zulatr

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 buck, but under 1400 I pretty much had to shift down.

He looked at my log file (which I don’t know how to share here), 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.

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 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 also used my infrared non-contact temp gun to shoot the manifold to look for temp differences. In general each bank was about the same 1,3,5,7 all looked about the same but 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, but don't know the right way to do that with a push button start car and an electronic throttle - I asked in Tech and Perf, we'll see what kind of opinions I get.

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?

zulatr

forgot to mention...

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.

jim2092

Go to the HP Tuner forum (below) and post your tune and a log file with a copy of the above and you will get serious help. You will want "Engine, Gas (Gen 4)"

http://www.hptuners.com/forum/

zulatr

That was my plan during lunch.

Here's the thread... with log files.

http://www.hptuners.com/forum/showth...629#post383629

thanks for the help, here or in HPT.

mike

jim2092

Good to see 2 highly respected tuners agree with what I was thinking - it is not a problem.