum. say at wot and milling, also what is a good way to hold a steady tank pressure? one way check valve, .020 orfice in pressure tap line, our engines are allways running rich and lean depending if you are milling or wot or half throttle.

The pressure exerted on a rigid fuel tank at 20,000 RPM, through a 1/8" ID line, can easily reach 120+ inches of water (4.336 psi). I have measured this pressure in various tuned pipe .45 cu in to 1.5 cu in engines during dynamometer tests. The pressure will be determined at any instant by the engines RPM, the total volume of the tuned pipe, the stingers ID & the stingers length. A water gauge connected in the line between the tuned pipe & the carburetor shows that this pressure closely follows the engine's RPM. It should also be noted that what ever pressure is read will be the same at any point along the tuned pipes length. I use this pressure to provide an adequate fuel flow through large ID (.625"+ bore), straight through bore carburetors.

Jim Allen

 

[Andy Brown]

what pressures are our fuel tanks running at in psi or inches of water collum. say at wot and milling, also what is a good way to hold a steady tank pressure? one way check valve, .020 orfice in pressure tap line, our engines are allways running rich and lean depending if you are milling or wot or half throttle.

What if... you ran a small air pump to pressurize the fuel tank instead of pipe pressure? You could have variable speed control of the pump to regulate tank pressure. Tie it in with your throttle servo, more throttle more voltage to the pump thus more tank pressure.

My random thought of the day.

Andrew

Now you guys are asking questions to issues that can be completely controlled by the fuel delivery system that we currently use.

The fuel line size, carb bore, venturi length, spraybar bore and stinger size can control the fuel mixture strength from low rpm to high rpm, 100%. When all of those parts are correctly matched the engine will idle smoothly on the beach. The engine will mill at slow speed for extended period. The fuel delivery from low rpm to full power will match the requirements of the engine. All of this without the need for a low speed needle in the carb, on board ignition or other devices.

Also controlled by these items is the mixture strength as the engine accelerates off the corner. Many of you have seen an engine accelerate strongly off the corner only to go rich at half track. If the needle is leaned to get the engine to clean out at top rpm the the engine would be too lean as it accelerates off the corner.

All of that can be completely controlled by adjustment the above mentioned items. When it is all correctly matched the engine will pull strong off the corner and accelerate to peak rpm at any point on the straight that you want it to. Overall pipe design will have an effect on this also.

 

[RC 90 Boat]

thanks for the information jim allen and andy very usefull .

 

[Jim Allen]

I should also tell you that a smaller size engine, typically .45 cu in, will have a lower maximum pressure of approximately 90 inches of water (3.252 psi). The stingers ID used is .3438" (11/32") X 5.000" long with a Silver Bullet pipe at it's shortest length. Engine RPM's are 29,000+.

JA

 

[PaulHail]

It's amazing that the fuel systems can deliver fuel at various carburetor openings and rpm with a single needle and pressure feed. If you look at a carburetor used in a two stroke motorcycle, it has multiple fuel circuits to cover the range of rpm operation and the amount the carb is opened.

Perhaps this will given an appreciation for why the fuel line and hence pressure drop of the fuel delivery is important in the overall picture of how fuel needs to be metered so that the engine will be able to properly deliver fuel when launching, at the end of the straight, coming out of a turn and quickly accelerate after milling.

 

[brad nichols]

With the needle in the carb you are doing the samething as the barrel rotates the opening gets bigger as it moves away from the needle, Same as on the slide carbs on R/C cars. Some people say the slide is better than the rotary as it is more lineal in acceleration.

Brad

 

[dr gary turner]

My pre needle tank is a LARGE DUBRO(silver) fuel filter w/o the screens. Seems to work

 

[mr_fons]

Out of curiosity has anyone every measured the pressures coming out

of the pipe to pressurize the fuel tank at different throttle setting?

Also would a check value work on our pressure line to help keep constant

pressure to the fuel tank.

Like this......

:

Cheers

Fons

 

[dwilfong]

Here is a little some thing I did to help with the flow on a BV needle.

 

[PaulHail]

Provided the check valve seals, it should maintain tank pressure. That said, you probably don't want to do that as it could end up causing a host of new problems.

The tank won't reach full pressure until the engine is at peak rpm at peak load. Revving the engine on the beach before launching will likely result in much less pressure in the tank so with the needle properly set to pull away from the beach will result in an engine that runs richer at those rpm's once the tank reaches full pressure. Not to mention your pressurized tank is going to drain the tank into the carb once the engine is killed or dies.

 

[Ray Sametz]

Tried a check valve once, if engine does not start right away it flooded the motor. OMHO

 

[Andy Brown]

Some of you are going astray with the thought of devices beyond a simple dump tube carb.

Read Jim Allen's post #22 and my posts #19 & #23 very carefully and think about the information there.

The information is GOLD if you learn to understand it and apply it.

 

[Michael Costanzo]

With the needle in the carb you are doing the samething as the barrel rotates the opening gets bigger as it moves away from the needle, Same as on the slide carbs on R/C cars. Some people say the slide is better than the rotary as it is more lineal in acceleration.

