I see no problem with looping the rail, but it will not solve the problem of #8 not getting flow. The problem there is the location of the fuel feed. If you look at the rail, the fuel comes into it at a 90° angle. Just beside this feed, and oriented at another 90° angle is the feed for the #8 injector. I made a video, which I need to upload, where I took a looped rail, and put a garden hose into the fuel feed point. I had 7 little bubbling geysers of water coming out of injector bungs 1-7. Almost nothing at all coming out of #8. You could look down into the bung, and see the water rushing past the opening! I added a new -6 fitting to the center, rear of the rail, as a new fuel feed into the rail, and blocked off the original. Running the same test, the issue went away completely.

My final fuel system uses a fuel pressure regulator at the firewall, just behind the left hand airbox. I took the two fittings used originally for the loop, and made separate feeds back to the regulator, which then feeds back to the tank. This is a static (not boost compensated), post rail regulator system. This ensures that Injector #1 and #4 see the full, regulated amount of pressure, which was the goal of the loop. Regulating before the rail in almost any application runs the risk of supplying the injectors closest to the feed with full pressure, and a little less at the injectors furthest down the line. TBH, I've rarely heard of that being a real problem. The main fuel feed now has a -6 fitting attached to the factory steel line, and then to the new center feed on the rear of the fuel rail. I will try to get some pics, but this is more simple than it probably sounds.

So there are two schools of thought on the regulation. This first is what I have described above, with post-rail regulation, ensuring good fuel distribution. The other is pre-rail regulation, which will setup a system closest to the factory, but moves the regulation closer to the engine. The advantage of pre-rail regulation is less heating of the fuel, as the fuel will not run through the rails, on a hot engine, picking up heat, and then bring that heat back to the tank, thereby slowly heating all of the fuel up. Some people have issues with the fuel warming up, and causing pump issues.

Before anyone asks, I have no fuel heating issues, pump temperature issues, etc. I live in Florida. If it was going to be a problem, I'd know it by now. From past experience on FWD turbo cars, with similarly plumbed fuel systems, this has never been an issue. However, I have read that some in this very community have had issues. YMMV.

Back to your original question, I modified my first looped rail, by taking the original stub for testing fuel pressure, and welding it back to the rail at more or less a 90° angle, just behind injector #1. It seemed to fit fine, and access wasn't too bad. For my current setup, I run a permanently installed, liquid filled gauge right on the regulator.

 

Do you have any pics of your fuel rail and post regulator setup? Do you still have stock fuel pumps and feed lines? Sounds awesome!

 

Mine is not an E class, so my fuel pump situation is very different. I run an SL55/65 Pierburg pump. I have to do some work that will require the plenums to come off in the next couple of days, so I'll get some pics.

 

Hello, I have an 06' CLS55 but essentially an E-class for powertrain. Thanks in advance for pics!