NPR 4wd conversion and camper build thread.
- Thread starter javajoe79
- Start date
Matt decided on 4.56 for his NPR due to the gas engine. RPM calculations for his 35" tires are posted in the thread if you want to see them.
A 6BT is usually happiest about 1600 - 1900 RPM. A 37" tire runs well on a 4.10 in a diesel with an average O/D, so a 3.73 would be OK with a 35" tire, which I believe is what is on that truck. Quick copy and paste from my notes, so you have an idea of what you are looking at: (numbers in parenthesis are O/D ratios):
245/70R19.5 (33”
tire with 4.10s:
55mph = 1400 rpm (.61) ------- 1610rpm (.70) ------- 1790rpm (.78) ------- 2300 rpm (1)
65mph = 1654 rpm (.61) ------- 1900rpm (.70) ------- 2120rpm (.78) ------- 2715 rpm (1)
75mph = 1910 rpm (.61) ------- 2200rpm (.70) ------- 2440rpm (.78) ------- 3130 rpm (1)
285/70R19.50 (35” tire with 4.30s:
55mph = 1410 rpm (.61) -------- 1640 rpm (.70) -------- 1760rpm (.78) -------- 2260 rpm (1)
65mph = 1670 rpm (.61) -------- 1943 rpm (.70) -------- 2080rpm (.78) -------- 2670 rpm (1)
75mph = 1925 rpm (.61) -------- 2240 rpm (.70) -------- 2400rpm (.78) -------- 3080 rpm (1)
Allison Six Speed turning a 265/70R19.50 (34” tire with 4.10s:
55mph = 1355 rpm (.61) -------- 1577 rpm (.71) -------- 2222 rpm (1)
65mph = 1602 rpm (.61) -------- 1864 rpm (.71) -------- 2626 rpm (1)
75mph = 1848 rpm (.61) -------- 2151 rpm (.71) -------- 3030 rpm (1)
Allison Six Speed turning a 275/80R20 MPT with 4.88s: (565 revs per mile)
55mph = 1541 rpm (.61) -------- 1791 rpm (.71)
65mph = 1822 rpm (.61) -------- 2120 rpm (.71)
75mph = 2100 rpm (.61) -------- 2444 rpm (.71)
A 6BT is usually happiest about 1600 - 1900 RPM. A 37" tire runs well on a 4.10 in a diesel with an average O/D, so a 3.73 would be OK with a 35" tire, which I believe is what is on that truck. Quick copy and paste from my notes, so you have an idea of what you are looking at: (numbers in parenthesis are O/D ratios):
245/70R19.5 (33”
tire with 4.10s:55mph = 1400 rpm (.61) ------- 1610rpm (.70) ------- 1790rpm (.78) ------- 2300 rpm (1)
65mph = 1654 rpm (.61) ------- 1900rpm (.70) ------- 2120rpm (.78) ------- 2715 rpm (1)
75mph = 1910 rpm (.61) ------- 2200rpm (.70) ------- 2440rpm (.78) ------- 3130 rpm (1)
285/70R19.50 (35”
tire with 4.30s:55mph = 1410 rpm (.61) -------- 1640 rpm (.70) -------- 1760rpm (.78) -------- 2260 rpm (1)
65mph = 1670 rpm (.61) -------- 1943 rpm (.70) -------- 2080rpm (.78) -------- 2670 rpm (1)
75mph = 1925 rpm (.61) -------- 2240 rpm (.70) -------- 2400rpm (.78) -------- 3080 rpm (1)
Allison Six Speed turning a 265/70R19.50 (34”
tire with 4.10s: 55mph = 1355 rpm (.61) -------- 1577 rpm (.71) -------- 2222 rpm (1)
65mph = 1602 rpm (.61) -------- 1864 rpm (.71) -------- 2626 rpm (1)
75mph = 1848 rpm (.61) -------- 2151 rpm (.71) -------- 3030 rpm (1)
Allison Six Speed turning a 275/80R20 MPT with 4.88s: (565 revs per mile)
55mph = 1541 rpm (.61) -------- 1791 rpm (.71)
65mph = 1822 rpm (.61) -------- 2120 rpm (.71)
75mph = 2100 rpm (.61) -------- 2444 rpm (.71)
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6BT stock or turned up? Concerned about MPG?
The stock engines in these things do not like high speed or high RPM, at least not for passing power or good MPG. With a 6BT and Allison, that is a different story. I think a 37" tire is perfect for these trucks, if for no other reason than to bring the RPM's down some and fill in the massive wheel gap. That is not as bad on a converted truck as it is on the raised frame FG with a center diff. Why MFTB does that is a mystery to me....
