need help on 4bt cummins in a 1970 bronco - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 14 (permalink) Old 07-28-2011, 11:07 PM Thread Starter
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need help on 4bt cummins in a 1970 bronco

I have a 1970 bronco that has a 302 in it and a 4 speed. It was my first ride and I bought it back from my brother. I am going to put some money into it and make it a daily driver but dont want to get killed on poor mpg. So the thought of a 4bt cummins looks like the best bet. I know that the nv4500 will bolt up to it. But I was thinking about a zf5/zf6 and a ford tcase would be a lil cheaper and easier to find. I want to stay with a standard tranny. Now on to my other question... I am going to be running 4.10 gears and 315/70/17 tires. What would be the best tranny/transfer case combo to get best mpg? I have heard the 4bt is most happy at 1900 rpm. I want to be able to cruise at 65 on highway. If anyone has any input thanks in advance.

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post #2 of 14 (permalink) Old 07-31-2011, 04:02 AM
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The ZF5/or 6 will work just fine but you will have to get an adapter plate those run about $750.00 but the availability of parts for the ZF's is much better than the nv4500's so it might be worth that extra cost. You can even go with an Eaton 6 or 10 speed if you have the room. Are you looking for a divorced or married transfer case? I think you might be ok with a late model NP271 but there may be others, lots of folks use NP205's There are a few older broncos with 4bt's in them so it can be done. There is a little more weight on the cummins than that 302 so you might want to check the suspension and axels.

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post #3 of 14 (permalink) Old 07-31-2011, 04:39 AM
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If you by a Destroked type trans adapter, Make sure you by the one for the Gasser ZF6. They are much easier to find and they are the same bolt pastern as the 6.0 trans.

My F250 with a ZF6 and 3.73 running 265/70/16 is turning 1800 rpms at 65mph. I think you may like 3.55 for your bronco 4bt.

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post #4 of 14 (permalink) Old 07-31-2011, 03:22 PM
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So the eaton 10 spd will bolt up to the 4 and 6bt?

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post #5 of 14 (permalink) Old 08-01-2011, 05:47 AM
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So the eaton 10 spd will bolt up to the 4 and 6bt?
yes all you need is to come up with a #2 flywheel housing for a Cummins B series.

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post #6 of 14 (permalink) Old 08-01-2011, 01:14 PM Thread Starter
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Why would I need an adapter plate? I can get a zf5 already bolted to it out of a bread truck...Axles and suspension are just fine I put 8 lug dana 44 up front and a 9in 8 lug in back.

Can so one do the math with 4.10s about how far about what will the 35in tires drop the gear ration to? 3.73? Lower?

Jon K
2003 F250 CC LB 4x4 PSD stock for now

1999 "Freight shaker" Century direct 10 Detroit 12.7
Pulling a 2001 Utility 53ft airride logistics Trailer LED all around.

sold 94 supercab LB 4x4 PSD ZF5 XLT , 357,000 miles(original motor) Bully Dog 4 pos chip, diy intake, 3in DP into 3in straight pipe, egt gauge, levelin kit and 265/75r16 on 16x8 rims

sold 2003 Mercedes SLK32 AMG Tax write off lol...

Last edited by Kasprzak85; 08-01-2011 at 01:21 PM.
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post #7 of 14 (permalink) Old 08-02-2011, 06:42 PM
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Quote:
Originally Posted by Kasprzak85 View Post
Why would I need an adapter plate? I can get a zf5 already bolted to it out of a bread truck...Axles and suspension are just fine I put 8 lug dana 44 up front and a 9in 8 lug in back.

Can so one do the math with 4.10s about how far about what will the 35in tires drop the gear ration to? 3.73? Lower?
I'm sorry you did not mention you have the ZF5 with the motor, and you were talking about putting an NV4500 on it, I assumed it was a Dodge NV4500 and you had the adaptor for that. If you want a ZF6 then you will have to make sure it is the same size bell housing, but I don't think you gain much with a ZF6 but a lower granny gear so it will not help with MPG's. I would just stick with the ZF5 if you have it/can get it.

Sounds like you are on the right track.

Jakob

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post #8 of 14 (permalink) Old 08-02-2011, 10:31 PM
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Quote:
Originally Posted by Kasprzak85 View Post
Can so one do the math with 4.10s about how far about what will the 35in tires drop the gear ration to? 3.73? Lower?
What it the final drive ratio for the transmission, .80 or .78 or .72 or maybe .64 from there it is no problem to figure the speed.
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post #9 of 14 (permalink) Old 08-03-2011, 01:09 AM Thread Starter
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.76 if I read the specs right...

Jon K
2003 F250 CC LB 4x4 PSD stock for now

1999 "Freight shaker" Century direct 10 Detroit 12.7
Pulling a 2001 Utility 53ft airride logistics Trailer LED all around.

sold 94 supercab LB 4x4 PSD ZF5 XLT , 357,000 miles(original motor) Bully Dog 4 pos chip, diy intake, 3in DP into 3in straight pipe, egt gauge, levelin kit and 265/75r16 on 16x8 rims

sold 2003 Mercedes SLK32 AMG Tax write off lol...
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post #10 of 14 (permalink) Old 08-03-2011, 01:49 PM
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1900 RPM, .76 OD, 4.10 gears, 35 in. tires. you will be going 64MPH.
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