Ive asked this question before, but no one has responded. How do we raise fuel pressure on an 84 (Ljetronic) by only using 1 rising rate regulator? Ive suggested the clamping off of the fuel return line on the other stock regulator, but we dont know exactly how to do that, as the return line is supposed to be non presurized. Im sure we could bend the fuel regulator opening shut and have some effect.
Dave at 928 specialists has suggested one regulator (RRFR) and a stock damper, but that makes no sense as the damper would just flow fuel back to the tank, with no restriction. using a S4 regulator on the opposite side would probably do nothing. since the S4 uses only one and the S2 uses 2 regulators, my guess is the S2 regulator has a highter spring tension. Hey, maybe the S4 folks can replace their fuel regulators with a single S3 regulator and raise pressure. (just a guess, but an educated one)

what do you all think?

Scot and I have installed one rising rate regulator and are debating what to do with the other side. Obviously, an other regulator will solve the problem, but why spend the extra $150 if you dont have to.


mk

 

Mark you should be able to block one return line both ways and the remaining adjustable will set the pressure . The balance pipe and feed for the cold start injector is designed to keep both fuel rails at similar pressures . I am not sure why Porsche thought it needed two when the later cars only use one ??

 

Mark,

I have a complete L-Jet fuel delivery system "update" in the final stages.

There will be 3 (perhaps 4) iterations - so that folks can buy depending on their needs. The update package(s) address the limitations of the factory L-Jet system.

It will probably be ~2 more weeks for final release (barring the unforseen). Crappy weather here in the midwest and being unseasonably busy at my other business are slowing me down.

PM me if you (or anyone else) wants to off-line this.

Greg

 

mark,

why raise fuel pressure, are there other mods to this car?

 

[QUOTE]Originally posted by mark kibort
[B]Ive asked this question before, but no one has responded. How do we raise fuel pressure on an 84 (Ljetronic) by only using 1 rising rate regulator? Ive suggested the clamping off of the fuel return line on the other stock regulator...............

Mark, I am sure you already thought of this, however, here goes:

When installing the Super FMU (for a supercharging application), the return line comes off the back of the FMU and the front of the FMU is plumbed to the back of each fuel rail using a "t" connection, with the original fuel damper(s) in series on one side (and directly to the fuel rail on the other side). This arrangement applies the pressure regulation to the whole fueling system, immediately before the return line, as the OEM system was designed. The original dampers would seem to have no effect on this configuration except as a component of conveyance.

I am sure you know this, so what am I missing here about your application?

 

when you go to the dyno and your ratios are near stoich (ie 14.7:1) you need to raise the fuel pressure. we are actually near this and worse, so we are throwing away a lot of power. at some points ,we were in the 15s and 16s. Mods were only headers and euro intake. the power goes away much faster as you go lean, than rich.

Mk

 

FMU? anyway, the dampers have no regulation to pressure, so if that was to replace the stock fuel reg on one side, then the fuel would have a clear and unobstructed path back to the tank. (thus, bleeding off all the fuel rail pressure.) So, I dont think that is what you mean. the thing that I think will work , would be to clamp off the return port to the fuel tank, and this would just connect the fuel rail lines as they were stock with the stock regulator. this way, if nothing can return to the fuel tank, the fuel pressure is soley regulated by the single regulator on the other side. either stock or adjustable.

I mentioned that the S4 regulator could be tested for its spring tension. I would imagine, now that I think about it, that the S4 regulator has a stiffer spring vs the dual Ljet regulators. (becuase the two are used in parallel) one reglator from the Ljet, may have such a weak spring that the pressure would actually go down with one used alone. so, maybe the single S4 regulator could be used in replacement of the two single regulators for some gain in pressure if you were on the boarder.
the S4 regulator could be used on the opposite side now that I think about that too, as if it is much stiffer than the stock regulator on the Ljet, than the adjustable on one side and the s4 on the other could do the trick. The question, would be whether the adjustable reg could have any effect beyond the spring tension of the stock s4 regulator. If the answer is , No, then it would just be better to run two S4 regulators, or go full circle here, and clamp off the return line of the stock Ljet reg, and use a single adjustable reg, as we have on scots car now. (not running yet)

MK


MK

 

It probably was ill advised that I try and add something here.

I think you have forgotten more than I will ever know about this stuff.

While I am likely not using the correct terms to describe the installation of a Super Fuel Management Unit, it is in fact installed in the same configuration as the stock fuel pressure regulator in the S4 cars.

For the few who may not know, the difference is that the Super FMU has the ability to raise the fuel pressure under boost conditions as well as the normal vacume conditions.

 

Mark the fuel pressure regulators bleed off the excess pressure into the RETURN line ; blocking the main return will give you SO MUCH pressure that the injectors will open BY PRESSURE ONLY . The fuel pump will be very stressed . As I commented above just blocking the return from one stock regulator will allow your adjustable to set the system pressure . The balance pipe / cold start feed line conects both fuel rails . With the stock two regulator system the "weaker " of the two bleeds off the excess . I assume we are still discussing the 1980-1984 USA

 

MK,

Tried to PM you, but your inbox is full.

Greg

 

Yes, you understand I know how they work, the question, which i think you kind of answered was, what would happend if you block one of the regulators of the dual system,and what the single adjustable regualtor now on the opposite side, would be able to regulate. I imagine it would work fine, but wanted to understand other options, such as using a S4 regualtor on the other side to increase the pressure. Yes, we are talking about 84 US Ljet systems

Point understood about blocking the return like on a single. heck, if the fuel flow of the pump wasnt matched, the pressure could build to the max of the pump and probably blow lines off the injectors before they were forced to open. Not an experiement I would like to engage in.

thanks,

MK

 

Mark,

Just buy the later pressure damper and an adjustable regulator. Then plug the extra return line. You can remove the dual fuel pressure regulators that came on it stock, replace one with the new regulator and the other with the damper.

I did this on the Bastard and run 50 psig static with 914 injectors and all is well. That is about 39 lb/hour worth of injector and the L-Jet has mapping to allow the car to run superbly even off of idle.

Now, how about a hot wire and calibrator for that car of yours?

 

Thanks John..

Isnt that idea below the same as what I was saying? a damper (later) or fuel reg (early), both will have return lines going to the tank. close it off, plug it, etc, and you get the same effect, right?? the early damper has a line out that could be the return line, as it is normally the entrance line for fuel in.

Ive clamped off the stiff metal end of the return line on a stock 84 regulator. it is pretty well closed shut. now, ill trim it to fit the return line and that should do it right?

thanks, (on scot's behalf)

MK