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