Silverado 2500 6.0L E78 Mid RPM / Mid throttle timing

[clinter36]

I am chasing down a hesitation, bucking, chuggle, that has bothered me since this truck was new. Its most noticeable around 1300 - 1800 rpm while applying moderate throttle but not enough to kick down a gear. For example cruising up a hill around 40 mph, or rolling on throttle cruising through light that just turned green (before you had to stop). I was told by the dealer that it was the Tcc locking and unlocking. I was also recently given some good advice to replace the plugs and wires and open up the plugs to .045"...which I did, and it definitely helped, but the bucking is still there.

I did some logging with the Tcc commanded to lock and you can feel the bucking even more pronounced (obviously) and I can see the RPMs fluctuate.

After reading Banish's book, he mentions that if timing is not advanced enough during cruise the engine can misfire and create a bucking or "trailer-hitching".....hmmm that sound familiar. I believe that he even recommends bringing the timing up to MBT.

So.. I pulled up my timing tables and the Maximum Torque table and the MBT is 15, 18, 13, 14, etc in this range. The main High Octane timing table is 10,11, etc in this range. I also notice that its a relatively narrow curve between 2000 - 3200 rpm with any amount of spark.

Why so little advance with this engine? I believe even vacuum advance chevy's run quite a bit more advance....

Any input on pushing the advance up in this area would be appreciated.. Will it help my hesitation?

Thanks in "Advance" (sorry)

clint

[schpenxel]

Vacuum advanced Chevy's probably have cylinder heads about as efficient as my dog too... the more efficient they are, the less timing you need/want

Not to say that it can't be made better. On my L92 truck I initially had problems with knock retard in lower RPM/mid throttle. It would pull timing, and you'd feel the power go down, timing would start getting added back and you'd feel it pick back up, KR again, timing pulled, rinse/repeat. It was pretty annoying and sounds similar

I ended up changing spark plugs/wires, desensitizing the knock tables a little bit under 2K RPM's or so. I ran 93 pump + a can of torco and it made zero difference on the "knock" it was seeing so I think it's false.

I also made some changes to the transmission tune which made a huge difference in how it felt.

If you can post a tune and log if it happening that would help us take a look.

[clinter36]

Here is the log and the tune. On the log "timeline" (at the bottom), you want to watch the last 1/3 of the 3rd incline (probably 4th gear) where the rpms are around 1500 - 1700

There is nothing done to the engine yet, the only changes are the Tcc lockup tables and moved a few shift points up by a couple mph.

I have yet to see any KR, but I am not sure if I am logging it correctly.

thanks
clint
Silverado V1.hpt
n2.hpl

[eficalibrator]

Just food for thought. GM released these trucks for use with regular (87 R+M/2) gasoline, which is why you might see lower values in the "high octane" table compared to the MBT table in some places. If you know you're going to run Premium fuel, then you can start walking the high octane map closer to the MBT surface in those areas as long as you don't start getting knock that may damage components.

Just make sure that you air models (MAF and GMVE/VirtualVE) are right, so that the resulting g/cyl number is also right and you're actually grabbing the right value from the spark table for your current condition. If you still run a stock airbox, chances are good that the stock MAF curve is right. If you have a new CAI, let's hope you've updated the MAF Hz. vs Airflow table appropriately since it affects both fueling AND spark.

[clinter36]

Thanks for the input....

I had a Volant CAI and it made things worse, so its been in a box for the last 4 years.... However, en light of new found knowledge, I pulled it out of the box and inspected it to figure out why it gets such a bad rap. What I found was that the ID of the tube where the MAF mounts is 3" ID vs 3.5" ID for the stock intake tube along with a few bumps for mounting screws and a grommet for the pressure sensor. Clearly, everything about it would screw up the calibration. So.... I am working on a little modification and replacing the section of the tube where the MAF mounts. But as long as we are discussing it... I am wondering if I am really going to get any gains or if I am better of with the stock intake with the wave canceling chambers...?

I do have a magnaflow muffler with the stock 3.5" exhaust tubing. After I figure out the intake I was going to start working on the VVE and MAF tables.

I really don't want to run 93 octane. but regardless of the high/low octane table I am getting around 10 deg of advanced spark where I am feeling the hesitation. My gut tells me I can run a little more advance... but I have so little experience its really nothing more than a guess. If you recommend sticking with stock there, then so be it....

thanks
clint