We often get asked whether or not you should add transmission tuning to your Cummins 68RFE
And the short answer is:
Yes! Without a doubt.
Even sitting behind a stock Cummins the 68RFE transmission tuning is subpar and has room for improvement. Throw a delete, a power tune or an upgraded turbocharger and you’ve got yourself a recipe for needing tuning. We’ll break down the shortcomings of the 68 and the improvements that tuning can do, but first, let’s break down what the 68 is.
The 68RFE that comes behind the 6.7 Cummins can be a great unit and offers a few benefits over the G56/Aisin transmissions. Don’t get us wrong a manual transmission behind a Cummins can be a blast, but you lose boost between shifts and you have to upgrade the clutch before you can start making more power. On the other hand, the Aisin is a stout transmission that holds fairly strong, however, when there are a few upgrades to the engine power, failure rates start to become more common and similar to that of the 68RFE.
The 68 stands on its own with options because it has a wide variety of transmission tuning available, huge aftermarket support and can bang through gears a lot faster than the big brother Aisin. Some companies have been experimenting with tuning the Aisin so we’re excited for the results that will come from that!
Reliability:
Oftentimes people say the 68 is rated to about 1hp less than the Cummins makes from the factory. So throwing a tune, lift kit and tires at the 6.7 might have the clutches inside your transmission screaming for mercy the first time you hit the throttle! Part of this is due to the line pressure that holds the clutches together. In factory form, the transmission only produces around 160psi of pressure, whereas a transmission tune will produce 180psi. This greater pressure decreases the chances of slipping, allowing you to extend the life of your transmission or have it hold more power.
Lockup:
The torque converter lockup on the 68 doesn’t start until 4th gear, which causes a unique shifting situation in the 3-4 shift called a flare shift. This happens when the truck's RPM will be at say 2000 rpm in 3rd, it’ll then flare up to 2200 rpm or so before slushing itself into 4th gear. This flare can become more prominent with high throttle, heavy loads, or mileage which makes the shifting feel….uneasy (crap).
By default, a tuned transmission will utilize a 3rd gear lockup, meaning the 3-4-5-6 shift will be crisp and clean as you go through the gears. These gear changes will feel more direct making the whole experience feel more crisp. You will notice a “shift” in 3rd gear when the torque converter locks up, but it’s not overly noticeable. This also makes the acceleration feel more instantaneous above city speeds because there will be no delay through the converter.
Shift Points
A 6.7L Cummins with a VGT turbo is a behemoth motor, making mountains of torque off idle allowing you to make big power down low! That doesn’t mean they’re limited in RPM, open up one of these motors with a tune that fuels more in the top end and a healthy boot from the right foot and you’ve got yourself something that will make monster power up top!
If only the factory transmission shift points knew that….
In stock form, the 68 constantly tries to grab the highest gear possible, which sounds like it would work for the low rpm torque, but then when you go to roll into it and accelerate it’ll awkwardly downshift at all the wrong times. It’ll initially hold the gear at very little throttle input which means you’ll get no acceleration, causing you to roll into it further until it’ll then downshift 2 gears jumping up the turbo and engine speed to have you accelerate far more than your throttle input would indicate. For some reason, it seldomly shifts down 1 gear.
Standard transmission tuning will optimize the low-end torque by allowing you to roll into the power far more without it commanding a downshift, so you can build up that torque, or if you select a high shift point tune that will downshift sooner so it can get up above the turbo quickly without having to wait for the standard transmission tunings awkward double downshift. A mid-shift point tune will give you the middle of both worlds.
Modifications
Big turbos require rpm to spool, big tires lug an engine as though it’s starting in a higher gear. Many aftermarket modifications require the engine to spin at higher rpms to drive the same way as it did when it was stock, this can only be done by tuning the transmission. If you’ve got an oversized turbo that is non-vgt and a lift kit with oversized tires then you’re having to really setup your truck differently to stay on top of that.
Raised/High shift point tunes will do that in two ways where it will downshift earlier on to get on top of turbo’s and it’ll hold a gear longer so that it doesn’t drop the rpms too low as it selects the next gear.
Some drivers appreciate having the high shift points on a truck where it still has the stock VGT or stock tire size, it can make the truck feel more responsive than the lower shift point tunes as well so the raised tunes are not exclusive to modified trucks.
Conclusion
Running the transmission tuning stock leaves reliability and driving dynamics on the table that are nothing more than a flash away. We highly recommend tuning for anyone considering, and are happy to answer any questions you might have by reaching out to us at DirtyDieselCustoms.ca
If you prefer to watch, we also have a video discussing and showcasing some of these characteristics here:
