Anatomy of an Engine Swap

If you’ve been following ECOA social media (if you’re not you should be) you’ve no doubt seen me drop a few clues that the LJ is off the road and undergoing an engine swap.  While the details regarding the progress of the process are limited for now (long story) I thought I would at least take a moment to talk about why I’m doing the swap.

While built for back roads, and that’s where I want to spent my time,
she still logs her fair share of highway miles.
The time has come to do something about the stock motor and transmission

Motors come in all shapes and sizes. Some are better than others.  The 4.0L inline six cylinder in the LJ is a great motor, if not a bit antiquated in its design.  That said, I’m not really doing an engine swap.  I love the 4.0.  I think you can’t get any more “Jeep” than the venerable straight-six.  That said, mated to the engine in my LJ is the 42RLE four-speed automatic transmission.  It’s junk.  So I’m not doing an engine swap so much as I’m going a transmission swap.  Read on for details…

Achilles’ Heel

In greek myth, despite his physical prowess, Achilles had only one weakness; his heel. The reason for this weakness vary depending on what myth you read.  However the end result is always the same.  Achilles dies from an arrow shot through his heel.  It’s why the Achilles Tendon is named what it is.  The phrase “Achilles’ Heel” has also become synonymous with weakness and is a metaphor for that one fatal flaw in an otherwise great design.  The Achilles’ Heel of the Death Star was its womp-rat sized exhaust port.  The Achilles’ Heel of my ’04 Jeep Wrangler Unlimited is its transmission.
Truth be told the 42RLE has no business being in a Jeep.  As typical of Chrysler transmissions it’s grossly under-designed for a four-wheel-drive vehicle.  The thin torque converted, poorly programed valve body shift solenoid pack, and weak gear clutches leave a lot to be desired.  Coupled with a shallow overdrive ratio of 0.69:1 the transmission is plagued with piss-poor shifting and prone to overheating.  And that’s in a stock Jeep.
Once you add bigger tires to a vehicle you’re next modification better be gears.  Deeper gears will help compensate for the size and weight difference of the larger more aggressive tires preferred by off-road enthusiasts.  It’s usually a safe rule of thumb that for every tire size you go up you should go down a gear ratio.  So with my ’04 LJ being equipped from the factory with 30″ tall tires and 3.73 gears conventional wisdom would be running 4.56 gears with 33″ tall tires.  However, given the deep overdrive and the weakness of the transmission as a hole many of my fellow late-model Wrangler owners have found they’re better off going one ration deeper than normal to compensate for the lack-luster 42RLE.  This is also beneficial for towing and maintaining the use of overdrive.  Those that drive manual transmissions are lucky they don’t have these kinds of issues.  With the factory 6-speed manual conventional wisdom works.
My experiences with the LJ are overall good.  I have no regrets about buying it or modifying it for overland travel.  For the most part it does the job it’s intended to do.  However on multiple occasions my patience has been put to the test dealing with that f***ing piece of s*** transmission.  I’ve done my best with gearing going with 4.88’s as well as adding both an auxiliary transmission cooler and a transmission temperature gauge.  Those modifications have helped the LJ on some great adventures including multiple cross-country drives.  That said, its time has come.


