The Dodge Challenger Message Board

Hi,

I will be needing a new fuel pump and have a couple of questions. As my mechanic tells me, an electrical is superior in many ways, but if I'd install one, it has to be placed below the tank and I'd also have to install new fuel lines (and som wiring obviously), which favours a mechanical.

Isn't there an electric pump available which manages to suck the fuel from the engine bay, rather than pump it from the gas tank? The ones I've looked at are all recommended to be placed below the tank. All mechanical pumps operate above and 2-3 meters in front of the tank, so why can't an eletrically driven manage this?

How much power loss does a mechanical fuel pump imply?

Thanks
Fredde

There are HP losses with both kinds. Some mechanics don't realise that loading the alternator output creates losses as well.

A mechanical pump sucks fuel from the tank. An electric pump pushes fuel from the tank.

So there is no way around this. You have to mount the electric pump at the tank.

I personally do not like electric pumps. They are often noisy and they are not as reliable as mechanical pumps. The only advantage to them is they can flow more fuel. But how much fuel flow do you need?

However if you need to flow more than 120gpm, have fuel injection, or run NOS you HAVE to use an electric pump.

A 120gph mechanical pump is OK for anything up to 550hp as long as you use 3/8ths or 1/2-inch fuel line from the tank and 3/8ths or -8AN lines to the carbs. Make sure your fuel filter flows enough as well.

freddesjunne wrote:How much power loss does a mechanical fuel pump imply?

It should be very maginal, if you ask me. 1 hp? The main consideration is the flow potential as Dave already has written. Right?

The engine will leave around 500hp, so I suppose both types will suffice. But why do they differ when it comes to suck/push? They're both based on some kind of rotation, so they should both be able to perform the same work, shouldn't they?

If it is as little as 1hp or so, all is well and I'll go for the mechanical.

/Fredde

freddesjunne wrote:The engine will leave around 500hp, so I suppose both types will suffice. But why do they differ when it comes to suck/push? They're both based on some kind of rotation, so they should both be able to perform the same work, shouldn't they?

If it is as little as 1hp or so, all is well and I'll go for the mechanical.

/Fredde

It's easier to push than pull for electrical pumps. Also most O.E.M. pumps are cooled by immersion as a side benefit.

freddesjunne wrote:But why do they differ when it comes to suck/push? They're both based on some kind of rotation,

Nope. The electric pump is based on rotation.

The mechanical pump works like a human heart. That is it expands-contracts and has a couple of valves under it. One is open one way and the other the opposite. So fuel is drawn in through one valve and pushed out of the other.

If a heart worked like an electric pump you would need one in each foot.

The mechanical pump is a positive displacement diaphram type pump. Whenever the diaphram is pushed to one side of the housing it creates a vacuum on the oposite side. This rubber diaphram is usually fails with age or use. The electric pump uses a impellor to push the fluid. There is no vacuum created, therefore the fluid has to be supplied by gravity. This is why the electric pump has to be below the fuel level in the tank. You should install some kind of safety device to automatically shut off an electric pump in case of crash. The mechanical pump shuts off as soon as the engine dies with very little residual pressure.

Just my 2 cents worth.
Bob

That is better than my simple explanation Bob.

I like the hearts-in-feet explanation.

So these are two different types of pumps. But they both have a rotational movement driving them. They just turn the rotational work into two different kinds of lateral work, don't they? It shouldn't have anything to do with whether the original rotation comes from an electrical rotor/stator or if it comes from a pulley driven construction.

I sometimes feel like I 'need' to understand things. Then I'm like a child who asks until there is no doubt about how stuff works. Then what I need is for someone to tell me to be quiet and do as I'm told, so if anyone could do that please

I think you missed the point there. The electric pump drives the fuel foward with rotational effort from an impellor. There is no lateral movement at all in any part of it.

There is NO rotation at all in a mechanical pump. The pump drive is purely lateral. Up and down. Driven by your camshaft same as your valves.

PS

Be quiet and do as you're told.

Just one last question - promise Since this is my last shot, I'll be a bit over-explicit.

The mechanical pump itself never actually rotates - fair enough, I didn't know that. But then the camshaft turns it's rotational movement into lateral and exposes this movement to the pump. Then the mechanical pump sucks from a location above the tank. Like a little piglet lying on top of the hog mom feeding.

Then why shouldn't you be able to take an electrical motor, turn it's rotational movement into a lateral, and then connect this to the same type of mechanical construction as in the mechanical pump, and place it in the engine compartment, above the tank? Why should this puppy have to lie below the dog mom and have it stuffed when the piglet lies comfortably sucking on top? It's not fair!

It is not required that the electrical pump is placed below the tank but it is the optimal placement if you only have flow in mind. In real life (if you don´t build a dragracer or something) you won´t be able to move the tank to another position. The tank is placed where it is so to say. To mount the pumps below it will cause ground clearence problems. What to do then? Try to find an acceptable compromise! To mount it as close to the tank as possible, but not necessarily below it, is my suggestion.

Christer is right. Alongside the tank is fine. They usually mount to the chassis.

As for driving a mechanical pump with an electrical motor.

Try moving the pump lever by hand. Then you will realise how powerful an electric motor you would need for that type of pump!

You may as well fix an electric motor to each of the valves as well while you are doing that.

And yes. I know that has been tried before.

The electric pumps work at high speed so the motors do not need to be so powerful.