Flat Spot

[Rob's 73]

I have been trying to figure out what is causing a flat spot. When slowly accelerating the car falls flat when I hit 1800rpms and then picks up again before 1950. My WB stays true but is in Bank 1 only. I have looked for timing issues but have never seen where timing is being pulled or anything. I think I have finally found an indicator. My STFT for Bank 2 spikes right after this. I am starting to think that Bank 2 runs lean momentarily until the STFTs can catch up. The attached DOC shows a screen shot of this. As soon as the RPMs get to ~2000 the Bank 2 STFTs drop back down. The flat spot is more severe the higher the IATs get. If I take off faster this is never noticed. It is very annoying though when I shift to second and the RPMs end up right at 1800.

Background: 03 Z06 with Magnacharger, 2 bar OS. This problem existed with the standard MAF tune as well. I also believe that it was there before the SC install. Just not as noticeable.

Any help is appreciated.

New O2 sensors installed. Same before and after.

added info: The more I started thinking about this and reviewing some logs. If I cruise in this RPM range my bank2 LTFTs go to the 20% range and the STFT are -2 to +2. I guess this is the LT learning what's needed. As soo as I get to about 1950 rpms the Long and Shorts go back to normal.

Is this a fuel injector issue?

[MNR-0]

Sounds like you just need a fuel shot. I doubt its the injectors - but you can always clean them in solvent just to be sure.

I use the n-alpha VE modifier to give me the fuel shot on stabbing the throttle. Also, I wouldn't run O2s on any cammed or blown car. Open Loop is the shiznit when it comes to absolute control - out of the box thinking you can control fuel better under varying load than any O2 sensor can.

[Rob's 73]

How do you add a fuel shot? Where is the n-alpha ve modifier?

Thanks

Rob

[Rob's 73]

TTT

Any help please? Point in the right direction?

[jimmyblue]

I do not see FTC (Fuel Trim Cell) logged, this is something you
may want to take a look at. Some models hade really stupid FTC
boundaries and this makes each one a grab bag of crap, they
accumulate trim "impulses" from too broad and discrepant a
range of operation. Stepping between them can be jerky and
the bottom vs the top, only one end can be anything like right.
Try and make the "bins" all-in and covering only closed loop
MAP, RPM areas; try and put the boundaries in places where
you do not spend much time (e.g. just above idle, below and
above normal highway cruise RPM; above idle MAP, below PE
enable MAP and split the difference for the middle "cut line".

I think the modifier referred to is only in the speed density OSes?

[SideStep]

Rob,

Jimmy is referring to the fields defined in "Engine, Fuel Control, OL & CL, LTFT Boundaries" There is a RPM and MAP button...

Look at this pic (the top half of it)... notice where the boundaries are between the MAP and RPM ranges. These areas between the boundaries are the "bins" Jimmy is referring to. (NOTE: the boundaries shown in this pic (MAP vs. RPM) are also your current settings)...

I do not want to speak for Jimmy, BUT I believe he is saying the specific "moments" of transition from one cell area to another are the least accurate. I am not sure WHY this only happens on Bank 2.

FYI: this pic is from another software package you will not see this in HPTuner...

[SideStep]

After looking at your histogram pic...I am not sure this is the answer... It appears you are just cruising along right in the middle of FTC 6, with no transition. Then Bank 2 STFT just up and jumps up to +25... I know you said you have new O2 sensors... I am going to go out on a limb and say (for a start) swap the injectors from side to side... see if this problem follows to the other bank... If so, there is your problem. If NOT, you can at least mark them (the injectors) off the list and keep going....

You said this only happens in CL, never in OL, right??? (a direction to start looking I think)...

[MNR-0]

n-alpha is available with the speed density enhanced OS licence. It sits right next to the VE table. It multiplies the VE number based on TPS.

So... if you stab it from cruise, you exceed 100kPa in an instant at 2000rpm, but your NA VE values are lean as sh!t and you get a flat spot. Throw in the VE multiplier at, say 1.5 or even 2.0 at 2000RPM from 85% TPS onwards, and there you have it! Instant fuel shot dialled in just at the point you are running lean.

[vetteman3]

Well what do you know, I guess EC tune was right. I posted a similiar problem with my '01 Magnacharged Vette a couple of weeks ago and EC tune said he had observed this problem and believes it's due to a harmonic being set up by the blower. My car has THE EXCACT SAME PROBLEM, the right bank goes way lean!! Mine does it from about 1900 to 2050 RPM, and is worse when hot. I was planning on pulling the blower tomorrow and check the intake gaskets, but now after hearing your story, I'll probalby just be waisting my time. We need to email the Magnacharger folks and see if there is some sort of fix for this. Hey, is your blower an intercooled or non, and what size pulley are you running, and what size injectors?? Mine is a non-intercooled but has the smaller 3.1" pulley to get 7# of boost with 42 lb/hr injectors.

[Rob's 73]

I just put in new flow matched injectors and the problem is still there. It must be harmonics. I swapped my pulley from a 3.4 to a 3.1 and the problem seems to have shifted by about 150 rpms. Can I shield the injectors with something?

The problem does get worse as the temps go up.

I/C Maggie 42 lb injectors

[jimmyblue]

Do you guys think the problem is MAP pulsation ("harmonics")
adding kookiness to the airflow measurements? I do believe my
car has an improper slope of VE vs MAP because I see MAP
oscillations driving fuel oscillation. If you can see a lot of MAP
"activity" but the throttle position and RPM are sitting fairly
still, maybe this is part of the trouble. One possible fix would
be to relocate the MAP sensor, with some "acoustic" filtering
interposed (hose, can full of screen, more hose, etc.). This
would damp the pusation that reaches the MAP sensor. You
could also do electrical filtering on the output, RC network,
to try and cut out any high frequency stuff before it hits the
PCM. Putting an oscilloscope to the MAP output might be an
interesting thing to do, the Magnusson blower has to put out
pretty high frequency while the scan tool is very limited in its
data rate.