As I said, I’m not going an engine swap so much as I am doing a transmission swap.  I considered multiple options when it came to selecting a new transmission. The easiest solution would be switching to an OEM manual transmission.  Being a 6-speed it would a great option were it not for me having blown my knees out training for the Army in college.  While I can drive stick for short periods of time (god how I miss banging gears in my supercharged Thunderbird) I just can’t manage it on long drives, especially in traffic, and doubly so off-road.  Another option would be the unicorn of Jeep four-speed automatic transmissions which would be an OBDII version of the AW4.  Hands down one of the best transmissions to ever be installed in a Jeep, the AW4 would be great in an LJ… if I could find one.  Beyond that, there really aren’t any good OEM transmissions for me to swap into the LJ.
The 42RLE just can’t handle any more.
As far as non-OEM brand transmissions the only ones that are common, cheap, and easy to swap — as well as have oodles of aftermarket support — are GM transmissions.  The top of the list for this is the 700R4.  It’s a four-speed automatic that was designed for four-wheel drive trucks and SUV’s.  Many people have found homes for them in their Jeeps over the years.  Companies like Advance Adapters and Novak Conversions make the necessary parts to make installation a breeze.  However, the 700R4 is an older transmission that has been succeeded by newer versions like the 4L60E.
Now, I could just swap in a 700R4 or 4L60E and call it done.  However that still leaves me with a stock OEM motor.  A tired one at that.  Recently the LJ rolled over 200,000 miles; the last 60k of which were all by me and I know they weren’t easy miles.  I always try to do my best to maintain my vehicles as best I can.  However things still happen.  The motor was overheated a few times (first when the radiator went out, another time when the fan clutch went out, and again when the replacement radiator developed a leak).  The motor is also prone to some quirky problems relating to the modern emissions systems (such as THREE catalytic converters, FOUR oxygen sensors, and one dumb EVAP system) and the late-model 4.0 coil-pack ignition system.  Chrysler did their best to modernize the 4.0 and help it meet rising EPA standards but in the process they compromised what made the 4.0 so great – its simplicity and its reliability.

More Power

To complicate matters I honestly need a little bit more power out of the LJ.  At 190 HP the 4.0 inline size leaves a little bit to be desired on the highway.  Yes, I know I’m an overland enthusiast.  And yes, I know I write a book series about avoiding highways.  However sometimes they are a necessary evil.  As such, when towing my camping trailer at highway speed I honestly need more power.  She’ll do 65-70 mph but that’s at 95% throttle (or as Deadpool would say, “Maximum Effort”) and a whopping 8-9 mpg.  Yeah, not great.  I have no additional power when it comes to hills, headwinds, or passing traffic.  Not good.
Going for Maximum Effort all day every day.
Now an easy option would be to throw on a turbo or a supercharger, right?  Well, remember when I said it’s a tired old 4.0 with over 200k miles and prone to quirky emissions and ignition issues… yeah, not a real viable option.  I could rebuild the motor, but that is a considerable expense.  Same for the transmission.  I could just make an easy call to rebuild the 4.0 and 42RLE and call it done.  But then I’m still stuck with a 190 HP motor and a crappy ass-tastic transmission with a ridiculous overdrive ratio.  Plus, if I’m going to go through all the trouble to swap the motor and transmission, which at this point needs to happen one way or another, I might as well upgrade in the process.

More than a Transmission Swap

With the realization I need to swap the engine and transmission one way or another I opened myself up to other options beyond just a rebuilt OEM powertrain.  I figured by going with a totally upgraded powertrain, and specifically a V8 one, I would not only have more power on the highway but I’d also have a more appropriate transmission for what I’m going.  That only left one question, which V8?
Ford, GM, and Chrysler all make great V8’s. In their respective modern arsenals are the 5.0 Coyote, the proven LS, and last but not least the Hemi.  Each have their champions and their haters, but none of them are bad motors.  What it boils down to for me is cost, availability, and ease of install.  The Coyote was sadly the first one to drop off the list for me.  While I got my automotive start with Fords (first a 3.8 n/a Thunderbird then a supercharged one, and a 4.0 Ranger) it’s just too pricey a swap.  This is in part due to it’s limited availability and it’s not an overly common swap yet.  The most logical option is of course a Hemi swap.  It’s OEM brand and a very common motor.  Sadly Chrysler locks down their PCM’s pretty tight and their vehicles are wired via a “Totally Integrated Power Module System” which is an absolute pain in the d*** to work with.  While professional shops like AEV make Hemi swaps look easy, they just aren’t that easy for the solo shade-tree mechanic.  Enter the LS.
Yeah, yeah, yeah… I know.  Everyone else is doing it.
LS swaps are common, popular, and cheap.  Those things reinforce each other.  They’re cheap because the LS motor is in pretty much everything GM makes.  It’s in their trucks, SUVs, and even their vans.  You can’t sneeze in a parking lot without hitting a vehicle with an LS in it.  They’re also an easy swap because this commonality makes them so popular thus creating an entire aftermarket support network to facility LS swaps.  This popularity also makes is cheaper since most of the hard work has already been done.  This is also made easier on GM’s part because their engine and transmission systems are, for the most part, stand-alone and fully unlocked (Chrysler, take notes).

