zenrx8

iTrader: (4)

Been doing some studying on FI systems, a book recommended by Kane. Just got finished with a segment on fuel pumps recommending the pump have a a stainless return line to the tank to prevent the pump from working against static pressure, say, at at idle. To my knowledge, the RX8 pump does not have a return. Someone please enlighten me how this works, as the text pointedly makes out that a non-return system leads to premature pump wear.

olddragger

iTrader: (3)

search newb!!

No, really, pumps have to run all the time, it maintains a certain pressure in the system governed by the fuel pressure regulator ( ours is in the fuel pump assembly). While the pump is not flowing to the engine it is flowing fuel to operate the venturi system that balances the tanks we have.
While there may be some static pressure the pump is operating agaisnt that is not going to be relieved by a return system. The line to the engine has to remain pressurized regardless.
Maybe they are speaking of a carb engine and not fuel injected?

We also have a have a two speed pump.

OD

oltmann

I can't provide much insight, but I'm quite certain that the GM LT1 uses a return line.

Also, I ran out of gas in my friends Dodge, and I remember wishing it had a return line. Had to disconnect the fuel line from the rail because the pump couldn't clear the air.

zenrx8

iTrader: (4)

True, Denny, the fuel line/rail must remain pressurized, true the two speed pump. The question I wanted to answer was how this system dealt with what appeared to be a stone wall at the injector rail; you're saying the venturi system robs some of the pump output to maintain equal fuel distribution? I can see that but it wasn't an option in the book Kane recommended to me; it was adamant that some type of return system be available to prevent excessive wear on the pump. A pressure regulator at the pump could send excess pressure to the venturi equalization system, right?

Search, my ***. Good thing college and job training don't run like that. Or military ops, for that matter.

Loki

iTrader: (1)

I don't know about RX8 specifically, but in general return-less systems there is a pressure regulator within the pump/tank assembly that maintains line pressure either by sending a signal to the ECU to modulate the speed of the pump, or by opening a bypass integrated into the pump. Or both.

oltmann

http://www.rx7club.com/showpost.php?...31&postcount=6

Found that.

zenrx8

iTrader: (4)

That's a good piece of info. Now I have to ask the Mother of All Stupid Newb Questions: Why can't fuel pumps be designed to be infinitely variable instead of on speed, two speed, the heating issues of return fuel. It would have to have a pressurized reservoir to accomodate immediate hi demand situations, but why else would this not work?

Loki

iTrader: (1)

I don't know the answer, but I suspect there is no benefit to be had over a 2/3speed pump, or the benefit doesn't justify the cost. The current systems are able to safely maintain the needed pressure in the fuel system well enough, so I guess they just leave it.

Plus, when you start playing with pump speed response and the resulting fluctuations in fuel pressure, you start getting unreliable fuel injector delivery, so it might actually be safer for everyone involved to run a stable pump speed to let the injectors deliver a predictable amount of fuel each time.

nycgps

iTrader: (1)

Cuz there is no point to make an infinite adjustable fuel pump.

zenrx8

iTrader: (4)

I'm willing to accept that, but not without the nagging question of "why?"

EricMeyer

I believe the fuel demand of the engine does not require variation and to measure and then adjust fuel pump pressure would be a "measure something and then adjust it". By the time the adjusted fuel pressure gets to the motor the motor could easily be in an entirely differnent rpm range, gear, load. We have found operating a rotary in a higher rpm range (6,000 to 8,500 for an hour at a time) requires lots of fuel and high pressure). I say this because dips in fuel pressure (and therefore fuel delivery) results in big temperature spikes as measure just off the exhaust ports. The engine is already running at 1,500+F at the exhaust port---it doesn't like high temps.

So.........constant fuel pressure in a return system allows the motor a solid supply to the injectors and they have access to instanteneous actuation. We run the tried and true Bosch 044 fuel pump which the majority of professional road race cars use. Run it at approx 90 psi measured at the fuel rail.

Hope that helps.

olddragger

iTrader: (3)

wouldnt long term trims show any dips in fuel pressure and theoretically lean/rich spikes?
Some pumps operate with a pulse width modulation system.
Agree with Eric--highly tuned engines need the constant pressure that a return system is suppose to give.--it is not without its own set of problems though.

HiFlite999

iTrader: (2)

A continuous flow through the fuel pump (either by having a return line or the bypass as on the RX-8) is needed to keep the pump cooled and lubricated. This is why it's a really bad idea to run out of gas with a modern car; running the fp dry even for a short time can result in pump failure. Some models are more resistant to this than others, but I know for older K-tronic injection Audis in particular, it was a big no-no.