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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One Comment

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

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