Voltage takes a big dump all over your chest
 

Under boost (up to and over 30psi), my system voltage takes a big shit and drops from 14v down to 12v. I swapped out the alternator and it still does it. The only electrical thing I have changed is adding a second Bosch 044 fuel pump.

Note that I have an NB alternator which is ECU controlled. I'm using the plain ol' alternator control settings from FM.

Why it do this?

Faeflora the Fecal Fornicator

Possibly swap in an NA 1.8 alternator and see if you still have the problems.

Install a 2nd alternator.
Do it.

Instead of a coldside superchalternator, you need a hotside turbochalternator. Then you can stop running shitty band-aids like a 5th spark plug to provide extra current.

But for cereal, is the voltage drop rpm dependent?

Yes RPM dependent. Let me check if it is kpa dependent too.

If I had to guess I would say that the alternator is having trouble with the twin fuel pumps at high fuel pressures.

What the hell kind of pumps is he running?!

To pull a spec at random, a Walbro GSL392 (aka 255 HP) maxes out at 20A, delivering 85 gallons per hour at 140 PSI. Two of them in parallel would draw an absolute maximum of 40A, while emptying the entire contents of your fuel tank in just under 4 minutes, against a restriction of ten atmospheres.

(That kind of fuel flow, incidentally, would support a little over three thousand horsepower.)

https://www.miataturbo.net/attachmen...ine=1320082867

I have a really hard time imagining this to be the case.

Take it into an auto parts store and have them test the alternator on the car. If its under 50 amps at 2000 rpm, replace it.

What ECU are you using. If it controls the alternator, then that is the first suspect that must be eliminated. For example, from what I have read on other forums, Hydra 2.7 has 'less than pretty' code in the alternator control.

Yeah, that's what I am thinking. Now how should I deal with it?

Two Bosch 044 pumps wired in parallel using stock relay and wiring.

Um you mean drive it to Advance Auto and say test my shit bitch? Is that it?

Hydra 2.6 EMS.

Oh BTW I have an AFPR now. I set the AFPR to be 70psi at idle.

Yes that is it.

Check your grounds. High RPM vibration can make a loose connection them lose contact when they would otherwise be okay.

Well, not quite. It delivers 85gph at 0psi of restriction, but only 5gph at 140psi of restriction (~10atm). If the pumps were in series then that would change, but faefauna is running them in parallel, so the flow capacity goes up but not the pressure capacity.

70psi base + 30psi boost = ~100psi of fuel pressure, so the pumps alone are dragging 15A each (more if the 044s draw more power, which IIRC they do). I'll just assume you have each one on a dedicated relay with 14awg wire straight to a dedicated fuse?

You say you swapped the alternator -- what did you swap it WITH? New? Rebuilt? Or "known good out of another car"?

This is exactly the failure mode my alternator died with, at high revs the current output (and thus voltage) would sag. Not enough to kill the car, but enough to notice the headlights dim and (once it got worse) enough to shut down the boost control solenoid.

JasonC tested it out on my car, IIRC we jumpered the alternator control to push it to full power and watched the current/voltage output on his scope. Above 4000 RPM, it topped out at 30 amps and about 11.5 volts.

So if it was "known good from another car", maybe it wasn't actually known good after all.

Another thing to check is the voltmeter built into the Hydra. Mine is off by almost a volt, as compared to what I see with a couple of external DVMs.

--Ian

Yeah, I rushed through that and didn't think.

Key point: even if both pumps were operating at full-tilt-boogie, they're not drawing enough power to pull down a reasonably healthy alternator.

And that pretty much guarantees that they're not operating at full-tilt-boogie. The stock fuel pump feed comes from the main relay, which is fused at 30A, and that fuse is also powering the injectors, ECU, etc. Not to mention that the wiring itself would be getting pretty toasty at 40A or anywhere near that.

This just seems odd to me.

:idea:

Don't charge the dildo while stepping on the gas?


Perhaps you can try tossing FM a pm and confirming your alternator outputs are good?

What he said. Can you guys switch to another bigger internally regulated alternator? RX7?

Also, rewire your fuel pumps. The fuel pump wiring was designed for a tiny maybe 10 amp pump. Your setup is asking for trouble. In the domestic world, it would have fried your harness long ago.