Fuel Return Line Ideas?
 

I have an '06 MkIII chassis and am installing EFI. I need to put a bulkhead fitting on the top of the tank for my fuel return.

I have the access panel in the trunk out and found the fuel sender plate the is about 3.5" in diameter. It is not big enough to get a fitting into without rebuilding the entire thing (I could pull it off). It would be every easy to drill into the top of the tank right next to the sending unit but I don't really won't to blow up the car and the garage so that's not an option.

I'm thinking possibly a punch and die setup to make the hole or maybe I'm missing something more obvious.

Thoughts?

Thanks!

Pull the sending unit out and put the return fitting in it. The return line can be 1/2 the size of the feed line.

To reduce fuel aeration have the return line with a line going to the bottom of the tank.

That was the original plan and I have the fittings and the tube to get to the bottom of the tank (-6).

It will not fit in the sending unit though. There is simply not enough room without heavily modifying it by moving the sending unit over on the mounting plate, re-soldering mounting tabs, etc, which I can do but that doesn't seem like the right way to do it.

I'm not sure that there is anything smaller than a -6 fitting that would do the job.

Can’t find my photos, but saw a guy who repurposed one of the senders mounting bolt locations as a return line point. Used 2 small circular plate extended segments on the adjoining bolts to maintain a seal on the sending unit. I will keep looking.
Also saw a guy who made a piece on the filler inlet where the rubber hose attaches to the filler spout. Lots of room to play around there. Best design of the two options.
Blas

Fuel line sizing can be a challenging task for both the feed and return side lines. Something to keep in mind as you plumb the car is the engine power level and the impact it has on line sizing.

From a design stand point there are some significant differences between mechanical and electronic system fueling demands. While the engine's appetite for fuel is similar for similar engines and similar power levels the plumbing can differ significantly. Mechanical pumps reach max flow at max engine rpm. Electric pumps reach max flow the moment you turn them on.

This flow difference has a significant impact on the minimum mandated line sizes for the two systems. In the mechanical system at low engine speed the mechanical pump is also turning at low speed and producing, a predictable low fuel volume. As a result even if there is a bypass requirement back to the tank (and there always is) it is small because the pump output is small.

The EFI system with an electric pump is quite different. Electric pumps pump their max flow rate the moment you turn them on. If you have a 250 HP capable fuel system and a 200 HP engine, when you are driving around town using 25 HP to move the car, you actually return 225 HP worth of fuel (250-25) to the tank. That means a prudent fuel system designer would provide a return capability nearly equal to the feed capability.

The larger the engine power level the more significant this problem becomes. If you have a 575 HP engine with a 600 HP fuel system, for a little headroom, while you are motoring around town using 25 HP your fuel system needs to be capable of handling a 575 HP return of fuel to the tank. That 575 HP fuel return requires plumbing identical to that needed to feed a 575 HP engine at max power.

So what happens if you undersize a return line? You end up faking out your fuel pressure regulator creating an unanticipated rich condition for the engine. While it can be tuned around by a sharp tuner, the engine will be more responsive to the gas pedal and easier to tune for the tuner if the return line is properly sized so it is not giving the fuel pressure regulator a head fake.

Aeroquip has an excellent page on fluid velocity (volume) through their hose and how to select the proper hose sizing to optimize this dynamic. I have attached the page from their tech section with the nomograph that they use for this along with the formula they used to build their nomograph and how to use both of them.

Alas and alack! The Club Cobra vBulletin software has been set up to limit the size of uploaded pdf files to 39K. The Aeroquip page even when downsized exceeds this limit with a size of 81K.

My recommendation is to go to their site, down load their latest catalog which is the 2013 edition and go to their technical data section page #54. They are speaking to oil velocities and sizing of oil lines however, the data is applicable to all fluids.


Ed

This might give you a return line location / adapter. Call Craig not sure if he's still making them. Worked on #2584 and I did something similar on #1855.

Fuel Return Line

Return lines should be free of restriction.

Jim

Thanks for the input Ed. I am putting in the Holley system so that's easy. They specify -6 with 3/8" lines on both sides so not a problem. You could step down a size on the return but I have no issue with 3/8".

Can you post a picture of your sender, and the surrounding area, top of tank etc?

I need to figure out how to do image hosting and then link a photo as I'm not having any luck with uploads.

I think I have the return sorted out though. I'm either going to use a variant of Craig's method in the link above and go into the passenger side tank vent or modify the sender plate.

There is not much room in the fuel sender plate but I'm visiting a friend with a hydraulic supply house tomorrow and he thinks he can put something together that will fit. Possibly -4.

Don't go down to -4AN, your fuel pressure will not adjust easily, especially with a big pump.

This would be a mistake of considerable proportions.


Ed

Got it. We came up with a solution with all -6 hardware going through the sender plate. I have the parts and just need to make sure it works and will post.

Sounds like you will have a happy result.

Photos please.

Gary

Will do. I've been too busy running down small parts to start putting it back together.

It also looks like I have a solution for my coolant temp sensor. Roush put a billet extension block between the block and the t stat housing. Perfect to drill and tap for the CTS.

Sounds like you getting somewhere now.

Gary

Just my two cents. If you have a stock Superformance tank with high HP, it will be an issue. I had mine carbureted with 850 Demon and a black Holley fuel pump. Once I got just below 1/2 tank, the pick up would suck air under hard acceleration. I had a few people say it wasn't possible. BS with 600 HP plus. The pickup is maybe 5/16" laying flat in the bottom of the tank facing backwards cut at a 45 deg angle with a tiny box around it.

We will find out. I inspected the baffle in the tank and have it up and running.

Let us know!!