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Ok this is my first time posting on this site. I’ve been reading a lot of forums to try to get a better understanding of doing a diesel swap. I’m not ready for the swap yet and this is more of looking for more clarity on what direction would be best to take for what I want to use the truck for. It was an ex military/ fire department truck has 9000 original miles never put on the road since put in private hands. I am wanting it to use on my property and go down the highway as well no huge trailer pulling. A mix of good MPG and highway speeds 60 to 65 is where I’m at. It is all stock. I currently own a Bobbed M35a2 so no stranger to the work that will be involved. I will have the truck home between now and March.
 

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What about firewall clearance and intercooler? I would be stuck with using an electric cooling fan as well am I correct due to engine bay size?What MPGs am I looking at? I am wanting to run a Spicer 3053a being that I am extremely familiar with it and do not care for the NV4500 or the GeTrag a Spicer is way better as far as strength and price except the 1000$ flywheel paywall with an SAE#3
 

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I personally would want a 6bt(cheaper too), but a 4bt will do the job. Firewall can be modified if youre handy with sheetmetal. 3053a is a good trans but have a bad 3rd to 4th spread for turbo. A close ratio ESO would be better for diesel. High teens low 20mpgs should be possible. Dodge 1st gen area. What is weight of vehicle? Dont rule out the zf 6 speeds, they are good transmissions but pricey. You should be able to pick up a SAE3 flywheel at a 1/4(1000) if you do some scouting before build.

Cummins FLYWHEEL 4939064 | eBay 1/2 price
 

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It weighs around 6500lbs GVW 10000lbs If I can locate a used flywheel I’d be golden but ya the 3053a has to have that flywheel in order to mate everything. The other concern with running a 6bt is the frame will need to have extra bracing and possibly an np205 transfer case stock one is the NP201. The stock gears in axles are 4.88s
 

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What about firewall clearance and intercooler? I would be stuck with using an electric cooling fan as well am I correct due to engine bay size?
This is an installation dwg for a 6bt marine. Some of the accessories don't apply but you can at least gauge the size and determine fitment.

https://www.sbmar.com/wp-content/uploads/2016/01/6BTA-Tier-2-high-mount-aux-Nov-05.pdf

MPG? That's a wild ass guess because your aerodynamics are unique & vehicle weight/rolling resistance is as well.
 

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Thank you for the diagram just wondering if it’s worth it with having to add more bracing to the frame to cope with the torque of the 6bt or if tuning a 4bt would be ideal won’t need more than about 160hp to 175hp. The gear ratio for the type of Dana 70s I have that I can go to are 4.10s anything else I’ll have to swap out the axles. I have thought a lot on this because the axles in mine particularly the front axle is almost identical to the stock M715 axles. There is no longer any disc brake conversions being made for them. So definitely have a lot of decisions to make once I get to that point.
 

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Thank you for the diagram just wondering if it’s worth it with having to add more bracing to the frame to cope with the torque of the 6bt or if tuning a 4bt would be ideal won’t need more than about 160hp to 175hp. The gear ratio for the type of Dana 70s I have that I can go to are 4.10s anything else I’ll have to swap out the axles. I have thought a lot on this because the axles in mine particularly the front axle is almost identical to the stock M715 axles. There is no longer any disc brake conversions being made for them. So definitely have a lot of decisions to make once I get to that point.
Thank you for the diagram just wondering if it’s worth it with having to add more bracing to the frame to cope with the torque of the 6bt or if tuning a 4bt would be ideal won’t need more than about 160hp to 175hp. The gear ratio for the type of Dana 70s I have that I can go to are 4.10s anything else I’ll have to swap out the axles. I have thought a lot on this because the axles in mine particularly the front axle is almost identical to the stock M715 axles. There is no longer any disc brake conversions being made for them. So definitely have a lot of decisions to make once I get to that point.
 

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Discussion Starter #10
I haven’t checked yet to see if all the frames from this era are the same or not across all models or if there are any differences if it’s the same as any of the other w series it will need bracing but will have to do some research The other reason I’m kind of not wanting to do the 6bt is I’m not sure the NP201 will hold up to it and I’d like to avoid unnecessary spending divorced 205s are not cheap and neither are electric fans or the alternators and batteries needed to run them.
 

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I know the 4bts are more expensive so will have to weigh the cost of one over the other and labor involved with each.
The extra cost of the 4 may be made up with later and labor savings. If youre dead set on a 4, do it. The Spicer has a minimal OD(.8?). I dont know what state you live in(sp limits) or the ratios that are in your vehicle but BTs like the 16-1900 rpm range for torque. Unless youre running 40s, youre going to want a deeper OD or taller gearing. If youre not familiar with this calc, you soon will be... Gear Ratio Calculator Its addictive. Check out the 3rd/4th shift, it sucks but many trans have this bad 3/4 spread. A better turbo and a 366 gov spring is the best fix besides a close ratio trans.

This could save you time and money...https://denver.craigslist.org/pts/d/loveland-1971-d100-dodge-cummins/7033505291.html
 

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You don't need more batteries to run an electric fan nor do you need megawatts of alternator. 105A does just fine.

You are going to spend money on getting 160-170hp out of a 4bt where this is a stock power level of a 6bt.

If you are concerned about the 201 at 170hp what difference does it make how many cylinders deliver that power? If anything a 6bt is gentler on drivetrain because torque is integrated over 6 pulses instead of 4.
 

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If you are concerned about the 201 at 170hp what difference does it make how many cylinders deliver that power? If anything a 6bt is gentler on drivetrain because torque is integrated over 6 pulses instead of 4.
True dat
 

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Actually, the way a 4bt fires vs the 6bt can have a damaging effect on drive train parts even if both have the same HP. A good example is the Gear Vendor OD unit. They will hold up fine with a 6bt but a 4bt will beat one to death. Whatever engine you go with, a SAE3 setup should be easy to find. Many 6bt's in medium duty trucks used that. The 4bt used it in industrial applications had them but I believe the Eaton used on the road trucks was SAE2. Might consider adding a Brownie OD in addition to the 3053a to get better open road gears. Not sure how that cost would compare to changing gear ratios in the differentials.
 

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Discussion Starter #18
Ya the current gears are 4.88s and the lowest I can go are 4.10s on the stock axles because of pinion location on them. Nothing is off the table at this point and I am considering an axle swap if I need to go lower in the diffs I’ve looked at whole trucks as well to make the swap cheaper. I think the Brownies are in the 1500$ range but again another idea to add to the pot. I am reading s threads on here now regarding the 6bt swap.
 

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The 1st Gen auto trans 3.07 axles, 3053a and 32" tires puts you at a nice rpm. They are getting hard to find though. Dont know where you reside but found several pairs in CA for a fair price. A 5831C in Salinas. C is a .85, the 5831/A/E are .73 od.
 

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Discussion Starter #20
I am in Arkansas larger parts I’ll try to locate hopefully in a reasonable driving distance. Once my Bobbed deuce sells this project will move forward a lot faster. I just want to at this point get a general direction to go in. It seems a whole truck might be the way to go because I need to relocate the gas tank as well as it’s in the cab and I never liked the idea of that for obvious reasons. 😅
 
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