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October 2011 Willie’s Workbench

Posted in Features on October 1, 2011
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This topic has been covered many a time by both Four Wheeler and the other 4x4 magazines. Bear with me through this Workbench; I am sure most of my readers have their own opinion on the subject, but it’s always a good discussion-starter when sitting around the campfire after a long day on the trail. I picked it as a subject this month mainly because, by chance, two people recently asked me the question.

Okay, just which one is the better choice? There is no such thing as a correct answer. I have always been a bit biased—or so it seems, because of the 20 or so off-road vehicles that I’ve owned, only four have had automatics. However, three of my four present rides have automatic transmissions. Even my flatfender has an auto-shifter. When I originally started building it, I used an early Chevy 420 Muncie four-speed with its 7:1 First gear. Problems with designing a new clutch linkage made me switch to a TH700R4. Yes, a Turbo 350 would have been a lot easier to install due to its smaller size, and a 400 would have been stronger. Mine has been built to handle 650 lb-ft of torque. I picked the 700R4 because of its great 3.06:1 First gear versus the 350’s 2.48:1 or the 400’s 2.52:1 ratio. Now that doesn’t sound like an enormous difference until one remembers that a torque converter, when starting out and slipping, generally multiplies the ratio by about 2:1 or better, and a big plus of the 700R4 is the Overdrive Fourth gear of 0.70:1. This makes those 5.38:1 axles an Interstate highway-friendly 3.76:1 ratio.

Let’s go over some of the facts before making up our minds in favor of one over the other:

Driver preference, availability, transmission gear ratios, overall gearing, and how the vehicle is to be used as well as a possible engine swap are major considerations.

Driver preference is a consideration if a person has learned to drive with an automatic transmission and has limited experience with a manual shift. Sometimes, it’s not the lack of ability but true preference to driving with an automatic.

When it comes to making an engine or transmission swap, there are several influencing factors. The main factor is whether the transmission is up to the job strength. The second consideration is the match-up. Obviously, it’s much simpler if, for instance, you match a Chevy engine to a Chevy transmission. However, with the wide variety of adapters available from companies such as Novak, Advance Adapters, and Auto Fab, the mix-and-match combination is quite varied. Do keep in mind that automatics are longer, wider, heavier, and bulkier than manuals, and the added length may cause rear driveshaft angularity problems. They also require a fluid cooler of some type, whether it’s in the bottom of the radiator, or a separate cooler. Manual transmissions, while being considerably smaller, require the intricacies of clutch linkage, which can be difficult to properly engineer at times.

Passenger-car manual transmissions generally make poor off-road gearboxes. Some are super-strong but lack proper gearing; in other words, a minimum 4.00:1 First gear is generally needed. (Yes we “jeeped” for a lot of years with the T-90 transmission’s 2.79:1 First gear, but also had those 5.38:1 gears in the differentials to make up for it.)

Low axle ratios are great, and this is where an Overdrive top-gear transmission really comes in handy. With, say, a 25-percent gear reduction in Overdrive, engine speed can drop down into the 1,800- to 2,000 rpm range for easy freeway cruising. A 3:1 or 4:1 drop gear in the transfer case can easily bring the overall gearing into the 80:1 to 100:1 range. No, not everybody needs this type of gearing. It’s just the ticket if you’re a Rubicon rockcrawler or need to pull heavy loads. If you play mainly in the sand, or on dirt and gravel roads, the higher-ratio, quick-shifting car trans would be the way to go.

This brings us to vehicle use, the biggest question of all. A manual transmission offers complete control of gear choice, whether shifting up or down. Most automatics, unless modified, don’t allow this. They will only shift down to a lower gear at a preset engine speed. Some of the automatics “hunt”—that is, shift back and forth between First and Second during low-speed off-road travel with no driver control. Going down a very steep, loose, rocky hill with an automatic transmission can be quite a thrill (backing down is even worse!). The transmission offers very little engine compression braking, so you have to rely on brakes a lot more to slow the descent. Steering control can become quite erratic when one of the wheels locks up and slides. With a manual that has low enough gears, steep descents are not a problem. Stay off the brakes and just point and shoot.

Most off-road race trucks use automatic transmissions, mainly because the shifting and selecting of the proper gear is one less thing to do and more concentration can be given to the terrain. Automatics make rockcrawling a cinch—no shaky clutch foot here. But properly geared manual boxes can be nearly as good, and in some instances even better. Do you drive in heavy-rush hour traffic with the 4x4 serving double duty as a commuting vehicle? Then it’s hands-up for the automatic. Pulling a trailer or heavy load? An automatic is easier for an inexperienced driver.

Which will live longer, the automatic or the manual? A lot depends on use, how well the vehicle is maintained, and of course, the driver. Actually, the manual transmission is a hands-down winner, and a rebuild usually consists of nothing more than bearing replacement if it’s never been abused. However, this again depends on the transmission itself, what it was designed for, and how it’s used. This also applies to the clutch, with it being the first thing needing to be replaced.

Nope, there is no easy answer. Still, maybe because I’m lazy, maybe I’m getting old, or maybe overall the automatic is easier to drive, I am leaning toward the auto-shifter. It’s your choice.

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