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The Ultimate Bronco Drivetrain

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Craig Perronne
| Brand Manager, Dirt Sports & Off Road
Posted June 1, 2001

Getting the Best of Both Worlds

Step By Step

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  • This is where you need to start the whole process. The original drivetrain needs to be gutted, and so does the exhaust system. If you are scratching your head on how to get to this point then it is probably best to let a shop handle the install.

  • After getting the flywheel machined, a Centerforce clutch was bolted into position. It is specially made for this conversion and provides a nice pedal feel with a much easier pull than the original.

  • The NV4500 uses the original bellhousing through the use of an adapter. It simply bolts into position.

  • While the Wild Horses kit is designed with the driveway installer in mind, we decided to bask in the luxuries of having Off Road Unlimited’s shop at our disposal. Using the shop forklift and a tranny jack made raising the NV4500 into position (with the Atlas temporarily fitted to the back of it) easy.

  • The Wild Horses crossmember can then be bolted into position. It uses the existing holes, so no drilling is required. An adapter plate and transmission mount (also supplied by Wild Horses) then bolts the tranny in place.

  • With the crossmember and bellhousing bolts snugged up, it was time to permanently attach the Atlas. A nice feature of the Atlas is that it is clockable. We decided to index it as high up as possible to get maximum clearance.

  • A concern to us was how close the Atlas rests to the framerail. The Atlas is wider than the Dana 20 and our clearance was even a little tighter due to the fact that we had 1/4-inch-thick extended radius-arm brackets welded into place right where the T-case was. We talked to Wild Horses and other Bronco folks running Atlas T-cases and they reported no problems with this amount of clearance.

  • Whoever installed our original NP435 must have been a hack, as the Bronco’s floor was absolutely butchered. For vehicles with a 2-inch body lift, a small hole can be cut for the shifter and then the shifter boot supplied with the kit can be used. However, to cover up the carnage of our floor, ORU made this small plate.

  • When doing a swap, it is usually the small things that can confound you. What’s nice about the Wild Horses kit is that it supplies all these small things so you don’t have to make them or waste time trying to find them. One of these is the tunnel cover used to install an NV4500 on a Bronco with no body lift.

  • Another nice small-thing part is this shifter. It screws right onto the NV4500 and was in a perfect position for us.

  • Also included in the Wild Horses kit is this speedometer cable and adapter. It is correctly geared to take into account tire size and gear ratio so you get a true reading.

  • Perhaps the biggest pain is adjusting the shifters on the Atlas. They are very sensitive to adjustment but after a few tries we got the T-case shifting smoothly. Once adjusted, we ran the twin-stick shifter through the side of the tunnel.

  • Here’s the completed install. You can see how high the Atlas sits. The lowest point is now the crossmember and extended radius-arm brackets.

Running a vehicle on both the trail and the highway has its drawbacks. Usually it means running high (numerically lower) gears for the highway. That results in a vehicle geared way too high for trail work. Of course, the other alternative is to run low gears and let the engine scream at highway speeds. That means you run in the slow lane with the rest of the folks too scared to drive at speeds of more than 55 miles per hour.

One of the common solutions to this problem has been to swap in a transmission with an integral overdrive. For owners of early Broncos, choices in this area have been limited. Most of them involve swapping to a later-model Ford automatic four-speed. None of the Ford overdrives have reputations for being stout transmissions that can take a lot of abuse. Another bummer is that with these transmissions, no additional crawling ability is gained. That’s because of their lack of a low first-gear ratio.

So here we were with a guinea-pig ’69 Bronco that needed help in the transmission department. This Bronco was equipped with a swapped-in NP435 four-speed manual transmission and the factory Dana 20 transfer case. The NP435 is a good tranny but ours had seen better days, as a lot of double-clutching and grinding was necessary to get going. Plus its lack of an overdrive left our poor 302 V-8 screaming at high revs when running at freeway speeds. Additionally, shifting the old Dana 20 had become a chore. We told ourselves that its linkage just needed an adjustment, but in the back of our minds, we knew better. While rebuilding both units was an option, we wanted to try something different. Our quest was to find the ultimate in Bronco drivetrains, something that would allow us to cruise at above-legal speeds on the highway—not that we would, mind you—and crawl slower than a sleepy baby over nasty sections of trail. We also demanded strength from our drivetrain since we tend to be, uh, very hard on parts.

The answer to our drivetrain dilemma came in the form of the Wild Horses NV4500 conversion kit for ’66-’77 Broncos. This kit is designed to make upgrading to an NV4500 a bolt-in affair, with all the little details taken care of for you. What makes the NV4500 five-speed manual so desirable? First of all, it comes with a 0.73:1 overdrive that makes freeway cruising not a problem. The unique feature of the NV4500, though, is that it combines the fifth gear with a super-low 5.61:1 first gear, giving it the ability to also crawl. As if that isn’t enough, the NV4500 is a very stout transmission that we’ll never break. Finally, Wild Horses uses brand-new (not rebuilt) NV4500s in its kits. With this transmission our drivetrain dilemma was solved.

However, we wanted even more. To make our Bronco an awesome crawler, we decided to mate the NV4500 to an Atlas II transfer case from Advance Adapters. These transfer cases are extremely strong and offer a 3.03, 3.8, or 4.3:1 low-range. We opted for the 4.3:1 to get the lowest crawl ratio possible. With an Atlas II backing up our NV4500 we now had the ultimate drivetrain.

Not all bolt-in kits are always bolt-in, so swapping can be a nightmare. The Wild Horses kit is designed to adapt an NV4500 to either the stock Dana 20 or an Atlas II transfer case and we found it to be very complete. It is usually the smaller details that can cause problems when performing a swap. What we found to be nice about the Wild Horses kit is that all the necessary parts to take care of all those pesky little details are included in it. On top of that, the folks at Wild Horses were plenty helpful in answering our questions. So was the swap really worth it? Did we reach our goal of low freeway rpm and super-slow crawling? Yes to both, is the answer. Our rpm at 75 mph with the original drivetrain was 3,000 rpm. With the NV4500 in place and in fifth gear, rpm at 75 mph dropped to 2,200 rpm. This makes for a much happier 302 V-8, and means we might actually be able to make it to Moab and back without our engine coming apart. On top of that, the NV4500 is easier to shift than our NP435 was, even when it was in good shape.

Teaming the NV4500 to the Atlas II has turned our Bronco into an extremely capable crawler. Our crawl ratio has jumped from 67:1 to 99:1. This is just right for a vehicle with a manual transmission, and we no longer have to touch the clutch at all. We can now creep over rough terrain in a very slow and controlled manner. Previously, we’d smoke our clutch and spin our tires over these same sections. But no more. So, our verdict? It’s combining the ability to go both slow and fast with unmatched strength that makes the NV4500/Atlas II combo the ultimate Bronco drivetrain.


Flowmaster Inc.
Santa Rosa, CA 95401
High Angle Driveline
Paradise, CA 95969
Off Road Unlimited
Wild Horses
Advanced Adapters
Paso Robles, CA 93446
Centerforce Clutches & Flywheels
Prescott, AZ 86301
Enoch’s Muffler
Burbank, CA 91506