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Thread: How much fluctuation is normal?

  1. #1
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    Default How much fluctuation is normal?

    Hi everybody. I have a bored over 302 with Edelbrock Performer heads & cam. I can post the specs if it's relevant, but not sure that it is.
    After getting some miles in on the current tune, I took a close look at a datalog recorded at 70 MPH. I've noticed some fluctuations in AFR an fuel flow. Is there a nominal range that's typical? I'm only achieving 13.5 MPG now and would like to lean out the Target AFR table, but I'm concerned about going too lean. My concern is how conservative I need to be, so I don't set a value that might put the fuel flow in a hazardous range. I'm still using the clamp on WBO2 bung supplied with the kit but have just ordered a weld in longer reach bung. I will also replace the header collector gasket as it is the only one between the WBO2 sensor and the engine. If there is a better way to post data results, please advice. Thanks for any advice! -Kevin

  2. #2

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    If you're in Closed Loop operation, it's normal for the AFR fuel flow to vary, as the Sniper EFI is constantly making minor adjustments to maintain your Target AFR. You don't mention what that is, I would assume at cruise, you have it set somewhere around 14.5 AFR? I've seen some on this board telling of very lean 15+ AFR ratios at light throttle cruise settings.

  3. #3
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    With a MAP of 50-55 kPa & RPM between 2500 & 3000 RPM, I have an Target AFR of 15.5. All other areas in the cruise segment are 15.0.
    50-55 kPa MAP is coincidental with 8-10% TPS, so lower BMEP cruise (not that we can read that). So that's where I'm trying to lean for fuel economy.
    Thanks! -Kevin

    I guess I should've uploaded this earlier:
    Attachment 2851

    http://i777.photobucket.com/albums/y...ps34fgimxb.jpg Maybe this is better?

  4. #4
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    Can't tell much from a picture. A datalog is much better. FYI: Going too lean will reduce fuel economy just as much as going too rich.

  5. #5
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    Here's the datalog with two GCFs. I can't remember which config I was using, but the datalog would be the same for both. I zipped the files and copied them to Google Drive. I think that's how to post these? Thanks, Kevin
    https://drive.google.com/file/d/1MZe...ew?usp=sharing
    https://drive.google.com/file/d/1OJx...ew?usp=sharing
    https://drive.google.com/file/d/1FMp...ew?usp=sharing

  6. #6
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    My first thoughts when I see an AFR trace that's erratic like yours, is air leaks in the exhaust or the engine is not happy.
    So then I look at some things in the datalog, like MAP, IPW, Target AFR and so on. Then if you use the overlay function and look at where the engine is running in the different maps, you'll start to see some issues. You Target AFR in your cruise area is jumping around. Your Base Fuel Table is no were near smooth in your cruise area. All this make it difficult for it to run smooth. Look at your Base Fuel Table in graph form, and you'll see it's running right in the middle of a big hole when it's at cruise. Look at your MAP & IPW trace in the datalog, they're up & down. I would say the engine is surging. I would get the engine running well at a richer Target AFR and once it is, and the Base Fuel Table has been smoothed, then you can work on Leaning to see how far you can go.

  7. #7
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    Thanks for taking the time to look at that. I just received a weld-in O2 bung and hope to get that in this weekend. After that, I'll start over and follow your instructions about enriching the AFR and smoothing out the Base Fuel Fable before leaning things out. Thanks again! -Kevin

  8. #8

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    I made some changes to your GCF/tune, it should help: https://drive.google.com/file/d/1u-s...ew?usp=sharing

    Also, if you want to calculate your live fuel mileage do (MPH x 6.3)/Fuel Flow in lbs/hr. 6.3 for gas, 6.6 for E85.
    According to your datalog: 70 MPH x 6.3 = 441/28.7 lbs/hr = 15.4 MPG.

  9. #9
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    Quote Originally Posted by ChevyC10 View Post
    I made some changes to your GCF/tune, it should help: https://drive.google.com/file/d/1u-s...ew?usp=sharing
    Also, if you want to calculate your live fuel mileage do (MPH x 6.3)/Fuel Flow in lbs/hr. 6.3 for gas, 6.6 for E85.
    According to your datalog: 70 MPH x 6.3 = 441/28.7 lbs/hr = 15.4 MPG.
    Much better Base Fuel Table to use as a starting point. I may have been temped to start with a little richer Target AFR. 13.5 idle, 14.2 cruise. I like your calculation for MPG. I will have to write that down.

  10. #10
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    Quote Originally Posted by 81 TransAm View Post
    I like your calculation for MPG. I will have to write that down.
    https://forums.holley.com/showthread...5032#post75032 (See "Instant MPG")
    May God's grace bless you in the Lord Jesus Christ.

    '78 BRONCO: 508" stroker, TFS heads, 11:1 comp ratio, Dominator MPFI & DIS, cold air induction, Spal dual 12" fans/aluminum radiator, dual 3" exhaust/Magnaflow mufflers, Moroso vacuum pump, Accusump, engine oil & trans fluid coolers, 100HP progressive dry direct-port NOS, A/C, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, traction bars, 4" Skyjacker lift, 35" mud tires

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