Driveline / Q&A

Ask Away! with Jeff Smith: Modern GM Transmission Upgrade Options for an LS-Swapped 1970 Chevy Nova

I’m looking into which late model overdrive automatic would be the best choice behind the 6.2L LS in my ’70 Nova. Chevrolet Performance has just announced availability of the 6L80E transmission package that will work behind either a Gen IV or Gen V LS engine. This adds two more gears over what might otherwise be a 4L65E. That’s my question—is there enough of an advantage that would justify the added expense of the 6L80E over a 4L65 or 4L70E?

B.L.

There’s a ton of material here to consider so let’s just dive right in. We’ve included gear ratio charts for the 4L60E, 4L80E, 6L80E and even the 8L80E transmissions for comparison purposes. There are multiple advantages to the transmissions with additional gear ratios.

GM Modern Automatic Transmission Gear Ratios

Transmission1st Gear2nd Gear3rd Gear4th Gear5th Gear6th Gear7th Gear8th Gear
4L60E3.061.631.000.70
4L80E2.481.481.00
6L80E4.022.361.531.150.850.67
8L90E4.562.972.081.691.271.000.850.68

If we go back in time far enough, the earliest successful GM automatic was really the Powerglide, a two-speed box. As history shows, we’ve evolved from two, to three, to four, to six and GM now has both eight- and 10-speed gearboxes. The advantage is that the engine now stays in its power band as shifts progress with less rpm drop between each gear. The ‘Glide would drop 1,500 rpm between First and High gear (depending upon the converter used) while late model boxes now offer very minimal drops.

This improves even part-throttle acceleration because the engine doesn’t experience a severe rpm drop between gear changes and acceleration is smoother despite the fact that the transmission will shift much more often.

The 6L80E is rated at 650 lb.-ft. of torque which is dramatically better than the 4L70E’s 496 lb.-ft. With the much deeper First gear ratio in the 6L80E, this will have an effect on a rear gear ratio choice.

Let’s look at some examples. With the six-speed’s 4.02:1 First gear, if the rear axle ratio was a 2.48:1, this would be the exact same First gear ratio as a TH400 or 4L80E with a 4.02:1 rear gear! However, when the 6L90E reaches overdrive, you are now faced with an extremely tall 2.48:1 x 0.67 (Sixth gear) = 1.66:1 (which is an impractical ratio).

This means you would need something closer to a 3.31:1 rear gear in order to have a somewhat practical overdrive ratio in Sixth gear. In this case it would be 3.31 x 0.67 = 2.21:1, which is somewhat useful but still pretty tall to be able to pull at 70 mph. As an example, at 70 mph in Sixth gear and the converter locked up with a 26 inch tall rear tire, the cruise rpm would be right at 1,999 or rounded off to 2,000 rpm—which is acceptable as long as your engine isn’t equipped with a big cam.

The advantages to the six-speed trans is that you now have four full forward gears at or below 1:1 while the 4L70E is limited to three gears. With the deeper ratios and the added gear, our simulations show that a typical 430 hp LS with enough tire to eliminate tire spin with 2,400 rpm stall speed converter and a 4L60E would run 12.30 at 110.3 mph. Just changing to the six-speed auto, the additional leverage from the ratios improved the Chevy’s simulated time to 11.93 at 114.30 mph. That’s an improvement of almost 0.4 second and four mph! This shows you what the additional ratios are worth.

This simulation does not take into account the additional weight of the six-speed which is roughly about 50 pounds heavier than the 4L60E. It’s also likely that the 6L80E converter is slightly heavier as well, so these issues might reduce the six-speed’s advantage slightly—but only slightly.

The 6L80E is the only transmission in Chevrolet’s lineup of gearboxes that places the transmission controller in the oil pan where it is subject to high oil temperatures if abused. The later eight- and 10-speed gearboxes moved the ECU back out of the transmission and the six-speed is essentially its own design—completely different from all previous and subsequent transmissions.

Chevrolet Performance part number NAL-19366637 includes a 2,400 to 2,600 rpm converter; part number NAL-19417102 has a looser 3,000 to 3,400 rpm stall speed. These all come with a controller harness and calibration already set up for the street. The harness will also accommodate a paddle shifter.

Overall, the new Chevy six-speed appears to be a fairly strong package, and it’s priced not that much more than the cost of a 4L70E transmission and converter.  

This is the Chevrolet Performance 6L80-E six-speed transmission. The bellhousing pattern reveals that this transmission will work with either the older LS bolt pattern or the newer LT style pattern. The later 8- and 10-speeds only accommodate the newer LT bellhousing pattern. (Image/Chevrolet Performance)

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5 Comments

  1. Hi Jeff, I purchased a 2003 gmc envoy for an ls swap in my 68 camaro. It has an all aluminum 5.3 which I am going to leave stock, it runs awesome. So I have all the computers an wiring. I have a 2011 6L80E trans I want to put behind the 5.3. Will the computer from the envoy run this trans or what do I have to do to control it? It has a 373 posi rear end. Thanks for your help. Dan

  2. Hi Jeff, I have a 95 C1500 WT I want to swap the V6 out to a V8. It has a NV3500 transmission so I’m not seeing a huge issue other that the clutch is larger for the V8 match up. The computer should be fine right?

  3. Larry Marcoux says:

    Does a 2400 rpm stall converter mean that in any gear – or, let’s say top gear at 70mph in your 2000rpms at 70mph example – if you depress the gas pedal enough to make decent torque, the rpms will go up to 2400? Do they generally lock in top gear? Or other gears?

  4. I would like to put a 6l80e trans into my 07 Saturn sky with my 5.3 ls swap can it be done ?

  5. Philip Allen says:

    I don’t understand why no mention of the vehicle its going into was brought up. In a 70 nova a 4l60 will fit without tunnel modification. The 6l absolutely will not fit with out cutting your trans tunnel.

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