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Thread: Avgas A/F ratios

  1. #1
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    Avgas A/F ratios

    Hello everyone,
    I have a feeling xr7Dave will be the main audience, but anyone with info please chime in. I finally picked up an innovate DLG-1 and LC-2 wideband o2 sensor. Still need to finish up tucking everything away but did the calibration and started the car today.

    Are the idle ratios an indication of top end performance? She idles in the high 16's- low 17's. Didnt drive it yet, hopefully tomorrow. I am running 100LL avgas.

    Hopefully on my way to a badly needed tune in the near future.

    Other recent add ons, finally got the oem MAF sensor, along with a 73 mm MAC housing (and matching sample tube) from SCP.

    Thanks for all the help,

    JJ
    Last edited by james5275; 05-24-2017 at 11:55 AM.

  2. #2
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    I am running that same wideband in my junkyard wars race car. I have the sensor plugged into the exhaust about 12" after the downpipe turns horizontal. My car has a completely stock 1995 long block with the exception of mustang headers and a 1989 blower pulley on the blower. It typically idles off the scale lean but seems to run fine at part throttle. The reading you get at idle has little to do with how it will run at wide open throttle. The computer uses what it has learned at part throttle to guess what fuel the engine needs at wide open but your AF ratio at wide open is influenced heavily by your mass air meter, your fuel injectors and your fuel pressure.. At wide open, my car runs best in the low 12s and it is fastest around there. I like to run it in the high 11s just for a bit of safety. It becomes a bit of a turd and drops mph in the quarter at anything under about 11.2:1 AF ratio. All of my numbers are on Sunoco 104 octane unleaded or VP racing 109 octane unleaded.
    Last edited by Broncojohnny; 05-24-2017 at 09:47 AM.

  3. #3
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    What I come up with is 15.1 or 14.9:1. That being said the engine should be running richer if the car was tuned for e10 fuel which is 14.1:1 Stoich. Or should idle about the same if it has the original tune from the 90's because non ethanol gas is 14.7:1.

    In conclusion I would not hammer on the throttle until you get a proper tune. It shouldn't take very long for the EEC to adjust for the fuel at part throttle and idle if the O2 sensors are working properly. But it won't adjust at WOT and you will go boom.
    ~Chris
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  4. #4
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    Boom is bad.

    Thanks for the input. Being on the safe side I'm running the Avgas since it's easily available for me and I know I'm throwing too much air at it. Car only sees about 1000 miles/year. Yes I have a lot of bolt-ons, reason for the wideband. Does Dave still do the mail tunes? Goal is to scoop up some headers, the tune will be my next step.

    Thanks,
    JJ

  5. #5
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    Quote Originally Posted by Broncojohnny View Post
    The reading you get at idle has little to do with how it will run at wide open throttle.
    Agreed! But....

    Quote Originally Posted by Broncojohnny View Post
    The computer uses what it has learned at part throttle to guess what fuel the engine needs at wide open but your AF ratio at wide open is influenced heavily by your mass air meter, your fuel injectors and your fuel pressure..
    At WOT the EEC ignores all learned trends. The EEC will use your MAF curve, injector settings and other settings to calculate your injector pulsewidth and thereby your end AFR. It does not used learned trends, nor does it feed-back the O2 sensor readings to adjust the WOT AFR...it looks at the air coming into the motor (MAF) and calculates the injector pulsewidth...and that's it!

    If you're thinking that the MAF curve must be extremely important for WOT performance...you're right.

    One thing that's easy to forget is that adaptive learning can hide tune (MAF curve) or minor hardware problems during part-throttle driving and even idle. When tuning, the ultimate goal is to have the car run as commanded without the help of short-term or long-term adjustments...that takes a good bit of driving time and adjustments though.
    Matt Haub
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  6. #6
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    JJ - do you plan to always use 100LL? You should tune for whatever you plan to use as different fuels will yield different results.

