Flat spot in acceleration

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

  1. GTSnake

    GTSnake Viper Owner

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    So that's $2k for the AEM and another $900 for the pro racer package?

    How does that compare to the Pro EFI that I've heard about?
     
  2. Viper Specialty

    Viper Specialty Viper Owner

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    Nope, you have it all confused.

    -The Pro-Racer has nothing to do with AEM, it is PCM flash tuning software from SCT so that you can edit your own factory computer, in the same ways that we can here.

    -The AEM is a stand-alone computer and replaces the OEM computer altogether, and it's software is free/comes with it.
     
    Last edited: Sep 19, 2008
  3. GTSnake

    GTSnake Viper Owner

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    Ahhh, got it.
     
  4. RedSrt007

    RedSrt007 Viper Owner

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    UPDATE: Replaced o2 Sensors, problem still there....
    Raised o2 sensors out of exhaust path (1" spacers)..problem still there.

    Disconnected o2 sensors completely, made sure vec has a/f's @ 14-15:1 cruise and problem solved...no more flat spot! Hmmmm 02 sensor issue, or PCM issue. Seems PCM response time or some sort of a/f adjustment happening.
     
  5. Viper Specialty

    Viper Specialty Viper Owner

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    PCM response rate due to base maps being incorrect... have said that before.
     
  6. ViperTony

    ViperTony Enthusiast

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    What does "made sure vec has a/f's @14-15:1 cruise" mean? I'm confused. :)
     
  7. uvbnbit

    uvbnbit Viper Owner

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    Yeah, are you talking about "at cruise" or "in acceleration" now. Also, when you said "disconnected 02's", did you unplug sensor or take it out of exhaust pipe so it's getting a downstream "fresh air" reading?? any error codes?
     
  8. Viper Specialty

    Viper Specialty Viper Owner

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    He is refering to disconnecting the O2's entirely. Doing so will force the PCM into Open Loop, and it will read off the fuel maps only, no longer correcting the trims. While that may make the "flat spot" go away, it is in no way the correct way to do it. The "flat spot" is from a peak in the fuel map that causes the trims to be pulled negative, and then due to the slow PCM response rate, they overcorrect a "valley" into a lean condition when it really should be a positive correction. When disconnecting the O2's, it follows the fuel map precisely-while the map may still be 10-15% incorrect, it keeps the system from overcorrecting and becoming 25-35% incorrect or more, which you will certainly feel. Re-writing the maps will allow the trims to remain active on top of a correct fuel map, so that they can do their job and correct out the smaller map problems/conditions/changes that occur when driving.
     
  9. uvbnbit

    uvbnbit Viper Owner

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    So what he was saying, is that under cruise (closed loop), all is well (yet, he said "problem solved")?:dunno:. That's what confused me was the "problem solved" part at cruise. But, does that "solve problem" at acceleration ?. (now I'm really confused):crazy2:
    Nevermind, I think I know what he was checking a/f at cruise for now.
     
    Last edited: Sep 24, 2008
  10. Viper Specialty

    Viper Specialty Viper Owner

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    A flat spot will not occur at cruise, only at accelleration. In order for the trim shift to become an issue, you have to be moving accross the base map from cell to cell. In short, the problem in his case may "appear" to have gone away, but it really isnt gone, it just isnt showing itself as a symptom anymore.
     
  11. KenH

    KenH Enthusiast

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    Daniel,
    I have the same issue. If I accelerate slowly, the car will go very lean in one spot at low RPMs which results in a flat spot in acceleration just as you describe.

    Do you know if this is consisent enough in the stock mapping to just offer a set tune to fix it with the SCT or is each car different?

    If I man up and get the programming module, what all can I fiddle with? Can I correct the temperature compensated fuel offset rather than upgrading to a VEC 3?

    I can't find an example of the SCT programming interface. How easy it is for someone that basically knows how to tune a VEC? Any screen shots available or list of available programmable functions?

    Since these are locked to the PCM SN are there any issues with programming a '96 PCM in an '01?

    Do you know of anyone that has completely replaced the VEC in a Roe SC setup using just the SCT programming on the stock PCM? Can the PCM directly handle the bigger low impedance injectors?

