STFT Imbalance - LS7 - newbie with issues!

[miikezohsix]

Back ground:
2008 Z06, pretty pampered before I got it. < 20,000 miles. I did the valve guides as a preventative, and
'while I was in there" I did a cam as recommended by Pat G., the specs are below. I've done a couple of LS engine teardown before, but I am in no means an expert. Stock manifolds, down pipes, O2 sensors and cats. Only other non-stock part is a vararam.

The car started right up after the teardown, no issues. I bought a tune package from Pat G. along with HPTuner Pro. I have zero experience with tuning.

I sent my first log file to Pat, with comments that the idle was way more cammy than I expected, and that under 1700 under light load, cruising conditions, the car would sometimes have a mild surge, but under 1400 I pretty much had to shift down.

He looked at my log file and said I have a 5% - 7% STFT imbalance in the low load and idle parts of the log. Easy to see, I understand what he?s looking at. Note he disables the LTFT to help him with his remote tuning. He pretty much said fix what's wrong, and he'll then work on the tune. The only thing he offered was grounds or a bad plug or wire. Two log files are attached, one just sitting after doing the plugs, the other a longer file with a couple of pulls in it.

So I set off to make single changes in the car, read the logs after each change, hoping that something would turn up. To date, this is what I've done:

-With the help of Bill Dearborn chased down all the engine grounds and confirmed them good.
-Replaced the original plugs with some NGK TR6 Pat recommended
-Swapped the plug wires from left to right
-Swapped the injectors from left to right
-Removed the intake and replaced the intake manifold gaskets concerned I might have a leak - no change
-Carefully looked at all the exhaust flanges - stock manifold to head, and manifold to down pipe - all looked clean, no leaks

I did the cylinder balance test in HPT a couple of times as I've gone thru this. The numbers come out a little different every time, but never more than 3% different between high and low, and they are never the same cylinder - moves around.

I also used my infrared non-contact temp gun to shoot the manifold to look for temp differences. In general each cylinder in a bank was about the same 1,3,5,7 all looked about the same and 2,4,6,8 were all about the same, but 20 higher than bank 1. I will say that the readings were all over the place depending on how far from the pipe you are, and how far away from the head you make the measurement. I did think cyl 1 was 40 degree lower at one point, but found a different spot and it fell into line with the rest of the bank - shrug.

Today I did absolute battle with the car on my short lift, wanting to swap the O2 sensors left to right. I got the right one loose with a 7/8 box wrench that I cut a slot in to pass the wires thru. I could not get the driver's side loose with my cut wrench too springy, an open end, or my O2 sensor socket, so I eventually bit the bullet and dropped the down pipe off the manifold, and loosened the pipe up at the H pipe. Finally was able to use the O2 sensor socket and an offset box wrench to get it loose.
Removing the O2 sensor wires on the drivers side was easy enough, not so on the passenger side. What a bitch, but I won - three hours later, both O2 sensors are out, tomorrow I find out that they probably won?t fix it either.

My next step is a compression test.

Cam specs -

Cam recommendation: 2008 Corvette Z06 w/stock 427 LS7 short block, 69cc stock, slightly milled LS7 heads, .051? gasket, stock LS7 intake manifold with 90mm TB, stock exhaust manifolds with dual cats, manual trans, 3.42 gear, 6700 rpm shift points, 7100 rpm rev limiter.
226/244 .639?/.639? 116 LSA +4 advance (with 1.8 ratio rockers)
EPS HiRev/HiRev lobes on 8620 steel 3 bolt cam core
EPS26-1/EPS244-1 HR116 LSA +4 advance (112 installed intake centerline)

Cam Valve Events .050" .050"
Intake Duration 226 Intake Valve opens 1 BTDC (- indicates ATDC)
Exhaust Duration 244 Intake Valve closes 45 ABDC
Lobe Seperation Angle 116 Exhaust Valve Opens 62 BBDC
Intake Centerline 112 Exhaust Valve Closes 2 ATDC (- indicates BTDC)
Exhaust Centerline 120
Overlap 3 degrees



So -

-How big a deal is this imbalance I'm chasing - internet searches were not fruitful.
- What else should I be doing to chase this down?

