Flat spot in acceleration

Discussion in 'Gen II' started by GTSnake, Aug 15, 2008.

  1. GTSnake

    GTSnake Viper Owner

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    When I accelerate I have a slight hesitation/flat spot around 2,000 rpm. Sometimes not even noticeable, sometimes very abrupt. I don't believe the car was tuned all that well so I'm trying to learn it myself and hopefully tune it correctly.

    Is it because there's too much fuel? Not enought timing? How do I go about smoothing it out? I haven't hooked up the VEC to data log yet and I've been meaning to do it. One of these days......:rolleyes:
     
  2. SYNFULL

    SYNFULL Viper Owner

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  3. 2001 GTS

    2001 GTS Viper Owner

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    Gary, Tom has 2 of the Innovative LC1s on each side of the car, but don't know if has the setup for logging with the VEC or not.
     
  4. GTSnake

    GTSnake Viper Owner

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    Yup, I've got the LC-1's on both sides. Only one is hooked up so far. I know it's not completly finished to be able to data log but I was just trying to get a feel for why it's happening.

    Hey Steve, hope you had a great time in Motown! Too bad you're not coming back for VOI.
     
  5. 2001 GTS

    2001 GTS Viper Owner

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    Had a great time up there Tom!! Just got back to SC last night.

    Too bad my car wasn't done yet or it would have made it up there. And I never noticed any hesitation when you had the car sideways 1st through 3rd:D.
     
  6. GTSnake

    GTSnake Viper Owner

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    That's because I was standing on the gas :rolaugh: It only shows up under moderate to light throttle.

    I guess I'm going to have to get the data logger hooked up before I can figure this out. I'm starting to think I should go to an AEM eventually though.
     
  7. SYNFULL

    SYNFULL Viper Owner

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    I'm still a novice in this area and I use the vec for logging but you can log direct to a laptop with the logworks program that comes with the LC-1's.
     
  8. Viper Specialty

    Viper Specialty Viper Owner

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    A flat spot in power is likely due to NOT enough fuel. The 2000-3000RPM range on the Viper seems to be where most problems come out when scaling larger injectors or changing heads/cam. The flat spots seem to come out in light/moderate throttle before power enrichment occurs. If this is the culprit, you are likely going to need a reflash in order to correct the base map, it will be difficult to tune out otherwise.
     
  9. GTSnake

    GTSnake Viper Owner

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    What do you mean by a "reflash"? I'm using the VEC2 for controlling the turbos.

    You mean reflash the stock PCM? What if I went to an AEM?
     
  10. Viper Specialty

    Viper Specialty Viper Owner

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    Reflash= SCT. Using the VEC2 works fine most of the time, but when you make changes to the engine that will affect the fuel tables at the base level, the VEC-2 can and will often fall on its face for closed loop operations.

    The AEM route works as well, but quite a bit pricier.
     
  11. RedSrt007

    RedSrt007 Viper Owner

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    I have the same situation...from 2k-3k in vac, I have the slightest surging. You wouldn't know its there as a passenger, but driving it, I can feel it. When data loging it, I see the a/f's go from 13-16 (HP/TQ fluctuate a lot also), but RPM / Vac / Timing is all steady. Most noticable in 1st / 2nd, not as noticeable the rest of the gears.

    ST/LT's are withing 5-10% (might not be refreshing quick enough to log it)

    Im going to try adding more fuel in the vac (16"-12") and see if that helps...not sure if PCM is going to try to revert back though.
     
  12. Viper Specialty

    Viper Specialty Viper Owner

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    Unless you have the ability to turn off your LTFT's, dont even bother trying to play ring-around-the-rosie with the STFT's. Unfortunately however, the "compact" fuel map issues that occur in one area dont show up correctly on LTFT's- they are pulled in one direction, then pulled back in the other direction as soon as the event passes. Using LTFT's on anything but a good map that just needs to be scaled is very difficult.
     
  13. SYNFULL

    SYNFULL Viper Owner

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    So Dan, the solution would be to reflash the PCM with a custom made tune based on your mods, or just a reflash of the stock PCM?

    Thanks
    Gary
     
  14. Viper Specialty

    Viper Specialty Viper Owner

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    Reflash based on your modifications.
     
  15. RedSrt007

    RedSrt007 Viper Owner

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    UPDATE:

    Ok,
    Per some recommendations, I checked out both 02's today, logged votages, and the way they fluctuate. I noticed Bank 2 (pass. side) seemed to be delayed, and slower then the driver-side bank. So I went to the parts store, picked up a new sensor, and 90% of the problem is gone! Now both sensors read correctly and pretty much dead-on (drivers side is now a little slower then the new passenger side one). So I still have a SLIGHT flat spot, but hopefully the adaptives pick that up with some driving (it currently has 5 miles since I cleared PCM).
     
