2014 Sierra with 6.2 ecotec
Diablosport In-Tune

I notice the 2 things and have questions:

why does the stock tcm tune reduce the shift point rpm in the higher gears? Just for fuel economy? Is the optimum to have all gear shift near the peak hp (around 5800 in this case)?

Also, I notice if I have a shift point programmed at 5800 rpm in the In-Tune and run a WOT data log, the replay of the log shows the shift at around 5400+ rpm. Which is correct and why a difference?

 

Does anyone else with Diablosport notice their log shift points to be lower than their programmed shift points? Can anyone with Diablosport confirm their log shift points to be the same as their programmed shift points?

 

The drop in RPM from 1st to 2nd is greater than in other gears because of the gear ratio.

 

Optimal shift point, as in 1/4 mile ET/TS optimal...?

Shift at the the highest rpm the engine can survive.

 

Yes, that's what I meant. Optimal for performance...1/4 mile. etc. So is there an advantage to reving past the peak hp rpm? HP will start to drop off after that, but I guess the other side is, after the shift, you would be higher up on the hp curve for the next gear. 2nd question, why does the factory TCM program have the shift point dropping lower and lower for each higher gear? Fuel economy? They have 1-2 shift near peak hp but then start dropping. The factory could just as easily programmed each gear shift the same, near peak hp.

 

Yes, higher up on the TQ curve.

IDK... maybe because trans might not be able to handle high rpm.

 

I've told you in a post above, it's dependant on the gear ratios. You want to drop to the best part of your torque curve after the shift, the gear ratios are not evenly spaced.

 

I'm trying to comprehend that thought process. Let ask this; with a straight shift, the gears are not spaced evenly either are they? So if you have a straight shift and you want to run the best times in the 1/4 mile. Are you going to shift at near peak hp (near redline) for each shift or are you going to shift at lower and lower rpms as you go through the gears?

 

And let me ask the tuners that are selecting their shift points. Are you selecting the shift point the same in all gears or lower and lower rpms with each higher gear?

 

A rule of thumb is to shift say 500 rpm above peak HP. I don't know what transmission you have but often first to second is a bigger jump than second to third and third to fourth. You compensate by winding out a little further in first. Look for some graphs that show the gears in relation to the torque curve for your setup. Look to see what RPM to shift at so that in the next gear you are at close to the flat part at the top. The torque curve can be quite wide on some engines. The trans will not necessarily shift at the RPM you set it at because of delays, you will probably have to do some logging to find out exactly when it shifts.

 

OK, that is starting to make some sense to me. I have a 2014 GMC Sierra 6.2, 6L80. I probably cant find torque and gear ratio curve for it. But I did find this:
http://www.welltall.com/ymc/discovery/car/shiftpt.html

It's a lot of number crunching but is educational. I'll see if I can plug my numbers into these equations. I'm betting optimal shift point is near 6,000 rpm for all gears +/-.

This seems to agree with that theory:

http://www.allpar.com/eek/hp-vs-torque.html

The stock tune has the following shift points. Approx. I'm going from memory right now but close to this:

1st to 2nd 5800
2nd to 3rd 5600
3rd to 4th 5500
4th to 5th 5400
5th to 6th 5200

The dyno graphs I have seen for this engine makes peak HP at 5800 rpm. I have not seen a torque curve yet.

Others feel free to add your comments or information on my set up.

 

Yes, you are on the right track. The only real way is to test at the strip.

 

Here's an interactive on-line optimal shift program. Amazing what you can find on the internet.
http://www.bgsoflex.com/shifter.html

It only has 5 gears, but that's enough to tell you what you need to know.

I could not find a good hp curve for the L86 (6.2 EcoTech) but did find several for the LT-1 6.2 in the 2014 Corvette. With my modifications to intake, exhaust and tune, my actual hp curve is probably closer to the LT-1.

Using the data from that curve and plugging the numbers into the above program, this is what I get:

Input Parameters Are the Following:• Transmission Ratio One = 4.03 to 1
• Transmission Ratio Two = 2.36 to 1
• Transmission Ratio Three = 1.53 to 1
• Transmission Ratio Four = 1.15 to 1
• Horsepower = 200 at 2000 RPM
• Horsepower = 240 at 3000 RPM
• Horsepower = 290 at 3500 RPM
• Horsepower = 340 at 4000 RPM
• Horsepower = 390 at 4500 RPM
• Horsepower = 420 at 5000 RPM
• Horsepower = 440 at 5500 RPM
• Horsepower = 460 at 6000 RPM
• Horsepower = 420 at 6400 RPM


Maximizing the Area Under the Horsepower Curve:

Finding Best Shift Placement Points For Ratio 4.03 to 2.36:
•Found optimum shift RPM at 6395, which achieves maximum area under horsepower curve, and lowers the RPM to 3745 after shift

Finding Best Shift Placement Points For Ratio 2.36 to 1.53:
•Found optimum shift RPM at 6394, which achieves maximum area under horsepower curve, and lowers the RPM to 4145 after shift

Finding Best Shift Placement Points For Ratio 1.53 to 1.15:
•Found optimum shift RPM at 6399, which achieves maximum area under horsepower curve, and lowers the RPM to 4810 after shift

Given the L86 has different intake runners to bring torque on lower in the rpm range and less peak HP, my optimal shift rpm may be 200 - 300 rpm less as a guess.

I'm going to program my TCM for 6,000 rpm shift points for all gears at WOT.

 

Shift as high as your engine can spin to.

 

My 2008 corvette with the LS3 I shifted at 6500 rpms the fuel cut off point at that thing was still pulling hard. I would shift at 6400 rpms.