My engine showed up with a plug in the end of one fuel rail and AN fittings on the other three ends, indicating that B&B "dead-headed" the fuel rails (feed the end of rail 1, loop rail 1 to rail 2, dead-end rail 2), and there didn't seem to be any problem with power.

Is there benefit to Y-ing the feed line to both rails and then Y-ing back to a pressure regulator as opposed to the scenario above where the pressure regulator would come before the fuel rails?

Obviously dead-ending one of the rails results in quite a savings in fittings, but is it really a "bad" thing? Is the main concern that the injectors on the end of the last rail might starve for fuel under high demand?

I've seen people set their cars up both ways. BTW, I have bank-bank injection with my SEFI-8LO, so sequential injection isn't a factor in my case.

 

jim, i dont have any facts to back this up.. but I would Y it in, that is how I have done every "big" fuel system, and never had a single hurt motor (including my 1000hp turbo lt1). Like you, I am concerned about flow to some of those injectors.

 

That would sound logical to me to have both supplied parallel with a Y.

Im sure you could mathematically figure it out right? Based off fuel flow and subtracting how much each injector consumes of that initial amount. It seems to me if you are running huge injectors which Im sure you are, there wouldnt be a lot of fuel/pressure left at the last injector at high Rs

 

With your present set up,is your regulator in the pressure line before it gets to the rails?
I have mine y'ed #8 to the rails from the pump and two lines running to the regulator out the back of the rails #8,hooking to both inlets on the regulator and a line from the reg to tank for return #6. Don't have any problens with it being n/a.
Don't really know which is best.

 

I would have thought that it wouldn't matter, since the pressure should be the same throughout the system.

I'm not giving an opinion, I'm just talking out loud. Wouldn't mind hearing the thoughts of one of the fluid dynamics guys though...

 

The objective of the "rail" is to give all the injectors as close as possible to the exact same pressure.... that way each injector flows the same for a given pulse width. As the flow rate to each injector goes up, the rail is going to start experiencing significant pressure loss from one injector to the next. By the time you go all the way "down" one rail, cross over and come back "up" the other one, the pressure difference between the first and last injector could become significant. I would assume with the FAST that you could "trim" individual injectors in a sequential system (I know the MoTeC can), so it would be possible to alter the trim to each injector to compensate for the falling rail pressure. If your engine builder has increased the inside diameter of the rail, it would tend to minimize the magnitude of the pressure loss.

Another way of minimizing the impact of the pressure loss is to run the fuel system at a higher pressure level, so the loss represents a smaller percentage of the total pressure. A 2psi loss on a 3bar fuel system would represent a 2.3% flow reduction, while the same loss on a 4bar system would only account for a 1.7% flow discrepancy between injectors.

In theory, the dead-end system is going to reduce the pressure loss through the rails as well, since the rails don't have to handle the 20% of the fuel flow that has to flow through the rails and back to the tank in a fully recirculated system. The less fuel flows through them, the less pressure loss.

In all honesty, I have never tried to measure the actual pressure loss in a given rail diameter and length, for various HP levels. But just applying simple logic, splitting the rails by using a "Y" on the feed end is going to reduced the pressure loss in the rails by a factor of 2. I've seen that approach support over 1,100 blown HP on stock LT1 rails. I have mine set up that way, with a -6AN into the back of each rail, and a -6AN return taken off the diagonal cross-over pipe and run to an AFPR under the cowl. But then I'm only making about 800HP w/N2O.

 

I have the same dilemma with a debate over the setup of injection for a 500 wedge.
One view is the feed from the pump going into the regulator[one inlet two outlets] then to the rails[making the regulator a "Y"] then the other end of the rails are two outlets plumbed to a common return.
The owner says this is the way the manufacturer [Indy] said to set it up.
The other view is to have the feed y'ed to the rails, then the other end of both rails y'ed to the regulator, with a return.
**does the fuel need to be regulated before or after the rails?**
I point out my camaro, where the fuel in the rails is the correct pressure[read of the schraeder valve on the rail] set by the regulator.
Or do i have it backwards?
The regulator is a Holley that looks like a carb style[bottom feed in-two side outlets-tower with adjusting nut] but came with the Indy setup.

This seemed to me very similar to jimlabs question,which is why i posted in this thread,I apologize if it is offtopic and will edit it.

 

Bringing this thread from back aways in an effort to answer a question I have. I'm considering the "Y" Parallel approach that Injuneer suggests, however I have a 93 and the crossover tube is in the rear of the rails. The input AN fitting would be welded to the "rear" of the rails, and the only place to put the return is on this crossover, similar again to what Fred did. I looked and because of the way the rails are on the manifold there isn't room to weld fittings on the front of the rails. The MAP sensor is in the way on one side and a boss for the throttle cable bracket is in the way on the other. Would it be OK to do a return setup using a rear crossover. Intuitively it seems like fuel would tend to go right out the crossover connection, without any fuel getting to the front of the rail. After thinking about it further I believe the regulator wouldn't allow fuel to go through the return until the rail is completely pressurized, but I still think it would cause trouble.

The only other alternative that I can see would be to weld fittings to the top of the rails near the front.

 

why not weld an fittings in the front of the rail in a different spot then the end or weld a 90 in? I have dine a few diff. set ups with various cars and there has always been some way to get the fittings in the front.

 

Something to throw out there for all to think about. Since we are talking pressure here, where does the pressure begin to drop and how is the pressure created? The pump creates the pressure by the volume of fuel its moving GPH. The regulator drops the pressure via the opening of the plug. The amount of fuel allowed to pass through the plug and seat of the regulator will determine how fast the pressure drops The GPH will determine how fast the pressure recovers. The inner size of the fuel rails will determine how much fuel can be moved through the rails.

If you used a back pressure regulator like one for a carb, then you would see significant pressure losses in rail two. But we use a upstream regulator in these systems. So the regulator will not open until it sees the upstream pressure high enough to overcome the spring pressure. Your entire rail system will have to be pressurized to achieve this. Otherwise the fuel will flow to the lower pressure in the system.

Now if your pump volume is too small or the port size/ flow rate (CV) of the regulator too large, then you would see fluctuations in fuel pressure. Granted internal drag in the rails could lower pressure, but thats why you use a large ID rail and the length of then rails is not significant enough to produce drastic presssure drops. So in answer to your question, if your pump and regulator are not balanced for the system, a parallel feed to the rail may offer some hope, but you would more than likely see pressure variables through out the system anyway.