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Old 07-18-2014, 08:06 AM   #1
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Default Conversion of Returnless Fuel

Someone please guide me to a write-up on converting a returnless fuel system to a referenced FPR style. I am starting to plan my winter projects. 1999.
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Old 07-18-2014, 09:41 AM   #2
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Old 07-18-2014, 10:03 AM   #3
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easy peasy

I'm curious why you want to do this though. planning to bump the pressure up for better atomization?
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Old 07-18-2014, 10:12 AM   #4
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Schuyler, Thanks. I knew I had seen this, but apparently used the wrong words in my searches.

Vlad,
See the many discussions on idle AFR swings with hot re-starts.

The second reason is seen with the posts here and on MS forums about the crazy Non-Linear sections of the RX8 Yellow injectors. No matter what I do, if the MS ever sends a pulse width between 1.51 and 1.59mS, then the car goes way lean. Basically the injectors don't open in that range, but do at, say 1.4mS. Now it is true that I don't see those numbers except during deceleration (idle is about 1.75mS), I hate the fact that the AFR swings all over the place. Is it hurting anything? No. But I think you can relate to the OCD desire for things doing what you want them to do.

Third, I get rid of the First damper and the loop de loop. I know you pulled yours, but I am leary to do so with the returnless system in use. Naturally, when I go to the return, this garbage will be cleaned from the engine compartment.

tl:dr
To complete the experiment on hot restart lean and will it go away, or at least be greatly reduced by moving to a return system.
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Old 07-18-2014, 10:17 AM   #5
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Fair enough, looks like you have more reasons to do this than me.
Though you could just get ID1K's and not have any weird issues
I think flounders way posted above is among the most simple ways, its not bad.
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Old 07-18-2014, 01:59 PM   #6
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Finally Found the Answer: nb-fpr-into-na

Now I have a further question.

If I use the FPR stock from NA Miata (I'll have to decide which year... straight or curved fuel outlet), will it max out at 43 PSI, even at boost? Or is it fully vacuum and boost referenced MAP with a fixed 43 psi differential?

In other words, Flounder felt the need to also add an AFPR, and I would not plan to go that route.

My goal is a 43 psi MAP reference that would supply fuel at 33 psi when MAP = 30 kPA, 43 psi when MAP = 100kPA, and 58 psi when MAP = 200kPA (15 psi boost) etc. Does the stock FPR respond in that manner?
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Old 07-18-2014, 02:08 PM   #7
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I ran stock 91 FPR for up to 400whp at something like 26psi. I have no idea if it was actually maintaining 43psi differential at 26psi or not, but it worked a-ok.
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Old 07-18-2014, 02:14 PM   #8
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Quote:
Originally Posted by soviet View Post
I ran stock 91 FPR for up to 400whp at something like 26psi. I have no idea if it was actually maintaining 43psi differential at 26psi or not, but it worked a-ok.
I'm sure it was fine with 400hp worth of fuel going out the injectors. The thing to worry about would have been 400+hp worth of fuel trying to make it through the FPR with only 4hp with of fuel going through the injectors.
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Old 07-19-2014, 04:54 AM   #9
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I like Leafy's idea from that thread of adding a barb fitting to the in tank FPR when you move it to the fuel rail to have a pressure referenced FPR with 60 psi base pressure.

Has anyone actually done that?

I am contemplating the swap to a return system for the same reason... 99 with lean hot restarts with MS3 (never had problem with stock ECU). It isn't heat soak of the air temp sensor, so the theory that heat soaked injectors are increasing dead time may be the issue.

Keith
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Old 07-21-2014, 02:57 PM   #10
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Keith,

Could be interesting. I'm thinking that I should be able to make my max HP with a 43 Reference. My logic is that I calculated things based on 60psi at tank and max flow would be about 55-58 at rail. Max boost should be about 15 with my turbo. So 43 + 15 at the rail gets me back to 58. If I needed more flow, then modifying and moving the NB regulator might be worth a try.
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Old 07-25-2014, 09:08 PM   #11
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Keith: also, part of the value of the return system at low loads is to increase the open time of the injectors. This accomplishes 2 things: first, keeps the injectors out of the non-linear range; second, it makes the change in dead times due to temperature have less percentage effect. The lower the fuel pressure at low loads, the longer the low load pulses, hence most effect. Going to a higher referenced FPR reduces this advantage.

What would be good is a rising rate FPR (rather than a 1:1), but I want to stay with the original fuel rail mounted design.
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Old 07-27-2014, 04:45 PM   #12
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I am running a 2003 NB2 VVT engine in a 97 chassis. I have the NB2 injectors, NA FPR, MS3, and the lean AFRs on hot restart problem. I just recently moved my intake to the cold side and switched to the plastic bodied GM IAT sensor from the brass bodied IAT sensor with open plastic tip. I also installed louvers in the hood to help with under hood temps.

I installed a fuel pressure gauge and confirmed that the baseline pressure is 43psi and that it varies with vacuum. I also changed my MAT table to only remove about 25% from the ideal gas law baseline. After going back through a VEAL/tuning session, my VE values range from about 50-98 (with required fuel set to 11.3). Injector duty cycle maxes out around 75%.

My IAT's are 20-40deg less now and the new IAT sensor quickly shows lower temps once the car starts to move.

So overall, I am much happier with this setup and tune combination. However, I still see signs of the lean on hot restart issue.

I will be able to look through my latest logs next weekend from running the car yesterday and will post up some screenshots of the hot restart behavior. I am really starting to believe that the issue is caused by longer dead times once the injectors sit without fuel flowing through them. However, these latest logs will prove it one way or the other.

Good luck with your conversions.
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Old 08-30-2014, 10:00 PM   #13
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Reviving an old thread. I just did this today on my 99 nb. Because I'm replacing my motor with a jdm bp5a I have the returned fuel rail. I picked up an na metal fuel feed line and install it in place of the empty slot in the holders. The na metal fuel line is perfect and some bending is needed. I picked up some fuel line connectors from oriely auto parts.


Dorman part - 5/16 quick clip to 5/16 barb straight 800-080 qty2

Dorman part- 5/16 quick clip to 5/16 barb with 90 800-081 qty2

Used high pressure line 5/16 gates fuel injection hose with one straight and one 90 for the fuel filter outlet to fuel line feed.

3 ft of 5/16 fuel injection hose for fuel return. 1 90 quick connector is needed for the pump and ran the 3ft to the newly installed na feed line. Na line doesn't require a quick connector.


I wish I had pictures but the conversion was easy and straight forward. Again the stock miata line fits and the nb fuel line holders have an extra spot for it.

Although I haven't turbo yet I like to be prepared. With the jdm fuel rail and 60psi fpr I'll be set for now in stock trim. I did removed the stock fpr in the tank. That was pretty straight forward also.
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