Ask Away! with Jeff Smith: Upgrading 6L80/6L90 Transmissions for Boost

This Sonnax illustration shows the stock clutch apply version on the left and the improved Sonnax apply piston, piston dam, and apply ring on the right side. By creating broader apply area to the clutches, the clutch plates do not deflect and therefore are more efficiently applied. This makes them last longer. (Image/Sonnax)

I have a 2015 Silverado with a 5.3L with long tube headers and I’ve installed a Whipple supercharger. My 6-speed trans is now slipping. Can you give me any direction on what to use as a replacement or builder to hold up to the extra horsepower thanks. — T.F.

Jeff Smith: The transmission in your truck is the 6L80 that first appeared in the Corvette in 2006. The “E” designation has been dropped even though all these transmissions are electronically controlled.

This is a much different transmission from the earlier generation 4L60/4L80/4L90E four-speed overdrives. This newer transmission is what is described as a clutch-to-clutch automatic, meaning that there is no overlap between gear where a band is applied or released.

We’ve worked with the Sonnax brand of transmission upgrade components in a previous rebuild of a 4L60E that is now running happily behind a 640 lb-ft of torque big-block El Camino so we know the Sonnax parts can do the job.

We did some investigating on this new line of 6-speeds and found that Sonnax is already on top of upgrading these transmissions. You mentioned your trans is suffering from slipping that we will assume is delayed shifts due to slipping clutches. This has probably been amplified by the addition of that Whipple supercharger!

Sonnax has a line of upgrade parts that address some of the 6L80 and 6L90 transmission weaknesses.

Foremost on the list is the Sonnax clutch apply kit. The factory clutch apply piston has a sharp, square edge that does not completely apply force across the entire clutch piston. This creates an uneven force on the clutches that tends to improperly load them. The Sonnax kit uses a redesigned piston and piston dam along with an upgraded base ring that applies much more even force across the entire clutch face for each clutch. With even apply load, the clutches will hold much more torque without slipping.

This improvement is for fourth, fifth, and six gears where much of the driving load is placed. There’s also a 4-5-6 clutch boost valve that improves pressure apply to the clutches in these gears.

6L80E Transmission Gear Ratios

1st Gear4.02:1
2nd Gear2.36:1
3rd Gear1.53:1
4th Gear1.15:1
5th Gear0.85:1
6th Gear0.67:1

Note: There’s no direct 1:1 gear as with most transmissions. 

There’s also a line booster kit for the 6L80 trans that helps with improved line pressure.

Finally, Sonnax offers what they call the Zip Kit (learn more about the Zip Kit here) which includes a series of valve body components that are common wear items or pieces that can be improved.

The combination of these Sonnax upgrade parts along with a standard rebuild kit with high quality clutches and seals would add quite a bit of life to your transmission. The 6L80 and 6L90 transmissions seem to have a fairly good reputation for durability and with these upgrade components will cut the slip and add the grip from your transmission.

6L80 Parts List

Description PN
Sonnax Clutch Apply Pak 104960-01K
Sonnax Clutch Boost Valve 104740-12
Sonnax Line Booster Kit 6L80-LB1
Sonnax ZipKit 6L45-6L90-ZIP
Sonnax High Line Pressure Spring 104534-HP

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  1. Hello Jeff I’m hopeful that you will help help me understand what is happening with my 6l80 I have 14 model 1500 chev 4×4 and hit at around 124000 miles and the factory converter started sounding like it had a big oval shaped steel ball in it and bout to jump outta truck well turns out conv craped out and I replaced it with billet heavy duty direct replacement all’s well but now trans is fumbling lightly for 1st gear after other than that it shifts wonderful what do ya think thanks Joel.

  2. Torque converter failures seem to now be more common with the 6l80 most likely due to material failures of key components in the converter.
    We have two 2016 Suburbans both failed catastrophically without warning this year with 88k & 75k miles and no towing or unusual driving stress.

    From what we saw when the trans were rebuilt the clutch material seems to be nothing more than a paper based friction material that also does not seem to be able to hold up to any converter failure. Both of our trans had to not only have replacement torque converter but also clutch replacements.

    I would guess you have the same issue now so will see clutch failure.

    There is no confidence now that these trans will hold up especially since neither vehicle was used to tow and only has the 5.3 engine with no reason for these failures.

    I personally drove a 2001 Yukon 425k before with never a hint of an issue and 2007 Tahoe 150k same result so this trans is a huge disappointment good luck with yours, check the fluid.

    Constantly check fluid for burning and small metal fines before any long trips since that was obvious in the fluid when the converters failed.

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