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Thread: Engine spasms when Manifold Pressure reaches atmospheric.

  1. #21

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    Quote Originally Posted by 81 TransAm View Post
    To add to Gen2Van's comments, I had a ZZ3 crate engine that would not rev above 5200 in first gear. It ended up being valve float. Who would have thought, on a new engine.
    Yes, my guess is that the OP has a mechanical problem and not a Sniper problem. Most likely valve float.

  2. #22

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    Quote Originally Posted by Gen2Van View Post
    I had a SBC engine that had a low performance cam and poor flowing heads. It hit a brick wall at 4500 rpm. It just didn't want to spool up any higher. I then got into superchargers and used that engine to learn on. The supercharger made it worse after 4500 RPM. Needless to say a higher performance cam and better heads with heavier valve springs solved the RPM limitation. I also had a few straight 6s, the Ford 4.9L and Jeep 4.0L, they were all turds after 5000 RPM. Typically they likes to stay below 4500. Especially the Ford 4.9L.
    This engine has had a lot of work done to it. I worked closely with the builder, who specializes in high end racing engines. The heads are professionally ported and polished. Enlarged valves, and high pressure springs to handle the 0.504" lift on the cam. It flows really well.

    If it's running great up to a specific RPM, and especially if manifold pressure equals zero at that RPM, you may simply be at the capacity for that engine to pump air.
    Another thought. Do you have a rev limiter? If so, bypass it and see what that does. Some HEI modules have built in rev limiter.
    Also being HEI, it's easy to adjust the timing curve. Make sure you use manifold vacuum for vacuum advance. Verify your mechanical advance is all in by 3000 RPM. You can also limit the advance for mechanical and vacuum to allow more initial timing.
    Quote Originally Posted by 81 TransAm View Post
    To add to Gen2Van's comments, I had a ZZ3 crate engine that would not rev above 5200 in first gear. It ended up being valve float. Who would have thought, on a new engine.
    The "Rev limiter" is in my head, as I just haven't gotten comfortable with taking it much over 5,000, although I've had it up to 5,500 and it still pulled hard. The cam and springs are rated to at least 6,500 RPM. I just haven't gone that high, mostly out of being timid with it.

    My comments about the RPM were more along the lines of how hard I was pushing on the throttle. If I only bring MAP to around 80 - 85 and hold the throttle, it'll scream up to as high of RPM as I feel comfortable with no issues. But if I push the throttle harder, so the MAP hits atmospheric (around 92 - 94), that's when the bucking/jerking spasm occurs.

    Quote Originally Posted by travisch View Post
    Ah, didn't realize that you were NOT using the Sniper to control timing. I'd just advance the distributor a couple degrees then and go test to see if it eliminated the problem.
    I'd like to use the Sniper to control timing, but (as far as I understand) would have to completely swap out the ignition for a new one.
    I'll recheck the timing curve and give this a try. Right now, initial timing is at 12° advance, which is where DUI told me to set it after putting a custom curve on it.

    Quote Originally Posted by andyf View Post
    Yes, my guess is that the OP has a mechanical problem and not a Sniper problem. Most likely valve float.
    This problem generally occurs at lower RPM. The higher the RPM, the less evident, to almost non-existent it is. I normally don't notice it unless it's under 2000 - 2500 RPM. 4000+ and it's much less. So, here's something that I have noticed. This problem likes to occur in the "hot spot" on the VE map. Looking over several logs, when the Sniper enters this area on the map, it goes extremely rich, and that's when it bucks and spasms. If I remove the hot spot and smooth it back out, it'll relearn it and put it right back! Even though it's too rich. Any ideas what would cause this? I've included my current config and a datalog file that shows the rich spikes. It isn't bucking during these because the MAP isn't high enough, but it's the same scenario on the fuel map. Sniper.zip
    1985 Ford Bronco. 300 (305) ci inline 6. 9.1:1 compression, P&P head with enlarged valves, high flow exhaust. 32" BFG A/T tires. 5 speed ZF5 manual transmission with 3.55 rear end. Holley 550-850 2300 Sniper EFI.

  3. #23
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    There are two things I would change in your tune. You're using the Simple method for your Target AFR Table, Change it to 2D Table and set a proper AFR table. And also your Cruise target of 16.5 could have something to do with it. The second thing is your Base Fuel Table. Look at it in Graph form, it has way to many hills & valleys.

  4. #24
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    Quote Originally Posted by 81 TransAm View Post
    There are two things I would change in your tune. You're using the Simple method for your Target AFR Table, Change it to 2D Table and set a proper AFR table. And also your Cruise target of 16.5 could have something to do with it.
    Originally Posted by Danny Cabral
    The Target Air/Fuel Ratio Table & Base Fuel/Timing Table (and MAP kPa & RPM axes/scales) should be configured using the Sniper EFI software:
    In the Sniper EFI software (LINK), Target Air/Fuel Ratio Table (Fuel ICF) & Base Timing Table (Spark ICF), select "2D Table" Type (not "Simple").
    The Target Air/Fuel Ratio Table & Base Timing Table can then be properly configured & tuned for any engine.
    https://www.holley.com/support/resou...Fuel_Injection (Sniper EFI Software & Holley 558-443 CAN/USB Dongle)
    The second thing is your Base Fuel Table. Look at it in Graph form, it has way to many hills & valleys.
    Read 4th & 5th paragraphs: http://forums.holley.com/showthread....2523#post62523 (Holley EFI Tuning Tips & Information)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  5. #25

