Hello everyone.
I really need some experienced advice so I can decide which route to go with my restoration.

I have a 1972 U152 with a stock 302 and 3-speed manual transmission. I will be putting in a crate 347 long block with around 350 to 400 horsepower.

I have been reading and looking into all the options for replacing the stock transmission with a 5-speed manual (NV4500, NV3550, AX15).
The question is though, can the standard transmission handle the power?
If not does anyone know what transmission can handle the power?

I have tried to find this information on my own but am getting a lot of conflicting information.
Please advise if you have the knowledge.

Thank you and Happy New Year!

The nv4500 definitely can. Those are put behind bigger motors. Similar to the ZF5's. Those are true truck transmissions.

The nv3550, ax-15, and the m5odr2 are popular for the fact that they are more "car" like. If you are running it hard, I'm sure you'll break one but the m5odr2 was put in the f150's and you don't hear of many issues. I know they don't have 400hp stock though.

I bet there are rebuilds that will take the HP.

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Thank you phyler, any thoughts on the original 3-speed transmission?

I had a transmission shop tell me that the original 3-speed transmission really could not handle much more then the stock 140 horsepower that the 1970 302 produced. I know this is not true but have been digging to find someway to know what a safe horsepower is. The reality is, I’m not a serious off roader but will get the Bronco dirt of course. Most the driving will be around town under 60mph.

Thank you.

If you keep your foot out of it, I'm sure it would work.

If you are just swapping the engine right now, no harm in trying it out.

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That three speed won’t last long with a healthy engine but if you aren’t abusing it, probably last longer then you think with new a new pressure plate and clutch.

Personally I would prefer a zf5 or nv4500, not only for the strength but the overdrive. Either will hold up easily to your new engine. Downside is the additional cost and fabrication involved.

If you just want to drive it, use the three speed while you figure it out.

If you truly are going to have 400hp and 350 ft lbs torque at the crank you will probably want to change the tail housing out on the Dana20 to a heavy duty unit as a preventive measure. It’s pretty weak when you start introducing more power.

No one wants to have power and not be able to use it. I don't have any idea what sort of HP the 3spds can handle. So you might want to start looking hard at 3/4 ton 5spds. The NP435 would probably be fine as well, but depending on your purpose for this Bronco, may not serve you well due to lack of OD.

Thank you everyone for the input.

Does anyone currently have a build that uses the stock 3-speed transmission and DANA20 transfer case?
If so, could you please share what the engine recipe is and how you feel about the build?

Sincerely…

Btw, I swapped from the 3 speed to the np435 20 years ago. That transmission is definitely bullet proof. It weighs a ton too. I have a ZF5 that I'll swap in. I wouldn't have planned to go to the 5 speed except I got a killer deal on it. The 435 is a great fit for a Bronco outside the fact it doesn't have overdrive.

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My former Bronco came with a stock 3 speed which I upgraded to a NP435 and my current one had a NP435 that was recently swapped for a ZF5. While I have no idea how much power a stock 3 speed will handle, the only reason I'd run one behind that engine would be to find out how long it would live, and I'd have a lot of fun finding out. My NP435s were great. They're bombproof and I used the granny 1rst all the time. It's handy for stop and go traffic in addition to rock crawling. Learning to double clutch is easy, and I still tend to do it when I shift into first out of habit as it makes shifting easier than waiting for syncros. The gear shift throw is long and it shifts like a garbage truck, but I kind of liked that.

All of that said, I'm super happy with my ZF5. Shifting is shorter and a little easier but still appropriately truck-like. Gear shifter comes out in an easier spot in relation to the dash, although tweaks are still needed. Overdrive is amazing. I ran down to the Springs yesterday and did 70-75 the whole way with zero drama. With 37's and 4.56 gears, mine turns about 2500rpm at 70, which fits the powerband of my 351w nicely for doing longer distances. If I were still running my 302 I'd probably want 4.88. The trans is noisy at idle, but no worse than my NP435s. The granny ratio is a little taller than the NP435, but not enough that it matters a ton, especially if you're not building a rock crawler. I use it more than I did on the NP435, as it's handy for traffic, parking garages, etc. I'd love slightly narrower gear ratios, but not enough to lose the low 1rst gear.

Having run all 3, I'd absolutely pick the ZF5. It's nicer to live with day-to-day, the extra gearing is the best of both worlds, and it's not hard to fit if you've got a small body lift. If you can find one, the 47 can handle a little more torque, but the 42 is likely fine for what you're doing. Also, I've heard that converting a 2wd trans to 4wd use results in a shorter transmission, but I've not read up on it in detail.