I don't have anywhere near the experience that other folks have that have posted, but I have noticed some things on my outings.
First, even ten feet of bad surface will totally reclassify an otherwise easy 10 mile trail. I know that I do not want to be going down any road at a pleasant minor speed with the engine at idle. I want the engine above idle by some amount no matter what because there will be (and have been) those short, ten foot stretches that need some consideration, slower speeds, and instant torque on occasion. This is not the place to be screwing with a clutch to inch it through, or doing the "one foot on the throttle, and one on the brake" act. The slowest speed at idle in your lowest gear should always be slower than anything you'll ever actually need so that you are manipulating the throttle only. Much nicer way to go.
Second, I know modern engines are doing some nice things, but 1600 rpm is not where most gas engines can cruise. Diesels? Yes. Gas? Not so much. Even the modern diesels have moved the peak torque band over 2000 rpm. My old 7.3 Power Stroke did 1600 rpm just fine. Not so with the subsequent 6.0 in my F-550. That engine wants to be 2200-2400 rpm. Point out any factory automotive gas engine that is designed to cruise at 1600 rpm and I'll retract my statement.
Next: too many gears! Unless I've mis-read the previous posts, the engine will be a power beast with a fairly broad power band. It's not some monster class 8 OTR Diesel with a power band 200 rpm wide. Those engines require LOTS of gears. Your proposed engine could very easily handle gears that are at least 20% apart. And, unlike a class 8 OTR truck, since you won't need all those gears, you also won't use all those gears. There were multiple sticks in an old Unimog I had. What a PITA! (A side note: the two first gears were so low that top speed was measured in meters per hour, not kilometers per hour! Anybody need to pull a set of plows?)
What I got from that experience was the TC, for example, should have gearing that can be thought of as "modes" of driving. Low = off road, High = highway, or along those lines anyway. Any more than one stick to manage just to do basic driving is an unnecessary PITA.
And, I have to ask, just how fast do you plan to be driving this rig on the highway? Now, obviously, I know you're not going to try to keep up with Bugattis. (The flying brick nature of a FSJ just doesn't lend itself well to such activities.
)
It is my opinion that maximum torque multiplication should be done in the rear axle, if at all possible. Doing it before the driveshaft means all those parts have to be bigger, stronger, and, importantly, heavier. Use some of the remaining torque for performance.
And, I would have to ask, just how much stuff you are planning on carrying along in the Jeep? I would have to think that a 14 bolt would be far more than enough for a rear axle. That axle is suitable for dually rigs with GVWs better than 10K lbs (11.5K lbs, IIRC). If you put even 3K pounds in the back and on the roof, you'll see sheet metal deformation take place. Personally, I've decided on a FF 14 bolt for my rig. Not because I need the load capacity that it will support (though it should mean that it will likely
never wear out), but parts are plentiful and cheap, and gear setup is just so simple. . .
Even disc brakes are straight forward on the 14 bolt. Get a later AAM axle and it will already have the disc brakes, as well!
Coincidentally, I pulled the cover off my recently acquired 14 bolt today. This is a 1st generation axle from a C&C SRW truck. I have no idea how many miles are on it, but the truck it came from was trash. Internal inspection shows literally zero wear. These axles have the reputation that they do for a reason. The only bitch you'll read about on these axles is their "limited" ground clearance.
Anyway, this is all intended to be in fun. Please don't take my writings as my pointing fingers, or passing judgement. I hate it when somebody says "oh, just do this" or "you don't need that". I understand the need to engineer things to death, and have done so ad-naseum. One piece of serious advice I would offer is to get the truck out and see what it is really like before committing to anything. Even since June when I started playing with the Waggy, I have determined that there have been a few expensive flaws in my plan. For example, the very day I finished putting the four-inch lift springs on the truck I could see that I should have gone SOA on the front. After buying new 15" BFG AT/TAs, I really wish I had bought 16" tires, etc., etc. There are many things I just could not foresee without actually taking it out and using it. Ah, the price of experience...
Chris
'84 GW, returning to service.
360, 727, Selec-Trac 229, TFI, Hydro-Boost, 4" all-spring BDS lift (what a PITA!), BFG/AT 31x10.5x15, 5125 Bilstiens
