How big is the return pipe on an IE fuel tank?

Bjorn Nilson

Bjorn Nilson

True Classic
My fuel pump has always been a little bit noisy. Today I started up my UT engine after winter hibernation. I have an EU carbed tank but I use it without modifications for my injected UT engine with a Bosch 046 high-capacity fuel pump. When starting up today, the fuel pump was screaming and I noticed a fuel leak at the fuel return inlet. I removed the fuel return hose from the tank and rerouted the return fuel to a bottle and the pump ran normal. The return pipe was clogged, but I cleared it with a steel thread and pressurized air. The fuel pump is now running much better but is still slightly noisy compared to running the return to a bottle, so the return pipe may be too narrow and cause too much back pressure.
What differs an Injected tank from a carbureted fuel tank? What is the diameter of the return pipe on an IE tank? On the carbed tank it is just 5-6mm but it seems like a wider pipe is needed to allow enough flow to reduce back pressure/noise.
 
carb tanks have the outlet and return tubes integrated into the sending unit. FI tanks have the outlet and return tubes welding into the lower corner of the tank. There is an internal baffle in the corner to help keep the fuel where it needs to be. I don't have have any measurements handy.

carb tank's bottom has no features.
carb_tank_4.JPG

FI tank has the baffle around the crusty fuel sock on the outlet. :)
IM002860.JPGIM002861.JPG

Lower corner of the stock tank, note the two pipes welded into the tank. The smaller tube on the left is the return.
tank_swap_2.JPG Fuel tanks and lines.jpg

Here is the FI sending unit above the carb unit. Note the FI unit has no pipes.
IM002961.JPG
 
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I believe the return on the carb engine is 6mm and the outlet is 8mm
I remember reading about someone swapping out and return lines and replacing the 6mm with a 10mm feed line.
However I didn't do that. I added a fuel swirl pot with a 10mm outlet to the fuel pump and an 8mm return from the fuel rail to the swirl pot. The return from the swirl to my carb tank goes to the standard 6mm return.

With the stock UT fuel pump no noise. More noise from my low pressure high flow fuel pump that feeds the swirl pot from the fuel tank.

Hope this helps
 
Fuel return lines are typically one hose size up from the inlet (pressure side) hose. If the inlet hose is 8mm ID, the return hose will be 10mm.

BTW, that Bosch 046 pump (the proven "race pump" really high fuel delivery rate) is sensitive to mounting direction, horizontal tends to run quieter, vertical makes more noise. Return restriction can add to the amount of noise produced.

Bernice
 
Rupunzell said:
Fuel return lines are typically one hose size up from the inlet (pressure side) hose.
Click to expand...

On the X1/9 FI cars, they use 3/8" (or is it 1/2"/12mm) from the tank to the inlet of the pump, and 8mm after the pump. 8mm is used as the return line after the pressure regulator all the way back to the tank.

I put in a pump similar to this but a direct replacement for the Uno Turbo - mounted vertically as OE in the X1/9 and so far so good as far as noise. Very quiet.

1712793709530.png
 
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Thanks for all replies. I will leave it as is for this season as it is working again. The pump is still noisy but not screaming or cavitating. I will consider rebuilding the tank to IE standard and replace inlet/outlet with bigger pipes with AN fittings at the bottom.
I didn't know that the pump mounting direction also has an impact on noise.
 
Bjorn, I just measured two tanks - one carb and one FI (both USA spec X1/9 tanks, not UT). Measurements are the OD of the metal tube (not to be confused with the ID dimension, in case that is where you are comparing them). But hose sizes correspond to the OD of the fitting.

Carb tank: 8mm feed, 6mm return.
FI tank: 12mm feed, 8mm return.

Something to keep in mind. This style Bosch pump comes with various sizes of inlet and outlet fittings. The X uses a 12mm inlet but much more common is either 13 or 14mm for many other applications that use this design pump. Therefore your "046" pump likely has a bigger inlet than the X (and FI X1/9 tank). I'd have to look at my Bosch spec sheets to remember the exact size of the "046". Depending on your specific setup, the return line size may need to be bigger as well.

