I have seen conflicting reviews on whether the AX15 or the Nv3550 is the 'stronger' transmission. They seem very comparable in design, gear ratio, factory applications.
No real conflict that I've seen if you read between the lines. Like you have to almost all the time on the internet!
If taken out of context the AX-15 has a higher torque rating. But if taken with the GVWR into account, they might be closer. I don't know what the mathematical factors would be that you'd need to take into account to figure that out, but it's probably somewhere out there.
Gear ratios are definitely different in first gear (3.80-ish vs 4.50-ish?). The 3550 has a lower gear, which is good for off-roading or for getting rolling in a heavier rig. Or one with bigger tires and taller gearing.
Nothing wrong with the gears in the AX, but the first gear is closer to a stock 3-speed's 2.90 (with close ratio) or 3.20 (standard wide ratio found in most EB's) so not much of an advantage for the off-roader without lower transfer case or differential gearing
Is it that they are to comparable to differentiate or is one or the other a little stronger, looking for opinion.
Be careful asking for an "opinion" from people. Because that might be exactly what you get. And likely why your first sentence was about finding conflicting information. That's what the forums are full of, because different people actually do have different experiences in different circumstances and even look at things a little differently from each other.
Stronger can be relative in the real world, but since you can't know what the others are like and what they use for a point of reference, you need to stick to the specifics put out by the manufactures.
I did see back in the day that the same or comparable transmissions with taller first gears (in the close-ratio versions) also had higher torque ratings.
Not sure if this was due to the ratio changes, or if the manufacturers also changed other components to make them stronger, but it was something I noticed.
Can't compare apples to oranges though, as a truck trans with a granny low might have a higher torque rating than a car transmission with smaller components. Truck-specific trannies are usually the beef.
The 3550 and AX are "comparable" then, and both are good suitable Bronco transmissions. But there are still some minor differences.
The only one I usually think of when talking about Broncos and these transmissions is the first gear ratio. Other than that I can't see any big reasons for going one over the other.
Oh, but the 3550 does require a very specific lubricant and nothing else works. If I'm not mistaken (and someone can correct me if I'm wrong) the AX can take a more standard type of gear lube. It's not hard to find the correct lube for the NV, but it's more expensive that what you can use in the AX.
So that might be considered a negative.
I have a 375 ft-400 ft# 351W planned for my 68, which wont see much offroad use.
Sounds nice. But remember that one of the biggest threats to a transmission is not just the engine's power (or the driver's right foot even) but the final gearing in the differentials and the tire size.
If you have tall gearing for the tires, and a lot of torque, guess what's stuck in between the engine and the road? The transmission (along with the transfer case and driveshaft of course).
Always factor in your personal willingness to push the go-pedal hard, and that's how to figure if a transmission is going to be good enough.
A lighter vehicle or less power is better for this. But so is better gearing. Which can make up for the other two.
Which all means don't scrimp on the gear if you want things to last longer. Or in other words, as they like to say around here, "don't fear the gear"!
After all, street driving can actually be harder on a drivetrain than going off road. More traction can be had, and more opportunities to utilize it.
But it still comes down to how you intend to drive it from the stoplights.
Sorry for the rambling. I just happen to think that both transmissions in question are good ones. There are just still some things to factor in.
Paul
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