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Discussion Starter #1
I have been lurking around here for quite some time and have decided upon a 6BT over the 4BT for my Wagoneer.

The project is a cherry 1991 Grand Wagoneer. I will be using Dodge Dana 61/71 axles with the 3.07 gears. Trans is a NV4500 and the tcase a NP205.
I'll be using Cherokee wide track front fenders and grafting Cherokee WT flares and wheel wells in the back. This will be a DD and mild trail useage vehicle built with fuel economy in mind.

The engine is a 91 Gen 1 non intercooled 6BT. It has 60,000 original miles on it and has been very well maintained. It will have the KDP, Gov Spring conversion, Pump turned up a bit, and an HX35 on it.

Before I install the axles, I have a question for you guys. With the 3.07's and 33" tires I'll be running at around 1620 rpms at 70 in 5th gear. I figured I would have plenty of torque at this rpm on the highway and fuel mileage would benefit from running at lower rpms. I am getting mixed signals from people I have discussed this project.

One camp thinks running at that low RPM at highway speeds will not be an issue and I may see somewhere around 25mpg. They believe the engine produces enough torque to handle this scenario.

The other camp thinks that with the poor aerodynamics of the Wagoneer I'll be lugging the engine, get poor fuel mileage and I will discover that the OD is useless. Some think 1800 rpms would be the perfect cruising rpm for this setup and others think I should be cruising at around 2100 rpms.

So, what do you guys think my ideal cruising rpm range for best fuel mileage should be with the Wagoneer?
 

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You'll get your best mileage when your cruising speed matches your peak torque. The engine will operate the most efficiently at that point.

We just changed tire size on our 24 Valve truck & I've seen a noticeable drop in mileage. It's a different animal compared to your 12 Valve though, ours is a '03 Common Rail 5.9, but I believe the principal still applies... our truck has 3.55 gears and the stock tires were 265's. We moved up to 315's and I was hoping to get better mileage, but IMO it seems like we're now lugging the engine & using more fuel when getting up too speed. Crusing RPM's are about 2000 RPM @ 75 MPH.

I'd think you'd be better off with a higher RPM range than with 3.07's, 33's and the NV4500. Perhaps 3.55's?
 

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1989 Jeep Wagoneer, 360v8, 727, stock for now,
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I'd say try it the way you think will work, the 33's, 3.07's and OD. If the mileage is not what you think it should be, or the EGT's are too high, you can easily go to a smaller diameter tyre. I would suggest you add the intercooler if you havent all ready thought of it, to keep the intake cool at those low rpms.
 

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My thoughts would be using the 31.5" LT265-75 tires with your 3.07 gears. Running at 1800 RPM this puts you at 55 MPH in 4th gear and gives you an "interstate highway" OD gear at a tad over 75 MPH in 5th gear. Sometime in the near future I plan on running the same set up in a Dodge crewcab. Lugging these engines while dumping fuel into them creates a smokey beast that will probably have constant EGT issues.
 

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I am planning on undertaking a similar project (65 wagoneer) and am curious how you decided between the 4 or 6 cylinder because I have to make that decision myself. If fuel economy is a goal the 4bt seems more of the way to go, what other factors were considered?
 

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Discussion Starter #6 (Edited)
I am planning on undertaking a similar project (65 wagoneer) and am curious how you decided between the 4 or 6 cylinder because I have to make that decision myself. If fuel economy is a goal the 4bt seems more of the way to go, what other factors were considered?
Noise and vibration on the 4BT were the primary reasons I went with the 6BT. Secondary reason is that I plan on towing my off road only M715 with this Wagoneer in mountainous country. No way is the 4BT up for that. The 1st Gen Cummins 6BT fits the bill perfectly for me. Plus I got a fleet maintained 60,000 original mile 6BT in excellent condition much cheaper than I could have bought a 4BT for. Also keep in mind I am not going coming into this blindly. Probably the best Cummins 6BT FSJ M715 in the country was built buy a good friend of mine and I was there during the whole buildup and understand it. This truck had massive HP/TQ, pulled in competitive events, and towed insane loads for the last 7 years. Plus it was a DD, so I have a good example to go buy. I have also done GM diesel swaps into FSJ's in the past with excellent results.

I have been doing a lot of resarch on the net about this and still have no conclusive answer but I am leaning towards 3.54's and an 1850 rpm cruise range. Apparently, according to TDR, 1st gen pumps generate best fuel economy around 1800 rpms.

I did find out that Dodge did offer the 3.07 axle WITH the overdrive transmissions as an option.
 

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the early 6bts from 89-93 used a different cam profile than the 94-98 12v's both have peak torque and optimal operating ranges of 1700 and 1750rpms the early 6bts from 89-93 functioned very well down to 1600 rpm. 89-93 , motors had a narrower power band from 1500-1900 rpm but it produces more torque in that range than the 94-98 12's did in the same range with less smoke, due to the different cam profile producing higher cylinde pressures.

i ran 3.07s with a Getrag on a Dodge 250 with 285/75 R16s and i loved it i originally had 3.54s' and got good mileage 20-22 mpg with the jump up to the 3.07s, i spent alot more time towing in direct witch was fine but my empty my mileage was 24-25 consistantly. with the smaller tires and 3.07's i saw mileage numbers identicle to the 3.54's and 285's since my rpms climbed back up. If you think a Waggy is a flying brick you should stick it in a wind tunnel with a 89 dodge i betcha they both would come in close to each others.

all of my first gens love 1700 rpm my later p7100 pumped trucks like 1750-1800 i dont know about TDR's numbers but my vp44 24v's liked 1900rpm for mileage though my common rail loves 1750-1800

As another note my 94 dodge did not have the torque like my 89 dodge did and the 94 did not like the lower rpms like my 89 did either . food for thought.
 

