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Thread: cummins 5.9 or ford 6.9

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    Default cummins 5.9 or ford 6.9

    Hay guys, what do you all think and this is a genuine question. I fitted a cummins 5.9 turbo and intercooler with an allison 545 auto box to my 89 cabriolet last year (not the easiest job) . The thing is while I'm happy with the engine itself I had an issue with the final drive, it's got no top end 55 flat out . now I'm not after a flying machine but at that speed if your doing any distance it can get to you. So I've just purchased a 1988 travel craft motor home with 6.9 diesel in it with 85,000 mile on the clock and I'm thinking of taking the cummins and box out and fit the Ford unit and box in the hope it makes the truck better for cruising at a more comfortable speed. So thats it, what do you all think. Will i achieve the top end final drive I'm looking for with the ford. And is the ford a good unit, I've never had one before.

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    Default Re: cummins 5.9 or ford 6.9

    The cummins is going to be the better choice long term. Sounds like you need to make some modifications to it. That is my vote.

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    Default Re: cummins 5.9 or ford 6.9

    Yer tried looking at a diff but that was a dead end, was told their just wasn't one that would solve the problem. Looked at an overdrive that would cost a small fortune, and the guy couldn't give me any figures on what the actual final drive would be. Got to say the cummins is so reliable when it runs. Mmm thanks for the reply.

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    Senior Member Committed Polarburg's Avatar
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    Default Re: cummins 5.9 or ford 6.9

    that trannys got to go .look into a overdrive unit and aftermarket computer to run it .the 6.9 and non o.d c6 trans will not be pleasant.no power,and crappy economy.

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    Default Re: cummins 5.9 or ford 6.9

    This is a purplexing predicament. I think the tranny is your limitation as well. I am looking at pulling out my 7.3 idi and installing a 12 valve 5.9 in my bronco and leave the A4OD ford tranny. This combination has been proven over and over again but you need to install a different transmission controller or you will be limited. The 5.9 makes peak torque at lower rpm than the 6.9 or 7.3 so the shifting band needs altered. I don't know much about your allision tranny though.

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    Default Re: cummins 5.9 or ford 6.9

    thanks for getting in touch guys, i think your right about the trans. i wish i could get my hands on a A4OD ford tranny not easy to find over here. its also a good point about the fuel economy, the cummins isnt to bad even with the diff ratio the way it is.

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    Senior Member Committed natterre's Avatar
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    Default Re: cummins 5.9 or ford 6.9

    If you are having a hard time finding an E4OD, you might look for one of ford's 4R100 models. The are basically the same thing as the E4OD, but they were used in the 1997 and later models. I don't know when they discontinued them, but about the only difference is they have an extra speed sensor on the output shaft of the transmission. Good luck, and keep us posted on how you solve the problem.

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    Default Re: cummins 5.9 or ford 6.9

    thanks for your ideas its certainly given me a few things to consider. im gonna do some more research on the transmissions you've mentioned. see whats out their, and i'll yer know.

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    Default Re: cummins 5.9 or ford 6.9

    You could look for an fso 8406a. These are pretty common on fleet trucks. There is also the fso 12 I believe. It is a less common swap done in the US in lieu of using some of Dodge's less than savory manuals. The fso should fit and will give you eight forward gears that will last as long as the truck

    You DO NOT want to put a 6.9 in the truck. Mine is a 6.9 and while it is a great engine it is very inefficient, will run hotter and is a pain to work on sometimes. The 6.9 was originally a gas engine that was build heavy enough to be used as a diesel. You just need to look at changing out some gearing. Keep in mind the 5.9 is most efficient in the ~2300-2400 RPM range.

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    Default Re: cummins 5.9 or ford 6.9

    you need 3.07 ratio gears for your dana 70 rear. you have a 4spd Allison so your 1st gear is lower than a 3spd auto. Dodge used dana 70 with single wheels in early 5.9 cummins pickups that came with the 727 trans w/o overdrive.

    6.9 was not a converted gas engine, LOL. that was a generation before, it looked huge compared to the 6.9. probably weighed 4>500lbs more. last one I saw was maybe 1981.

    good luck and keep up the news.

