Corvette Dual-Clutch Transmissions 101: RPO Codes M1L, M1M, and MLH

C8 Corvette dual-clutch transmissions (DCTs) are smart, lightning-fast, and more user-friendly than some folks might think.

You’re likely familiar with the fact that the eighth-generation Corvette is not available with a manual transmission. We know, we know. We’re sad about it, too. But it’s also not technically available with an automatic transmission, either. Say what?  Instead, the C8 Corvettes feature dual-clutch transmissions (DCTs), which in many ways are manual transmissions that act a lot like automatic transmissions.

And we’re here today to tell you not only what a DCT is, but what it’s like to drive and own one in a 2020+ Corvette.

C8 Corvette DCTs – The Basics

Modern automatic transmissions use a torque converter to transfer torque from the engine to the transmission, which then drives, in the case of a Corvette, the rear wheels. The transmission automatically shifts up and down gears based on controls from a computer, or driver inputs. Manual transmissions use a clutch to transfer torque from the engine to the transmission. To change gears, one must not only move a shifter, but also briefly disconnect the engine from the transmission by depressing the clutch pedal, which is sometimes called the third pedal.

A dual-clutch transmission, as you might expect, has two clutches. According to How Stuff Works, each clutch controls its own dedicated set of gears. In most cases, one clutch controls the odd gear numbers, while the second one fields the even gears. In simple terms, one can look at it as if it’s two manual transmissions operating side by side. The benefit here is that the next gear is always waiting and ready to go, so to speak, which results in blink-of-an-eye shifts.

This is why DCTs first became popular in race cars.

However, unlike a traditional manual transmission, there’s no clutch pedal or physical shifter. Instead, DCTs hydraulically or electrically actuate their clutches dramatically quicker than either manual or traditional automatics. Like an automatic, dual-clutch transmissions are also computer-controlled, allowing for a laid-back automatic daily driving experience for those who like to cruise.

But for those who like to shift manually — which, to be fair, is also a feature on many automatic transmissions — DCTs are typically paired with steering-wheel-mounted paddle shifters.

Tremec TR-9080

Every C8 Corvette utilizes a variant of an eight-speed DCT that’s built by Tremec. The TR-9080 DCT was “the first transaxle application from TREMEC that combines the transmission, differential and axle drive in one compact package.” It’s available with both a mechanical and an electronic limited-slip differential (LSD). The gear ratios are as follows —

• 1st – 2.91
• 2nd –  1.76
• 3rd – 1.22
• 4th – 0.88
• 5th – 0.65
• 6th – 0.51
• 7th –  0.40
• 8th – 0.33

With the mechanical LSD, the final gear ratio is 4.9:1. With the electronic LSD, it’s 5.2:1. The RPO code for the base Stingray transmission is M1L. The Z06 utilizes a very similar TR-9080 variant with the updated M1M RPO code and the following upgrades, as reported by GM Authority —

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The E-Ray also has a unique RPO code for its DCT variant — MLH — but it’s unclear what has changed physically. We do suspect, however, that additional tuning would be required for its DCT to coordinate with the E-Ray’s electronic front axle and hybrid systems. The E-Ray, for example, offers Stealth mode, which allows you to drive it in EV-only mode for very short distances. In this mode, the E-Ray’s V8 motor can power up while driving.

In general, the C8 Corvette dual-clutch transmissions impress owners and professional drivers with the ability to shift in less than 100 milliseconds, not to mention the way GM engineers tune it to behave differently in different driving modes. (More on this in a moment.)

However, these DCTs haven’t exactly been problem-free. Spend enough time here in the CorvetteForum — or anywhere Google will lead you — and you’re bound to find folks who have experienced owners with leaky, faulty, or broken TR-9080 DCTs. GM launched an investigation into the issues back in 2021, and has reportedly kept revising the transmissions over the years. Still, despite these growing pains, most owners do not experience mechanical or programming-related failures.

Are Corvette Dual-Clutch Transmissions Hard to Drive? Nope!

This is a surprisingly common question for those who haven’t driven a modern DCT.

Why the confusion? We have a theory. Twenty-ish years ago, if you drove an exotic with paddle shifters, there was a pretty steap learning curve involved. Why? Because those cars technically used automated or electronically controlled manual transmissions. So, just like with a manual, the clutch must be disengaged evenly as the throttle is applied. This made for clunky and lungey starts in some older cars.

There were also some bizarre and overly complex DCTs (like the Lexus LFA).

Fortunately, the C8 Corvette’s dual-clutch transmission is as easy to drive as a traditional automatic. The C8’s DCT features a creep mode that automatically disengages the clutches when the car stops (at stop lights, for example). When you release the brake pedal, the clutch automatically engages and the car creeps forward like a standard automatic. Then, as you apply the throttle, you smoothly accelerate as the clutch automatically disengages. This tech makes for smooth takeoffs and prevents excessive clutch wear while stopped since they disengage instead of partially slipping with the engine load fighting the brakes.

