Driveline / Q&A

Ask Away! with Jeff Smith: Explaining 6-, 8-, and 10-Speed Automatic Transmission Gearing

TCI six-speed transmission kit

(Image/Summit Racing)

I see where Summit Racing lists a TCI transmission that was originally a 4L80E – but they list it as a six-speed transmission. I’ve heard a little about this—did they add two more gears to the transmission? How is this different from the GM 6L80E transmission that is used in the Corvette or in the newer GM trucks? – R.F.

When Four Becomes Six

Jeff Smith: The TCI six-speed automatic is indeed a revamped or upgraded 4L80E. The factory 4L80E—and the stronger 4L85—is essentially a traditional TH-400 three-speed automatic equipped with a fourth, overdrive gear. The original first three gear ratios are in fact still the same as the TH-400. The overdrive ratio that’s added is 0.75:1. The 4L80E is a very large, heavy transmission but, like its original TH-400 cousin, it can absorb and transmit an enormous amount of engine torque.

The TCI six-speed version has a few special additions to it. Essentially what TCI did was to engage the overdrive at the back of the trans to split the gear spread between the original First and Second gear ratios and again between Second and Third. But in order to make this work properly, they began by making the overall First gear ratio deeper.

Where the original 4L80E First gear is 2.48:1, TCI created a 2.97:1 First gear ratio. Then to create a new Second gear, the overdrive is engaged 2.97 x 0.75 = 2.23:1. This creates the situation where the engine drops less rpm from First to Second gear. The new Third gear in the TCI trans is 1.57:1, which is then overdriven to create Fourth gear which is 1.57 x 0.75 = 1.18:1 which is just 18 percent away from direct drive 1:1, which becomes Fifth gear. Then, of course, Sixth gear is still 0.75:1 overdrive.

The idea behind this conversion offers multiple overlapping advantages.  The overall First gear ratio is deeper at 2.97:1 which means the car builder can leave a taller rear gear ratio in the rear end and still have great First gear acceleration. As an example, combining the TCI Six-speed’s 2.97:1 First gear with a 3.50:1 rear gear ratio is equal to a traditional TH-400 (or stock 4L80E) with a 4.19:1 rear gear.

Then, because the six-speed is splitting ratios, the rpm drop between gears is less because TCI has turned this into a five-speed automatic with an overdrive. By reducing the rpm drop between gears, the engine remains within its power band. Our simulations reveal that this is worth anywhere from 0.10 to perhaps 0.15-second in a quarter-mile comparison between a stock 4L80E and a TCI Six-Speed.

You also asked about GM’s new 6L80E. This is a completely new and different transmission compared to the 4L80E. The way the power is applied is different than the 4L80E / TH-400. It employs an even deeper First gear ratio of 4.02:1 with tighter gear splits but does not apply an overdrive for subsequent gears. All this is handled with application of planetary ratios.

GM Automatic Transmission Gear Ratios

GearsTH-400TCI 6-SpeedGM 6L80GM 8L80GM 10L60

GM now also offers both 8- and 10-speed automatics. The 10-speed first appeared in the 2017 Camaro ZL1 backing up the 650-horsepower, supercharged 6.2L engine. At the current time, the only way to use a GM 6 or 8-speed automatic is to tie it in with a GM engine like the LS3 and use the engine controller to work with transmission.

It’s interesting how the new 8- and 10-speed automatics have essentially turned the traditional powertrain ratio scheme 180 degrees.

Here’s what we mean: Let’s assume we have a TH-400 trans in our Chevelle with a 4.10:1 rear gear. The overall First gear ratio is 2.48 x 4.10 = 10.17:1. If we change to the latest GM 10-speed automatic it has a 4.70:1 First gear and is factory matched with a 2.85:1 rear gear.

See how the ratios have swapped places? The new 10-speed combo is 4.70 x 2.85 = 13.39:1.

Comparing that to our old TH-400 combo, we’d need 5.40:1 rear gear to equal the overall First Gear ratio in the new Corvette.

So it’s no wonder that the new cars accelerate so well—they’ve got a ridiculously deep overall First gear ratio and reduced rpm drops between the gear changes. It’s the best of both worlds.

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  1. What is a good torque converter today for a 4.3 high output 4 wheel drive Blazer?

