Fuel line diameter- Pros and Cons

[Jeffmaturo]

Couple schools of thought here...some like the idea of having a bigger inside diameter allowing more fuel to pass through.

Others like the smallest diameter to create more pressure.

What do you think?

 

[shoboat]

Jeff,

I believe in the volume, their are many motors that are starved

for fuel and cannot and will not perform without enough fuel.

Spraybar and carb design is another big item in this picture.

Good Topic,

Mark Sholund

 

[Andy Brown]

There is a CORRECT size. For me that tends to be bigger and shorter. Lots of variables involved here.

 

[Grimracer]

I did some testing on this..

Pressure line can be used to adjust as well as hole in pressure nipple.. But.. line to the carb.. the larger the better.

Regarding larger boats. A 90 boat can use up to 1.5oz fuel OR MORE per lap.

Grim

 

[Andy Brown]

I did some testing on this..

Pressure line can be used to adjust as well as hole in pressure nipple.. But.. line to the carb.. the larger the better.

Regarding larger boats. A 90 boat can use up to 1.5oz fuel OR MORE per lap.

Grim

Grim,

I disagree with "the larger the better".

In general, yes it is better to error to the larger size, but it is very possible to have too little resistance in the fuel delivery lines.

I have actually used fuel delivery line diameter and length at the lake as a temporary tuning tool until I can get back in the shop and make the correct hard parts.

There is much more to this subject than most could even imagine.

 

[RC 90 Boat]

I disagree that larger is better also, especially pressure tap line to tank when you let off the throttle the tank pressure is lost quicker due to larger id of the line. with the correct id line the tank holds pressure longer , I have noticed this when milling.

 

[brad nichols]

An orffice is a restriction, under pressure the fluid has to move quicker for the same volume. There is always proper sizes for everything but shorter is better. pressure is developed by the pipe and stinger size. Smaller stinger will give you more pressure but can hamper engine performance. Bernoulli's principle or fluids. It's all a balancing act!!

Brad

 

[PaulHail]

Not only does a smaller orifice increase velocity for a given flow rate, a given diameter of pipe/tubing can only flow so much fluid and the longer that line is, the less fluid that line can flow for a given diameter. To add to the complexity, our fuel and pressure lines are not rigid, so they will expand under pressure.

Stinger size and back pressure are a completely separate issue and in a way can be looked at similarly to leaning in the needle. Both of them will increase the temperature in the engine, and that can increase power, and going to far will lead to detonation, damage to the piston and sleeve and taken too far will melt the piston.

 

[dwilfong]

The line after the needle is where most of the sizing make the most difference. This works in conjunction with the spray bar diameter to control the pressure drop at the needle. Remember the needle is just a pressure drop control for the system.

The amount of fuel in the line before the needle has a mass to it. This mass needs to react to the drop in pressure that is created by the spray bar in the fuel line from the carb to the needle.

So how much head ( fuel volume by weight) you have before the needle and after the needle will determine how fast the carb reacts to the load.

Some do not use pressure at all to the tank and some also use a float tank in front of the needle.

A float tank is the best way to control the head that the needle see. It is constant all the time.

Tank pressure is not a constant so it make for extra tuning veritable.

A pressure tank is simple,compact and easy to set up so it is the rule of thumb for fuel system control.

The systems we use are all about pressure drop across a orifice. Thy are not a water spout on a sink. A bigger hose is not always the right way to go to get more flow.

 

[PaulHail]

It is not that simple. The fuel line also produces a pressure drop, so you need to look at the entire line from the pickup in the tank, hardline in the tank, tubing from tank to needle, tubing from needle to carb, pressure drop through the carb and spray bar. Small diameter tubing will have a greater pressure drop than large diameter tubing, longer tubing will have a greater pressure drop than shorter tubing, and the pressure drop is related to flow.

Why this is important is that engine fuel demand is based on engine rpm, load and the effect of the pipe (both on air/fuel flow through the engine and pressure on the fuel system). The carbs ability to supply fuel to the engine is based on the air flowing through the carb, the needle setting, the pressure of the fuel tank and the pressure drop from the fuel pickup in the tank all the way to the tip of the spray bar.

We have a dynamic system where the engine's fuel demand varies, and the fuel systems ability to supply fuel varies based on fuel demand and engine operation.

If our engines operated at steady state, it would be easy to tune the needle and the only requirement of the fuel system would be that it can flow sufficient fuel for that steady state demand. We however operate in a very dynamic system where the fuel system is a part of the equation of how the carburetor is able to supply fuel to the engine.

 

[RC 90 Boat]

From the remote needle to the spray bar the fuel is not a solid stream of fuel the fuel just kinda trickles through this section of fuel line so why does this section of line have to be large id? I allways liked the old style carbs were the needle is right at the spray bar I think they work better and mix the fuel better .

 

[Andy Brown]

It is not that simple. The fuel line also produces a pressure drop, so you need to look at the entire line from the pickup in the tank, hardline in the tank, tubing from tank to needle, tubing from needle to carb, pressure drop through the carb and spray bar. Small diameter tubing will have a greater pressure drop than large diameter tubing, longer tubing will have a greater pressure drop than shorter tubing, and the pressure drop is related to flow.

