4r70w or c4???

[kholding]

Building a '73 Bronco from the ground up and am stuck at the transmission. I want an auto trans that I can drive around town and be able to hit the trails with as well. What would be a better choice; the 4R70W or C4? What are the advantages of each and downfalls as well? An others I should be considering? Any help is appreciated...

 

[DanWheeler]

I dont know much about the C4 but I can tell you that the 4R70W is an awesome transmission for a Bronco if you have the $500 to spend on the baumannator controller you'll need.

Advantages:
- completely programmable shift points
- adjustable line pressure (smoother or harsher shifts)
- manual locking torque converter for hill descents (engine braking)
- overdrive
- 2 shift tables (i.e. flip the switch for sport mode)
- overdrive cancel
- set your lockup torque converter to auto/off/on

disadvantages:
- cost
- extra computer component that could break
- if you're not fuel injected, you'll have to retrofit a TPS which I found to be a pain (once you go fuel injected you just tap into the TPS on the throttle body)

I haven't heard of anyone having trouble with their computer. The transmission still works without th computer. I forget which gears are available with a computer failure. Pretty sure 1st and reverse at the very least.

do it and dont look back.

edit: just spend the extra $100 for the harness from baumann

 

[broncnaz]

On top of that to run the 4R70W you'll need lower axle gearing to effectively use the OD unless you already have 4.11 gears and run tires close to stock height definetly no bigger than 33.
With a C4 if your gearing is 3.50 the largest tire I would run is 33in. 4.11's and 35in tires would be about as big as I would go.

 

[kholding]

Thanks, I have 4:11 gears and am planning on 35's so maybe the C4 is the better choice? Wasn't sure if all around drivability would be better with the 4r70w and since I can get either right now, which way I should go.

 

[Amac70]

id say do the 4r70w and switch gears to 4.88. Be a much better driving combo then the c4.If you want to keep the cost down you could always do an aod with the the 4r70w internals. get some of the benifits of the 4r70w with out needing the computer.

 

[ScottC]

The C4 is not a better choice unless budget is the only reason. Since you're building from the ground up, you may as well do it right once. You'll be fine with 4.11s for now...and by fine, I mean it will drive. It will lug the rpms down pretty low and may be a bit tall for difficult trail, but it will drive. It's not too late to swap the 4.11 for 4.56 either.

If you don't want to shell out for the Baumanator to control it, you can swap the 4R70W internals into an older AOD housing and eliminate the electronics. This gets you the wide ratio gears but not the flexibility of the controller.

FWIW - I'm building a '77 with a 4R70W, 4.56, and 35s. This seems to be about the best all-around combo for my needs which will be 95% daily driver, and very capable on moderate trails.

Ha, Amac70 beat me to it. And I agree on the 4.88 for more trail oriented, but that would give you something like 2500rpm on the highway(ballpark guess). That's a little higher than I prefer. My '96 with 3.55 on 33s runs at 1700rpm on the highway which is a little low. The 4.56s and 35s should be somewhere around 2000-2100 which ought to be just right.

 

[blubuckaroo]

Building a '73 Bronco from the ground up and am stuck at the transmission. I want an auto trans that I can drive around town and be able to hit the trails with as well. What would be a better choice; the 4R70W or C4? What are the advantages of each and downfalls as well? An others I should be considering? Any help is appreciated...

Your stock drivelines will work with a C4. You will have to have new ones made for the 4R70W and AOD. The front driveline may even need to be made of smaller diameted material to clear the AOD & 4R70W. A C4 can be made more trailworthy by changing it to wide ratio planetary gears. BTW, a C4 is a tough little tranny and is a lot cheaper to overhaul than any of the overdrive options.

 

[Broncobowsher]

As mentioned, C4 is a usable low cost automatic. Works fine, plenty strong. Lacks a lock up converter so you need to gear highway driving for more RPM to keep the convertor into the coupling stage so it doesn't slip too much and make too much heat. 4.11s and 35's is a little light on the gearing for my taste. While the tall tires will bring down the highway RPM, it may bring it down too much.

The 4R70W is a hands down all around better transmission. With the lockup converter you can cruise at low engine speeds and not slip the converter (making bad heat for the trans and wasting fuel). The lower 1st gear will get off the line better and make it a blast to drive. Now axle gearing should be 4.56 or 4.88, I would lean toward the 4.88s. I have ran a 4R70W for years. Did lots of miles (a few Phoenix to Vegas road trips was part of that). My combo was a very torquey, built up 351W, the 4R70W, 4.88 gears and 33" tires. For the engine and tire I was running the 4.88 was a little low for freeway driving. Not bad, completly acceptable, just a few hundred RPM higher then what I felt it should be. I kept thinking that going to a set of 35" tires would make it geared perfect. Or, if it only had a 302 then it would have been geared perfect. So another factor is what engine are you running and how is it built?