And, of course, if you want that ultimate sense of control. (Or to pretend you’re at Le Mans or Daytona). You can operate C8 Corvette dual-clutch transmissions in one of two manual modes:

1. In D (Drive), simply shift up or down with the paddle shifters. You are now, temporarily in manual mode. You can go up and down through all of the gears, but the transmission will shift if you hit redline and this mode eventually times out and returns to Drive.
2. Tap the M button below the D to lock the C8 into manual mode. Revs will hang at redline and it doesn’t time out.

(You can also free-rev the motor at any time if you simultaneously pull both paddles! Or downshift to the lowest possible gear by holding the left paddle shifter.)

At the end of the day, don’t fear the dual-clutch. It’s a remarkable bit of engineering heaven and, we promise, it’s still a blast to drive. We love our manual cars, too. However, the C8 is still a phenomenal driving experience.

Bonus: C8 Corvette DCT Tips and Tricks

In a recent CorvetteForum thread, user JerryU listed out some interesting tricks and tidbits about the DCT transmission when a new buyer asked about the ins and outs of driving with this transmission. Here’s his full rundown we found VERY helpful for newer owners —

Our newbie owner may like the full DCT feature list I put together in 2020. This summary operation includes comments from GM’s DCT Controls Manager, a C8 forum post by Tadge Juechter (Executive Corvette Chief Engineer) and a very interesting comment by then Chief Corvette Engineer Ed Piatek (no Exec in front- lower pay grade!) Pretty impressive options.

• The C8 DCT transmission has different automatic shift strategies for the various drive modes, which adapt in real time. The more aggressive, the more spirited you drive, the more aggressive the car’s responds, -start to relax, the car starts to relax.
• The DCT uses latitudinal and longitudinal accelerometers and looks at information like throttle position and steering angle to gauge how the car is being driven, and react accordingly.
• In Track mode set to automatic, the car will downshift aggressively when the driver is braking hard into a corner, and hold upshifts until corner exit (My note same in Z Mode power set to track, which I find a great feature so don’t have to use the downshift paddle when making turns. Can but no need.)
• The C8 has two manual modes. If you pull a paddle while in Drive, you get a temporary manual mode, which automatically times out, or can be exited sooner by holding the upshift paddle. In this mode, the car will automatically upshift at redline. If you press the M button in the center console, you get full manual mode. There’s no time out, and the car won’t upshift at redline. (My note, an interesting issue as if I am in the only other Mode I use but Z Mode, My Mode-set to Sport- and downshift with the paddle, for 5 seconds after it’s in Manual Mode and I MUST upshift! When in Z Mode, no need!)
• Hold the downshift paddle, the DCT will serve up the lowest possible gear. Do that while braking, and the transmission will keep downshifting as engine speed allows. (My Note: I have fun not doing that and letting it downshift with aggressive braking thru 2 or 3 gears rev matching in each in the blink of an eye. If Tracking- I don’t – this is useful.)
• Pulling both paddles at the same time is equivalent to pushing in the clutch pedal on a manual car, which allows you to rev the C8’s new V-8 as much as you want.
• The paddles are directly wired to the transmission control module (TCM) for quicker response times. This doesn’t mean the paddles will give you a downshift that over-revs the engine—the TCM prevents that.
• You can avoid V4 mode using the “M” manual button or temporally for 5 seconds after pulling the last shift paddle.
• The new “Z” mode comes from the factory set up as an extra sporty mode including shift schedules pulled from the “Track” mode, so that will be V8 only. You can customize “Z” mode any way you want, so if you elect another shift pattern, V4 mode will return.
• An addition item with no details but is very interesting was made by GM’s Chief Engineer (at that time,) Ed Piatek: “We found that during very aggressive launches we can drive torque through both shafts/clutches simultaneously which improves the 0 to 60 times.” Road and Track interpretation of that statement was: “With the C8’s Performance Launch mode, the car will actually use the inertia of the engine coming down between revs to propel the car forward. Chevy calls these “Boosted Shifts,” and they’re only used with a Performance Launch.”

Overall, we can all agree that it would be a blast to take a C8 for a cruise with a manual transmission. We totally understand that rowing through your own gears adds an exciting element to spirited driving and experiences on a race track. However, the C8 DCT is optimized for performance. With its adaptive shifting and blink-of-an-eye gear changes, we can all but guarantee it’s a whole lot faster around a track than a manual one would be.

Let your thoughts be heard about the C8’s killer dual-clutch! Join the conversation at Corvette Forum!

Images: Chevrolet

Additional writing & images: Michael S. Palmer