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Discussion Starter · #1 ·
I attempted to reach W. Buchanan based on my reading of rpm/gear ratio's but since i am new it would not allow me to send him a message so now I will ask the forum for advice. I have read a lot of articles and respsonse on this site which looks to be amazing for information. I apologize for the dead horse question.


Here is my delima. 4.10 vs 3.73 gears. I am running zf5 trans .076 OD and 37's BFG ( 36" call it i guess )

1850 rpm the calculator shows 64mph and 2000rpm it shows 69mph.

with 3.73 gears.. 1850 rpm at 70mph and 76 mph at 2000rpm.

The zf5 has a big drop in gear from 3rd to 4th which is a drawback but that is what i have. I hear people talking about a bit more gear due to the square oven being pushed down the road and 37 tires so I am thinking 4.10 but 3.73 looks good as well.

Thanks for any thoughts or feedback on this.
 

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You missed some important info.
Do you tow?
What are you seeking to run at on the road.
Final use will have the most bearing on what you need.
 
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I have 3.54s, want 3.07s
 

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Discussion Starter · #4 ·
Thanks Blackduck.

I do not tow with this rig.

I woukd like to run 75mph.

Vehicle is being built for expedition type usage, off road and interstate driving.
 

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Discussion Starter · #5 ·
I actually already had my ARB lockers and it dawned on me that I would not be able.to go 3.73 without swapping the carriers out so tha leaves me with 4.10 gears.
 

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I actually already had my ARB lockers and it dawned on me that I would not be able.to go 3.73 without swapping the carriers out so tha leaves me with 4.10 gears.
You sure you have to change out the carriers? My 9.25" IFS ARB could use any ratio up/down w/o changing the carrier.
 

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Discussion Starter · #7 ·
You are correct, I just called to get this topic cleared in my head. I can go any gear.. haha. so now back to figuring out 37's on 4.10 or 3.73. Man its hard to make the decision when a few hundred RPM's make a big difference in how it operates.
 

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I'd think both gears could work OK. If you plan to spend a lot of time in the 70-75 MPH range I'd think the 3.73 would be best. After you pass 2000 RPM the torque starts to drop and so does the fuel economy. Peak torque on the 4bt is typically around 1700-1800 RPM. You've found that the ZF gearing isn't ideal for the Cummins and neither is the NV4500. They were both originally gas transmissions and have gotten adapted to diesels. The ZF that came behind the 7.3 diesel has much better gear ratios but costs more to adapt so most don't go there. The SB Ford ZF is just a bolt up if you have the Ford adapter plate on the engine. If you spend a lot of off road time the SB ZF has a lower 1st gear which helps there and the 4.10 gear might be better. Same is true for the NV4500. One weak link on the ZF can be clutch size if you crank up the power too much. With a single turbo you are probably safe there. With twins it would never hold. Member Eggman can verify that. Something else to keep in mind is a diesel produces power depending on stress. If you gear too low your turbo may not boost all that much since it senses you don't need it. I owned a '73 F100 4x4 that was geared 3.73. Got exactly the same fuel mileage, 13.7 MPG, whether I was in city, on highway, or crawling around in the boonies. With your big, heavy tires, I'd guess yours will hit in the 20-25 MPG range so long as you stay below 2000 RPM. Maybe on the low side of that since the P pump doesn't get quite the mileage of the VE but has greater power potential.
 

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Discussion Starter · #9 ·
Thank you sir. After using the calculator numbers this is what I get.
3.73 (36" tire)
[email protected]
[email protected]
[email protected]

4.10
65mph @1850rpm
70mph @2035rpm
75mph @2150rpm

I drove my friends p-pump 4bt on 35's 3.73 gears and 4th gear seems to be his go to gear for downshifting, etc.

I ordered THD fuel pin and 3200 spring for vepump. It had a wh1c turbo but my buddy who knows diesel a lot more swapped the exhaust onto the baby h1 so now it's like 46/60/12 baby h1c turbo. I also will be adding a intercooler and I assume adjusting the timing a hair and pump as needed.

