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Thread: Twin Turbo 408 Ford - Cruise AFR at 12.8:1?

  1. #1

    Default Twin Turbo 408 Ford - Cruise AFR at 12.8:1?

    Hey hey everyone. I'm running a 408 SBF in a Fox chassis, twin T4s, Lentech automatic with Transbrake & TTC Lockup. Don't ask why, LOL. Just a general question about tuning, as I'm still working on the tune. It starts/idles well, pulls good; just working on the fuel tables and getting into more boost. Looking for anyone's general thoughts on my cruise issue. At 2000 RPM, 4th gear and converter locked, at 13.5 AFR, it's got a slight, but noticeable rolling misfire; bumping the AFR to 12.8 makes it smooth out nicely. I've tuned a bunch of other (more mild) engines, and would never run that AFR at cruise, but this thing seems to need it. Timing from 34° to 39° makes no difference. Fuel & Timing tables are smooth in the cruise range as well. I have yet to mess with IEA, it's currently at -90°. Could that be the cause? I don't care about mileage, but I'm curious as to why it needs so much fuel to cruise smoothly?

  2. #2
    Join Date
    Dec 2009
    Location
    Connecticut
    Posts
    27,037

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    Sometimes it's a camshaft grind inefficiency phenomenon that happens only during a specific RPM range. (?) Does it happen with the TTC Lockup off & on?

  3. #3

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    It's only noticeable with the TCC locked up. If it's happening with the converter unlocked, I can't feel/hear it.

  4. #4
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
    1,973

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    Sounds like a similar symptom to some GM cars that have what feels like a misfire at cruise 50 MPH.
    Torque convertor clutch failing is the cause. Replace torque convertor.
    If you need to run your AFR into the 12s while the driveline is locked up (NO convertor slip), then you should be looking at individual cylinder AFRs. Your intake manifold could be the cause. I'd set the car up with new plugs, take it for the cruise where the surging occurs for five miles, then do a spark plug inspection, on the side of the road, or tow back to workshop. Gary

  5. #5

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    Yeah, I run an automotive repair and custom shop. I felt my share of bad converters over the years, this isn't the case. I like where you're going with the individual plug reading; maybe an intake problem or lazy injector. With a fresh set of plugs, I'll have the Ms. follow me in the flatbed and tow it back to check the plugs right after it's been acting up. Thanks.

  6. #6

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    I'm not one of the Wizards on here with Holley EFI, but if it was me, I'd for sure set Injector End Angle angle before going for other fixes. Allan50 did a spread sheet on here that is fantastic. Helped my ride a bunch. My 2 cents.

  7. #7

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    How big are the fuel injectors? They may be dropping below the Minimum Injector Opening Time. I had mine do that.

  8. #8

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    Does the engine have a PCV valve? PCV valves can cause issues with EFI engines at cruise. If the valve flutters a bit you'll feel it. Adding more fuel covers up a bunch of possible sins. Another likely suspect is the interaction between the cam grind and the low lift flow of the heads. Heads that flow really well at low lifts don't like certain cam lobes at cruise. Race cams work great at WOT, but sometimes they don't work very well at cruise speed. If the heads flow really well at low lift then sometimes the intake charge just blows out the exhaust at 2000 RPM. You'll never see it at the track or on the dyno, but it'll drive you nuts cruising down the freeway.

  9. #9
    Join Date
    Dec 2009
    Location
    Connecticut
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    Overlay the datalog onto your Global File, and pay special attention to the center paragraph in blue print:
    Originally Posted by Danny Cabral
    Quick Datalogger Information:
    Always provide data, not just symptoms. Is the AFR & ignition timing optimized? Overlay the datalog onto your Global File.
    Ensure the Target A/F Ratio table is properly programmed for your engine. Ensure the ECU is "Learning" (Learn Table LINK).
    Scroll through the datalog to the problem area. Then look exactly where this happens on the Base Fuel & Timing Tables. Scrutinize:
    RPM, MAP, MAT, TPS, CTS, IAC, Target AFR, AFR Left, CL Comp, Ignition Timing, Duty Cycle, Fuel Flow, Fuel Pressure & Battery.

    Using the Holley EFI software (LINK), look at where the live cursor moves to during this occurrence, and
    manually tune that area of the Base Fuel Table (enrich or lean) & Base Timing Table (optimize ignition timing).


    I find the most helpful datalog function, is overlaying the datalog on your Global File.
    I'm in the habit of using the datalog Overlay feature every time I review a datalog.
    Click on "Datalog" (on the top Toolbar), "Activate Overlay", then "Open Data Log".
    You can literally "playback" a recorded event, and watch it as it happened on any screen.
    Minimize (shrink) the datalog window, and move it to the bottom of any Global File screen.
    Then click & scroll anywhere on the datalog, and watch it playback on your Global File (EFI software).
    http://documents.holley.com/techlibr...9r10543rev.pdf (Holley EFI Datalogger Instructions)
    https://forums.holley.com/showthread...s-amp-Datalogs (Datalog & System Log Information - Read "NOTES")
    https://www.youtube.com/watch?v=iqHO...C6FEA0BB99AF17 (How To Record & Email A Datalog)
    https://www.youtube.com/watch?v=q7OZUXLRS1Q (How To Record A System Log & Diagnose A No-Start Condition)
    https://forums.holley.com/showthread...7531#post77531 (Closed Loop Datalog Tuning - Posts #2, #4 & #6)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 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 M/T Street Comp tires.

  10. #10

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    @Fordman351ci: I was wondering about that; went from -90° to -180° and although I haven't been able to drive it yet (raining here and no hood on it yet), it cold started and idled smoother than it ever has.
    @nopushrod: Never thought of that. I'm running 83 lb/hr injectors; I'll look at the PW at cruise; would explain why increasing the fuel at cruise made it better.
    @AndyF: No PCV yet; just both valve covers venting to draft tubes under the car. Vacuum pump system in the works over the winter. The cam is kinda snotty, so as DannyC mentioned, that might have something to do with it.
    @DannyC: I've read a lot of your posts/responses to guys and you obviously know your stuff. I read that response in another thread and have had the GF drive it while I watched the fuel and spark tables; flattened out the suspect areas already, trying to find the cause.
    Thanks for all the advice/suggestions. Once it dries up here I'm going to see how the IEA change affects it during cruise, and in the mean time, look at my datalogs to see what the injector PW was at during cruise.

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