    Thanks,
    --- Ken
     
  12. Viper Specialty

    Viper Specialty Viper Owner

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    Hi Ken,

    -You have the exact stereotypical "flat spot" in the power curve that we run accross all the time.

    -Unfortunately, every car is different, and every car needs to be logged have the trims corrected on an independant basis. The only time this can be effectively corrected otherwise would be if I happen to build two identical cars side by side... which is a very rare thing.

    -With the SCT & software, you can fiddle with everything from fuel, spark, idle, fan temps, IAT corrections, start fuel, cruise fuel, WOT fuel, Tip-In fuel, baro fuel corrections... the list goes on. Basically, everything.

    -I will try to get a few pictures of the interface. While it is quite easy to navigate when you get the hang of it, it is a bit overwhelming at first.

    -I would actually advise running the 01' PCM instead. The later PCM's are actually JTEC+ and have faster processing speeds. (96-98 are the slowest, 99-02 are in the middle, and 03-06 are the fastest of the JTEC controllers) I can always send you a screen shot of some parameters of a 96-99 PCM so you can copy them into the other year PCM to get the benefit of both.

    -Yes, it has been done many times, and is in fact the basis for the Gen-3 Roe Blowers. It has also been done on the SRT-10 Rams, both Roe and Paxton. The key is having a MAP sensor that can read boost and have the same voltage range as the OEM sensor, then scale all of the maps to include the boost range, and learn to ignore the "then incorrect" pressure heading on all of the maps. The only thing you will need the VEC for is to control the Methanol, unless you have another means of doing so... and there are plenty of other ways.

    -The PCM can directly handle the larger injectors so long as there is a Resistor Box, or better yet, a P&H Driver Box. I personally run 1000cc injectors on my SRT-10 at stock base fuel pressure completely under the control of an OEM PCM and a P&H Driver. The car runs as well as a stock car would in all circumstances.
     
  13. KenH

    KenH Enthusiast

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    I am running the '96 for correction of erroneous misfire codes. Can this be dealt with on the SCT if I put the '01 back in to get the other benefits?

    I'm really starting to like the idea of this approach. Since I haven't been able to munch my cast pistons after a couple of years with 6.5lbs and a VEC2, I'm beginning to think I need more things to play with that I can screw up :D
     
  14. GTSnake

    GTSnake Viper Owner

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    Hmmm the SCT route sounds like a good idea. The only downside that I can think of is getting the right tune if you're not familiar with tuning your own car.
     
  15. Viper Specialty

    Viper Specialty Viper Owner

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    Yes, you can adjust the misfire configuration w/SCT.

    PS- you can also opt for a set of saturated fuel injectors rather than P&H and get rid of the Resistor Box/P&H Driver altogether. The only downside is you will be limited to 72lbs or less on a Saturated before you start having response rate issues at idle. (the same applies in most cases to a resistor box driven P&H injector. A true P&H box is much, much faster.)
     
  16. Viper Specialty

    Viper Specialty Viper Owner

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    Yep, that would be the only downside, but we can offer some assistance if need be.
     
  17. RedSrt007

    RedSrt007 Viper Owner

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    Just to explain :D

    As VS stated, once you unplug the (front) O2 sensors, the car will not go into closed loop, so technically you are cruising / accelerating in Open loop the entire time. So with your Open loop map, you would normally run fairily rich is some areas, so to compensate for this I tuned my vac fuel map (vec2) so that it was 14.7 to mimic closed loop (trying to compare apples to apples fuel-wise). Problem / flat spot was not apparent anymore.

    Hope thats easier to understand :)


    Dan - Can you fix this responce rate in the PCM if a PCM is sent to you?
     
  18. Viper Specialty

    Viper Specialty Viper Owner

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    Hello,

    Actually, a properly working base map will be 14.7 in all cases, not rich. It does not become rich until the part and full throttle multipliers become active. (at which time, the system switches to Open Loop anyway)

    The issue with running Open Loop at all times is that the system has zero ability to compensate anything, including the normal parameters & changes that it should be able to tune out on its own. Also, when running a VEC on top of a base map in open loop, you cannot tell when/where the Part Throttle multipliers become active, again causing a possible situation where you inadvertantly "tune out" something that was intentional, costing yourself mid-range power.

