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88 bronco II 6-speed Transmission swap


PacoTaco1000

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I’ve been looking around trying to find anything on a transmission swap for a Bronco II and haven’t found anything. Has anyone done a 6-speed in a little b2? I’m currently running my stock FM145 5-speed on my freshly rebuilt 2.9l. I plan on swapping to the 4.0l after my motor goes and saw that they both are able to use the same trannies, any recommendations would be much appreciated, just looking to Maintain highway speeds for cheaper.
 


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PacoTaco1000

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Is this just something not common or completely ludicrous? Is there some sort of a mod i’d have to do to fit one?
 

Dirtman

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I know of no 6 speed transmissions that will fit a 2.9/4.0 bolt pattern. Even the last of the mustangs to use the 4.0 sohc never had a 6 speed transmission.

At the bare minimum a custom bell housing adapter would need to be made and then no idea how you'd find a proper clutch, fit the shifter, work out the mounting, slave cylinder etc. It's just a useless endeavor.

Also, a 6 speed transmission isn't going to change how the truck operates at highway speeds. A 5 speed and a 6 speed have the same or at least very similar final overdrive gears. Its not like a 6 speed has an "extra" overdrive gear. A 4 speed, 5 speed, 6 speed, 8 speed, all have roughly a .75:1 final gear.
 

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2.3 Duratec swap (or go whole hog and 2.0/2.3 Ecoboost swap) and then get a 6spd out of a Ecoboost Mustang... and lose 4wd.

I have heard rumors you can convert them to 4wd using Rover parts but have never really cared enough to look into it.

+1 to not much gain for the effort
 

PacoTaco1000

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Thanks for the input! Is there a tranny that has more efficient gearing than my FM145 or is it more an engine issue then? I reach 70 and i’m right at 2,500RPM’s and i’m just looking for better fuel economy and trying not to overwork my motor either on my drives down to Iowa
 

Dirtman

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2,500 RPMS at 70mph is excellent. Any lower and the engine would be lugging...

You're dealing with a vehicle made at a time when the nationwide maximum speed limit was 55mph. It was never even meant to do 70. :icon_rofl:

Getting higher (numerically lower) gears in the axles would increase overall speed to a lower RPM but again, 2,500rpm is not bad for 70mph, and secondly doing so would make the the engine work alot harder at lower speeds. It would drive like a slug until you get up to highway speeds.

Your dealing with a 33 year old vehicle with around 130 horsepower and zero aerodynamics. Short of putting a modern engine and transmission in it. Not a whole lot more you're gonna get out of it.
 

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2,500 RPMS at 70mph is excellent. Any lower and the engine would be lugging...

You're dealing with a vehicle made at a time when the nationwide maximum speed limit was 55mph. It was never even meant to do 70. :icon_rofl:

Getting higher (numerically lower) gears in the axles would increase overall speed to a lower RPM but again, 2,500rpm is not bad for 70mph, and secondly doing so would make the the engine work alot harder at lower speeds. It would drive like a slug until you get up to highway speeds.

Your dealing with a 33 year old vehicle with around 130 horsepower and zero aerodynamics. Short of putting a modern engine and transmission in it. Not a whole lot more you're gonna get out of it.
Even a more modern powertrain would be turning the same RPM.

My 2005 5-speed has the same overdrive as a '87.
 

Dirtman

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Even a more modern powertrain would be turning the same RPM.

My 2005 5-speed has the same overdrive as a '87.
Yea that's kinda my point. Overdrive is overdrive, they are all roughly .75:1 be it a old 4 speed or a new 6 speed. A different transmission won't do much if anything. Something like a modern 10 speed automatic would make a difference but good luck getting that to work in an 88 bronco 2.

A more technologically advanced modern motor would obviously get better fuel efficiency at the same RPM since it has 3 times the power, but your still doing 2500rpm in overdrive cruising on the highway because gearing dictates RPM/speed not power. Power is just how efficiently the engine will do it.
 

PacoTaco1000

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Well thanks for the help lol, I suppose i’d be expecting slightly better out of the 4.0 then huh?
 

Dirtman

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Well thanks for the help lol, I suppose i’d be expecting slightly better out of the 4.0 then huh?
Yes, the 4.0 ohv has around 30HP more than the 2.9 which doesn't sound like much by today's standards but it will make a noticeable improvement in power and some fuel economy. The 4.0 ohv is also a more reliable motor all around even though they are very similar to the 2.9.

Make sure you get the transmission with the 4.0. Your FM transmission will physically bolt to the 4.0 but it won't last long. They don't even last long behind the 2.9. The 4.0 will have an M5od behind it which is a much stronger transmission. (Again, identical overdrive, just stronger).
 
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PacoTaco1000

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Location
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Engine Type
2.9 V6
Transmission
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2WD / 4WD
4WD
Good to know. If I were to swap my FM145 for the M5od would that do much for my 2.9 my plan is to swap to the 4.0 regardless just a matter of the order in which i do
 

Dirtman

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2WD
Total Lift
It's up there.
Total Drop
It's down there.
Tire Size
Round.
My credo
I poop in the furnace.
There is no improvement except reliability. Both the 4.0 and 2.9 will happily bolt up to an m5od.
 

PacoTaco1000

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Good to know, thanks!
 


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