Do I sound like i.am missing or overlooking anything?

You mentioned something that caught my attention. Diesel produces power off stress. Humm. So pushing a old square bronco on 37's in the 1800-2000rpm should produce the boost it needs? Sounds like it.

Thanks again.
 

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I now have 3.55's and 33" tires and would really like gears in the 3.2x or as Red said 3.07's, with the right turbo the 4BT likes to work starting early on the uphill side of the torque curve compared to the respectable plateau beyond peak torque. Red had a double OD setup and it pulled well, IIRC engine cooling was the limiting factor he faced in double over.
I'd gear for 1,700 at interstate speed limit on OD if I were to start from scratch again...$.02
 

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Discussion Starter · #11 ·
Thanks Eggman. Yea not having real world experience with the 4bt and performance characteristics. I keep wondering about the drag the bronco will have with 37's and non aerodynamic body and how that affects the 4bt performance. I heard you don't want to lug the engines but I assume 1700-1800 is not considered lugging?

Thanks
 

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1,500-2,000 is where they are happiest, with where your turbo starts to make reasonable boost the deciding lower limit.
 

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I think you will find that turbo will still be a dog. Nothing you can do to that small H1C can solve its problem. The 12 cm2 housing helps some but it's no competition for an HX30W which has a 6 cm2 turbine housing. Any kind of performance mods tend to just make more heat but not much more power. Even with the smaller housing you probably can't get much over 20 PSI where the HX30W can go over 30 PSI. The H1C is just an obsolete unit.
 

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Discussion Starter · #15 ·
Thanks, yea I have been noticing people talking about the hx30. I guess I will look into pricing of this turbo.

Thanks
 

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Real world, my 69 Bronco weighs 5500lbs on 38s with 8-1/2" of lift. Its a brick, 16/17/18mpgs but have gotten better depending. I like my r's around 17/18. Chars right, lipstick wont fix that pig. I ran the HE221, HX27w and HX30W. Havent run the super 30. Have but havent tested a HX32W . I like the HE221 best so far, 35psi instantly with pedal. Ran HE221/WH1C compounds. Scary as hell and not practical for my use.
 

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Discussion Starter · #17 ·
Thanks for the feedback redheadbronco. I have not put this on the street yet but soon. I wanted to figure out what gears would be optimal for 70-75 plus on interstate. I know I didn't want to over gear it but based on many discussions and lots of reading it appears many people on 37's go 4.10. I see a lot on 35's go 3.73 so I was hoping to match those calculation numbers.
 

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Red has been there and done that with turbo combos and all he listed will make 15+ psi at 1,500 under a good load and at that psi you can feed it a good amount of fuel cleanly at that point you get the most from our narrow 500 RPM sweet spot, I have found that good early boost is much more useful / fun then great late boost.
The he221 was IMO the ideal single for a 200hp build, really good early boost that carries all the way to governed speed mine ran with a 500' tune up to 8,000' without any loss of drivability and all #'s still in the green just a great DD setup. The down side to the 221 is it's a T-25 flange but that is not a big deal once you take the plunge.
My RPM's are just lower than your 4.10's in post #9 I turn [email protected] gaining 5mph at that rpm would be ideal I "think"...
 

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From you numbers on the highway speeds, the 3.73 is about as perfect as you could hope for on a 4bt. If it was mainly going to be crawling around in off road places the 4.10 might get the nod. Would have to toy with the numbers to see how they stacked up. If 3.73 is the gear in place now, I'd run that and see how it works. Steve, your experience with the HE221W is probably why that turbo has seen heavy use in newer engines since the 1st era 4bts. Many of the industrials changed to the HX30W in the '90's but that was before the HE221W came into existence. It became the main turbo on the common rail engines and was in short supply. Now, the HE is common and reasonably priced. Has a few drawbacks with its odd compressor out and exhaust, but nothing that can't be overcome.
 

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Discussion Starter · #20 ·
Ok, so I sounds like I should just go ahead and bite the bullet and purchase a HE221W and just sale the h1c is anyone was interested in it.

Thanks.
 
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