So if you are using a carb tank with the stock tubes (through the sending unit at the top), the return side is 2mm smaller than on a FI tank. One possibility may be to use the existing 8mm supply tube (on the carb tank) as your return, and add a new 12mm (or whatever size your pump is) fitting for the supply.

By the way, the larger one is the supply from the tank to the fuel pump. And the smaller one is the return from the carb/injectors back to the tank. Not the other way around as some seem to think.
 
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There is a very real and technical reason for the fuel return line to be larger than the pressure/feed side (EFI systems), it is much about fuel return restriction at the fuel pressure regulator. The return line diameter could be as large as possible as this simply reduces the back pressure at the fuel pressure regulator.


Bernice
 
Take a look at the actual pipes, it is the other way around....return side is smaller than supply side.

Here's one example for the carb version. Follow them from the arrows to the other end to verify their functions:
9FU817__76122.jpg

The FI version is more difficult to get a good photo of due to being inside the tank. But think about it, the inlet to the fuel pump will be the same size as the supply hose barb on the tank (12mm). And the return from the fuel rail (pressure regulator) will be the same as the return hose barb on the tank (8mm). Sometimes reality cannot be ignored. :D
 
Dr.Jeff said:
Take a look at the actual pipes, it is the other way around....return side is smaller than supply side.

Here's one example for the carb version. Follow them from the arrows to the other end to verify their functions:
View attachment 83554
The FI version is more difficult to get a good photo of due to being inside the tank. But think about it, the inlet to the fuel pump will be the same size as the supply hose barb on the tank (12mm). And the return from the fuel rail (pressure regulator) will be the same as the return hose barb on the tank (8mm). Sometimes reality cannot be ignored. :D
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Yea sure, does not take a lot to remove that fitting then make it larger... or use a reducer going into the return fitting.
Bertone simply had to use what was designed, in inventory..

Bernice
 
No idea what the point was there.

The volume of return fuel will always be less than the volume of supply fuel, as long as the engine is running. Therefore the flow rate of the return side will always be less than on the supply side. Consequently the pipe size for each side will follow accordingly. The fuel pressure regulator is the restriction in the system and is located on the return side. Any pipe size larger than the restriction in the regulator is of no benefit, but that's not to say a larger one couldn't be used (which might be what the previous post was trying to say?). ;)

Regardless, pressure, volume and flow are facts of science, and what actual engineers go by - not what happens to be in inventory. :) It might be worth noting the size of the supply (feed) side is more important than the size of the return side. But it is the relationship between the two that we are talking about here.

Getting back to the original question, yes the return line on Bjorn's application might benefit slightly from a larger size than what he currently has. Not necessarily due to the volume and flow rate of the return fuel (the existing size is sufficient), but because of the larger fuel pump being a little noisy....a function of the increased pressure caused by a pump that has a greater capacity than the system really requires. The difference here can easily be handled by the Bosch pump, so it is merely a matter of reducing the noise level. In other words it is fine the way it is so long as the noise isn't too bothersome.
 
Dr.Jeff said:
The volume of return fuel will always be less than the volume of supply fuel, as long as the engine is running. Therefore the flow rate of the return side will always be less than on the supply side.
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That is true. When turning on the ignition, my ECU starts the pump to prime and pressurize injectors for a couple of seconds. When I checked the return line I got 2-3 dl of fuel in the bottle after priming. I assume that even if running the engine at full power (240 hp) wouldn't reduce the return flow much. Based on Bosch 046 specs (300 liter/hour@3 bar), it makes sense that the return pipe should be big enough to handle 300 l per hour without significant back pressure. Can the 5 mm return pipe handle that?
 