  2. Len Gilbert says:

    Hi Jeff,
    I have a 700 R4 trani with a dash mounted after market switch to engage or disengage my lock up.The trani has a 2500 stall & is behind a old school 383 stroker engine with 388 hp at the motor with a 3.73 Rr gear. It’s good for a 12.69 @ 111 mph in cool dry weather.I go down the track with the lock up switch off. Do you think that I might pick up a few more hundredths if I switch the lock on say when I hit second? Will this hurt the trani? Thanks Jeff. Always appreciate the great exchanges. Respectfully, Len Gilbert Kitchener ON Canada.

    • Jeff Smith says:

      That’s a great question. Conceptually, locking the converter should help the mph trap speed by reducing slippage. It probably won’t do much for the elapsed time which is all that really matters. Also, I’m not sure if the trans will allow converter lockup at WOT. Beyond that, it’s probably not a good idea even if it will allow lockup because that clutch surface area is limited. It’s only about a 3/4-inch band around the inside circumference of the converter. I’ve not done the math on total surface area, but it’s likely the clutch would not last long engaging 450 hp. So my suggestion is to not subject the lockup to that amount of abuse. Your car is quick! Running 12.60’s at 111 is a fun ride! Thanks for the question! The readers on this site are a pretty sharp bunch.

  3. Is the out put shaft and splines on a 4L80E the same as a turbo 400?

  4. Steven Bishop says:

    I have a 2wd 2000 regular cab stepside bed Silverado that used to make closer to 600-650 wheel horsepower with the D1-SC Procharger and 4L80E. But I was su unhappy with it I sold a bunch of parts and began changing the built. I am currently planning on a high compression LS2 and a progressive nitrous kit but before another 4L80E goes in there is a 6,8, or 10 speed not an option? I am making a great deal of changes to the truck from a Procharged 5.3 to a head, cam, Fast LSXRT intake LS2 on nitrous before we build a 3.9″ bore and stroke twin turbo 372ci setup. My problem with the 4L80E is it was always a workhorse and in the past 25-30 years trucks have gotten much quicker and versatile. I also have a 2wd 2006 Trailblazer SS with a forged 416ci stroked LS3 which makes around 600 hp or so with a Yank SS3600 stall converter. Now it has RPM Level 6 but if a 6L80E or 6L90E swap was introduced or even the 8 and 10 transmissions it would be a game changer. There are swaps for the 6L80 and 6L90 but they don’t support all the Trailblazers. Finally I am looking for a Chevrolet Caprice PPV to build into a fun road trip sedan for the family with either a LS3 or L92 engine block, stock LSA crank, rods, pistons, ARP hardware through out with CNC LS3/L92 heads and a pair of turbos with a goal of 750-950 horsepower at the wheels. I would like to keep the turbo small so it would spool extremely quickly and the compression high to be responsive. More or less a TT V8 german car so if the new 8 or 10 speed GM transmission would work where the 6L80E came originally and the CTS V, SS Sedan, and ZL1 Brembo brakes work it could be a great platform. So if there are any possible ways to upgrade the RPM Level 6 4L70E in my 416 CI Trailblazer, the 4L80E in my Silverado with a LS2 with heavily worked and milled 243s, Ported and polished Fast LSXRT 102mm intake, and the Caprice PPV I am looking for the right one

  5. Please help>> I have a 2013 GMC yukon. It is a dog in the summer time. I didn’t realize that it came equiped with such a lame gear ration 3.08. What is your recomendation for towing? I’m thinking 3.73. It has factory size tires 275/55/20. I use this vehicle to tow a 20 ft 10k trailer that I use to carry a 5500lb tractor. Thank You..

  6. Phillip Shaffer says:

    Jef, Swapping a LS3 and 6 spd trans in , In place of a 454 and a turbo 400 3 spd, I currently have posi rear, Big yolk drive shaft, after the conversion, will i need to mess with the rear gear, or will I be ok for around town and occasional red light duel. LS 3 has a Big Turbo on it, I am only worried about rear gear, rest I have help with, Car never went over 130 or it red lined with Big Block, Never was a highway car, It has a 373 or a 323 in it, Not a 411 I know that much, Thats all,

  7. I’m building a gen one Camaro and I’ve purchased lt4 supercharged motor for it I was wanting to know if you know the difference in size for 10 speed transmission to a turbo 400 I need to know how much to raise my floor pan at tunnel and about how much fire wall to cut out

  8. Dean Stutheit says:

    Hi Jeff, I have a 2016 Silverado with the 5.3 and the 8 speed transmission. Nothing but problems with it – big clunks, engine revs up before grabbing a gear. GM has been useless to say the least, and waited me out (now off warranty). Do you know – does the aftermarket have a fix? I would be willing to get it rebuilt, but want it to function like it should (not back to the way it came from GM).

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