By the Numbers

Ultimately it came down to the numbers for me.  The LS makes 130 HP more than 4.0 and weights 60 pounds less.  Talk about a killer power-to-weight ratio.  Aside from that power available where I’ll need it most on the highway, I’ve been told I can expect to double my fuel mileage.  So yeah.  130 more horses under the hood and double the fuel mileage in something that weighs less than my current motor?  Yeah, I’d be an idiot not to consider this swap.  But again, it’s not about the engine (so I keep telling myself) it’s about the transmission.
The ratios for the 42RLE are 2.84, 1.57, 1.00, and 0.69.  In contrast, the 4L60E ratios spread out to 3.06, 1.62, 1.0, and 0.70.  While overdrive is still a little on the shallow side for my liking, there’s a big difference between a transmission like the 4L60E coming out a truck and the 42RLE coming out of a car, not to mention a motor that can turn it.  It will handle towing, four-wheel-drive, and long highway drives a lot better due to it’s overall robustness and better construction.
I’ve run the numbers, and they come out in my favor.
Ultimately it came to one final number for me.  As I mentioned I was told by multiple people that I can expect to at least double my fuel mileage.  Honestly that’s not hard at a whopping 8-9 mpg towing and 11-13 mpg around town.  I keep pretty good track of the fuel mileage of the LJ and lets just call it an even 10 mpg average.  Given that I’ve driven the LJ 60,000 miles over the course of just over three years it’s easy to say I’ll probably maintain that average of 20,000 miles a year (and yes, I do enjoy driving).
If I do the math, 20,000 miles at an average of 10mpg works out to be 2,000 gallons of fuel a year.  At a PA state average of $2.50 right now that’s about $5,000 for fuel for the year.  If I double my fuel mileage that will cut my fuel budget in half.  That means over the course of a year I’d say $2,500.  That’s not something to scoff at.  Especially considering I only paid $800 for my LS donor motor.  Yes there are other associated expenses (more on that in a future article) but over the next three years (even if I don’t full double the fuel milage) I can still expect the LS swap to more than pay for itself financially.  Not to mention the drivability factor on the highway and the reliability factor of not having a crappy transmission holding me back.
The other added benefit of doubling my fuel mileage will be doubling my fuel range.  Right now at an average of 10mpg, with my 31.5 Gallon GenRight Safari Tank, I give myself a working range of 300 miles.  Double the fuel mileage means double the range.  Even if it’s not exactly double I can still expect, at minimum, 15-16 mpg which gives me an effective range near 500 miles.


I have to replace the engine and transmission in the LJ. That is an unavoidable fact.  At over 200k miles the time has come.  While there are many options for me to consider, the LS swap was the easy clear choice.  130 additional horsepower, 60 less pounds, mated to a much better transmissions, with an expectation of double the fuel milage, at a base investment of $800 all adds up to a very simple statement: It’s LS Swap Time!
Once the swap is complete I’ll have more information pertaining to the swap itself.  While I have no intention of writing a step-by-step guide to swapping an LS motor into a Jeep (or any other vehicle) I will at the very least outline the parts I used to make the swap possible, as well as rough estimates for the time and money I have invested in the swap.  I will also make sure to mention the unexpected things I encounter along the way.  Until then I hope you’ve enjoyed this look into my decision making process.