Talking Trackhawk With a Jeep SRT Engineer
Jeep vehicle integration manager Paul Mackiewicz’s baby, the 707-hp, Hellcat-engined Jeep Grand Cherokee Trackhawk, might have been the performance star of the New York International Auto Show if it hadn’t been for its tire-melting, wheelie-popping, limelight-stealing SRT sibling: the Challenger Demon. Still, we seized the opportunity to have the Trackhawk’s vehicle integration manager give us a walkaround to learn some of the details that might not have made it into the press release.
Motor Trend: Our initial story described the Grand Cherokee Trackhawk as essentially being a Grand Cherokee SRT8 into which Motor Trend’s Roadkill brothers Freiberger and Finnegan dropped a Hellcat engine. We know your integration efforts went well beyond that. Take us through some of the details your team sweated that Roadkill wouldn’t.
Mackiewicz: Obviously, like you mentioned, the most obvious body modification in front is that is we eliminated the foglamps so that we could basically start drawing air from that lower right corner. This feeds the air box, which is also larger than what we have on the SRT8. On the left side, we’ve got an oil cooler integrated into that former foglight opening. Basically, what we’re doing is pulling some of the heat away from the engine. We had an oil cooler before, but it was an oil-to-water cooler. Now we’ve got a separate oil-to-air cooler, and it’s away from the engine bay so it’s pulling that heat away from the engine. We’ve also got a low temp radiator, which is a dedicated cooling loop just for the supercharger.
MT: Where is that radiator?
Mackiewicz: That one is just forward of the primary cooling module breathing through the upper and lower grilles and through the mail slots that came in for 2017. We also had to change from the standard 6.2 that we used on the Challenger and Charger. We have different exhaust manifolds, mainly for packaging.
MT: I noticed the oil capacity went up. Was that just to feed the oil to air line?
Mackiewicz: Yep, the sump capacity is the same, although the oil pan is unique to this application because of packaging. Our sump is oriented opposite to what they use on the Charger. That’s really what it comes down to—and a revised thermostat housing. Once again, for packaging reasons. And obviously we had to reroute all the plumbing that comes along with those systems. That’s in a nutshell what had to happen under the hood. Obviously, some tweaks to AC lines or changed oil cool lines, things like that. Those are just adaptation pieces.
MT: There’s nothing about this Jeep engine compartment that looks much like a Challenger/Charger…
Mackiewicz: Exactly. So while the base engine is identical, all the adaptation pieces had to change along the way.
MT: Then moving back to the driveline, is this the only fitment of the Quadra-Trac MP 3015C transfer case?
MT: So does that upgrade from 3010 to 3015 nomenclature primarily indicate structural reinforcements?
Mackiewicz: It is actually a brand new transfer case.
MT: Okay. The press kit mentioned an upgrade to a forged steel chain sprockets pulling a wider chain. Is there anything else inside reinforcing it?
Mackiewicz: No, those are really the load carrying members. You’ve got the main shaft, you’ve got the sprockets, you’ve got the front output. In a nutshell, that’s it. Then there’s obviously a clutch pack, which how much torque you send to the front. It really wasn’t changed too much because it basically had enough capacity for what we were asking of it. We did change both front and rear case halves to make it a shorter transfer case. It’s a brand new transfer case for this application, so it’s not just changing a couple internals and then adapting it. The 8HP95 transmission is an evolution of the 8HP90 that is used on the Challenger Charger. This one just allows us to adapt for an all-wheel-drive system and has some changes as well for torque capacity demands of all-wheel drive.
MT: Did those changes make the transmission longer by adding additional friction and steel materials to the clutches?
Mackiewicz: Yes, but then we shortened up the transfer case so the overall driveline is the same length.
MT: I know the Jeep boasts way more towing capacity than the Charger and Challenger. Did that drive any of these revisions, or were they just to accommodate all-wheel drive?
Mackiewicz: It’s primarily for the all-wheel.
MT: I mean that would be a big difference, being able to tow 7,200 pounds or whatever. You can’t do that with a Charger.
Mackiewicz: Right, but the load that you get from towing a 7,200 pound trailer, whether with a 6.2 or a 6.4, is going to be very similar. It’s just the rate which you can accelerate. And you’re not going out and doing a 0 to 60 pull with a trailer.
MT: What kind of a drop in track times can you talk about between an SRT8 and a Trackhawk—either drag strip ETs or handling circuit times?
Mackiewicz: To be honest with you, we did very little back-to-back comparison at say a known track that we’ve done in the past. We haven’t done an A to B comparison between say this and the SRT8 6.4 at a track at the same time, on the same day, so I can’t really speculate…
MT: Your Demon colleagues managed to find time to get numbers on their dragstrip!
Mackiewicz: Well I can tell you dragstrip quarter-mile elapsed times: We ran a 12.08 on an SRT8 6.4 and this ran 11:06.
MT: OK, cool.
Mackiewicz: That’s a very straight comparison.
MT: Now what happens to weight distributions and so forth, other things that would affect handling? Does that change at all really?
Mackiewicz: You’re talking about 200 pounds up front, over the whole mass of this, so it really didn’t affect the weight distribution much at all.
Mackiewicz: That’s really the main difference between this and the 6.4L—200 pounds up front.
Mackiewicz: Obviously, we did the changes to the transmission, transfer case, so that 200-pound delta is over a long driveline.
MT: Tell me again, did the front tires change at all?
Mackiewicz: Not in size. They’re 295/45R20s all the way around. What we did have to do is update the speed rating of the tires so we went from W to [Y]. Which is obviously good to well over what this is capable of, which is 180 mph.
MT: Any other exterior changes?