Why this is important is that engine fuel demand is based on engine rpm, load and the effect of the pipe (both on air/fuel flow through the engine and pressure on the fuel system). The carbs ability to supply fuel to the engine is based on the air flowing through the carb, the needle setting, the pressure of the fuel tank and the pressure drop from the fuel pickup in the tank all the way to the tip of the spray bar.

We have a dynamic system where the engine's fuel demand varies, and the fuel systems ability to supply fuel varies based on fuel demand and engine operation.

If our engines operated at steady state, it would be easy to tune the needle and the only requirement of the fuel system would be that it can flow sufficient fuel for that steady state demand. We however operate in a very dynamic system where the fuel system is a part of the equation of how the carburetor is able to supply fuel to the engine.

Great post Paul!

 

[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.

 

[Grimracer]

Its all good.. to me.. I want line flow volume to be greater then the volume passing though the seat. No more no less. I have had them to small but never had them to large.

Length does make a difference. BUT.. what I have found is once tuned its not all that critical in the lengths we have to work with. IF however you change line its a good idea to match that working length to avoid a re-tune.

Works for me. your results may vary.

Grim

 

[Andy Brown]

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.

 

[johnsteltzer]

Has any body tried a gas carb on a nitro motor

 

[Jeffmaturo]

Just curious Andy, why would this make a difference?

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

 

[PaulHail]

It is not that simple. The fuel line also produces a pressure drop, so you need to look at the entire line from the pickup in the tank, hardline in the tank, tubing from tank to needle, tubing from needle to carb, pressure drop through the carb and spray bar. Small diameter tubing will have a greater pressure drop than large diameter tubing, longer tubing will have a greater pressure drop than shorter tubing, and the pressure drop is related to flow.

Why this is important is that engine fuel demand is based on engine rpm, load and the effect of the pipe (both on air/fuel flow through the engine and pressure on the fuel system). The carbs ability to supply fuel to the engine is based on the air flowing through the carb, the needle setting, the pressure of the fuel tank and the pressure drop from the fuel pickup in the tank all the way to the tip of the spray bar.

We have a dynamic system where the engine's fuel demand varies, and the fuel systems ability to supply fuel varies based on fuel demand and engine operation.

If our engines operated at steady state, it would be easy to tune the needle and the only requirement of the fuel system would be that it can flow sufficient fuel for that steady state demand. We however operate in a very dynamic system where the fuel system is a part of the equation of how the carburetor is able to supply fuel to the engine.

Great post Paul!

Thanks Andy

I've been teaching an industrial instrumentation class the past year and a half and have spent more time than usual thinking about flow so that I can convey to my students not just an understanding of how various flow instruments measure flow, but an understanding and appreciation for upstream and downstream conditions that affect flow through the instrument and that if you have an error with the instrument it might not be as simple as, the equipment is junk or the engineer that designed the plant was an idiot. Every component has limitations and conditions under which it performs well, and those where it will fall on it's face. If something had worked well for years or decades, it could actually be changes in the upstream and downstream flow that are causing problems.

I raced karts for several years and that gave me a great appreciation for the dynamic conditions under which our seemingly simple two stroke engines operate. Nothing like feeling the rush or acceleration as well as the sight, sound, smell and minimal lap times of an engine operating properly throughout it's rpm range. Also nothing like feeling a bog when something is wrong with the carb, fuel system and/or pipe not to mention the effect of the load, clutch settings in the case of single speed slipper clutch karts, or a prop on a boat.

 

[Andy Brown]

Just curious Andy, why would this make a difference?

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

Hi Jeff,

As Paul mentioned. it's Dynamics. The faster liquid must flow through a line the more resistance it feels.

Much more liquid must be delivered to the engine as the nitro percentage is increased. This means 70% nitro fuel must flow through the line at a higher speed.

This higher speed makes the line "feel" smaller to the 70% fuel. To open the needle more does not completely solve the problem.

The reason is that there is a larger differential in flow requirement between low engine speed and high engine speed when changing from 25% to 70%.

In other words, a 25% set up may flow 2 at low speed and flow 4 at high speed. The 70% might flow 3 at low speed but need to flow 10 at high speed. This requires a large line that will support the massive differential increase in liquid flow from low speed to high speed.

For most everyone there is no issues and knowing the physics behind it is not required because everyone is using a common set of variables that have been proven to work. Use X carb on Y engine with Q pipe and Z fuel in R boat at near sea level. Basically all of the combinations that work have been arrived at by the try, try, try method until something finally works.

When someone goes off the beaten path it becomes try,try,try again until something works. All of us know some combo that works, but most of us do not know why the combo works.

When one knows the WHY of the different systems such as the fuel delivery system, it becomes much easier to try something complete new and make it work.

Of coarse that is just one piece of the puzzle. So much to understand about our "toy" boats.

 

[RC 90 Boat]

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.