[DSteck]

That's not that bad of an imbalance. I'd write that off as normal.

[realcanuk]

I agree. I have a similar imbalance and I know everything is ok mechanically. I just use an average of the STFTs.

[will15]

looks normal to me, the imbalance is probably just the difference in the 02 sensors one looks a bit lazy.

[SultanHassanMasTuning]

normal...

[Michael_D]

Due to the firing order of the LS platform, you will always see the left/drivers/bank1 read richer than the other side at low lode/low rpm. The lumpier the cam, the worse it gets. I'd love to see my imbalance under 8%. It's usually around 10%. At idle however, they should be closer, but thats just idle air not calibrated yet. I'm surprised Pat G would think you have something mechanically wrong with those logs. He should know better.

[Eaglegoat]

Due to the firing order of the LS platform, you will always see the left/drivers/bank1 read richer than the other side at low lode/low rpm. The lumpier the cam, the worse it gets. I'd love to see my imbalance under 8%. It's usually around 10%. At idle however, they should be closer, but thats just idle air not calibrated yet. I'm surprised Pat G would think you have something mechanically wrong with those logs. He should know better.

Please elaborate.

[Michael_D]

Please elaborate.

It has to do with cyl #1 firing immediately after #3 and the open / shared plenum of the LS intake. 1 & 3 fight for a fresh air charge, as the entry of both runners are next to each other. So the combustion at lower rpm ends up being slightly rich in comparison to the remaining cylinders. The problem is exasperated with LT box headers where the primaries are equal length, and not timed into the collector.

[miikezohsix]

I have two new O2 sensors on the way.
Not going thru the hassle of changing them out again if the problem does move from Bank 1 to 2.

[miikezohsix]

Due to the firing order of the LS platform, you will always see the left/drivers/bank1 read richer than the other side at low lode/low rpm. The lumpier the cam, the worse it gets. I'd love to see my imbalance under 8%. It's usually around 10%. At idle however, they should be closer, but thats just idle air not calibrated yet. I'm surprised Pat G would think you have something mechanically wrong with those logs. He should know better.

Would you consider that a lumpy cam based on the specs above?
I went to two different tuners for cam recommendations and both came back with a very similar set of cam specs for a nice streetable cam.
Pat is pretty well respected from what I was able to read, and he picked up the phone when I called, and spent the time talking to me, which made me pretty comfortable with what he was offering.
I'll get back in touch with him when I've run out of things to check on the motor, and hopefully get going on the tune again and get my car to where it should be.

[schpenxel]

What injectors are you running? If you swapped them bank to bank then it wouldn't seem that is an issue, though I'm still curious

I had 5-8% bank to bank FT deltas using some cheapo Deka 60# injectors. I'm down to around (and many times under) 2% after going to ID1000's. My setup is basically a stock engine though, so I won't pretend to know whether something is or isn't wrong in your case..

Bank 1 is always higher numerically on mine

[miikezohsix]

What injectors are you running? If you swapped them bank to bank then it wouldn't seem that is an issue, though I'm still curious

I had 5-8% bank to bank FT deltas using some cheapo Deka 60# injectors. I'm down to around (and many times under) 2% after going to ID1000's. My setup is basically a stock engine though, so I won't pretend to know whether something is or isn't wrong in your case..

Bank 1 is always higher numerically on mine

I am running the stock injectors, they had never been out of the manifold before I switched them left for right.

[Michael_D]

Would you consider that a lumpy cam based on the specs above?
I went to two different tuners for cam recommendations and both came back with a very similar set of cam specs for a nice streetable cam.
Pat is pretty well respected from what I was able to read, and he picked up the phone when I called, and spent the time talking to me, which made me pretty comfortable with what he was offering.
I'll get back in touch with him when I've run out of things to check on the motor, and hopefully get going on the tune again and get my car to where it should be.

It's standard practice to rule out mechanical problems, before putting any time into a tune calibration. Dumb not to. So I suspect that's what Pat wants to do, make sure there isn't something mechanically wrong. A 5% imbalance though.....not much at all. This has been talked about on this forum before, numerous times, and damn near everyone who participated in the threads says they see bank 1 running richer than bank 2. That's what got me thinking about it, and asking myself "why".

