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Old 03-15-2007, 07:57 AM   #1
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Default Perfect air/fuel ratio?

Just wondering what a "perfect" air/fuel ratio on a greddy miata would be?

Also what is considered to be in the "GOOD" range for a greddy miata?

Thanks!
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Old 03-15-2007, 08:58 AM   #2
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Just wondering what a "perfect" air/fuel ratio on a greddy miata would be?
The one at which you produce maximum torque with minimum EGT and no pinging.
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Also what is considered to be in the "GOOD" range for a greddy miata?
Seriously though... Generally it's somewhere between 13:1 and 12:1. Depends on I/C and W/I efficiency (if installed), condition of engine, timing... Every engine seems to have its own preference, and every WBO2 gauge seems to read just a little different. It's not terribly important that you be accurate to six decimal places on this- even with a full replacement ECU it'll fluctuate a bit.
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Old 03-15-2007, 09:08 AM   #3
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I think most folks shoot for 12.5-12.8, seems to be the best compromise between power, black smoke, and that "damn I'm running rich" feeling.

I'm planning on 12.8AFR/.85-.87 Lambda (~15% richer than stoich.)
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Old 03-15-2007, 01:41 PM   #4
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Sweet! Thanks!
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Old 03-15-2007, 03:30 PM   #5
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I'd edge closer towards 12 flat if you don't have an IC.
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Old 03-16-2007, 04:30 PM   #6
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Alrighty so i just put the wideband in and i cant drive it around because there is 6" of snow on the ground here but i warmed it up and it is at about 11.5 on idle.

I have a vortech afpr and i have the 7:1 disk in there now so Im gana try a smaller ring to get the a/f up in the 12's.
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Old 03-16-2007, 04:35 PM   #7
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the vortec is not supposed to have any effect at idle. In fact, it shouldn't do anything until manifold pressure is positive over atmospheric.

the problem is probably elsewhere (or the WB is improperly calibrated). what's the rest of the system?

<edit> snow, huh? fuel mix is primarily determined by engine load, but ambient air temp and coolant temp also factor in. still you should idle in closed loop, somewhere around 15:1...

Last edited by Ben; 03-16-2007 at 04:55 PM.
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Old 03-16-2007, 04:54 PM   #8
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Originally Posted by Snowsurfer03 View Post
Alrighty so i just put the wideband in and i cant drive it around because there is 6" of snow on the ground here but i warmed it up and it is at about 11.5 on idle.

I have a vortech afpr and i have the 7:1 disk in there now so Im gana try a smaller ring to get the a/f up in the 12's.

what injectors are you running? the vortech wont do anything to AFRs at idle, unless it's stuck closed.
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Old 03-16-2007, 05:17 PM   #9
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i warmed it up and it is at about 11.5 on idle.
Nay.


The vortech should not be doing anything at idle, regardless of disc. Check (and then double-check) the vacuum line running to it. Make sure the fuel hoses aren't reversed. All that jive. But don't go changing discs yet.


Are you running a non-stock fuel pump?
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Old 03-16-2007, 06:17 PM   #10
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Heres my set-up.
https://www.miataturbo.net/forums/vb...do=view&id=642

1.8 tan injectors and a walbro 190HP pump.
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Old 03-16-2007, 06:40 PM   #11
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there's no reason to idle so rich.

remove the vortec from the fuel circuit and see what your afr at idle is.
next I'd check the o2 sensor and clamp.
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Old 03-16-2007, 06:42 PM   #12
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if worst comes to worse and you cannot solve it, drill into the small circle on the AFM. turn the screw that's underneath it counter-clockwise untill your idle reaches 14.7.

the car must be fully warmed up first with correct timing and idle speed.
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Old 03-16-2007, 06:45 PM   #13
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if worst comes to worse and you cannot solve it, drill into the small circle on the AFM. turn the screw that's underneath it counter-clockwise untill your idle reaches 14.7.
I don't think that's the correct solution.
Maybe he'll end up getting a near-stoich idle afr, but if there is an issue with vortec or o2 system, the car won't cruise well, even if your method fixes the idle mix.
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Old 03-16-2007, 07:42 PM   #14
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Alrighty so just took it for a drive and pulled my plugs.

driving normal at 3krpm my a/f is at 12-12.5 but my idle is still at 11-11.5

My spark plugs are black and have a bunch'a junk in'em.

They are platinum should i go w/ regular cold ngk's?

What else could I do?
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Old 03-16-2007, 07:52 PM   #15
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you need to do what I wrote a couple posts ago.

Your car should regulate itself around 14.7:1 when you are cruising. 12:1 is only appropriate for accelerating in positive mani pressures.

Do you have an O2 sensor installed?
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Old 03-16-2007, 08:09 PM   #16
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Will do later tonite. Yes to the o2 clamp!
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Old 03-16-2007, 08:14 PM   #17
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Well the evidence points to no O2 sensor feedback. I'd check it to see if its voltage output dithers from around .3-.7 Volts.

To test: Remove plug 2 from the ecu, with your fingernail flip open the little tabs on the side of the connector. Then the top of the connector will swing up, allowing access to the pins from the wire side. Use an open paperclip and push it in over the o2 wire's pin (2N I think, not sure, it's the center wire and has a gray shield around it). The o2 wire will release from the rest of the harness. Plug the harness back in, start the car, and test the o2 wire that you pulled out of the harness for voltage with your DMM.

Don't ask me how I know how to do this.
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Old 03-16-2007, 08:21 PM   #18
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To test: Remove plug 2 from the ecu, with your fingernail flip open the little tabs on the side of the connector. (etc etc)
Or, since this is a '91 with a single-wire sensor, just unplug the O2 sensor itself and measure there. Pulling pins out of the ECU connector is very hard, and you're likely to deform them to the point they cannot be re-inserted. Dont ask me how I know that.

Seriously though. If you follow the black wire coming off your O2 sensor you'll find that it terminates at a small black connector somewhere in the behind-the-head region. Don't worry, it's accessable. Unplug the connector and probe it here. Much easier.
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Old 03-16-2007, 08:29 PM   #19
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Will do later tonite. Yes to the o2 clamp!
If you have an O2 clamp installed incorrectly it may be feeding a lean signal to the ECU all the time. If you have mine, check to see that it is wired correctly. If you have FM's, it is possible that the switch is made while still in high vacuum.
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Old 03-16-2007, 08:31 PM   #20
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Or, since this is a '91 with a single-wire sensor, just unplug the O2 sensor itself and measure there.
yeah, but he's got an o2 clamp potentially in the circuit too. Though your point is very valid, grabbing the voltage pre clamp may not be as revealing as grabbing the voltage at the ecu, post clamp.
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