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Adjusting your air fuel on Spec Miatas

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Jim Drago

Jim Drago

    East Street Racing / 2 Time National Champion

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Air fuel adjustments on Spec Miata


I get this question probably 25x a year, so I decided to write a quick and easy tutorial for a quick reference…  This is for 99-05  up Spec Miatas



  1. Make certain you have an accurate a:f gauge before making adjustments. The ONLY accurate air:fuel gauge I have seen and I have tried pretty much all is the aim Lcu 1. All AEM, innovate and autometer are ALL incorrect! Worst of all, they are off in the wrong direction. It is relatively safe to assume if you have one of these gauges it is reading .3-.7 richer than you are actually running. Meaning if you see 12.5 on your gauge, you are likely 12.9-13.0
  2. On a 99-00 despite what you see indicated on a:f gauge, never go below 46 psi and on the VVT never go below 57 psi. Doing so can result in damaging your engine.  in extreme altitudes, these numbers likely will not apply, I do not have enough experience there to comment. 
  3. Make all a:f adjustments off of wide open throttle at operating temps only, not when under 180 degrees, I prefer 190 actually.  
  4. I choose to make all my adjustments referencing 6500 rpm in 4th gear as I know where that is on my curve and easy to see on track typically.
  5. To make fuel pressure adjustments. First off the lower the number, the richer your car is running, the higher the number the leaner your car is running.  Generally speaking you will want to see around 12.5 at 6500 in fourth gear at wot throttle and proper operating temps.  You may get varying ideal numbers, but that will be VERY close on all cars.    
  6. To richen your car.. you want to increase fuel pressure. You do this by tightening the adjuster on the adjustable fuel pressure regulator.  You should do this while the car is running, and you will see the pressure increase on the gauge. lock the jamb nut when finished.
  7. To lean the car you will do the opposite, you loosen the adjuster screw on your regulator and watch the pressure go down.
  8. GENERALLY speaking for every 3-3.5 psi change you make the car will lean or richen 1 full point. So if you are at 11.5 and want to get to 12.5, a good start would be to decrease your fuel pressure 3 psi. I am typically more cautious going down than going up. A little rich will not hurt anything.  If you are at 13.0, you will add 1.5-1.75 psi to get closer to your target number.
  9. Cars running with E10 mixtures(most of us) will run leaner than cars running with ethanol free fuel. Typically a 1.5 -2 psi difference.
  10. If you find yourself outside these parameters (99-00 cars (46-53 psi) on 01-05 (57-64 psi)) stop adjusting and check your car!!!! Most likely you have a faulty gauge and most likely doing more harm than good. I see far too many continuing to lean their cars due to a faulty gauge(s). Common sense needs to step in here at some point, stay within those parameters always! Before going outside of them, confirm with your engine builder. It will save everyone a lot of time and money.


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