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.

Even if the small return line affects fuel pressure by a little and you are running an adjustable fpr and a megasquirt your just going to tune it to the available fuel. Should not make a difference.

https://www.miataturbo.net/attachmen...ine=1452061955

It would be inconsistent and annoying. Just like the returnless system.

Why? It would consistently add the same resistance and lets hypothetically say it adds 10psi to the fuel, you would never know it but you would set the fuel pressure to 40psi or what ever you want as long as it was above 10psi. It would probably only affect it at all at idle.
I understand its just wrong but sometimes wrong works.

The problem is that it's not consistent. You're getting whatever the pump can deliver, and that varies with voltage, temperature, amount of fuel in the tank, and probably a few other things. So you turn the headlights on, system voltage drops by half a volt, and idle fuel pressure goes down by 5 psi (making up these numbers, but you see the point)

--Ian

Exactly.

Would that voltage issue not happen anyway with any return system? A smaller than ideal line would just act as a regulator, its its even a restriction. Regulators are static (except boost reference but not at idle).

I can yeah. I could use whatever I want, I have a spare -10AN port on my AFPR lol.

But if you use an intake referenced fuel pressure regulator (i.E. a regulator that keeps the fuel pressure at a certain level ABOVE intake), it would set the Fuel Pressure at the setpoint (IM + FP). The only time when the backpressure from the return line would affect the outcome is when the backpressure is higher than or close to the setpoint.

No, backpressure from the return line should not exist at all, and he's saying if some were to happen, it would cause all sorts of wacky behavior in the fuel system. He's right

Get a 10an plug with a 1/8npt hole. Buy an ebay pressure sender and wire it into the ms3

Yeah that'd be the plan. I'll order this, I can drill/tap the plug. The premade 1/8npt ones are like 4 times the price.

https://www.miataturbo.net/attachmen...ine=1452102147

Is that a linear sender? I use some 0-5v sensors I got I can link you. I paid like 8$ for mine. But drilling and tapping is fine too.

Should be linear according to the description, yeah. Always down for a cheaper sender though.

In a properly functioning fuel system, the pump is sized to deliver more flow/pressure than is required for the engine to operate in all conditions. Yes, in any system the output of the pump will vary with the voltage, temperature, etc, but that's what the regulator is for. It uses a spring and a diaphragm to measure the inlet pressure and drive a variable restriction in the outlet such that the inlet pressure matches the set point. If things are operating inside the proper range, then when the outlet of the pump drops a bit due to voltage drop, the regulator tightens up the outlet restriction so that the pressure in the rail remains the same.

The variable restriction in the regulator has a mechanical limit on how far it can open and if you hit the limit then the regulator stops being able to adjust the pressure. At that point, any variance in the flow output of the pump will be directly reflected as a change in the pressure in the rail. A return line that is too small for the desired fuel flow will cause pressure to back up into the regulator and put you into this state.

--Ian

Not cheaper. Just smaller.

Pressure Transducer or Sender 100PSI for Oil Fuel Diesel Gas Air Can&apos;T for Water | eBay

It would work as you described and be ok at idle too, so long as the regulator can open up enough to bypass enough fuel to hit the target PSI at idle. It depends if your regulator can make an adjustment larger than the pressure drop on the return line. My guess is a 5-10 PSI pressure drop on the return line wouldn't be a problem at all on an aftermarket regulator, not sure on the OEM one.

+1 ^ What Pat said!

Someone that sees the light. Agreed.

I did some testing yesterday on my new fuel pump setup, a Walbro 450 and a Walbro 255. I have 6AN return line with aftermarket regulator. At idle, I can turn on the 255, then turn on the 450. Fuel pressure stays within 1PSI of target regardless of which pump, or both, are on. Flowing 700 LPH, it's safe to say there's a few PSI pressure drop in that line. Regulator takes care of it.

No, that was a guess. There is no light until its actually tested, or math is done.

Pat has a 6an return line. We are discussing a 4an return line. That is over 50% smaller.

I should go hook up a smaller return line just for fun, I have all the fuel pumps to test this.

My guess is the regulator is the key. I would guess the factory 99 regulator just can't bypass enough flow with a larger pump, with the regulator being the problem not the return hose diameter.

Do it for science.

The current discussion is about using a 1/4 evap line as return. Not about the regulator.

My test would likely show that a 4AN return would be fine on my setup. I'm running an aftermarket regulator though, so it doesn't say anything about using a stock regulator.

I probably won't test it though as my setup works like it sits.

So let's do the math:
Assumptions
1) System Voltage not to exceed 13.5V (because a quick Google search yielded no flow data for higher voltages)
2) Lowest fuel rail pressure we may reasonably set is 40psi. (lowest rail pressure is worst case- flow rate drops at higher rail pressures)
3) In idle (worst case) essentially all fuel going into the rail also runs through the return line.
4) Length of return line is 7' (because Miata and I could only find 7' and 14')
5) ID of our 6mm evap line is 4mm
6) ID of 6an hose is 8.6mm

In this case Pats pumps would push approx. 665lph/176gph.
Walbro 400 (400lph @ 40psi):
https://www.miataturbo.net/attachmen...ine=1452195087
Walbro 255 (265lph @ 40psi):
https://www.miataturbo.net/attachmen...ine=1452195087

Now let's take that and plug it into the back-pressure curve for a 6an fuel line:
https://www.miataturbo.net/attachmen...ine=1452195087

That yields approximately 5.5psi pressure drop in the return line. Which apparently Pat's regulator can handle. Here we have an actual datapoint- we can completely tune out 5.5psi back-pressure, using Pat's regulator- whichever that may be.

So if we're running just a Walbro 255 at 70gph, A random internet calculator yields a pressure drop of 3.15 bar in a 7' long 4mm ID line. That's 46 psi. More than the previously assumed rail pressure- no good.

The same calculator with the same basic values, but an ID of 8.6mm for 6an hose gives us a pressure drop of 5.1psi which is pretty darn close to the 5.5psi pressure drop from the random internet graph, confirming that the assumptions for our back-pressure calculation agree with somebody else's reality.

So the full flow rate from a Walbro 255 seems too much for our NB evap lines. Again, making all the assumptions above and a few more.

Now somebody has a tank of gas, a Walbro 255, a pressure gauge and some evap line lying around to confirm?

I agree

So I think I have just given up and decided to run new fuel lines. Simple clean solution. How far is stock single feed fuel good for? 400whp on gas?
Need a duel feed option?
OBX ebay one junk as expected? I'd toss out hardware but the rail it self is a rail.

no one has maxed out a stock fuel rail on a BP that we know of

no one

I haven't found the limit of a stock rail yet, ~500whp on VD.

Still only feeding it from one side?

Correct. I put a goofy stock fitting to 6AN adapter on it and hooked a 6AN hose to it. It's an 01 fuel rail, they have a bit better internal flow design vs the 99 rail my car came with. Still a 99 rail would work fine on my setup, but I had both so I used the 01 rail.

OK time to order goofy adapter and some fuel lines. Lol thx Pat