Most dedicated mud rigs have exhaust systems that exit straight up with tractor-style caps to prevent mud from entering the engine. Its a waste of time routing pipes underneath the vehicle because chances are if they dont get ripped off in deep muck theyll become plugged, causing the engine to stall. The most common practice weve seen is to run zoomies or marine-type pipes that exit right through the hood.
Basically, run whatever your engine likes, but just make sure to keep it dry. Often, this means mounting the ignition inside the cab with you. Some hard-core mudders run dual ignition systems mounted inside the cab. That way if one gives out its merely a matter of reaching over and switching to the backup.
Real mud rigs run healthy engines with big, aggressive cams. However, one byproduct of a big cam is that theres no vacuum to run power brakes. Most people convert the master cylinder to manual brakes or convert to a hydroboost system and run four-wheel discs, since drum brakes quickly become clogged to the point where they begin to lose effectiveness. In addition to being lighter than drums, discs are somewhat self-cleaning.
Axles Mud running means big tires and big horsepower, so youll want axles that can stand the abuse. Most run 1-ton-and-up axles like Dana 60s, 70s, and 80s, GM 14-bolts, or even 2½- or 5-ton Rockwells. As for width, some prefer to run equal width front and rear axles so the rear tires can run in the trough made by the front tires, while some think a narrower rear axle width will make the truck track better in the mud.
Transfer Case Very few mudologists want really low gearing in the transfer case. Most commonly seen are iron geardrive transfer cases with a low range of around 2:1. These include the NP203, NP205, and various Rockwell transfer cases. Also somewhat popular are homemade chaindrive systems, such as the one shown. The double-row chain isnt likely to break, and if it does its easy enough to rebuild, even if the undercarriage is slathered in goo.
Overall Gearing Tire speed is critical. If your tires stop turning, youre going to bog down and get stuck. Depending on your engine output and tire size, gearing is usually much higher than that of rockcrawlers. If youve got the power to keep the tires spinning, you can get away with a numerically lower gear ratio, which will mean more tire speed. However, if youre running a weak V-8 or a V-6, youll need a numerically higher overall gear ratio, which will help the engine keep the tires spinning, though at a slower speed.
Like in drag racing, you want to run as much horsepower as you can. Its not uncommon to see a rig that looks like someone threw up on it with a 600-inch big-block under the hood. Some of the coolest mud engines weve seen displace somewhere in the neighborhood of 550 cubes and run 6-71 blowers with either dual carbs or fuel injection. As always, nitrous oxide is a welcome addition to any party.
Basically, you just want to make sure youve got something to sit on, a good safety belt, a rollbar, and a steering wheel. Some remove the windshield and replace it with steel mesh because once the glass gets coated theres nothing your wipers can do to help you see. Also, if youre among those with grande cajones who remove the doors, make sure your gauges are mounted where they wont get a brown coating.
To prevent the radiator from quickly getting clogged with mud and overheating, almost all mud rigs run one or two radiators in the middle-to-rear of the vehicle. Make sure to run electric fans that pull enough cfm to keep things cool, and make sure to route the coolant lines carefully to keep them from getting gouged or ripped off.
Pro: Wont overheat; brute strength
Con: Clutch can get packed with mud; loss of momentum when shifting
Pro: Quick shifts keep momentum; dirt-simple to drive
Con: Can overheat keep momentum and die; Coolant lines to drive can be vulnerable