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Preferred AFR for a '99

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#1
nballard76

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I am working on dialing in my new 99 engine and am wondering what AFR folks are shooting for at say 6500 RPM with a WOT.  Any advice or feedback will be welcomed.  Thanks!



#2
Tom Sager

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Roughly 12.5:1


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#3
Jim Drago

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Going to start this in another thread as well

 

 

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 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 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.  
  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.  You can fine tune or come up with whatever number you like, but, you will want to see around 12.5 at 6500 in fourth gear at wot throttle and operating temps.  You may get varying 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.
  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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nballard76

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Thanks very much Tom and Jim!  Fortunately, I am running the AiM Lcu 1 so it sounds like I have the right gauge.  I've struggled with power and have only ever been able to squeeze about 121 HP out of in on two different dynos.  Lately, the motor has seemingly been running a bit more powerful but I'm still getting readings of around 13.5 at 6500 RPM, WOT in 4th...this information is very helpful.  Thanks guys!



#5
Tom Sager

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Thanks very much Tom and Jim!  Fortunately, I am running the AiM Lcu 1 so it sounds like I have the right gauge.  I've struggled with power and have only ever been able to squeeze about 121 HP out of in on two different dynos.  Lately, the motor has seemingly been running a bit more powerful but I'm still getting readings of around 13.5 at 6500 RPM, WOT in 4th...this information is very helpful.  Thanks guys!

 

Great instructions from Jim.

 

Getting to 12.5 from 13.5 will make a meaningful difference.  Probably 3HP or thereabouts through most of the RPM range.  


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#6
gerglmuff2

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how do you confirm your AFR is reading correctly? my AEM uego matches the dyno i work with's AFR ... but who knows if thats any closer than anything else. how did you establish that AIM Lcu was correct? is there an ANSI or ISO standard/calibration somewhere?


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#7
Jim Drago

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how do you confirm your AFR is reading correctly? my AEM uego matches the dyno i work with's AFR ... but who knows if thats any closer than anything else. how did you establish that AIM Lcu was correct? is there an ANSI or ISO standard/calibration somewhere?

Than your dyno is likely wrong :(  I buy( as do many others) calibration gas from dynojet. It reads 13.0. We use it to confirm dyno afr as well as cars at the track. I have NEVER seen ANY of the gauges I mentioned read accurately.  Close as they ever come is .3 or so, 90% end up in .4-.6 range

Jim 

 

 


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#8
Steve Scheifler

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The problem I see most often is that the logger/ECU doesn’t match the dash gauge. You can adjust the logger/ECU voltage->AFR map if it wasn’t setup correctly, but if the dash gauge can’t be trusted then you’re kinda screwed without test gas or a known good sensor/gauge for comparison. Thanks for the heads-up, from now on I’ll put less trust in the supplied gauge.
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#9
Steve Scheifler

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Rereading my comments I see room for confusion. What I’m saying, and I think you’re saying, is that even with a “good” sensor the supplied display may read significantly wrong. That can be a “controller” problem or an actual gauge/display problem. The 0-5v output supplied as the input to an ECU or data logger is separate and the receiving device can typically be configured to correctly convert volts to AFR. (Some wideband controllers can also be configured to a custom output to match the connected device). I’ve generally assumed that the supplied gauge is as accurate as the sensor, at least with major brands. But if that’s not true then I don’t want to remap the logger/ECU to match the gauge unless it’s with knowledge that they are both wrong by some amount but at least in sync for review purposes. Ugh! For Spec Miatas and other mild builds we tune to whatever makes power then try to keep it there, so absolute accuracy isn’t critical, just consistency. With highly stressed and boosted engines you can’t necessarily do that so being able to stay close to the limits without going too far is critical.
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#10
Jim Drago

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I’ve generally assumed that the supplied gauge is as accurate as the sensor, at least with major brands. But if that’s not true 

 I assumed the same, but it is 100% not true. 


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#11
chris haldeman

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For those without access to dyno…..
51psi for 1999
59psi for vvt

These numbers from a known good gauge will not be off by more than .5 hp and are on the safe side
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#12
nballard76

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For those without access to dyno…..
51psi for 1999
59psi for vvt

These numbers from a known good gauge will not be off by more than .5 hp and are on the safe side

Thanks Chris.  Did you ever see the dyno runs from Nationals at Laguna?  I never did see mine...

 

If you have them, could you post what the top cars were running?  This would be a helpful relative number.  Thanks!



#13
Ron Alan

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Dyno numbers at Laguna in comparison to other dynos were low...NASA didn't make public anything I'm aware of. I only saw a few from people who paid to use the dyno. 

 

Your car runs fine Nate...chase the nut behind the wheel :) 

 

And btw...what if your FP gauge is off from the factory? :duck:


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#14
chris haldeman

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Ron you did see I said from a known good gauge correct?
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#15
Ron Alan

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Ron you did see I said from a known good gauge correct?

Was just pointing out the thread theme...what can you trust :) An attempt at i-ron-y....

 

Looked like Texas had a big party last weekend...that place was packed!


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