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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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.