Mackiewicz: Well, we did restyle the rear fascia for the quad tips, and we retuned the exhaust system to make it a little bit more aggressive than what we have on the 6.4L, give more of a presence. It’s not really any louder, it just has a different tone to it.
MT: OK, have you ever drag raced a Model X P90D Ludicrous?
Mackiewicz: I have never even seen one of those at a dragstrip that we’ve been at.
MT: OK, OK. I’m thinking that would be a pretty close drag race.
Mackiewicz: Can’t speculate.
MT: Sure. OK.
Mackiewicz: Obviously, it would be interesting in many ways over many times.
Mackiewicz: As you know, we have a very unique validation program that we go to on the SRT vehicles, which includes doing 100 runs on the dragstrip back to back.
MT: Right. I think that kind of thing is discouraged at Tesla.
Mackiewicz: Maybe. We also do multiple hot laps at dragstrips, we do a 24-hour test. Granted, we do that over the course of three days. That’s mainly to keep the driver safe, as we can only put our drivers through so much. We’re not actually running a 24-hour race. We’re doing a durability test.
MT: Sure. So did anything change inside?
Mackiewicz: Well there’s quite a bit inside. Hop in. OK, our drive modes have been updated for the 6.2L. Similar to what we had on the previous versions, we have the same ability to modify and customize the different modes accordingly and have a unique custom mode, where you can kind of tweak the different modes a little bit. Then there are the more performance based pages. The homepage itself is reconfigurable.
MT: Wow, how many different options are there?
Mackiewicz: I’ve never counted them. There’s at least two or three pages’ worth. You can reconfigure this page to what you want it to be. If you want to use all of them, they’re basically right here. You can page through three different pages here that are easily accessible. The nice thing here is when you basically click on one of these information channels, it gives you a running graph. This will give you a real time look at what’s going on, in this case, air to fuel ratio. Going to the timer screen, this is an adaptation of what we had in the past. It gives you very much a look of a classic dragstrip time slip. You can view your last run, best run, and you can save runs to a USB.
MT: Oh nice.
Mackiewicz: The engine screen gives you real time information on engine output for both torque, horsepower, and boost. It gives you a real time graph with about a 30-second block at a time. You can page forward and back as you want. Then do a snapshot. With any of these screens that I just pulled up, you can actually hit snapshot and actually save it to your USB.
Mackiewicz: This is the launch control. What you can do is you can preset your launch RPM, based on the surface that you’re on. The more grip you have, the more RPM you’ll want to hold.
MT: Right. What’s the max?
Mackiewicz: It will go up to a 3,500 rpm stall speed. You’ve kind of got to get the brakes a little warm before you do that. Typical launch is anywhere between 1,750 and 2,250 rpm, which will give you a really repeatable, excellent start. The other thing that launch control does, once you’ve enabled it, is it starts our torque reserve program. This basically changes the bypass valve behavior and alters fuel and spark—basically, it prefills the manifold so that as soon as you lift, you’re getting instantaneous. You’re not waiting for anything to build up.
Mackiewicz: It’s actually a perfect setup for this. That’s how we achieved our 3.5-second 0 to 60—which was a pretty violent start to be honest with you. The interesting thing is the difference between this vehicle on a prepped surface, say on a dragstrip, versus say the return road—minimal.
Mackiewicz: It actually prefers the return road over the prepped surface because it likes to have a little bit more slip.
MT: Four-wheel drive makes a big difference.
Mackiewicz: Yeah, it does. The prepped surface—obviously street tires don’t like prep surfaces as much, that’s where it actually benefits it on a nonprepped surface. We also gives you a shift light. Obviously with an automatic it’s kind of a moot point, but it’s useful if you’re going to be using your paddles—which you really don’t need to use if you’re in track mode. You start getting into our quickest and hardest shifts. The whole focus on our shift characteristics in track is how to make the car as fast as possible. It’s not about comfort. It’s not about having a cushy ride. It’s about making this thing quick. Either on the dragstrip or on the track, it will hold gear a little longer.
Mackiewicz: Sport mode is a little less aggressive—kind of backs everything down. Obviously more aggressive than auto. Then auto is your daily driver, comfortable. Shocks change, steering changes, transmission changes, engine maps all change. They all go accordingly. Track mode also gives the option of going to ESC full off. This is carried over from our previous 6.4L.
Mackiewicz: And similar to our other vehicles, we do have the Valet mode, which will limit the vehicle to about 200 horsepower. So if you are handing the car off to somebody that you don’t want using the full 707 horsepower, you can limit their usage.
MT: Right, probably a good idea. I see there’s an Eco button.
Mackiewicz: There is. Eco mode is available in auto only. All that does is change our fueling strategy, our trans shift strategy, and some of the other engine parameters. It tries to maximize fuel economy without having the same emphasis on performance.
MT: Any other interior aesthetic upgrades? Is this carbon-fiber stuff standard?
Mackiewicz: We’ve had carbon fiber in the past, but the Trackhawk gets a silver thread integrated into it.
MT: Really gives it a dimensionality.
Mackiewicz: Exactly. It gives it a lot of depth. Yeah. We’ve got two different radios available, like I mentioned. The Panasonic, which is a 19-speaker system and then an Alpine system as well, which is a nine-speaker system. Both of which have active noise canceling, which is used to kind of complement our exhaust tune, especially when you’re in a steady state going down the freeway. You’re blocking out some of the objectionable sounds. When you’re into the throttle, you really hear the exhaust still.
Mackiewicz: We’ve got the Trackhawk badging on the interior, on the steering wheel, and embossed into the seats. There’s also a Supercharged sill plate. And finally, we’ve got a pretty cool splash screen that pops up—just a subtle reminder of what you’re doing and what you’re driving.
MT: Yeah. Good stuff.