But regardless, I'm sure you'll get it straightened out. Pat does have a good reputation. And the EPS cam he speced out looks like a great all around grind. Not real aggressive, not real mild. Mid-range. The 116 lsa should build good power to 7K, and not cause too many drivability issues.

[schpenxel]

When you say bank 1 is typically "richer", do you mean fuel trim values are higher or lower?

[Michael_D]

When you say bank 1 is typically "richer", do you mean fuel trim values are higher or lower?

Sorry – my bad. No good excuse, just my personal mental image of the phenomena and seeing a green bar that’s higher than the other, and thinking "it's running richer"….... Positive trims = increased fueling. Negative trims = reduced fueling. I consistently see “higher” trims on bank 1. Which would indicate this bank is adding more fuel than the other bank. And in all honesty, I don’t really understand exactly what’s going on inside that plenum, or exactly how the ECM responds to it. I think with fuel injection, it might just be one of those chicken or egg things because you have one O2 sensor monitoring the exhaust stream of an entire bank, so it may be over compensating cylinders 5 and 7, but I don’t know for sure….

[schpenxel]

Sorry – my bad. No good excuse, just my personal mental image of the phenomena and seeing a green bar that’s higher than the other, and thinking "it's running richer"….... Positive trims = increased fueling. Negative trims = reduced fueling. I consistently see “higher” trims on bank 1. Which would indicate this bank is adding more fuel than the other bank. And in all honesty, I don’t really understand exactly what’s going on inside that plenum, or exactly how the ECM responds to it. I think with fuel injection, it might just be one of those chicken or egg things because you have one O2 sensor monitoring the exhaust stream of an entire bank, so it may be over compensating cylinders 5 and 7, but I don’t know for sure….

Gotcha.. yeah I think about it the opposite way. i.e. higher numerical fuel trim values means that bank is running leaner to me as it's needing MORE fuel to get back to stoich. I had a feeling that's what you guys meant though as bank 1 is a little leaner on mine too (higher fuel trim #'s). It's < 2% though on average so I don't worry about it much. Using cheapo injectors I was at 5-7% different pretty regularly which really seemed like too much to me. I was especially worried about #7, like I assume nearly everyone is..

[miikezohsix]

I replaced both O2 sensors and it made a significant difference in the imbalance.
Latest log is attached.

The car feels "crisper" STFTs were basically on top of each other for the first 45 seconds, then stay within 3%, often identical.
So, at least in this case, one of my 16,000 mile O2 sensors was either a little off from stock, or was possibly damaged during my head removal process.

I'm thinking I may have gotten anti-freeze in the large opening of the LS7 down-pipe. When I pulled the heads, I first pulled the manifold off, leaving the cat pipe in place, then cracked the heads from the block. Maybe some AF got down there, or who knows?

Either way, I'm happy, have my logs off with Pat G, who is above water in TX, and looking forward to zeroing in on an improved idle and low load conditions and leaning out my WOT conditions.

I'd like to thank everyone who spent the time to comment.
This is all new for me, and getting knowledgable input helped me keep all this in perspective.

[schpenxel]

I replaced both O2 sensors and it made a significant difference in the imbalance.
Latest log is attached.

The car feels "crisper" STFTs were basically on top of each other for the first 45 seconds, then stay within 3%, often identical.
So, at least in this case, one of my 16,000 mile O2 sensors was either a little off from stock, or was possibly damaged during my head removal process.

I'm thinking I may have gotten anti-freeze in the large opening of the LS7 down-pipe. When I pulled the heads, I first pulled the manifold off, leaving the cat pipe in place, then cracked the heads from the block. Maybe some AF got down there, or who knows?

Either way, I'm happy, have my logs off with Pat G, who is above water in TX, and looking forward to zeroing in on an improved idle and low load conditions and leaning out my WOT conditions.

I'd like to thank everyone who spent the time to comment.
This is all new for me, and getting knowledgable input helped me keep all this in perspective.

Good to know--thanks for updating.