STAR

iTrader: (1)

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?

FirebirdSS

iTrader: (7)

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.

Old Geezer

iTrader: (3)

Ne data logs for the pulls??

JDMC5

iTrader: (3)

without any scan data, it's a guess.

Gabbiani

iTrader: (20)

Knock retard (alot?) Or low timing there to keep it from koncking?
Need air/fuel data and some logs.

STAR

iTrader: (1)

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

Sid447

Possible valve-spring harmonic problem?

James Linder

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?

ysb02

Easiest thing to do is try another dyno / put some more weight in the back.

STAR

iTrader: (1)

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.