The Ultimate Bronco Drivetrain

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.