1800 RPM and 68 MPH sounds good to me. Remember these things are bricks going down the road
Keeping Donaldson type intake and high cab exhaust?
The stock engines in these things do not like high speed or high RPM, at least not for passing power or good MPG. With a 6BT and Allison, that is a different story. I think a 37" tire is perfect for these trucks, if for no other reason than to bring the RPM's down some and fill in the massive wheel gap. That is not as bad on a converted truck as it is on the raised frame FG with a center diff. Why MFTB does that is a mystery to me....
1800 RPM and 68 MPH sounds good to me. Remember these things are bricks going down the road
Keeping Donaldson type intake and high cab exhaust?
javajoe79
Fabricator
I'd be inclined to use a taller gear but I guess considering the final weight and size of this rig, I should be content with 68mph. I don't want it to be a total dog down low. They also told me they installed a fuel plate. I need to verify if true and which one. May consider a turbo upgrade too. But ultimately I'm most concerned about fuel economy and cruising speed. Even though my background is in racing I need to be less focused on upping my power level with this project but if upgrades can yield better mpg then that's another thing.
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Czechsix
Watching you from a ridge
Sounds like a great project, and who knows? It's one direction I might eventually go with the 2012 I've got, if I get disgusted enough with the stock system. I've got a 2001 6BTA mated to a NV4500 sitting in a Suburban right now. I was planning on p-pumping it anyway, so I'll be watching what you're doing quite closely.
Nice project!
Nice project!
justbecause
perpetually lost
I am glad I stumbled into this thread. I became aware of that project because someone on pirate said they wanted to duplicate it, but I never saw an update from him or spike TV.
that 350 rear axle is rated for several thousand lbs less than the stock zuu axle. That may not matter for hauling a camper, but I believe it was too small for their proposed build on extreme.
that 350 rear axle is rated for several thousand lbs less than the stock zuu axle. That may not matter for hauling a camper, but I believe it was too small for their proposed build on extreme.
javajoe79
Fabricator
Yeah it was going to be one heavy SOB if they had ever finished it. Based on what similar trucks end up weighing I think I will be fine with this axle combo. If not, I can always upgrade.
javajoe79
Fabricator
I haven't done much lately but I needed room in the shop so I put the camper on the NPR frame. It's blocked up at about the height it will be once it's mounted on the flatbed so it's a good mockup. I will be shortening the wheelbase to align the wheels approximately with the opening in the side of the camper. The rear suspension as pictured is at full compression.
javajoe79
Fabricator
I've made some progress lately. I got most of one side skinned then I flipped the camper over to skin the bottom of the bed area. I've been making cap rails for all the corners where pieces of the skin come together. Makes for a "soft" edge.
I also bought a set of hummer wheels and tires. I am recentering the wheels to bolt to the ford axles. I got one rear wheel done. I am using 3/8" thick plate steel centers from trail worthy fab. They normally weld to the inside wheel half but I machined them to fit the front half to achieve the offset I wanted. The front wheels will be a different offset to make the track widths match front to rear. I will be just over 80" wide outside the tires. I bought the cheaper old Goodyear MT this time around as it will be a while before the camper is complete. Supposedly these tires don't ride or balance too well. In the future I will upgrade to a better tire and maybe one that is rated for more weight. These are rated at 3850lbs each. I am thinking I won't be over that weight once complete but I would still prefer a larger margin of safety. I will also be buying the PVC beadlock inserts for these wheels.
I used an indicator to measure run out and the specs didn't change from before to after. About .010" is the most I measured. I took my time and moved around as I welded to avoid any distortion from the heat.
I also bought a set of hummer wheels and tires. I am recentering the wheels to bolt to the ford axles. I got one rear wheel done. I am using 3/8" thick plate steel centers from trail worthy fab. They normally weld to the inside wheel half but I machined them to fit the front half to achieve the offset I wanted. The front wheels will be a different offset to make the track widths match front to rear. I will be just over 80" wide outside the tires. I bought the cheaper old Goodyear MT this time around as it will be a while before the camper is complete. Supposedly these tires don't ride or balance too well. In the future I will upgrade to a better tire and maybe one that is rated for more weight. These are rated at 3850lbs each. I am thinking I won't be over that weight once complete but I would still prefer a larger margin of safety. I will also be buying the PVC beadlock inserts for these wheels.
I used an indicator to measure run out and the specs didn't change from before to after. About .010" is the most I measured. I took my time and moved around as I welded to avoid any distortion from the heat.
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