MoTeC Global Forum

Hey gang,

I'm stuck on a few fuel system design questions and I'm hoping for some help.

The main question is, will my OEM hard lines (3/8" OD Supply/ 1/4" OD Return), cause starvation at 15 psi boost?

Recap:

I'm installing a new 2.2L naturally aspirated motor, with plans to supercharge it in the near future (discretionary spending pending). All my current purchase/ installation decisions are about future proofing for an easy transition to SC.

My existing fuel system is a bit convoluted, but worked fine with the prior 2.2 NA motor (i.e. Fuel pressure, 46psi, was static at all RPMs):

The system starts with a modified OEM gas tank where I've welded in a 300ZX in-tank fuel pump (Bosch 67867). I can't seem to find any official flow data on this, but users say it can flow around 200-260 lph and 80+ psi, and can support applications up to 500-600 HP.

Fuel leaves the ZX pump via a -6 hose, then mates to the OEM hard fuel lines by way of a -6 to -4 conversion. This OEM supply hard line is 3/8" OD. Once this gets to the front of the car it jump back from a -4 fitting to -6 Earl fuel filter, then -6 hose all the way to the -6 fuel rail.

The end of the fuel rail has a non-referenced -6 fuel pressure regulator mounted on it (which will change to referenced, once I go SC).

It comes out of the regulator via -6 hose until it once again hits the return OEM hard line, which is only 1/4" OD.

At the back of the car it once again converts from -4 fitting to -6 hose up to the fuel pump return port.

I recently upgraded my old 31lb GM fuel injectors to a more precise set of Injector Dynamics ID725s. These should easily cover the target 375-400 RWHP I'm hoping to make. I also bought their ID F750 (5 micron) fuel filter with dual sensor (http://injectordynamics.com/id-f750-fuel-filter/), this has -8 inlets and outlets.

Since this new fuel filter has -8 inputs and outputs, and I'm upgrading my hose, I figure it's time to challenge my assumption on hose size (-6 has to be plenty for 400HP), not only in the engine compartment, but considering the possibility of replacing the OEM hard lines (the possible bottle neck).

I'm targeting 15psi of boost for the SC set up, so assuming I'll be running an effective injector pressure of 46psi again, I'd have to run the rail pressure at 61psi?

I'm thinking the 3/8" OD supply (5/16" ID?) could be fine but the 1/4" supply might need replacing?

I'd prefer to troubleshoot on paper vs during an expensive dyno session. Any thoughts or experience on how to solve for this?

Thanks,
Cory

CNDat510 wrote:Fuel leaves the ZX pump via a -6 hose, then mates to the OEM hard fuel lines by way of a -6 to -4 conversion. This OEM supply hard line is 3/8" OD. Once this gets to the front of the car it jump back from a -4 fitting to -6 Earl fuel filter, then -6 hose all the way to the -6 fuel rail.

You're all over the place here.

If you do not know for sure whether the pump is capable or not...find out or replace it.

-6 hose as a high pressure supply will easily feed 5...6..7..800hp etc, and then some.

-4 as a fuel line ? No, wise up. As a return line...again no. That isnt to say it may not work, you could try, but I wouldnt. Do it right from the start. -6 might be more than you need, but it makes things simple. Either that or 8mm OD hardline type of thing for both, although you then need to find fittings to adapt to whatever you're using at each end.

30lb injectors on a 4cyl engine....so I presume you mean 4 of them and 400hp ? Not a hope. 750cc's though yes 400 without issue.

stevieturbo wrote:

CNDat510 wrote:
You're all over the place here.

If you do not know for sure whether the pump is capable or not...find out or replace it.

-6 hose as a high pressure supply will easily feed 5...6..7..800hp etc, and then some.

-4 as a fuel line ? No, wise up. As a return line...again no. That isnt to say it may not work, you could try, but I wouldnt. Do it right from the start. -6 might be more than you need, but it makes things simple. Either that or 8mm OD hardline type of thing for both, although you then need to find fittings to adapt to whatever you're using at each end.

30lb injectors on a 4cyl engine....so I presume you mean 4 of them and 400hp ? Not a hope. 750cc's though yes 400 without issue.

Yeah after re-reading, my post is a bit confusing, I think you mis-read a few of my points, but I'll take the hit for that. Lot's of info to digest there, and obviously I must have caught you at a low blood sugar moment

My restoration was 16 years ago and frankly I have no idea why I went from 3/8 to -4 to -6 (must have been a valid reason at the time), but it worked without issue on the NA motor while maintaining steady pressure to the old GM injectors.

To your point, the -4 bottleneck likely needs to be addressed, but based on a bit more research the 3/8 supply line over all should be fine. Muscle car guys are pushing 500+ HP on 3/8" lines http://board.moparts.org/ubbthreads/ubbthreads.php/topics/604960/1.html.

The 1/4" return does seem too small, but Fuel Lab has an interesting article on this http://fuelab.com/fuel-line-size-vs-pressure-drop/. I'll likely run a larger line here but based on this article it'll need to be the same or slightly smaller so as not to affect the regulator. There's also an interesting discussion on the topic here http://www.pro-touring.com/archive/index.php/t-17846.html

The Z fuel pump will rate up to 600HP, so I seem to be good there.

I upgraded my injectors to ID725s because of the future HP requirement, as mentioned, they were more than fine for the NA version, but I know they will not work at higher HP.

Thanks for the reply either way.