[MaxPF]

I've taken the same trip towing the Casita using my 92 Dodge 5 speed manual, stock 5.9-160hp, with factory gearing, tires and it gets 3 mpg less. Dodge is turning 1920 rpm and Scout is turning 1870 rpm @ 65 mph. For me that's a big incentive to use the Scout more.

Interesting. I tow a pop-up camper behind my K5 on most trips. By the time you add in the weight of my porky truck, the loaded camper, gear loaded into the truck, and the dog I tip the scales right at 10,000lbs. Three years ago when I traveled to Big Lake I stayed right at the speed limit and didn't spend much time exploring off-road, and yielded a hair over 16mpg. That is with the engine turning about 2050rpm at 65mph. With the engine stock I was able to travel from the Bush/Beeline highway intersection to Payson without downshifting a single time (traffic cooperated on that one), and without dropping below 60mph. Climbing the rim required downshifting on numerous occasions due to traffic, but I was able to pull the grades in top gear if nobody slowed me down.

Added info for those who don't live in AZ: Mesa (where scout4bta and I live) is 1200ft. Big Lake is at 9000ft. Not sure of the round trip distance, but it's about a 4-5 hour trip one way, depending on speed and traffic.

 

[MaxPF]

that sketchy bearded dude is a notorious hack and a liar! haha.

LOL, next time I see him I'm gonna tell hin you said that!

while still third hand info, i know this guy as well. he has no reason to lie about the performance of his burb. he doesnt care what any of us think. it is one of the best conversions i have ever looked at. everything is well executed and top quality. there is no need to question his results. he is more than intelligent enough to do the mileage calcs and like i said, he doesnt give a poop if we believe him so he isnt going to lie.

Nick is very detailed, both in his build and his mileage/fuel logs. That much distance traveled means the error in average mpg is very low, so I fully trust his mpg claim. And no, he does not lie, exaggerate, or embellish his mileage claims. He has gotten crap mileage on a few tanks and readily admits it.

i wonder what the difference would be on his burb after the turbo swap? i would bet if he ran that trip again now that the turbo is more efficient he could do a little better.

I've wondered the same thing. I guess we need to tell him to do another road trip so we have more data

regardless of any of this, i think we can all agree that on a test stand the 4bt will win the consumption argument. if the 6 was going to be more efficient, would they have even made the 4? the differences will come from application and driver. so if the absolute bottom line is fuel consumption, put the 4 in it. just make sure it is set up to be efficient. if the same variables that we all have to consider when building a custom apply to this build as well, then there are more questions to be asked of the OP and more info needed.

Oh there is no doubt the 4 is more efficient. My entire argument is that for a heavy vehicle the relatively small difference in fuel economy is not worth the considerable difference in performance. That is my opinion, of course, but I have met very few people who feel differently. The OP's 7000lb van with a 4BT would have a WORSE power-weight ratio than a Hummer H1 with a 6.5L V-8, and I have yet to meet anyone who has driven an H1 (excluding the Alpha) who thought it had enough power.

 

[Dougal]

Yet you're perfectly fine comparing two completely different vehicles with completely different drivetrains

Massive amount? There are 28 trucks in the 1996 data. The rest have less. You cannot "guarantee" anything about gearing since nobody specifies their drivetrain. The highest probablility suggests they are largely stock, and many may have larger than stock tires which negatively impacts fuel economy. Were the trucks empty or loaded? City, highway, or mixed. Driving style?

The high (or even mid) 20's are not in heavy venicles. If they are, well, I will be polite and call the data extremely suspect.

There are a couple on the first page alone that I can compare my vehicle to. The first is in post 15:

Here's another one:

This guy lives in the same town I do, and that trip is similar to trips I take (although I am usually loaded heavier). Neither of those vehicles do much better than mine.

Yes I'm perfectly fine comparing two pickups of similar dimensions driven by the same guy in the same way. As I've said many times already they are the best results we have. Far from perfect, but still the best we have.

The fuelly data is 63 vehicles, 1997 data alone has 253 fill-ups, so it's an excellent sample size which I'd expect to cover the spectrum of vehicle use. Feel free to drill down through the data to individual vehicles and fill-ups for a check.

So you're good with 6BT's reporting 22mpg, but it's extremely suspect for a 4BT to report mid to high? That's some bias you've got there. It's perfectly okay to have a favourite engine. But you've got to realise that it's not the perfect engine for everyone else.

Yes there are genuine 30+MPG reports in there. This is what they have in common:
2wd vehicles which sit low to the ground.
Street tyres or tall and skinny ones pumped up hard.
Relatively high gearing.
Driven long distances at moderate speeds.

Don't expect anyone with 33" mud tyres in a taller 4wd to get respectable MPG. All that noise and wear the tyres suffer is energy wasted. Plus the aerodynamic losses, which alone make or break decent fuel economy.
That said, one of the Power Wagon owners did a genuine 30mpg run. But at quite low speed.