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Discussion Starter #8
the early 6bts from 89-93 used a different cam profile than the 94-98 12v's both have peak torque and optimal operating ranges of 1700 and 1750rpms the early 6bts from 89-93 functioned very well down to 1600 rpm. 89-93 , motors had a narrower power band from 1500-1900 rpm but it produces more torque in that range than the 94-98 12's did in the same range with less smoke, due to the different cam profile producing higher cylinde pressures.

i ran 3.07s with a Getrag on a Dodge 250 with 285/75 R16s and i loved it i originally had 3.54s' and got good mileage 20-22 mpg with the jump up to the 3.07s, i spent alot more time towing in direct witch was fine but my empty my mileage was 24-25 consistantly. with the smaller tires and 3.07's i saw mileage numbers identicle to the 3.54's and 285's since my rpms climbed back up. If you think a Waggy is a flying brick you should stick it in a wind tunnel with a 89 dodge i betcha they both would come in close to each others.

all of my first gens love 1700 rpm my later p7100 pumped trucks like 1750-1800 i dont know about TDR's numbers but my vp44 24v's liked 1900rpm for mileage though my common rail loves 1750-1800

As another note my 94 dodge did not have the torque like my 89 dodge did and the 94 did not like the lower rpms like my 89 did either . food for thought.
Thanks!

Since this is a fuel economy first project, I want to keep the rpms down as much as possible without lugging the engine. Since I have a set of 3.07's and 3.54 axles I'll experiement with the 3.07's based on your experience with them. I never planned on using 5th except on the highway at 70+ speeds anyway and expected to be using 4th a lot.

Is there anything else I can do to boost fuel economy in the 91 6BT. I have a 12 cm exhaust housing with an HX35 with 4" exhaust, would a different size housing give better fuel economy such as the 16cm? What about injectors? Would the smaller later injectors be better? I do not mind de-tuning it a bit if the fuel mileage increase was great enough to warrant it. I also do not mind spending money on mods that will give results. Primary goal for this engine is fuel economy and reliability. The 1st gen engine seemed perfect for this goal over the other models.

I'll be doing the axle swap this weekend and hopefully start the engine swap soon after that.
 

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You'll get your best mileage when your cruising speed matches your peak torque. The engine will operate the most efficiently at that point.

We just changed tire size on our 24 Valve truck & I've seen a noticeable drop in mileage. It's a different animal compared to your 12 Valve though, ours is a '03 Common Rail 5.9, but I believe the principal still applies... our truck has 3.55 gears and the stock tires were 265's. We moved up to 315's and I was hoping to get better mileage, but IMO it seems like we're now lugging the engine & using more fuel when getting up too speed. Crusing RPM's are about 2000 RPM @ 75 MPH.

I'd think you'd be better off with a higher RPM range than with 3.07's, 33's and the NV4500. Perhaps 3.55's?

if you look at the fuel consumption curves (there is one for a 4bt on the yahoo group maybe 6bt too) you will see that best fuel consumption and peak torque do not occur at the same rpms. at least not on the 4bt i was looking at. Just something to think about, i always thought it was where the HP and TQ curves intersected...

There really is a lot to fuel economy, what works in one vehicle may not work as well on another.
 

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... i always thought it was where the HP and TQ curves intersected...
No, those always intersect at 5252 rpm if using a 2 scale graph(commonly a single HP and torque scale on the left side, rpm at the bottom).

Some graphs use 3 scales, with HP on one side, rpm at bottom and torque on the opposite side of HP. This type of graph has the HP and torque scales sized to fit the curves to a similar area, thus where the lines cross is meaningless.

Regardless, HP = torque in foot-pounds at 5252rpm by definition(of HP).

Ken
 

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Welcome Pascal :)
 

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No, those always intersect at 5252 rpm if using a 2 scale graph(commonly a single HP and torque scale on the left side, rpm at the bottom).

Some graphs use 3 scales, with HP on one side, rpm at bottom and torque on the opposite side of HP. This type of graph has the HP and torque scales sized to fit the curves to a similar area, thus where the lines cross is meaningless.

Regardless, HP = torque in foot-pounds at 5252rpm by definition(of HP).

Ken
hahahah, you are right. i know the calculation but apparently i was a little :rasta: that day.
 

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I have a 01' 24V 5spd with 315's and 3.73's. I have the Banks Big Hoss Kit, AFE filter, 4" Flowmaster turbo back kit, and the Edge fuel box. I get a consent 19.5 unloaded and 14.2 loaded (8500 lbs). The only problem I see with my numbers is I live in the moutains, so I'm either pulling up or being pushed down a mountain.:dustin:
 
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