I just bought a 1984 bronco 4x4 with a 351w. Driving it home i noticed i was running about 2900-3100 rpms going 60-65 mph. I believe it has the c-6 transmission. since then due to other circumstances i have pulled the motor and in doing so the bell housing broke all across the top. Now that i have to replace the trans. is there swappable tranny that would put my rpm's lower at regular freeway speeds. Or does it sound like something was wrong with the trans. in the first place and just putting the same kind back in would be best. Thanks for your advice.

 

thats about normal rpm for those speeds with that tranny. you may think about locating a ZF 5speed manual tranny. Another option is to go with an AOD but this could get tricky to swap in properly and isnt a very strong trans.

 

What Vehicle or year model did the ZF come in?

 

Welcome to FTE and the Big Bronco Forum. Information will be the best we can collectively cull together for you.

The C6 is not an overdrive-equipped transmission so you are going to see higher RPM at highway speeds than you would with any transmission with that extra (overdrive) gear. Kem is right though about the options for automatics equipped with OD that will work in your truck. The AOD is actually a derivative of the old C4 transmission and not all that hefty along with a need for special shift control cable linkages to the throttle linkage. The M5OD manual is a light duty manual with overdrive but comparatively weak alongside the C6 auto that you have. The M5OD is also not all that great for crawling or digging in/out if you need to. (You CAN do about 30MPH in reverse though!)

The ZF 5-speed manual was available in the 3/4 ton and heavier trucks from about 1987-present. There are a few variants... the S5-42 and S5-47 with the "47" being available in the 95 and up trucks. Both were equipped with either a close ratio or wide ratio gearset. They were also drive-specific so you will need a 4WD version for your Bronco. The nice thing is that the transfer case in the Bronco now should bolt directly to it. The clutch assembly and linkage in 1984 was still mechanical but you will probably want to go with the newer style hydraulic clutch from the late 80's-early 90's for reasons listed below. I don't know how long a C6 is but the S5-42 is just under 24 3/8" from bell-housing to t-case flanges.

The S5-42 has an input torque rating of 420 lb./ft while the S5-47 is @ 470 lb./ft. Both are fully synchronized in all gears. Both can utilize the OEM hydraulic clutch setup found in the 1/2 ton trucks as well as the 3/4 ton and up. The S5-47 is a lot harder to come by and is extremely engine specific with different part numbers for the 4.9l (300), the 5.8l (351) and the 7.5l (460) engines and no listing for the 5.0l (302) power plant at all.

The gear ratios for the S5-42 are as follows:

Close Ratio
1st Gear:
4.14
2nd Gear:
2.37
3rd Gear:
1.42
4th Gear:
1.00
5th Gear:
0.77
Rev Gear:
3.79

Wide Ratio
1st Gear:
5.72
2nd Gear:
2.94
3rd Gear:
1.61
4th Gear:
1.00
5th Gear:
0.76
Rev Gear:
5.24

(Yes, you can climb damned near vertical walls with the right engine, axle gearing and the wide ratio S5-42).

The ratios for the S5-47 are:

Close Ratio
1st Gear:
5.08
2nd Gear:
2.60
3rd Gear:
1.53
4th Gear:
1.00
5th Gear:
0.77
Rev Gear:
4.66

Wide Ratio
1st Gear:
5.72
2nd Gear:
2.94
3rd Gear:
1.61
4th Gear:
1.00
5th Gear:
0.76
Rev Gear:
5.24

 

Wow, super informative. I thank you for this info. Im going to start researching. My motor is at the machine shop so i have a little time to get the tranny ready. Thanks agian