Brad

i have ran a ops slide valve .80 carb on a few of my .45's and 1 of my .67's and i really like the throttle response,much snappier. i wish i had the time to build a larger one for the bigger motors.i think as far as the cars(i run both 1/10th &1/8th scale on road) being more lineal, i think that also has to do with port timing,pipe choice.also like exh throttles like we use to run in control line carrier planes,super responsive..

 

[Michael Costanzo]

Here is a little some thing I did to help with the flow on a BV needle.

dave,did you do this only for the 1.05 motor??

 

[Michael Costanzo]

i must have way too much time on my hands but thought about this myself and wondered about if i should run bigger fuel line on my .91's. in the 35 years of nitro modeling i have always ran MED size in every thing from a .21 to .91 and have NEVER had an issue.tried the LARGE size on my .91 and it DID NOT make 1 difference in the way the boat ran from low speed to WOT. the motor is only going to take in a certain amount of fuel and then it is going to be metered by the spray bar.this goes for boats,planes,cars,etc.Andy,i was told by our friend Al Chinnelli many years ago about larger pipe fittings and larger fuel line to the pipe would make MORE power and have NEVER seen any difference at all in power or throttle response.PS sort of off topic,anyone who says you have to run the remote needle in front of carb is lost. 95% of all of my boats,the needle is BEHIND the motor.as far as the shorter the line is holds some merrit but some hull designs you cannot always get the neddle where you want it.

Hi Michael,

Your 90 did not see a difference because it is set up to run on a relatively small amount of fuel. For instance OPS, Picco, Rossi, CMB have supplied ALL of their marine engines with a 1.5mm spraybar bore for decades. That includes .21s and .90s

Boaters simply adapted to the small spray bars on .67 and larger engines by running small bore carbs , big head volume and small stinger bores to build more tank pressure.

This makes the 90 engine work with a wide variety of fuel delivery systems, restrictive or not so restrictive.

Some time in the late 70's or early 80's John Finch wrote an article on engines and stated that .90 engines ran better with smaller carbs. He stated a specific limit. I forget the size, but it was small. Maybe .390" or .435" max.

Aside from the MAC 67/84 being supplied with a .550" carb, no manufacture has supplied .90 engines with anything bigger.

A MAC 67 with small lines would have problems that would be noticeably cured with the use of large fuel lines.

The amount of nitro makes a difference too. 25% nitro, small lines are fine. 60-70% nitro, small not so good.

Aside from the inconvenience of having the long lines, the rear mounted mixture valve works fine. We have mounted it behind the engine on the Sg/SGX for the last 20 years.

However, we have recently provided more space between the engine and tank to mount the mixture valve in the newest SGX boats, because there is a benefit.

ok i understand what you are saying,but dave wilfong made my carb and spray bar bigger and also larger stinger (.475) (55-60%) for my .91 vac and still no change on the tubing size. not looking to set the world on fire(boat hauls ass anyway) but always looking to tinker and find more.i will always say,fast is in the pipe and prop and set up..

 

[Jim Allen]

The use of a check valve was successfully done in the past by a model boat builder named Ed Kalfus. The fuel system was pressurized from the twin exhaust system & then latter from the twin stainless steel tuned pipes Ed built. In between the check valve & the carburetor was a float that maintained a constant fuel level no matter how much pressure was exerted. Ed built the check valve & the float. Before the twin tuned pipes were added, both exhaust baffles & the carburetor's barrel (.625" straight through bore) were closed together for an excellent engine idle. When the boat was placed in the water, Ed would open the sub-piston porting before launching. ONE OF A KIND!!

Jim Allen

 

[Andy Brown]

Since 2001 we have exclusively used a simple dump tube (nitro style) carb on our Zenoah and Tiger King gasoline engines.

The power delivery is very smooth over a very wide range of rpm.

It works because the relationships between the carb bore, venturi length, spray bar bore and fuel line are correct.

Many gas boaters have observed us use this system at the races very successfully.

The following video is a break-in run of our TK 27 gas engine using this system.

Watch the full video and note the wide, smooth range of power. There are absolutely no peaky points in the power curve as in seen in many engines both gas and nitro.

This carb even uses a 3rd channel mixture valve. The beginning of the run was very rich and only part throttle, I leaned the mixture and gave short burst of full power down the front straight only as the run progressed.

In the second video is our Seaducer with CMB 91 Evo and also a simple dump tube carb. Watch the last laps as I demonstrate

the low speed mill and instant throttle response. The engine also idles very low and smooth on the stand.

 

[dwilfong]

Mike

I did this for my RS 1.01 first when I went with the 1/8 ID spray bar in the over sized oval bore CMB carb.

Also went with lager 1/8 ID tank lines on the outlet. This made the complete fuel system 1/8 ID from tank to spray bar with the extra large Durbo fuel line.

No need for larger one on the pressure. Air flows much easier in to the tank than nitro flows out.