My personal best for the 4BD1T is around 8.6 litres/100km (27.3 USMPG) in a full-time 4wd with A/T tyres while cruising with summer holiday traffic on a route with rolling hills.
But even this result is poor compared to the exact same vehicle factory fitted with a smaller engine (300tdi) with less power which turns faster (2500rpm at 100km/h).

what are you using to back the guarantee? third hand info?

you are using Lonno's trucks as a valid comparison, but they dont even have a similar drivetrain. 4bt is ve pumped. the 6bt is p-pumped. we all know the inline pump will hurt mileage compared to the ve just because of the static timing. these are completely invalid comparisons. both are modified in some way. that makes the comparison moot again. the same reason why dieseldudes trucks are not a for sure comparison either. they are close to the same, and i would take that as more valid comparison, but without knowing the build on both rigs and what they are it is just speculation as to why or what causes the difference.

this is why i question calling into view the data on an 4bd1-t info and NPS chassis. its like comparing a 5.0 mustang to a 5.0 camaro. one is a roach and the other works well. just because they are similar in size doesnt make them a good comparison. if we are going to the effort of calling technical data up to back an argument it needs to be technical data for the engine in question or it certainly cant be taken as evidence for one side or the other. it is just circumstantial.

agreed. this is why the question i ask is what is the absolute restriction on the build, happiness with end result or fuel economy, and what circumstance is that economy value going to be in question.

that converted camper rig is sweet. needs a desert tan paint job and it would be perfect.

that sketchy bearded dude is a notorious hack and a liar! haha.

while still third hand info, i know this guy as well. he has no reason to lie about the performance of his burb. he doesnt care what any of us think. it is one of the best conversions i have ever looked at. everything is well executed and top quality. there is no need to question his results. he is more than intelligent enough to do the mileage calcs and like i said, he doesnt give a poop if we believe him so he isnt going to lie.

i wonder what the difference would be on his burb after the turbo swap? i would bet if he ran that trip again now that the turbo is more efficient he could do a little better.

regardless of any of this, i think we can all agree that on a test stand the 4bt will win the consumption argument. if the 6 was going to be more efficient, would they have even made the 4? the differences will come from application and driver. so if the absolute bottom line is fuel consumption, put the 4 in it. just make sure it is set up to be efficient. if the same variables that we all have to consider when building a custom apply to this build as well, then there are more questions to be asked of the OP and more info needed.

Out of a sample of 63 pickups I'm quite confident I'd find trucks with raised gearing. Through regears or just bigger tyres. The effect is the same.

How many kw in drive power do you think it takes to run a ve pump vs p pump?
The 4BT motoring friction I've been quoting is for an A pump 4BT. Comparable drive power to a P pump. The fixed timing hindering efficiency is only a problem with P pumps at higher rpm. At cruising rpm (where fuel economy is made or lost) they are timed correctly.

Put two different drivers in the same vehicle over the same route and they'll get different fuel economy. You can see 20% difference from driver alone.
Throw in the vehicles being driven in completely different places and comparisons get tenuous.

Then we're into fuel economy reporting.
Some people round off the miles, round off the gallons and do the maths in their head. Good enough for their purposes.
Some people calculate the occasional trip or fill-up. Too much hassle to do it all the time.
Some people calculate every fill up but never bother to average.
Some people keep a complete log and calibrate odometers to make sure their records are correct.

As you can imagine, there can be significant differences in reported MPG for what should be exactly the same data. There can also be wildly different data coming in and the same MPG being reported at the end.

This is why Fuelly is good. It's a running log for each vehicle. Only really good for stockers though.

The newer NPR/NPS expedition builds are much more impressive. That was the only example I could find on the net of a camper body on a pre 94 factory fitted with the 4BD1T. Mitsubishi do a comparable Fuso with similar sized engine.

I've had 3 6bt powered 3/4 ton trucks that averaged over 20-22mpg. If a full size truck is only getting 25 with a 4b I cannot see any reason to use one.

Was it 20 or 22mpg? There's 10% difference right there.

25MPG vs 20MPG is a 25% improvement.
In a vehicle that goes a long way from fuel stations that 25% greater touring range can be extremely important.
In a vehicle that's used only occasionally and never strays far from paved roads it's unlikely to ever be a problem.

 

[MaxPF]

Yes I'm perfectly fine comparing two pickups of similar dimensions driven by the same guy in the same way. As I've said many times already they are the best results we have. Far from perfect, but still the best we have.

Of course you are fine with it. Two very disimilar vehicles which give a nice, wide spread in mileage figures. I also doubt they are driven in the same way. They are NOT the best results we have. Not even close. Your assertions that they are a typical difference between a 4 and 6 cylinder Cummins are downright absurd.

The fuelly data is 63 vehicles, 1997 data alone has 253 fill-ups, so it's an excellent sample size which I'd expect to cover the spectrum of vehicle use. Feel free to drill down through the data to individual vehicles and fill-ups for a check.