    We can certainly help you with your issue, but it would not be accomplished by sending the PCM here, as the engine configuration on your car is the issue, not the PCM- one does no good without the other. It would be accomplished instead by either purchasing the SCT tool for tune loading, and a Scanning Program so we can see your car's tune problems over e-mail and write tunes for your car back and forth until corrected, or we can help you learn how to tune yourself with a Pro-Racer Package and a decent scanning software.
     
  19. ViperTony

    ViperTony Enthusiast

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    Dan, which scanning software do you recommend?
     
  20. Viper Specialty

    Viper Specialty Viper Owner

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    Palmer Performance PCMSCAN works very, very well at a reasonable price.
     
  21. GTSnake

    GTSnake Viper Owner

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    Can't we just use the VEC logging feature?
     
  22. uvbnbit

    uvbnbit Viper Owner

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    What would removing (not disconnecting) rear o2's do? I've got a 98 w/6.5 Roe, B&B's, RR's, no cats. If LC-1 is accurate (seems to be), I run rich (11.5-13.9) at 1800-2200rpm cruise. Sean said (if I understood him correctly) that w/o the cats, the rear o2's may not be getting a big enough difference in sensing to make adjustment in PCM. ??? Again, I don't want to be mis-quoting Sean, I may have just misunderstood him. And no, I haven't gotten opportunity to disconnect rear o2's OR any datalogging yet:D. Soon though, I hope. Thanks guys for sharing ya'lls expertise:2tu:
     
    Last edited: Sep 27, 2008
  23. Viper Specialty

    Viper Specialty Viper Owner

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    No, the VEC has no capability of logging the factory PCM fuel trims, or any other internal PCM value for that matter.
     
  24. Viper Specialty

    Viper Specialty Viper Owner

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    The only thing removing the Rear O2's will do is keep the PCM from trying to protect the cat's in some situations, and make the car pass emissions for the cats. If the LC1 is in a single cylinder, you may just be running richon that particular one. If its down stream and can read all 5, then you are likely running into an area like everyone else is above (but opposite, rich instead of lean), or have run out of adjustment room on the trims.
     
  25. RTTTTed

    RTTTTed Viper Owner

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    Hmm, switching between simms and rear 02 sensors causes complete tune changes on my car. Carpping out past half throttle? Switch rear sensors and a while later it'll run OK. Etc. Weirdness.

    Ted
     
  26. RedSrt007

    RedSrt007 Viper Owner

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    I have heard of situation like this...makes no sense / relation, but have heard of it :dunno:
     
  27. Viper Specialty

    Viper Specialty Viper Owner

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    It is likely part of the Cat Temp Protect Configuration, which adds/removes fuel at certain loads and RPM's in order to keep the cats from overheating, and is based on what exactly the output of the Cat is. keep in mind, the cats need to be at stoich to function correctly. This can be turned off once the base maps are corrected if someone wishes.
     
  28. RedSrt007

    RedSrt007 Viper Owner

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    Dan, Do you think Drilling out front o2 sensors would help response time (STFT) with the closed loop situation?
     
  29. Viper Specialty

    Viper Specialty Viper Owner

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    Absolutely not, the O2's have nothing to do with the problem, it is the PCM that has the internal issue. The PCM is fighting a losing battle- there is no way around it, no short-cut, and no magic tricks that are going to solve a tuning issue other than fixing it the correct way.
     
  30. GTSnake

    GTSnake Viper Owner

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    "-I would actually advise running the 01' PCM instead. The later PCM's are actually JTEC+ and have faster processing speeds. (96-98 are the slowest, 99-02 are in the middle, and 03-06 are the fastest of the JTEC controllers) I can always send you a screen shot of some parameters of a 96-99 PCM so you can copy them into the other year PCM to get the benefit of both."

    Was the '01 PCM different than the '00 and '02?

    If I get an '01 are there any compatibility issues with my '97 car?
     

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