Bjorn Nilson said:
Based on Bosch 046 specs (300 liter/hour@3 bar), it makes sense that the return pipe should be big enough to handle 300 l per hour without significant back pressure. Can the 5 mm return pipe handle that?
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I agree. However remember that the return will actually be less than 300 l/h due to the fuel consumed by the engine. For the brief time the pump is running but the engine isn't running, the return volume isn't critical. So one question might be what will the return volume actually be when the engine is running? There's probibly a way to calculate that but I doubt it's really necessary to go that far; I think a good "estimated" return line size will be more than sufficient.

Therefore the next question is as you said, will a 5mm pipe handle it (I tend to doubt its ideal). That could also be calculated, but again I'm not sure it's necessary. Personally I'd change it to a larger size. As stated earlier, I'd swap the return to the existing 8mm pipe (currently the supply side), and replace the 5mm pipe to 12mm (or whatever size your 046 pump has feeding it) for the supply. The end of the 8mm pipe internally (inside the tank) may need to be modified slightly - to make sure it doesn't interfere with the new supply pick-up. And the new 12(?)mm pipe (pick-up) will need to extend all the way to the bottom of the tank (if it comes from the top).

Here's another way to answer your question. Referring back to post #5 by @myronx19:
"On the X1/9 FI cars, they use 3/8" (or is it 1/2"/12mm) from the tank to the inlet of the pump, and 8mm after the pump. 8mm is used as the return line after the pressure regulator all the way back to the tank."
Therefore stock X1/9 pump has a 12mm hose going into it, but a 8mm hose coming out of it. Then another 8mm line for the return. In other words the size of the output side of the pump (to the engine) matches the size of the return line from the engine (back to the tank). What is the size of the output side of your 046 Bosch pump? Use that as a guide for the size of your return line.
 
Given your exxe has a turbo uno conversion with increased power and Bosch 046 (044?) pump, few of the oem Bosch EFI fuel system designed in elements apply (for nee well under 100hp). What is important here is not following the as delivered oem/as delivered hose and fitting sizes as religious dogma as most of all that no longer applies..

The basic Bosch EFI fuel system:
Bosch EFI fuel system, basic.jpg


The Bosch 046 (044?) pump feeds the injector fuel rail, once the pressure at the fuel rail has reached the pressure regulators static set point, the return port will open allowing fuel to return back to the tank. The manifold vacuum inlet at the regulator acts as a "variable spring" adder to modify the fuel pressure based on intake manifold vacuum. Based on how this fuel system functions, it illustrates how a return flow restriction will increase fuel rail pressure beyond the designed in static pressure setting of the regulator. This is also why the largest diameter fuel return line is preferred.

It is assured the stock oem less than 8mm tube/hose will produce some amount back pressure on the pressure regulator altering it's designed in static pressure set point.

Notable, the Bosch pump in discussion is likely a 044, not 046 and has been replaced by the 200 pump. Single pump is rated to support nee 700bhp.
www.highflowfuel.com

Genuine Bosch 0580464200 (supersedes -044) 200LPH Inline Fuel Pump, BOS-0580464200, Replaces 058025044

FREE 2-DAY SHIPPING, EASY INSTALL from the AUTHORITY on OEM replacement Fuel Pumps. SAVE on parts for your vehicle with BOS-0580464200
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Direct experience with this Bosch 200 pump happened on the Triumph TR6 as it was used to replace a Walbro marine fuel pump and previously a Facet cylinder pump. Both the Wallbro & Facet fuel pumps failed.. which resulted in fitment of the Bosch 200 pump and complete alteration of the fuel system. The TR6 has twin stromberg carburetors that cannot be fueled at more than 3psi... Question for the "peanut gallery" how was this done? How was a high pressure/high flow rate EFI pump made to work with two carburetors that cannot function above 3psi (fuel bowl way overflowing and needle valve stuck open above 3psi) and very modest fuel flow. This is where the discovery these Bosch "race" pumps run quieter horizontal -vs- vertical...

Anytime the fuel system is altered/modified to accommodate a higher capacity fuel pump and flow rate capacity, all of the original design system parts come into question and often no longer apply to the revised/modified system.. Again, the oem design must not be followed as religious dogma..


Bernice
 
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