Fuel pump fitting position

[24OZ]

Hi guys,

 

Need some help please.

 

I have just purchased a new BOSCH fuel pump and I am having my 240Z tank fitted with baffles and a surge tray, also having a bigger return and supply fitted.

 

The picture below shows how my original pump was fitted:

 

 

I am considering fitting my new Bosch pump in the same position with the fuel coming in from the top and out of the bottom.

 

I heard that the BOSCH pump doesnt like to suck so i am worried that it may not be mounted low enough in the position above. What do you guys think, will i be ok?

 

I also need to mount a fuel filter in between the tank and the pump.

 

Thanks.

[Pop N Wood]

I hope that is OK. That is about where I mounted my fuel pump. I did get it an inch or so lower, but I didn't want to extend much below the bottom of the control arm bushings. I used 1/2 inch stuff from the tank to the pump, including a 1/2 inlet and outlet in line filter, even though everything else is 3/8 inch. I haven't run mine yet so hope it all works.

 

This is an interesting thread

 

http://forums.hybridz.org/showthread.php?t=120415

 

The guy went through hell and with all the changes he did at once it is hard to tell what actually fixed his problem. But if you look at some of the pictures his pump was only a little above yours and he had no end of problems. So IMO the lower the better and the less restriction in the pump inlet plumbing the better.

 

Another intersting thing I have noticed is the braided fuel line designed to work with those slick looking AN fittings has an inner diameter that is a good bit smaller than the el cheapo and/or twist lock black hose. I used the braided stuff for all the high pressue stuff simply because I had already bought it, but used non braided on the pump inlet side.

[24OZ]

Thank you for directing me to that thread, it certainly was an interesting read.

 

I will bracket the pump as low as possible and try and get a filter in between and hopefully will be OK... (fingers crossed!!)

 

I have run a new copper/nickel hard line supply of 3/8" (10mm) and have a 5/16" (8mm) return. I have also asked the shop to make the fittings on the tank bigger, Return is 5/16" now and the outlet will be 12mm.

 

I have another quick question.

 

Below is a picture of the same pump i will be using:

 

 

 

I am thinking that i need to mount it upside down from how the picture is, as i believe the fitting with the black plastic cap is the outlet and that should be orientated to the bottom when i fit it. But there is no lip to help it from staying in position, the lip is on the other side.

 

(Does that make any sense?) Should this pump therefore be better mounted on its side?

 

Thanks.

[ktm]

The fuel pump is going to 'suck' no matter where you put it. What you want to do, though, is limit the head (head is the cumulative losses, friction, elevation, fittings, etc., that the pump must overcome to move the fluid) against which the pump has to suck. I believe this is what people are referring to when they say that the Bosche pumps do not like to 'suck'.

 

Placing the pump all the way to the ground is not going to help you, as your fittings are not changing location. The lower you put your pump, the greater the suction head (miniscule difference given the lengths we are talking about). As a matter of fact, to reduce the head, you would want your pump inlet as close (vertically) to the tank outlet fitting as possible.

 

Given the location of the fittings, the pump has to rely on the siphon effect to supply the fuel (not a bad thing). The only way you could possible further reduce the suction head would be to relocate your fittings to the bottom of the tank and then move your pump inlet to a position slightly lower. In this way the pump feed would be assisted by gravity.

 

Edit: Why not have the shop make the return and supply the same size? I recently had a shop modify my tank as well and had them install 3/8" SS feed and return lines fitted with -6 AN SS fittings.

[24OZ]

Thanks for your input, I never really thought about the "head" and how it effects the pumps performance.

 

As for why i didn't ask the shop to mount the same size fittings on the tank, my reason was that my return hard line is 5/16" and I was thinking, that it would be easier to use the same size pipe for the return on the tank as I would only need to use the same ID rubber fuel hose (to connect the two up) without having to use an expander/reducer.

[Pop N Wood]

Placing the pump all the way to the ground is not going to help you, as your fittings are not changing location.

 

....

 

Given the location of the fittings, the pump has to rely on the siphon effect to supply the fuel (not a bad thing). The only way you could possible further reduce the suction head would be to relocate your fittings to the bottom of the tank and then move your pump inlet to a position slightly lower. In this way the pump feed would be assisted by gravity.

 

I have to disagree with those statements. Whether the pick up tube comes out of the bottom of the tank or loops up 10 feet and back down, the static head at the pump inlet is going to be based on the difference in height between the fuel level in the tank and pump inlet. The only way the 10 foot loop will hurt you is if the lines have air in them or if the fuel is flowing through restrictive lines causing a pressure loss (head loss) through the line.

