Written by Mike McGlothlin
The choice between manual and automatic transmissions is an age-old stand-off. With a manual transmission, its operation has to be carried out by the driver making the shifts manually. With an automatic, the shifts (both up and down) occur automatically, requiring virtually zero input from the driver. On pavement, the manual vs. automatic transmission debate has largely been settled by the free market’s preference for automatic gear changes over manual ones (i.e. car sales). However, in the world of off-roading you’ll find something much closer to a 50/50 split between manual and automatic transmissions.
Without question, choosing a manual or automatic transmission in for an off-road vehicle boils down to the driver’s personal preference. Those comfortable using a clutch and preferring to select their own gears opt for the manual. Others, who would rather the transmission do the work for them, simply go with the automatic. But it goes much deeper than that in an off-road application. Depending on the type of terrain you’re covering, the driving conditions you’ll be facing, or your specific needs, both the manual and the automatic transmission have their pros and cons—and we’re covering all of them here.
Proven, Trail-Ready Manual Transmissions
-NV4500 (five-speed)
-NP435 (four-speed)
-NV5600 (six-speed)
-ZF S6-650 or ZF S6-750 (ZF-6)(six-speed)
-Muncie SM465 (four-speed)
-BorgWarner T-18 (four-speed)
-Muncie SM420 (four-speed)
-Aisin AX15 (five-speed)
Rugged, Off-Road-Ready Automatic Transmissions
-GM TH400 (three-speed)
-GM TH350 (three-speed)
-Ford C6 (three-speed)
-GM 4L80E (four-speed)
-Chrysler TorqueFlite 727 (three-speed)
-Aisin-Warner AW4 (four-speed)
-ZF 8HP (eight-speed)
Photos:
ZF 2004 S6-650 (ML6) Six Speed RWD Manual Transmission for GMC 2500 HD Sierra, GMC 3500 Sierra, Chevrolet Silverado.
Anatomy Of A Manual Transmission
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While both a manual and an automatic transmission serve the same basic purpose within a vehicle (and although some internal parts are similar), the mechanical makeup is vastly different between the two. Below, you’ll find the key components found in a standard shift transmission.
*Clutch
*Pressure plate
*Flywheel (single or dual mass)
*Input shaft
*Counter shaft
*Output shaft
*Gear box with individual gears (1st, 2nd, 3rd gear, Reverse gear, etc.)
*Shift fork
*Synchronizer(s)
*Gear shifter (located on top of the transmission)
Anatomy Of An Automatic Transmission
Although automatic and manual transmissions both utilize an input shaft and output shaft, the ingredients contained within the transmission case are very different. For example, in place of a clutch and flywheel used in a manual, you’ll find a torque converter and flex plate employed in an automatic. An automatic’s key components are as follows:
*Torque converter
*Flex plate
*Transmission pump
*Input shaft
*Intermediate shaft
*Output shaft
*Clutch packs
*Planetary gearsets
*Valve body
General Advantages Of A Manual Transmission
Off-road or on pavement, both manual and automatic transmissions have their benefits, but we’ll start with the biggest pros of a manual transmission. Starting with the fact that they produce significantly less heat, it stands to reason that a manual transmission, by and large, enjoys superior long-term durability over an automatic. Other advantages of the manual gearbox include a comparatively lighter overall curb weight, better fuel economy, and being cheaper to rebuild. A manual is also typically the better option in mud (no torque converter delay or loss of wheel speed due to an upshift), especially when wheel speed is needed to clean out your tires’ tread blocks.
General Advantages Of An Automatic Transmission
First and foremost, with an automatic you don’t have to worry about stalling the engine with the
transmission in gear. Secondly, an automatic transmission is a fully sealed unit, including the torque converter. This means it’s safer to submerge them in water during river and creek crossings, or deep mud. By contrast, the clutch in a manual transmission doesn’t enjoy the same protection and can begin to slip when exposed to moisture. And finally, automatic transmissions generally perform better in soft sand thanks to the smooth, quick gear transitions they provide.
