Fuel systems...
 

Looks like the 6758 will be here soon enough I’m going to have to figure out a fuel system. It’s a 2004 MazdaSpeed with built 1.9l 1000CC injectors and that’s where my fuel system thoughts seems to have stopped. I’ll be running pump gas. Looking for 350whp and I’m betting it be much easier to run a return system than keep the dead head system in the car now.

Thinking I can use current feed line for return and just run a new feed line. Will a 255 fill my needs or should I look at 320/340 that are now flooding the market? I’ve seen that a 255 should in some threads and is might not in some. Then I just need a basic regulator and and some fitting from pump through the tank lid and from fuel line to stock fuel line feed.

What you guys running with comparable power?

Pump gas a 255 should be fine. @Schuyler is using his evap line for return, and stock for feed.

Thanks, I messaged him last night about his setup.

I would run a dw300 and upgraded fpr

I agree with this.

It's not much added cost at this point either.

Thoughts on if the use the stock feel line and use the EVAP line for a return will be big enough?

See discussion here: https://www.miataturbo.net/diy-turbo...l-style-74629/

I'm gonna put it this way:

no one has found the limit yet

.....so, yeah.

And theoretically the more fuel you use, the less that line will flow. So only someone like me would find that limit (stock 1.6 turbo+Walbro450), lots of fuel, not much used.

Truth.

Has anybody actually checked if the restriction presented by the converted evap line keeps the back-pressure in idle too high?
High flow pumps feeding lots of fuel through small lines could create quite a lot of back-pressure. I seem to remember some musings about that somewhere, but couldn't find anything.

I've looked at the size of the orifice in my FPR. I seriously doubt a fuel line could cause more backpressure than that at low flowrates.

But a return-style system will have all the flow not needed by the injectors going back through the return line. That could be quite a lot. Could be as high as 70gph at 40psi.

we'll find out :) good on schuyler for trying it.

A rough calculation making LOTS of assumptions gives me a pressure drop of 14psi in the return line at 70gph. Quite something but not so high as to keep the pressure above setpoint.

I would go with a bigger pump if you're going to install an aftermarket regulator. Might as well and have the headroom, cost between pumps won't be that big of a diff.

The problem with using the 1/4 evap line as the return is that when it clogs you get the same problem as when you're overwhelming the stock regulator. Since it won't flow the full rate, the regulator can't work properly, and your idle fuel pressure varies based upon voltage, temperature, etc. You don't know it's happening unless you're data logging fuel pressure, it just shows up as a car that idles poorly.

IMHO 1/4 is too small for a big flow pump.

--Ian

@Schuyler you're gonna log fuel pressure right? You can do it for like <$30

The fuel rail outlet is small, i think its a 1/8npt adapter, not to happy about that, but its the only adapter for our rail.

I also have an 04 Mazdaspeed and hopefully will be putting an EFR (6258 for now) on it once the TSE manifolds ship. I've been trying to read as much as I can to plan the fuel system and had similar question about the 1/4" evap line being a bit small for a return. I'm wanting to go E85/flex fuel too so that will be just that much more fuel to return at idle. Already got a DW300 setting in the basement and was thinking I want a 1:1 regulator mounted close to the fuel rail to avoid having a ridiculously long (vulnerable) signal line like you'd have to use if you mounted it like FM does by the factory fuel filter.

I'd really like hardlines if possible instead of running braided. Thought about pulling the factory evap and either making a 5/16 myself or having a local shop do it. Maybe 2 new in 3/8" if that worked out but that might be overkill.

Also wondering if maybe the 5/16" NA return is the same shape and would fit? Thought I read where someone was going to try and use another NB feed line off a donor to replace the smaller evap but never saw where someone has actually done it. Are they close enough in shape that it could be bent to fit? Not a lot of miatas around here so probably not going to be easy to find a junk yard one to pull though.

Sorry to ramble. Probably way overthinking all of this but nothing better to do while I wait on manifold and MS3.......

Just get another NB line. NA line probably won't fit.