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    Default Re: cummins 5.9 or ford 6.9

    Thanks for your comments guys I really appreciate it. I will be looking out for those boxes and that rear end for sure because I will be sticking with the cummins. The donor vehicle I bought has now itself become a project, as I decided to stick with the cummins like I said and the motorhome mechanically was good but interior and body was shot I've striped the interior out and removed the skin to leave me with a steel frame, that has now been chopped and altered and I'm making a custom style rig similar to the cabriolet. I'm building this one more like a mini rig I can actually camp in as it will have cooking facilities, running water and hopefully a toilet. The sleeping side of it will either be a pullout sofa style bed or fixed bunk depending on the final layout. Anyway thanks again guys.

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    Default Re: cummins 5.9 or ford 6.9

    Quote Originally Posted by natterre View Post
    This is a purplexing predicament. I think the tranny is your limitation as well. I am looking at pulling out my 7.3 idi and installing a 12 valve 5.9 in my bronco and leave the A4OD ford tranny. This combination has been proven over and over again but you need to install a different transmission controller or you will be limited. The 5.9 makes peak torque at lower rpm than the 6.9 or 7.3 so the shifting band needs altered. I don't know much about your allision tranny though.

    AOD tranny are not up to the task of a diesel without heavy mods. Even after all the mods done the Cummins may still tare it up. You'd be better off with a E4OD/4R100 or C6 with a added OD.
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    Default Re: cummins 5.9 or ford 6.9

    Those boxes youve mentioned are the ones that do seem to be the most popular. It's really frustrating tho trying to source bits like that here in the uk, very limited. I'm just gonna have to keep an eye on ebay and hope something pops up. Thanks for all the suggestions I'll keep you posted.

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    Default Re: cummins 5.9 or ford 6.9

    Quote Originally Posted by 68Mercury250Ranger View Post
    you need 3.07 ratio gears for your dana 70 rear. you have a 4spd Allison so your 1st gear is lower than a 3spd auto. Dodge used dana 70 with single wheels in early 5.9 cummins pickups that came with the 727 trans w/o overdrive.

    6.9 was not a converted gas engine, LOL. that was a generation before, it looked huge compared to the 6.9. probably weighed 4>500lbs more. last one I saw was maybe 1981.

    good luck and keep up the news.
    The block was initially designed to be a gas engine. Development changed towards a diesel when IH and Ford signed an agreement in 81 to develop the diesel. It was never sold as a gas engine but that is where the block developed from. The same was true of the GM 6.2. Both originally NA. I don't know what engine you're talking about as the previous though.

    You're right on the 3.07 though. That would help considerably. Would put you at 81MPH in fourth gear @ 2400RPM on 16 inch (880mm ) tires.

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    Default Re: cummins 5.9 or ford 6.9

    Right guys I think the easiest and best way is the 3.07 gears as suggested by so many of you. I've gotta say thanks again for your help. Next thing how do I recognise the diff if I find one as a 3,07, are they marked up somewhere or are the diff gears stamped somewhere?

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    Default Re: cummins 5.9 or ford 6.9

    Look through the "Helpful links" on this thread post.

    http://www.rme4x4.com/showthread.php...-Dana-70-Bible

    Specifically this guy:
    http://www2.dana.com/pdf/XGI.pdf

    This will give you the BOM numbers you're looking for. There are tags on the rear ends that match up. Or you can just google the number.

    Keep in mind you're going to lose a little bit of low end torque by switching to 3.07 gearing. But there are a lot of ratios between 3.73 (Or 4.10 if that's what you have) and 3.07. 3.54, 3.45, 3.31, 3.27. All are pretty common as the Dana 70 was widely used across a broad spectrum of manufacturers. Hopefully this will only end up being a couple hundred dollars for you. If you have trouble try looking on off roading and jeep forums. They like the 60's and 70's.

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    Default Re: cummins 5.9 or ford 6.9

    That's great thanks for the info , I've been in touch with a guy who specialises in diffs. So now I'm gonna give him all the details you've given and hope he can find me something. I'll let everyone know

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    Default Re: cummins 5.9 or ford 6.9

    Do this..



    It has a nv4500

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