steve-d

iTrader: (7)

The graph shows the HP and TQ dipping at 5300rpm. It lasts for 150 rpm where it picks up where the curve would be if there hadn't been a dip.

I asked if there was knock and pulling of advance and they said no.

What could be the cause of the dip. I don't want to lose power momentum.

Any suggestions as to what to look for or appropriate Questions to ask.

Steve

Car:

'02 LS1 TA
Heads Cam Headers, 85 mm MAF, Harland Sharp RR, Fast 90mm intake and TB

Right now it peaks at 425 RWHP; 393 RWTorque

jimmyblue

iTrader: (11)

You can get into resonances on the dyno that
just freak out the machine. If the line is jagged
rather than smooth this is likely. The dynos I
think have to a lot of "funny massaging" of the
load roller data to make things even look clean,
anyway. Though it would be interesting to see
the dyno curve lined up with wideband meter
and any logger data for spark etc. my first guess
is just the mechanical resonance or something like.

steve-d

iTrader: (7)

I would post the graph, but its too big a file. I could email it if it'd make a diff in your input.

thanks much for your time.

Regards
steve

NOBR8KSS

iTrader: (12)

Check your MAF sensor...make sure the screen is free of debris and that the resistors don't have filter oil on them.

jimmyblue

iTrader: (11)

I looked at the pic, the AFR is not perturbed. I
still suspect it's the dyno getting jiggy and the
smoothed result is just off. Is this a repeatable
characteristic? Does the tuner give you any log
files to review for PCM abnormalities / complaints?

WAHUSKER

iTrader: (4)

Bent pushrods can cause this. But judging by your setup, you must have chromolys.

steve-d

iTrader: (7)

Yes, they are cro-mo comp cams push rods.

Steve

Another_User

Burst knock?

jimmyblue

iTrader: (11)

Burst knock should only trigger on a large
delta CylAir and this is not where I'd expect
to see it (steady wide open, the only thing
that can change CylAir is a very sudden
change in volumetric efficiency).

Though timing logs sure would have value in
this case.

gzero

What kind of dyno? I used to have a dynojet with inductive pickup and it would do all kinds of things like that. Whats wierd on the dynojet is if the inductive pickup signal
hicups both the torque and hp curves are effected. I now have a mustang eddy current dyno and we use only optical pickup, which is very accurate and noise free
with no more problems like you mentioned. I am not putting down dynojet, but run the dyno in horsepower mode only, no pickup and if the problem goes away you in.

Zeueses97ramair

Mine has the same prob changed everything even got an msd 6a its the opti.... ill send you some dyno graphs after my 410.s installed my dyno sheet was 309 338 tq... this one was 298 328torque after 410s I compared and same dip results....oh yea i have all boltons, norr no heads or cam...

Nine Ball

iTrader: (38)

I can't believe nobody has said it, but I'm betting its valve float. This is quite common in LS1s with weak/worn valvesprings, and always happens at 5300. I've had it happen twice with other LS1 combos I've had and it was always valve float.

You might do a search on "Dyno Dip", possibly in the internal engine forum.

Another_User

Good call. I forgot about that one. Better yet, valvetrain geometry and/or harmonics.

steve-d

iTrader: (7)

Originally Posted by Nine Ball
I can't believe nobody has said it, but I'm betting its valve float. This is quite common in LS1s with weak/worn valvesprings, and always happens at 5300. I've had it happen twice with other LS1 combos I've had and it was always valve float.

Thanks much for the input. Your great. I hadn't considered valve float, especially since the curve picks up past the dip and continues to 6400 rpm. I suppose it'd only take one or two valves to do that at a specific rpm to have an effect.

Harmonics, I suppose it could be a factor. How to check? Swap out balancers? Prior pulls months ago w/the same balancer didn't show any dip. Isn't it possible for them to "wear out" depending on the material in them?

Valve geometry? When I installed the Harlan Sharp RR and the hardened push rods, I made sure the roller tip was properly centered over the valve stem as the engine was rotated over by hand. I also used a push rod length checker. They appear to be the proper length. But I could check them (16!!) again if anyone feels there's the likelihood of a bent rod.

Thanks again. Much appreciated. If anyone needs to see the graph I could email it. I just can't post it.

Steve

Another_User

I didn't mean engine harmonics. I think I heard that springs can do weird things too based on harmonics.

Gman2002Z06

It's called spring surge...I had it on my '00 FRC...At 5600 rpms, the hp would fall off 25 rwhp, for about 200 rpms, then it would be back just as fast as it went...before I figured out what it was, I had spent alot of money on different mods and Dyno tuning...

To solve the proplem, I bought a set of LS6 valve springs...Replaced them in the driveway and the hp dip was gone...There had to be one of the 16 valve springs that was acting up...And thus caused one cylinder to drop its power for just a fraction of a second...But long enough to be noticealbe under WOT on the road...

May want to try a different style or set of springs...

Peace...Gman

Another_User

That's part of why they have the dual springs right? Because most springs have an RPM that they wig out at and with the dual springs it happens at two different times so you don't see it?

gameover

99% it is the valve springs. It is likely only one cylinder bugging out, but thats all it takes.

If you log your O2's, you will likely see lean dips on the bank that is floating.

Chris...

mikey

iTrader: (6)

Steve, I am the one who tuned your car. If I remember, you had an HS rocker C-clip break correct? I would look at the rockers first. I'm not a big fan of the HS rockers. Try and put the stockers back on. Takes 1 hour to do. Make sure the pre-load is correct. Then look at the valve springs. They are SLPs? SLP uses Manley singles which I've seen work just fine. If the rockers do not cure the problem, replace the springs with with Comp 918s or Crane Duals. I've seen float before, this is not float but rather a harmonic of the present valvetrain.

There were no air/fuel or timing anomolies that caused this. It is 100% valvetrain related.

Mike

gojo

iTrader: (12)

I replaced the HS rollers with the stockers rebuilt by HS and a dip I had at 6200 went away.