Various Performance Modifications - Add Yours

[89 MJ]

Let's talk about some of the various performance modifications people have done to their Eagles. It doesn't have to be just engine upgrades either, handling upgrades are perfect for this thread too.

Motorcraft 2150 carb - this seemed to add quite a bit of power to my car, but I did it at the same time as an HEI ignition, so I can't say which made more of a difference. I did start the car and let it run some after the carb swap and I can say that it seemed to run smoother, but I cannot speak for any power gains. Some of that difference could be the difference between a brand new carb and a 30+ year old carb that sat for 20 years and had never been rebuilt. I think this is worth doing if emissions don't matter.

HEI ignition - mine is a used unit from DUI. It came with the car, but uninstalled along with a set of Live Wires plug wires. My other experiences with HEI leads me to believe that I did gain some power with this modification, but I did do it at the same time as the carburetor, so I can't speak for sure. It took my car from not being able to spin the tires to being able to do an okay burnout. Anything I own in the future with a 258 will be getting an HEI.

Axles/Gearing - I went from the stock AMC 15 with 2.72 gears to a Chrylser 8.25 out of an XJ with 3.55 gears. It made the car way more drivable in town or on the backroads. I really didn't lose any drivability on the freeways either. The car will still run 80 mph, but its turning a lot more rpm at that speed. This is definitely the best way to get better acceleration on a budget, in my opinion.

Transfer case swap - my car came with an NP128. I swapped to an NP231J for the low range because my car does see some off road use. When I did this swap, the transfer case and the rear driveshaft length were the only two things to change. The car seriously felt like it picked up 30 horsepower after that swap. With the NP128, you could floor it from a stop and it would leisurely accelerate. With the NP231J, it will spin the tires for about 10 feet if its floored from a stop. And if you try to power brake the car to do a burnout, it will do it with ease now, whereas before the car had to work do get a tire spinning. I did it for the low range, but it definitely made the car feel way sportier too.
Does the NP129 seem to take a lot of power too?

Add-a-leaf - my factory rear springs were sagging, so I added a leaf out of another random leaf pack laying around at my house. It did make the car quite a bit stiffer in the rear, but this helped a lot with the handling too. I used some pretty heavy duty springs, so if comfort is a priority, a thinner spring would be a good choice.

Does anyone have anything to add to the list?

[CaliEagle97]

What shifter did you use for the NP231 swap? Do you have any photos of it?

Lift - I have a mild lift on my wagon using Javelin springs up front with rubber spacers, and an add-a-leaf in the rear as well. Eventually I would like to get my 8.25" Chrysler axle in the rear, but without a garage & the time to do the work, it remains a future goal.

HEI - I also have an HEI distributor swap, done before I bought the vehicle.

Fuel Tank - Back when I was lifting the eagle, I also opted for a larger fuel tank. Picked up the AMC1 from Spectra, a 22 gallon tank to replace the 15 gallon tank it came with.

Cooling - Last year I discovered coolant boiling over inside the engine. I replaced the thermostat, assuming it went out, but to no avail. This put me on for swapping the whole radiator, thermostat & housing, using a 3 core aluminum rad. Currently working on getting this done.

NP231 - Up next is the process of my NP231 swap. I got the T/C out of a '98 TJ. Installed an SYE kit, just for the giggles, while rebuilding the transfer case, and I am using the YJ shifter bell-crank, along with Advance Adapters' shifter kit (715545, 715545H & 715545C). Got it all prepped & ready, just saving up for the driveshafts now.

[89 MJ]

What shifter did you use for the NP231 swap? Do you have any photos of it?

Lift - I have a mild lift on my wagon using Javelin springs up front with rubber spacers, and an add-a-leaf in the rear as well. Eventually I would like to get my 8.25" Chrysler axle in the rear, but without a garage & the time to do the work, it remains a future goal.

HEI - I also have an HEI distributor swap, done before I bought the vehicle.

Fuel Tank - Back when I was lifting the eagle, I also opted for a larger fuel tank. Picked up the AMC1 from Spectra, a 22 gallon tank to replace the 15 gallon tank it came with.

Cooling - Last year I discovered coolant boiling over inside the engine. I replaced the thermostat, assuming it went out, but to no avail. This put me on for swapping the whole radiator, thermostat & housing, using a 3 core aluminum rad. Currently working on getting this done.

NP231 - Up next is the process of my NP231 swap. I got the T/C out of a '98 TJ. Installed an SYE kit, just for the giggles, while rebuilding the transfer case, and I am using the YJ shifter bell-crank, along with Advance Adapters' shifter kit (715545, 715545H & 715545C). Got it all prepped & ready, just saving up for the driveshafts now.

I used the JB Custom Fab cable shifter for mine. It did require a fair bit of floor trimming to use the shifter, but I like it a lot. I can't get photos to be added to my replies here, so I'll link annother build thread that I've got and you can see it there. I'll also add the link to the website I got it from. I went with the flush mount with vinyl boot and the 48" cable length. The shifter is way smoother than any of the factory YJ or XJ/MJ stuff I've used. You should be able to reuse the front driveshaft too.
https://jb-custom-fabrication.myshopify.com/products/cable-shifter-single-np231-billet-style-p-n-np231j-1


Roughly 1/3rd of the way down the page on page 9.
https://comancheclub.com/topic/66355-1986-amc-eagle-plus-eagle-axle-swap-and-other-tech-info/page/9/#comments

[CaliEagle97]

One more thing I am concerned of when I get the NP231 installed;

How tight of a fit is the speedometer cable? I noticed the angle of the speedometer housing on the tail-housing of the transfer cases differ quite a bit, but looking at the skid plate on my NP129, i'm not sure if the cable will have enough room to connect without a 90 degree adapter.

[89 MJ]

One more thing I am concerned of when I get the NP231 installed;

How tight of a fit is the speedometer cable? I noticed the angle of the speedometer housing on the tail-housing of the transfer cases differ quite a bit, but looking at the skid plate on my NP129, i'm not sure if the cable will have enough room to connect without a 90 degree adapter.

From the research that I did, you will need a 90 degree adapter because the speedo cable will hit the floor. I ended up going with a slip yoke eliminator because they are a little bit stronger and because then the cable will not interfere with the floor. You then can use an 84 Jeep Cherokee XJ speedo cable. You will need a different driveshaft (roughly 4" longer than stock), but then the driveshaft will have the slip joint, which I prefer.