Great Eight: The Grand Cherokee SRT® receives an 8-speed transmission for 2014.
A capable transmission can transform a good sports car into an extraordinary machine. The new 8-speed transmission builds on the proven capabilities of the Grand Cherokee SRT® further enhancing its confidence-inspiring abilities.
"With the new 8-speed, the sky's the limit," says Jeff Roselli, Lead Development Engineer for the Grand Cherokee SRT. "The transmission has 99 different shift maps available, which gives us a lot of options."
The new transmission does not have the old AutoStick shift feature on the center console shift lever. Manually changing gears is now accomplished with the paddle shifters on the steering wheel. The console shift lever has a useful new "S" (Sport) position below "D" (Drive). Selecting "S" firms up the shifts and changes the shift map for more aggressive performance.
Transmission settings are managed by the Selec-Track® rotary control in the center console with the familiar Auto, Sport and Track programs on the performance side, and Snow and Tow on the capability side of the dial. But that's where the similarity ends. Because of the large number of available maps, each selection has more customized responses available than ever before to tailor the vehicle's response to various conditions. For example, in Snow mode when the vehicle hits ice, the control map will change.
Shift paddles further enhance the drive, giving drivers to shift gears when they want. "When the driver goes to paddles we try to deliver a faster, firmer shift, because we are assuming they want to get involved and are driving in a sportier manner," says Roselli. "But that's not the case in tow mode because when you are using the paddles in tow mode to get engine braking going down a hill, we don't crank up shift speed. We have lots of options within the transmission tool set to tailor the experience to the driver."
When you start the car it always defaults to the Auto mode. From there you can use the Selec-Track rotary control to, for example, select the Sport mode, which offers more rear bias, firms shifting, increases suspension firmness and reduces stability control intrusion. However, the driver can now use the shift lever on the console to change the shift map while leaving the other Selec-Track settings intact. With the D and S positions on the shift lever, you can toggle in and out of the sport shift mode and have comfy shifts but retain the other Sport settings. Same thing in the Auto mode: You can have the soft Auto settings with the firmer Sport shift program by selecting S on the shift lever. When you go into Track or Sport mode you are automatically in the S shift mode, but you can toggle out to D and have normal shifting.
The new 8-speed has a lower first gear than the previous 5-speed so the car can be quicker off the line. The 2012 and 2013 had a pretty low first gear, a 3.6:1 ratio, but the new first gear is 4.7:1. If the old car was ever wanting for any punch the new lower first gear certainly delivers it now. "The nice thing about the 6.4L HEMI® engine is that it has a very flat torque band," says Roselli, "but, with the 5-speed, once you got into third gear it was a really long pull. With the new 8-speed, it's really fastshifting and you are staying right in the heart of the power band. The car doesn't appear to be working any harder, it's just living in the power band, accelerating harder always."
Off-the-line performance is an immediately evident benefit for 2014, but there is also cruising, especially engine RPM at cruising speed. The 5-speed transmission had the engine rotating at 2400 RPM at 70 mph, but with the new transmission we are at 1900. "So there are fuel economy benefits to be had," according to Roselli, "and the general cruising speed behavior is less frantic. There's plenty of torque available at that speed, so where a vehicle with a less-powerful engine will get occasional downshifts on grades, the SRT has so much more torque — as much as 170 more lb.-ft. available — it just goes into eighth gear and cruises along without downshifting so much on gentle grades."
There's an "Eco" button on the dash. "Yes, even SRT has an "Eco" button," says Roselli. "It's a sign of the times: Even hot rodders have to look at fuel economy. The ECO mode will cause the car to start in second gear, which it will also do in "Snow" mode for a gentler launch, which lessens the chance of wheel spin.
"Track mode does not use eighthgear, because we don't really assume that people will use track mode on normal roads," says Roselli.
Rev-matched downshifting is a meaningful (and fun) transmission improvement. In the Sport and Track mode the transmission control system will raise engine speed during a downshift so it matches the gear ratio in the selected lower gear. This means no gear-engagement lurching that could upset the balance going into a turn. "With rev-match, you can go down an extra gear without fear of instability from dragging the rear wheels," says Roselli. Speaking of racetrack driving, Roselli says, "In transmission calibration development on the track we were getting a half-dozen extra shifts that we just didn't want with the 8-speed. We had to work really hard on the calibration to re-tune Track mode to make sure it held gear and anticipated what the driver was going to do. We really got an appreciation of what you can do as far as holding a gear."
Further improvements involve a unique torque converter with a higher stall speed (2800 RPM) than other 8-speed applications, which go only to 1900 RPM. The others are designed to lock up early and to prioritize fuel economy and more linear response. The SRT converter gives a more forceful launch when you need it.
The new 8-speed keeps the 6.4L HEMI engine in the heart of its power band, says Roselli. "When we do 50-75 mph passing, the mode that also comes into play while merging on an expressway, the 5-speed car did it in 4.7 seconds the 8-speed does it in 3.9. When you go on road trips and you end up on two-lane highways, passing cars can be accomplished with ridiculous ease. Your exposure in the oncoming lane is nothing when you have that kind of performance."Other new features include:
- Auto mode has a 40-60 torque split front to rear
- Sport remains 35-65.
- Track is 30-70 (previously 35/65)
- Snow and Tow remain at 50-50.