Rock Crawling
Advantage: Automatic
As previously stated, the type of terrain you encounter in your off-road endeavors can dictate the type of transmission you need, or prefer. When it comes to traversing large obstacles at low speed such as rocks, boulders, and other sizeable objects, the automatic transmission shines. A key benefit to the automatic’s success here is that, unlike with a manual transmission, very little torque is applied right off idle. This “delay” or absence of all-or-nothing torque actually aids your ability to crawl over large hurdles. Additionally, it helps an automatic conquer steep uphill ascents more effectively.
Disadvantage: Manual
To be sure, rock climbing and low-speed maneuvering can be done with the use of a manual transmission. However, a clutch doesn’t have the inherent slip a torque multiplying fluid coupler (i.e. torque converter) does at off-idle engine speeds. This means herky-jerky action can often be the result when navigating big rocks, boulders, and other large obstacles you encounter along the trail. While experienced manual owners can get through rocky sections relatively smoothly, significantly more driver input is required, and more wear and tear if often placed on the clutch.
Steep Uphill Grades
Advantage: Automatic
The automatic transmission scores another win in this category, and here’s why. For starters, steep angle ascents are safer with an automatic. This is because there is no worrying about running out of gear or having to physically shift gears on the fly. Performing a gear change during a hill climb with a manual transmission can be dangerous. However, the automatic doesn’t completely dominate this segment, primarily due to it being more difficult to limit wheel spin with an automatic as opposed to a manual (picture climbing a snow-covered hill).
Disadvantage: Manual
In a hill climb situation, you are limited to one gear with a manual transmission—so you better pick the right one. Go too low in your selection and being stuck in a single gear can limit you to a crawl. Select a gear ratio that’s too high and you could risk stalling the engine. It’s also important to note that, winching up a hill or steep incline will call for working the clutch and winch simultaneously. As you can imagine, things can get tricky (and busy) very fast in that sort of situation.
Sharp Downhill Grades
Advantage: Manual
On the opposite side of the hill climb comes the descent, and when it comes to descending hills manual transmissions rule the roost. Thanks to the ability to lock a manual gear box in first gear (often combined with low range in the vehicle’s transfer case), you can descend even the steepest of hills as slowly as you wish. Plus, most manual transmissions feature a substantially lower first gear ratio than their automatic counterparts, which means the engine braking effect will be that much stronger. Combine a low first gear and the low side of a transfer case with a diesel engine, and you get the best of all worlds.
Disadvantage: Automatic
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Excluding modern day downhill assist features (such as on late-model, auto-equipped JL Jeeps, where this function works extremely well), hard angle declines aren’t the automatic transmission’s forte. With this being the case, hill descents will call for notably more braking input than is required with a manual transmission. Engine braking isn’t a strong suit associated with the automatic either, namely because they aren’t geared as low as a manual but also because some automatics will upshift with excessive engine speed—even with a low gear having been manually selected. And, with excessive service brake input you can never rule out the potential for the vehicle to experience brake fade, which can be highly dangerous.
Manuals—The Doomsday Transmission Of Choice
When it comes to ensuring you make it safely home or back to camp, the manual transmission is extremely hard to beat. A true end-of-days transmission option, a vehicle equipped with a manual transmission can be tow-started or push-started in the case of a dead battery (whereas an automatic-equipped rig will require a tow). On top of that, even a damaged manual transmission will afford you the ability to limp the vehicle home (picture losing a gear…). You cannot do that with an automatic, as catastrophic internal failures typically contaminate and disable the entire unit. And, as previously mentioned, the lack of heat a manual transmission experiences lends itself to long-lasting durability.
The Automatic’s Biggest Disadvantage
The number one killer of automatic transmissions is heat. Even with the transfer case locked in low range (to provide gear reduction), transmission fluid temperature can easily crest 220 degrees F in deep mud, sand, or due to low vehicle speeds (i.e. not enough airflow moving across the transmission cooler). Every automatic transmission can handle a flash temperature scenario (a brief instance of extreme heat), but once ATF temps get too high too many times, the fluid will oxidize and begin to breakdown, eventually leading to outright transmission failure. Worse yet, in direct comparison to manual transmissions, automatics are considerably more expensive to overhaul.
