A 2021 Jeep Wrangler’s 3.6-litre V6 engine apparently tried revving to eight times its redline after the owner left the vehicle’s transfer case in low-range while flat-towing, The Drive reports. The resulting stresses tore the engine apart, yielding a scene that looks as if it might have been caused by a grenade.
The car website spoke with the foreman of a north Florida shop to learn more about the ruined white 2021 Jeep Wrangler Unlimited Rubicon that had been flat-towed behind an RV. From The Drive:
Tuten told The Drive that when the techs began diagnosing the vehicle, they noticed it had been left in “4-Low,” which is what you’d want when traversing rough terrain at low speeds, or if stuck in a rut — but most definitely not while being towed at highway speeds. For reference, the JL Wrangler manual explicitly states not to exceed 40 km per hour with 4-Low engaged.
Video of the mechanical disaster that once was a dual overhead cam, 3.6-litre, naturally aspirated V6, made it to TikTok and YouTube. Prepare your souls for destruction:
As you can see in the clip, the crankshaft is broken, the block and pan are in pieces, there’s no flywheel or clutch kit to be found, the catalytic converter is destroyed, the bell housing is toast, the manual transmission input shaft has been sheared, and — though it’s hard to tell — I’m fairly sure the passenger’s side upper control arm was bent by the flying debris. It’s just a huge mess.
But that’s what happens when you have lots of reciprocating mass moving far faster than it was ever designed to. The Pentastar Upgrade was really only ever meant to consistently rev to roughly 6,600 RPM. With the Rubicon coming stock with a 4.10:1 axle ratio and a 4:1 transfer case low range ratio, and with first gear at 5.13:1, the vehicle’s crawl ratio is 84.13:1. This means the engine spins 84.13 times as fast as the rear wheels do.
If we assume that the RV was towing this Jeep at 97 km/h (we don’t really know how fast it was travelling; the shop foreman that The Drive spoke to guessed 55), we can use the fact that the Jeep’s stock 285/70R17 BF Goodrich KO2s are rated to turn 645 revolutions per mile to learn that, at that speed, the vehicle’s tires were spinning at 645 rpm. Here’s my maths for that, in case you’re curious:
The 84.13:1 crawl ratio tells us that, when in first gear and low range, the engine spins 84.13 times as fast as the wheels. This means that, with the RV towing the Jeep at 97 km/h, the Jeep’s wheels were trying to force its engine to spin at 54,264 RPM. That’s 8.22 times the engine’s 6,600 RPM redline. No wonder it blew up (If I had to guess, it happened well before 50,000 RPM).
Towing a vehicle is a tricky process that requires thoroughly reading the owner’s manual. There are lots of factors in play, including ensuring that certain components spin and others don’t, and making sure that what is spinning is getting proper lubrication.
Read the Jeep Wrangler JL owner’s manual, and you’ll see that it firmly denounces towing a four-wheel drive vehicle on a two-wheel dolly, and it spells out specific rules for flat-towing (see above).
You’ll see that vehicles equipped with a manual transmission must be towed with the transfer case in neutral and the transmission in gear. Why is this the case? I reached out to two driveline engineers to find out, and the good news is: They both came to the same conclusion. Engineer 1:
The only thing that comes to mind is that parasitic drag within the case might still allow some rotation to transfer to the output shaft and thus still spin the transmission over. By putting it in gear, you prevent that.
I have no idea if that’s really the case.
Engineer 2 talks about an automatic, which Jeep says should be put into “Park” during flat-towing. But the same reasoning applies to the manual transmission:
My understanding is that with the transfer case in neutral, the wheels will still drive the propshafts, but there will be no torque transfer to the transmission. Putting the transmission in Park (I am assuming you are talking about the Auto?) would stop the transmission internals rotating. From my experience on other transmissions, often neutral isn’t always a complete disconnection, as there is often drag from the internals or oil churn, which could transfer some torque into the transmission, and on an auto that would mean it would turn (albeit at a low torque) without lubrication with the engine off. Putting it in park would stop this from happening.
So the short answer is: Putting the transfer case into neutral technically disconnects the wheels from the transmission, but the reality is that bearing drag and oil churning can still create a torque on the transmission output shaft. You don’t want the transmission output shaft spinning on its own for too long of a duration, because the input shaft must spin in order to properly lubricate the gearbox.
By towing the Jeep in gear, you hook the input and output shaft together and couple them both to the engine — the inertia of these connected parts should easily be able to resist whatever torque is sent to the transmission output shaft as a result of transfer case mechanical drag. Ergo, by being in gear, you can prevent a scenario where only your output shaft spins without the lubrication created by a spinning input shaft.
Tremec breaks down the lubrication issue on its blog, where the transmission company actually recommends disconnecting the driveshaft leading to the manual transmission (note: this likely refers to two-wheel drive vehicles or ones without a neutral option in the transfer case):
If you will be flat towing a vehicle that you’ve installed an aftermarket transmission into such as the TREMEC TR-4050 5-speed or Magnum 6-speed, you will need to disconnect or remove the driveshaft. Even with the transmission in neutral, if the driveshaft is not disconnected or removed, the mainshaft in the transmission will turn with the rear wheels. However, the cluster will not turn and lubrication will not be fed to critical transmission parts.
This will lead to damaged needle bearings under the speed gears and/or the pocket bearing between the mainshaft and input shaft. This is why it’s crucial to remove the driveshaft if you’re having to flat tow your TREMEC-equipped vehicle. If you can’t remove the driveshaft before flat towing, don’t flat tow the vehicle.
Though you may initially have thought upon seeing the blown up engine: “How the hell did an owner keep the Jeep both in four-low and in gear?” it should be clear by now that making such a mistake really isn’t that difficult do. You’re supposed to tow the vehicle in gear. The only mistake the owner made, as far as I can tell, is keeping the vehicle in four-low. But just look at the shift knob above, and notice how the four-low shift lever position is just below the neutral position. It’s very possible that the driver simply pulled the lever too far, and didn’t realise that the vehicle wasn’t in neutral, but rather in low range.
If that’s what happened, it was tremendously costly, but seemingly easy mistake to make, and I feel for the owner.