It’s tempting to dismiss the 2018 Jeep Grand Cherokee Trackhawk as simply a Grand Cherokee with a big Hellcat engine under the hood (because that is, in essence, what it is). But shoving 707 horsepower into a Grand Cherokee takes a lot more engineering effort than you might think.
As Uncle Ben from Spiderman once said “With great power come great axial, torsional and flexural loads.” (Or at least I think that’s what he said). And if he hadn’t died in that film, Uncle Ben almost certainly would have continued: “And if that power comes from a Hellcat engine, you’d better start beefing up your drivetrain components.”
Jeep understands Uncle Ben’s wisdom, because unlike that small block Chevy-swapped Wrangler you thought was a good idea in high school, the Trackhawk is a production vehicle. It can’t immediately overheat, grenade its transmission, snap its transfer case’s chain, shear its axle shafts, and leave you calling your parents for help from the schoolyard—it’s got to actually be designed to take a beating from that brawny motor day in and day out.
To accomplish this goal, Jeep strengthened the crap out of its flagship SUV’s drivetrain. The transmission is now an 8HP95 instead of the 8HP70 that’s in the standard 475 horsepower SRT, meaning it can handle 950 newton-meters of torque (700 lb-ft) instead of just 700 (516 lb-ft).
Since the second-generation TorqueFlite transmission—which gets a new torque converter with brazed blades—is longer than the wimpy 475-pony SRT’s, the Quadra Trac transfer case had to be shortened by 30 millimeters. But despite its smaller size, the T-case has actually gotten stronger, with its innards now including a pair of forged sprockets wrapped with a wider chain.
Here’s a closer look at that wider chain:
That single-speed transfer case—with a front clutch that has a 1,400 lb-ft capacity—sends power through a stronger driveshaft with a 3/4-millimeter higher wall thickness than the standard SRT’s. That driveshaft bolts up to the rear differential, which gets a new housing with four mounting points to spread out the load, whereas the standard Grand Cherokee SRT’s rear drive module has only three.
Here’s another angle of that rear diff:
Inside that differential housing sits a 230mm ring gear, which—along with its pinion—has a revised tooth geometry. The pinion gear also gets a “double-pass shot peening” treatment, which is a cold-working process meant to increase the gear teeth’s resistance to bending fatigue.
That ring gear bolts to a carrier with four spider gears instead of just the two found in the lesser SRT’s diff, which—in theory—means the loads resulting from that Hellcat trying to move 5,400 pounds of steel gets spread out, yielding less stress on each individual gear.
Once that limited slip rear differential (which is capable of handling almost 2,000 lb-ft of torque) turns the driveshaft’s rotation 90 degrees, the power then goes through a new 35-spline, vacuum-melted low alloy steel shaft, and then to an eight-ball constant-velocity joint (replacing a six-ball design on the standard SRT8), ultimately transferring power to those rear wheels to propel the Jeep forward to 60 mph in as little as 3.5 seconds.
Despite the significant revisions to the transmission and to the rear part of the drivetrain, the portion of the drivetrain twisting the front wheels—in other words, the front driveshaft, differential, and axle shafts—is the same as that of the current Grand Cherokee SRT. That’s because, as Jeep engineers told me, peak drivetrain stresses occur during track driving, which is normally done in the transfer case’s “track mode.”
Since the track mode setting splits the engine’s power 70 percent rear, 30 percent front, Jeep says it has determined via analysis and testing that the front drivetrain out of the standard SRT can handle without issue the high stresses imposed by that mighty Hellcat.
But, of course, Hellcat-ing a Grand Cherokee SRT took more than just toughening up some drivetrain goodies. The cooling system also had to be modified. You’ll notice that fog lights are absent on the Trackhawk’s lower fascia. In their stead sits an engine oil cooler, tasked with keeping engine temperatures down during track driving. And on the other side there’s an air scoop feeding 1,000 CFM of air (at full engine power) to the supercharger.
There’s also a new full-face low temperature radiator at the front of the cooling module. Its job it is to keep the air charge temperature entering the engine below 140 degrees Fahrenheit at all times to maximize engine power.
As hot post-supercharger air flows through the engine-mounted charge air cooler (seen below), heat gets transferred to the coolant in the tubes. An electric pump then sends that hot coolant to the low temperature radiator at the front of the Jeep, where that liquid is cooled off by ambient air. That now-cooled antifreeze then flows back into the engine’s charge air cooler at a rate of about six gallons per minute, where it takes heat away from the post-supercharger air charge, and the cycle begins again.
The end result is denser air flowing into the engine at a pressure of up to 11.6 psi, ultimately yielding the enormous power figures that get this Jeep through the quarter mile in 11.6 seconds.
On top of the bolstered drivetrain and the new cooling system, the engine also gets a new oil pan, a new catalytic converter setup, and new exhaust manifolds in order to fit into the underhood packaging space (the latter two explain the Trackhawk’s 5 lb-ft deficit versus the Charger and Challenger).
Plus, FCA had to re-tune the engine mounts for the heavier engine, recalibrate the active damping system to handle the higher loads, and tweak the springs and shocks.
On top of that, to bring all that power and weight (this thing weighs 5,400 pounds) to a halt, the new Trackhawk sports huge 15.75-inch front brake rotors with six-piston calipers, and a pair of 13.78-inch rear dinner plates squeezed by four-piston calipers. All this is enough to get the the Jeep from 60 mph to zero in only 114 feet.
So with the 2018 Grand Cherokee Trackhawk, Jeep has bolstered the drivetrain, made some engine alterations to account for packaging constraints, changed up the cooling system, and retuned the brakes and suspension.
Those are just the hardware changes. Add to that all the validation testing—which FCA engineers told me included a 24-hour on-track durability cycle and a punishing high RPM/high load track test wherein the Jeep had to undergo five 20-minute hot laps around a racetrack at temperatures greater than 100 Fahrenheit—and that motor swap begins to sound a lot more arduous than all those small-block Chevy projects you’ve watched on YouTube.
So while it may look like Fiat Chrysler just slapped the big Hellcat engine into its standard Grand Cherokee and called the result The Trackhawk, there’s a whole lot more going on under the sheetmetal of this SUV than just a straight engine swap. It turns out, “slapping an engine” into something meant for volume production that has a warranty isn’t nearly as simple as it sounds.
Source : https://jalopnik.com/how-jeep-made-the-grand-cherokee-trackhawk-strong-enoug-1798516698