    Chris (Blown38) is right too! It may run seemingly okay during normal driving, since the adaptive learning function can adjust errors automatically...again though....those learned adjustments are ignored during WOT and the MAF curve could be pretty far off.
    Matt Haub
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    Black '89 5spd - 281K miles, MPx, E85, Alky-Inj RWHP: ? RWTQ: ? Best 1/4mi:12.521, MPH:107.23mph
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  7. #7
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    Don't plan to keep the 100LL. Only using it because it will ping at wot due to mods. Od on a late model blower, injector size, etc. I plan to go back with premium once I'm ready to tune.

    Who's the latest and greatest for tuning now days? Is Dave still the man?

    Thanks again for all the input! Understood on the MAF curve, that all makes perfect sense.

    JJ

  8. #8
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    Took it for a drive today, this wideband is cool!!!

    WOT high rpm pulls in 3rd show about 11.4-11.9. Fuel pressure holds steady at 55-60 ish.

    WOT low rpm/high load pulls are definitely where my concern has been, 13-14 ish.

    TUNE TUNE TUNE!

    JJ

  9. #9
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    Quote Originally Posted by james5275 View Post
    Took it for a drive today, this wideband is cool!!!

    WOT high rpm pulls in 3rd show about 11.4-11.9. Fuel pressure holds steady at 55-60 ish.

    WOT low rpm/high load pulls are definitely where my concern has been, 13-14 ish.

    TUNE TUNE TUNE!

    JJ
    Area of greatest concern is peak torque rpm which would be 3-4500 for most engines. 13+ AFR doesn't hurt anything especially with 100 octane fuel. It's not right of course, but it won't hurt anything. 12.5 on the other hand might.
    Quote Originally Posted by Miller View Post
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  10. #10
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    This may seem like a silly question but is there any reason why you are tuning for AFR instead of lambda? I have my WBO2 sensor setup to report Lambda and from what I've read (ex: http://tunertools.com/articles/AFR-Tuning.asp) a Lambda of 1 is always stoich no matter what fuel is being used.

    The goal from my reading (and what i tuned to) was to make sure that the Lambda was .95 (slightly rich).

    -g

  11. #11
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    Quote Originally Posted by S4gunn View Post
    This may seem like a silly question but is there any reason why you are tuning for AFR instead of lambda? I have my WBO2 sensor setup to report Lambda and from what I've read (ex: http://tunertools.com/articles/AFR-Tuning.asp) a Lambda of 1 is always stoich no matter what fuel is being used.

    The goal from my reading (and what i tuned to) was to make sure that the Lambda was .95 (slightly rich).

    -g
    Because I don't know what I'm doing?!?! Im learning as I go, the lambda option is new to me, I just became aware of it when I bought the DLG-1. I'll switch and see what it reads.

    DAVE, (or anyone) what's my next step to achieve a proper tune? A "rewritable" chip right? (EEPROM?) I'm extremely happy to see that the supporting fuel delivery mods are providing enough fuel. Thanks to everyone in the past for helping me get this far.

    JJ

  12. #12
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    Lambda and AFR are just a matter of what you understand better. If you are using a gasoline based fuel (up to E20) you can use the gas scale. If you are using an ethanol based fuel (E60-E90) you can still use the gas scale but you will want to tune a bit richer on WOT.

    Lambda is a representation of ideal (stoich) being 1.000 and everything else being either leaner (higher than 1.000) or richer (lower than 1.000). Since all fuels have a stoich AFR value, you can always tune part throttle to 1.000 lambda and so for the scientifically minded, this is the correct way. For everyone else it means you have to learn this system of relative "units". Your EEC only understands lambda so if you want to be more "in tune" with your EEC then you'll need to learn it.

