PCM Diagnostics & Tuning HP Tuners | Holley | Diablo

big dip in dyno graph-have a look!

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Old Feb 20, 2011 | 10:23 AM
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Default big dip in dyno graph-have a look!

I recently hit the dyno with the some changes being made to my combo and overall very happy except for a dip that occurs on the graph. I'm not sure if its related to the torque converter, dyno pick up interference or possibly in the tune.
The combo is basically a stock bottom LS2 with ported heads and a cam. The power is transfered to the rear through a built 4L60 and a Circle D 3200 stall combined with 4.10 gears. The dyno tuning was done with the converter unlocked with zero knock and air fuel at 12.9-13.0.
Does anyone have any ideas what's causing the dip?



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Old Feb 20, 2011 | 10:28 AM
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Have a KB vette in the shop that has the same problem, haven't had a chance to take a look at it yet. Subscribed.
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Old Feb 21, 2011 | 06:37 PM
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Ne data logs for the pulls??
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Old Feb 21, 2011 | 06:43 PM
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without any scan data, it's a guess.
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Old Feb 21, 2011 | 10:35 PM
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Knock retard (alot?) Or low timing there to keep it from koncking?
Need air/fuel data and some logs.
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Old Feb 21, 2011 | 11:18 PM
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Don't have the logs, but I can tell you there was zero knock retard. The air fuel was 12.9-13.0 and the timing is 25*. We added up to 29* with no change in power and left at 25*.
Do you guys think it may be tire spin? If the dyno pull was started at 3800 rpms it pulls clean with no dip. Weird
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Old Feb 22, 2011 | 11:09 AM
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Possible valve-spring harmonic problem?
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Old Feb 22, 2011 | 01:27 PM
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One of these professional tuners on this forum should be able to tell you what this is in a heart beat. Oddest thing I've ever seen dipping and resuming back where you would expect. Makes you wonder if the measurement equipment is having an issue?
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Old Feb 22, 2011 | 03:26 PM
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Easiest thing to do is try another dyno / put some more weight in the back.
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Old Feb 22, 2011 | 04:07 PM
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The funny thing is we locked the converter for one pull and the graph was smooth! I don't think its tire spin because when locked it made more power. Also, on some other pulls the dip occurs 1000 rpms sooner. It looks like I need to put it back on the dyno and make some logs.
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