Now the 4R70W parts into an AOD. Yes it can be done. Gets you 2/3's of the way to a 4R70W. Costwise it comes in nearly the same as you now need 2 transmissions to start with and rebult one with half the parts from the other. It will get you some of the benifits of the 4R70W (wider band, gears, mechanical diode) but you still won't get some of the better aspects of the 4R70. You can't control the lock up of the converter, you can't dial in each of the shifts, you can't hold 2nd gear (without another costly upgrade to a new custom valve body). So there are several limitations to doing the reworked AOD.

Ground up resto, all the good stuff, just do the 4R70W and be done with it.
On a budget, want to get it together fast and cheap, C4

 

[DanWheeler]

I have 4.88s with 35s and on the highway when I dare to go 70+ I think my RPMs could be a little lower.

on the trail, i have a 4:1 Atlas so the difference between 4.56 and 4.88 is negligible but in theory, you'd want 4.88 over 4.56 for the trail. Either 4.56 or 4.88 with 35s will work great.

 

[Apogee]

I have 4.88s with 35s and on the highway when I dare to go 70+ I think my RPMs could be a little lower.

on the trail, i have a 4:1 Atlas so the difference between 4.56 and 4.88 is negligible but in theory, you'd want 4.88 over 4.56 for the trail. Either 4.56 or 4.88 with 35s will work great.

That sounds like an argument for 37's if I've ever heard one ;D

 

[DanWheeler]

That sounds like an argument for 37's if I've ever heard one ;D

actually I'll be going 5.38s in a Dana 60 with 39s so we'll see how that works out with the 4R70W. Should be almost identical to the 4.88s with 35s but slightly lower RPMs.

 

[DanWheeler]

overdrive on the 4R70W is .67:1 so you can multiply your number on the chart by .67 to get your RPMs while cruising in OD. This wouldn't be accurate without the lockup torque converter. Otherwise your RPMs would depend on slippage of the TC.

so in overdrive with my 4.88s and 35s i'm running at about 2046RPMs.

wait... is that right? that seems low.

 

[kholding]

Great responses. Thanks everyone. Looks like the 4R70W is probably the way to go for me.

 

[Broncobowsher]

overdrive on the 4R70W is .67:1 so you can multiply your number on the chart by .67 to get your RPMs while cruising in OD. This wouldn't be accurate without the lockup torque converter. Otherwise your RPMs would depend on slippage of the TC.

so in overdrive with my 4.88s and 35s i'm running at about 2046RPMs.

wait... is that right? that seems low.

AOD is the .67 (true 2/3).
The wide gearset of the 4R70W is only a .70 OD.
Not that the .03 matters that much, but it is the correct ratio. It has been misquoted as .67 many times, but that doesn't change what it really is.

I remember 60 MPH = about 2000 RPM (give some tollerance stack up for some number errors) was good with 4.88 and 33's. I know the speedo was close (withen 2-3MPH at 60) but I never did an accuracy check with the tach. So a perfect math 2046 RPM on 4.88 (the 9" will really be 4.86, D44 4.89) and a perfect 35" tire sounds correct.

Low range (3.15 tera lows) in overdrive was 2000 RPM for 20 MPH, after that it was better to shift back into high range if driving any distance.

 

[Steve]

I can't believe nobody has asked what transfer case the OP is going to run, as that makes a difference, especially in what axle gears to run. Could even make the C4 a decent choice.

 

[DanWheeler]

I can't believe nobody has asked what transfer case the OP is going to run, as that makes a difference, especially in what axle gears to run. Could even make the C4 a decent choice.

good point, putting a Dana 20 behind a 4R70W will cost you another $500 for the adapter but as far as gearing, i dont think it makes a difference.

The 4R70W may also require new driveshafts. I ordered a custom driveshaft for my rear after the conversion then moved my old rear shaft to the front after having a shop take a couple inches off.

 

[Steve]

good point, putting a Dana 20 behind a 4R70W will cost you another $500 for the adapter but as far as gearing, i dont think it makes a difference.

Sure it does. An EB D20 has a low gear of 2.46:1. You can get an Atlas with 3.8 or 4.3, or a STaK offers other options. The difference between 2.46 and 4.3 is huge on the final crawl ratio.

 

[blubuckaroo]

good point, putting a Dana 20 behind a 4R70W will cost you another $500 for the adapter but as far as gearing, i dont think it makes a difference.

The 4R70W may also require new driveshafts. I ordered a custom driveshaft for my rear after the conversion then moved my old rear shaft to the front after having a shop take a couple inches off.

Will the new front driveline hit the transmission?

 

[kholding]

What are good transfer case options for the 4r70w? I was planning on a Dana 20 but that is because I'm not really sure of all the options out there?

 

[bronco-billy-66]

I can't think of a single reason, other than money;D, to not go with the 4R70W. I wish I had one. I do plan on it in the distant future.
I say do it, take lots of pics and post up