GEARED TO CRAWL
Jon's Bronco has enough gearing to make a farm tractor jealous. Behind the Cummins engine Jon swapped in a ZF five-speed manual transmission that's mated to an aftermarket Stak transfer case made specifically for early Broncos. The ZF transmission has a First gear ratio of 5.72:1 and an Overdrive ratio of 0.67:1. When coupled to the three-speed Stak transfer case with a 4.33:1 low range, 1:1 direct drive, and a 0.79:1 overdrive, Jon has almost infinite gearing options. Why so many gears to choose from? Simple, Jon's Bronco also runs a set of Mercedes Unimog 404 axles that have an effective drive ratio of 7.56:1. With axle gears that low, John needed double overdrive just to get the Bronco up to highway speed. Plus, the low gearing (down to 187:1) gives Jon amazing control of the Bronco as he crawls over radical off-road obstacles.
BEHIND THE WHEEL
Driving a diesel-powered Bronco off-road with 39.5-inch tires is kind of like having your own personal Bigfoot. With the low gearing, front and rear locking differentials, and enormous ground clearance, the little Bronco will go anywhere the driver is brave enough to take it. For our test drive, the tires were aired-down to 20 psi and we crept our way up and over rock ledges and straddled boulders that only the Unimog axle's portal hub design would allow. With the doors off, the driver's visibility was impressive. The 4BT engine churned away unfazed by the mass that it was moving. Getting on the throttle brought a quick response that you just don't get out of an electronically controlled diesel. With the air filter poking prominently up through the hood, you can really hear the turbo drawing in the fresh air. What a ride!
There's an '84 3.9L Cummins 4BTA with a modified Holset H1C/E hybrid turbocharger sitting under the hood. The 4BT runs a 3,200-rpm governor spring and re-clocked fuel pin in the VE injection pump to feed the POD Stage II injectors. The Holset turbo now has a 12cm turbine housing, and the compressor side pumps air through an intercooler from a '99 Freightliner MT35 truck. | 
The portal axles came from a Mercedes 404 Unimog. These axles are desirable because the extra reduction at each wheel means the centerline of the axle is above the centerline of the tire. This provides better ground clearance than a conventional 1-ton axle. | 
Jon told us that people have ruined a lot of gauges trying to duplicate his Bronco's speedometer. In order to blend the classic Bronco looks with modern accuracy, a 160 mph Auto Meter Ultra-Lite gauge was cut up and grafted into a factory Bronco gauge cluster. |