 

The static pressure at the bottom of a tank is proportional to the height and density of the liquid above it. Gravity does not care what the shape of the tank is, it works on the liquid the same way. With no fuel flow the shape and location of the fittings are irrelavant.

 

What you need to do is mount the pump so the impeller is as low as possible. I am not familar with that type of pump and whether the impeller is closer to the inlet or outlet. But I would think if you mount is horizontally you will be covered. You should be able to build a bracket off the rear diff cross member pretty easily. I make an L shaped piece of sheet metal that was sandwiched between the rear diff cross member and verticle plate mounting just above that rear control arm bushing. Trust me when I tell you you don't want to try and drill into that verticle plate.

[ktm]

You are forgetting frictional losses, which are significant in small diameter pipes/hoses/tubes. The 10 foot loop will hurt you when fuel is flowing. You can not talk about static conditions. By your logic, you could put the pump in the engine bay at the same elevation as the fittings (10 foot loop just straightened). Friction loss is proportional to the square of the velocity.

 

Admittedly, I forgot about the siphon effect. If the inlet is too high, then you lose the benefits of the siphon once the fuel level drops below the inlet of the pump. The single most important pre-requisite is that the discharge be lower than the liquid level. I was basing my earlier post without thinking about the siphon effect, which is significant.

 

Let's just say that we both know what we are talking about, but I am a poor communicator.

 

My point about (theoretically) reducing the suction stands though. At some point the fuel level will drop below the inlet of the pump, at which time it must overcome that elevation difference. By placing the outlet at the bottom of the tank and the inlet slightly below that, you have reduced the required 'suction'.

 

Edit: We are being nitpicky. Simply placing the fuel pump close to the tank will accomplish what you want to do 24OZ. The difference in elevation is inconsequential. The reason people were saying that the pump does not like to suck were referring to placing it in the engine bay as opposed to close to the tank.

[Pop N Wood]

The difference in elevation is inconsequential.

 

Well, no, I disagree with the statement above. I say the elevation of the pump definitely matters. I personally think that was one of the contributors to TonyC's problem.

 

The bigger worry is if the pump is higher than the level of the fuel, the fill line could drain back into the tank and the pump will lose it's prime.

[24OZ]

Thanks guys, I really appreciate the input.

 

I like the idea of mounting the pump on its side, I didn't even know if it were possible. Benefits of that to me are pretty obvious as it will be lower, which is a good thing. Especially if my L shaped bracket is on the reverse side of the ledge that its sitting on in the top pic above.

 

I will also have to drill that horizontal bar and fit a bracket for my Filter. hopefully it should all work out.

[ktm]

Well, no, I disagree with the statement above. I say the elevation of the pump definitely matters. I personally think that was one of the contributors to TonyC's problem.

 

You're being nitpicky again and taking me out of context. In the context of 24OZ's original statement about BOSCH pumps not liking to 'suck', the difference in elevation regarding reducing the 'suction' required is inconsequential compared to reducing the length of piping/hose required. Or are you saying that pipe diameter and pipe length have no bearing on head loss?

 

Elevation is important if you want to take advantage of the siphon effect. If the inlet is too high, then as you (and I) mentioned, you lose the siphon effect. However, the siphon effect is only an issue if you do not have a pump. If you have a pump, you have a mechanical means of lifting/moving/pumping the fuel.

 

Edit: I read the thread about TonyC's problem. I do not think it was an elevation problem. He even states "I thought about the pump height too but it does it with a full tank also." It was either a restriction (he mentioned that some crap was knocked loose) or maybe something to do with the vent lines.

[Pop N Wood]

You're being nitpicky again and taking me out of context.

 

Well, no actually I simply don't agree with you. But in the end it doesn't really matter. I have said what I have to say.

[ktm]

Look, if I am wrong I will gladly admit it. I view everything as a learning exercise. However, you have yet to back up your statements. You have not said anything.

 

You say that elevation is important otherwise fuel will flow backwards from the pump to the tank. You are correct....unless your pump is operating. You are thinking of static conditions, but a pumping system is dynamic.

 

You can place your pump inlet around 25 feet (33 theoretically, but you have losses and pump inefficiencies) above the water level, but you will need to prime your pump. You may be thinking of a self priming setup, using the principle of a siphon. Once the pump is operating and pushing the liquid, you do not need a siphon. The pump is doing its job.

 

Here is a great resource regarding pumps and how they work:

http://www.pacificliquid.com/pumpintro.pdf

 

Edit: If I am coming across too strong or like an ass, let me know. If I am wrong, please let me know and show me why.