    However, since I fully comprehend and am comfortable with either value, I can give you the perspective that for most people, it's just easier to understand AFR. 14.64 for stoich, 12.6 for max power N/A, 11.5 for boosted power, and 11.0 for E85 power. It's just easier for me to attach meaning to those numbers rather than 1.000, .86, .79, .75. If someone says to me "I saw 10.6 AFR on my gauge" I can process that much quicker than if someone says " I saw .91 lambda". I'd have to do the math in my head to figure out exactly what ".91" lambda is. It's like speaking Spanish as a second language. Most people aren't really fluent and so they end up trying to translate in their heads to attach meaning to what is being said. Most of us are naturally more comfortable thinking in AFR.

    One thing you probably don't want to do is start talking in exact AFR for the fuel you are running. Since the exact AFR varies for each kind of fuel, you end up confusing everyone in the room. So either use Lambda or Gasoline AFR for discussion purposes. More people will understand Gasoline AFR and you'll spend less time trying to explain yourself and you'll receive less deer-in-the-headlights stares.
    Quote Originally Posted by Miller View Post
    Ya thats why i tape mine down. People think its bc i dont have a moonroof seal (which is true) but its really to keep my roof from ripping off .
    Email me here.

  13. #13
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    Quote Originally Posted by XR7 Dave View Post
    However, since I fully comprehend and am comfortable with either value, I can give you the perspective that for most people, it's just easier to understand AFR. 14.64 for stoich, 12.6 for max power N/A, 11.5 for boosted power, and 11.0 for E85 power. It's just easier for me to attach meaning to those numbers rather than 1.000, .86, .79, .75. If someone says to me "I saw 10.6 AFR on my gauge" I can process that much quicker than if someone says " I saw .91 lambda". I'd have to do the math in my head to figure out exactly what ".91" lambda is. It's like speaking Spanish as a second language. Most people aren't really fluent and so they end up trying to translate in their heads to attach meaning to what is being said. Most of us are naturally more comfortable thinking in AFR.

    One thing you probably don't want to do is start talking in exact AFR for the fuel you are running. Since the exact AFR varies for each kind of fuel, you end up confusing everyone in the room. So either use Lambda or Gasoline AFR for discussion purposes. More people will understand Gasoline AFR and you'll spend less time trying to explain yourself and you'll receive less deer-in-the-headlights stares.
    If I understand your system correctly, you are setting the stoich at an arbitrary number (14.64) but changing the target AFR depending on the fuel/type.

    I would think your system still offers confusion b/c just like how AFR is different for each fuel (14.64 for 100% gasoline, 14.13 for E10 -- what we have in california, and 9.86 for E85), any feedback given by people online to a statement "I see xxx AFR on my gauge") will be skewed by the fuel type you are tuning for -- or they think you are tuning for.
    Q: How would the OP report "gasoline AFR" if he's not running gasoline?
    Lambda is absolute.

    Esp if you live in a place that has E10 more often than not or as in the OP's case they are using a fuel that I'm willing to bet most people have little/no experience with (100LL avgas), switching to Lambda (to me) makes far more sense.

    Q: How low lead is "LL"? I wonder how quickly it will kill your O2 and WBO2 sensors. Depending on how cheaply you get your AVGAS (maybe it's free for you), I wonder if the cost of these O2 sensors sooner than normal will offset the extra value of a little more knock protection.
    -g

  14. #14
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    its not that it is an arbitrary number... its the gasoline scale. You can think of it this way, the AFR gauge is reading in lamda, but is displaying that reading on the gasoline scale. So stoich will display 14.7 (or 1 lamda) no matter what fuel is used. I think it is pretty common for the AFR gauges to display the gasoline scale, so thats why most people here use it.
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  15. #15
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    Yup, I'll get the avgas out of there so we can all compare apples to apples. It's good to know there's more than one way to skin this cat. I tend to be old school, lambda scale seems to be more like the metric system, so I claim AFR. I'm with DAVE, I understand afr. Using Lambda I would have to retrain my thought process.

    It could be worse, our pilots manually tune in flight by EGT. No specific degree, just 20 degrees on the lean side of peak EGT.

    JJ

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