Conversion to return-less fuel?
 

My fuels and emissions class at CC over winter put all kinds of ideas in my head. One of them is the idea of converting Lucy to a returnless-style fuel system, for the reasons why manufacturers go to a returnless.

Now, what would be involved, to my view, is at the simplest;
-Capping the aux fuel rail outet, remove the vac line from the regulator.
-Reroute vac line to a new FPR just outside the tank.
-Return line back into tank, cover assembly with a debris plate.

Or am I hopelessly underestimating the complexity of my proposal, as well as any unforseen circumstances?

You're not really making a returnless fuel system as much as you are a really really long fuel rail. You'll just end up with greater flucuations of your fuel pressure because of the delay in the FPR correcting over that long of a line. I haven't personally seen it done so I can't say if the delay is actually long enough to effect your car and you could probably make up for it with the tip-in tables.

Aren't you supposed to loose the vac line to the FPR all together with a returnless and you use another sensor at the rail to monitor pressure? Then you can either have the ECU cut voltage to the pump to reduce flow or just get an ECU that takes into account rail pressure when deciding how long to open the injector for.

there's more than one design for returnless fuel systems and a lot of them don't use actual fuel pressue sensors on the fuel rail (although direct injected motors pretty much always do). The FPR in the tank isn't directly referenced to manifold pressure like on a return-style system, so the ECU has ways of compensatin for that.

Here's some stuff on the C5 returnless fuel system:

http://www.google.com/url?sa=t&sourc...Ywmfuw&cad=rja

and here's a diagram of the 2009+ Rx-8 fuel system:

https://www.rx7club.com/attachment.p...1&d=1307449401

So what would be the advantage of such a system ...other than of course the "TTT" factor...?

Improved fuel economy. A return fuel system pours hot fuel back into the tank, increases pressure and evaporated fuel, possibility of filler valve venting pressure. A returnless system keeps all the heat away from the tank. And especially in a RX-7, I'm guessing the fuel going back to the tank is scalding.

senario, what if you got an extremly hot day(texas style), you been drivin for 3-4hrs, pull into a resturant,shut off engine , come out 1/2hr later and the fuel in the rails has boiled into vapor?

alot of cranking to purge vapor from rails before liqiud fuel reaches injectors, called old fashioned vapor lock!

I'm all for innovation, especially if it yields some quantifiable results.

Interesting, never really researched that, kinda filed it next to hydro-locking in the "not gonna worry about it" folder

Vapor lock is avoided by increasing fuel pressure. On a return fuel system the hot start solenoid cuts vacuum to the FPR. On a return system the fuel pump control module or ECU can raise the fuel pressure.