  16. Joseph Dell

    Joseph Dell Enthusiast

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    The issue is the VEC and nothing else. An AEM or the PCM Re-flash that Dan is speaking of would fix it. The problem is that a) you are running a passive injector box (not an active driver), and b) the VEC, as a piggy-back, isn't responsive enough. and where it is LEAST responsive is at tip-in (when you transition from vacuum to boost). when you floor it just under or at 2k RPM, your VEC can't keep up. I've seen this on dozens of cars and the only fix is to get rid of the VEC. There is nothing wrong w/ the VEC, but what you want it to help you with, well, it isn't designed to do.

    my .02...

    JD
     
  17. Viper Specialty

    Viper Specialty Viper Owner

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    Hi JD,

    I have to agree, though point in a slightly different direction. The problem here sounds to me like it is base-map related, not just VEC related. While the VEC COULD dial this problem out, it is also an incomplete solution due to its inability to correct one small section of the map, while not effectively "overcorrecting" other areas- make it a 2-dimensional correction if you will, when a 3-dimensional correction is what is needed. As I have seen over, and over again, the base maps tend to change drastically in the 2-3K RPM range when changing injector sizes and/or cam profiles, and on a boosted car, correcting this at the source in the only real solution. The VEC's work great- but IMHO *only* when added on top of a properly scaled and corrected base map setup in the factory PCM. We can even adust tip-in fuel and PE as well if need be. The more you ask a VEC to do, the more problems it can cause. Add fuel and pull timing? no problem. Have it do that as well as injector scaling, offsets, fuel/timing in vacuum, etc... and it all goes down hill from there, proportionate to the increase in injector size of level cam profile you are running.
     
  18. GTSnake

    GTSnake Viper Owner

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    Sounds like AEM for me...
     
  19. Viper Specialty

    Viper Specialty Viper Owner

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    Not at all. We have yet to find an application that an SCT/VEC combination cant handle. While its nice sometimes to have the AEM "all in one" packaging, it does have some downsides as well. The VEC does work very well- so long as it isnt asked to do certain things.
     
  20. GTSnake

    GTSnake Viper Owner

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    What are the downsides of the AEM besides price and finding someone to tune it?
     
  21. plumcrazy

    plumcrazy Viper Owner

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    so much better for ME to bring it to a qualified tuner and let HIM tune it for me.....but hey, thats just me.
     
  22. Viper Specialty

    Viper Specialty Viper Owner

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    -Not OBD-2 compatible, no fault finding, and impossible to make pass emissions in some states, such as NY (Excl. NYC)

    -For a clean install, it is a disaster. Yeah, you can make it "look" OK, but its still a mess no matter how you slice it. Needing to retain OEM PCM is an after-thought... just plain lazy engineering IMHO.
     
  23. GTSnake

    GTSnake Viper Owner

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    Hmmm, I don't care about the emission compliance but if there is a fault it won't give you any error codes?
     
  24. Viper Specialty

    Viper Specialty Viper Owner

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    Correct.

    But consider the emssion compliance- if your state is plug-in like here, that means you have to rip everything out and put OEM back in, and find a way to make it pass every year. If your state doesnt plug in, then you dont have a problem.
     
  25. GTSnake

    GTSnake Viper Owner

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    Nope, no emission checks here. Good ol' Automotive lobbyists in Mi government:2tu:

    So wouldn't the stock PCM throw the code? Or does the AEM block it?
     
  26. Viper Specialty

    Viper Specialty Viper Owner

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    Stock PCM isnt connected to anything in that capacity when its in an AEM setup.
     
  27. GTSnake

    GTSnake Viper Owner

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    Hmmm, having second thoughts now. I've also heard that they've had some reliability issues with units crapping out and you have to remove it and send it back for repairs.
     
  28. Viper Specialty

    Viper Specialty Viper Owner

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    Correct. And if it makes any difference to you, the SCT Pro-Racer package just came WAY down in price recently- literally half of what it used to be. [this is the package that allows you to do your own tuning]

    Price was 1700.00, now it is 900.00 including the flash tool.
     
  29. GTSnake

    GTSnake Viper Owner

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    What's included in this Pro racer package?
     
  30. Viper Specialty

    Viper Specialty Viper Owner

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    (1) SCT Flash Tool, (1) Security Dongle, and (1) Software Disc. Just add a USB A-B cable (printer cable) available from any best-buy/radioshack, etc... and you are tuning. It is the same software that we dealers use to write our tunes, and has all the same capability. The only difference is that it is restricted to work with your car's PCM, and your SCT tool only.
     

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