Miata Air Flow Meter Location

Question (by Dick Hancock #39 SM - Knoxville, TN):
Bought your adjustable FP regulator for my 1.6 Miata recently, now have it installed along with a stock AFM, Marren calibrated injectors all @ 60.6 cc/min, NGK AFX air fuel sensor, and a new Actron fuel pressure test guage with schrader valve connection on the high pressure side of the fuel rail. After installation I checked/adjusted the 5x regulator pressure at idle (950 rpm showing on OEM tach) - with the regulator vacuum hose attached I set it @ 40psi, but then noticed that if I disconnect the vacuum line it drops to around 35psi. My assumption is that I should test pressure with the vaucuum line connected - is that correct? I plan on a dyno session next week to see what pressure range gives me a 12.8 to 13.2 AFR. Also, I have a straight air filter to AFM inlet - do you recommend mounting the unmodified AFM near vertical, with the electrical connect pointing up?

5XR Answer:
Vacuum hose: disconnecting the vacuum hose is going to simulate the fuel pressure under full throttle conditions while idling, as there is no vacuum when wide open. You can tune either way, and while my father tunes his with the hose off and recommends this method because it's representing the actual "full load" pressure (he's got an engineering mind), I actually tune mine with the vacuum hose on because it is easier for my wife or mother to not have to pull off the hose when they adjust it when I pull into pit lane during a practice! (I have a "shortcut" mind). The only difference to me is in the reference numbers used for your tuning. I will say that the "hose off" method will most likely provide a more stable gauge reading, but in my opinion I want to be able to tell whoever is adjusting it for me when I pull into the pits not to have to pull off the hose before they adjust it. It's your preference really.
AFM: This is a tricky little thing. Mine AFM is tuned from the days of non-adjustable FPR's, so it is already set lean from that tuning. Here is what I know about turning the AFM on it's axis:

  • Plug pointing strait up is going to put your AF/R very rich, as gravity is helping the "trap door" inside the meter open up, actually keeping it open most of the time, even during off throttle coasting during the race. I've experimented with the meter like this and could not dial enough fuel out of it to reach 12.8-13.0, so I adjusted the meter back around.
  • Plug pointing strait down is going to lean your AF/R out as much as allowed by your AFM. Unfortunately, this setup allows gravity to work against the trap door, restricting air passage through the intake tract. This setup will be the easiest to use for tuning though, because you can turn the meter up and it will give you a leaner mix, then dial in more fuel with the regulator.
  • Keeping the meter flat is not recommended in my opinion, as the trap door is going to get pulled around by gravity through the g-forces experienced during turns. This will cause a fluctuating reading from the meter.


    Here's what I would try on the dyno: Plan to turn the meter with the plug pointing a bit more than halfway up, maybe at 10-11 o'clock so you get the benefits of gravity helping the door, but not the full load of strait down gravity. If that makes sense!