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    Quote Originally Posted by 81 TransAm View Post
    There are two things I would change in your tune. You're using the Simple method for your Target AFR Table, Change it to 2D Table and set a proper AFR table. And also your Cruise target of 16.5 could have something to do with it. The second thing is your Base Fuel Table. Look at it in Graph form, it has way to many hills & valleys.
    I didn't realize that was a setting. I thought 2D just allowed you to fine tune it once you've set your base calibrations in "Simple" mode. What is the difference? I've setup my own custom Target AFR table before, with a lot smoother transitions, but it got reset at some point when I started over from scratch. I've run with a cruise between 15.0 and 16.5. It doesn't seem to make much difference under hard acceleration (which I wouldn't expect it to).

    And yeah, I've looked at it in Graph form many times, and done a lot of smoothing. However, those hills & valleys will "self learn" themselves right back on my first drive after, even if it creates rich and lean conditions. It's frustrating, and I don't know why it does it. I know it shouldn't be that way, but don't know what to do to stop it from doing it.

    Quote Originally Posted by Danny Cabral View Post
    Originally Posted by Danny Cabral
    The Target Air/Fuel Ratio Table & Base Fuel/Timing Table (and MAP kPa & RPM axes/scales) should be configured using the Sniper EFI software:
    In the Sniper EFI software (LINK), Target Air/Fuel Ratio Table (Fuel ICF) & Base Timing Table (Spark ICF), select "2D Table" Type (not "Simple").
    The Target Air/Fuel Ratio Table & Base Timing Table can then be properly configured & tuned for any engine.
    https://www.holley.com/support/resou...Fuel_Injection (Sniper EFI Software & Holley 558-443 CAN/USB Dongle)
    Read 4th & 5th paragraphs: http://forums.holley.com/showthread....2523#post62523 (Holley EFI Tuning Tips & Information)
    Thanks for the links. I've read those over many times (as well as a lot of your other useful information).
    As stated above, I've smoothed the table a lot, but it relearned itself back to jagged with all the hills and valleys.
    1985 Ford Bronco. 300 (305) ci inline 6. 9.1:1 compression, P&P head with enlarged valves, high flow exhaust. 32" BFG A/T tires. 5 speed ZF5 manual transmission with 3.55 rear end. Holley 550-850 2300 Sniper EFI.

  6. #26
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    Quote Originally Posted by 85 Phoenix View Post
    I didn't realize that was a setting. I thought 2D just allowed you to fine tune it once you've set your base calibrations in "Simple" mode. What is the difference?
    The "2D Table" is a 31x31 size table based on MAP kPa & RPM. "Simple" adjusts an average value for the three "Idle", "Cruise" & "WOT" areas.
    See page 25: http://documents.holley.com/techlibrary_199r11259.pdf (Holley Sniper EFI Help Manual - EFI Software, Tuning & Datalogging)

    Thanks for the links. I've read those over many times (as well as a lot of your other useful information).
    You're welcome.

    As stated above, I've smoothed the table a lot, but it relearned itself back to jagged with all the hills and valleys.
    Then it simply reflects your Target A/F Ratio table programming (right or wrong).
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  7. #27

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    Quote Originally Posted by 85 Phoenix View Post
    I've run with a cruise between 15.0 and 16.5.
    Isn't that cruise AFR too lean?

  8. #28
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    Not necessarily, it depends on the engine/vehicle application.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: Dominator MPFI & DIS, A/C, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/X-pipe/shorty headers, 100HP progressive dry direct-port NOS, 160A 3G alternator, Optima Red battery, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  9. #29
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    Quote Originally Posted by gregs69rs View Post
    Isn't that cruise AFR too lean?
    I can run at 16.5 AFR in light cruise, but he's running 16.5 in the moderate to heavy load areas, which could be a problem.

  10. #30

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    For me, cruise is around 2500 RPM @ roughly 65 - 70 MAP (just about where the default table switches from cruise to mid-range), so it's a really fine line/balance. This is a full sized SUV with 32" tires, and sits round 5000 lbs, so it's cruise takes a little more throttle. Still, that might explain some of the abnormalities, but it wouldn't explain the ultra rich condition that causes the engine to buck and spasm under full acceleration. Even though the AFR table is set to 12.5, it can dip down as low as 9:1 - 10:1. I thought maybe it was the acceleration enrichment, but even on a datalog of a full drive to work (about 30 minutes), the AE vs. TPS Rate of Change and the AE vs. MAP Rate of Change charts barely blip, if at all, when it's going rich and having issues.
    1985 Ford Bronco. 300 (305) ci inline 6. 9.1:1 compression, P&P head with enlarged valves, high flow exhaust. 32" BFG A/T tires. 5 speed ZF5 manual transmission with 3.55 rear end. Holley 550-850 2300 Sniper EFI.

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