They are stock 3/4 and 1 ton Dodge pickups, which are known to have less than optimal gearing and tuning for fuel economy. They are also heavier than the typical vehicle that receives a 4BT.

So you're good with 6BT's reporting 22mpg, but it's extremely suspect for a 4BT to report mid to high? That's some bias you've got there. It's perfectly okay to have a favourite engine. But you've got to realise that it's not the perfect engine for everyone else.

I'm fine with mid 20's from a 4BT in certain vehicles. I find most high 20's claims suspect, particularly given the weight, aerodynamics, and tire sizes of many vehicles sporting such claims. I have very good reasons for such doubts. One example was a guy with a K5 that was similar to mine (37" tires, manual trans) claiming 28mpg highway. Having used a G-Tech dyno to measure the overall drag of my truck, I knew how much HP was required to travel 65mph on flat ground. By using the BSFC curves (which, as you pointed out, are best-case) for his 4BT I concluded that 28mpg was physically impossible. Best case, he would get 22.5. The only way to get 28 was to travel continuously downhill. Or with a hurricane tailwind. Or being towed by another vehicle. Take your pick.

Yes there are genuine 30+MPG reports in there. This is what they have in common:
2wd vehicles which sit low to the ground.
Street tyres or tall and skinny ones pumped up hard.
Relatively high gearing.
Driven long distances at moderate speeds.

Don't forget halfway decent aerodynamics and light weight. Not exactly a valid comparison against a 7k+ lb brick $#!thouse on mudders flying down the highway at 65+mph.

Don't expect anyone with 33" mud tyres in a taller 4wd to get respectable MPG. All that noise and wear the tyres suffer is energy wasted. Plus the aerodynamic losses, which alone make or break decent fuel economy.

Yet, for some folks the intended use of their vehicle requires mud tires, a lift, and a vehicle with less than stellar aerodynamics. You know, something like what the OP is proposing.

That said, one of the Power Wagon owners did a genuine 30mpg run. But at quite low speed.

Which makes it irrelevant to the discussion at hand.

My personal best for the 4BD1T is around 8.6 litres/100km (27.3 USMPG) in a full-time 4wd with A/T tyres while cruising with summer holiday traffic on a route with rolling hills.

Good for you.

But even this result is poor compared to the exact same vehicle factory fitted with a smaller engine (300tdi) with less power which turns faster (2500rpm at 100km/h).

Perhaps you shoudl run that engine then? You seem to not have an issue with woefully inadequate power as long as you can wring every last bit of fuel economy from the vehicle.

Out of a sample of 63 pickups I'm quite confident I'd find trucks with raised gearing. Through regears or just bigger tyres. The effect is the same.

Good grief! Do you not know anything about physics? Bigger tires may achieve the same rpm results as taller gears, but they add rotating mass which has to be accelerated by the engine. They are also often wider, adding more rolling resistance. Bigger tires almost invariably reduce mileage, not increase it. Thus the OVERALL effect is NOT the same. Not even close. Thanks for playing though...

 

[roachie]

Was it 20 or 22mpg? There's 10% difference right there.

25MPG vs 20MPG is a 25% improvement.
In a vehicle that goes a long way from fuel stations that 25% greater touring range can be extremely important.
In a vehicle that's used only occasionally and never strays far from paved roads it's unlikely to ever be a problem.

3 different trucks
4 different gear ratios
3 different tire sizes
same transmission
3 different fuel systems (VE, P3000, VP44)


All averaged 20-22 depending on the activities used in that week. All had correct speedo's, all hand calculated, all filled up from the same place each time over the course of the last 7 years. Winter, mileage dropped to 18ish on the low side. The interesting part is the HP, the stock VE truck (except for the HE351 and 4" exhaust) is averaging 22. The VP truck had right at 400RWHP, also averaged 22 regularly. The P3000 truck had a slight decrease in mileage going from 4.10 to 3.31 gears, tires had no effect, still would average 20 if I kept the speed reasonable.

I dont see a truck with a 4b, loaded heavy, and going offroad getting a true 25% advantage in fuel usage.

On a personal experience note, every 4BT and 4cyl Izuzu I've driven in a stock application is a gutless wonder. Most of the driving experience was spent hoping to god you didnt get caught at a light on a hill.

 

[K204DR]

the 18 I get out of my truck, is done with a 4" lift and 35's. sometimes I get 19-20 but 18 is the normal. doesnt matter how I drive it, or who is driving it.

 

[scout4bta]

Interesting. I tow a pop-up camper behind my K5 on most trips. By the time you add in the weight of my porky truck, the loaded camper, gear loaded into the truck, and the dog I tip the scales right at 10,000lbs. Three years ago when I traveled to Big Lake I stayed right at the speed limit and didn't spend much time exploring off-road, and yielded a hair over 16mpg. That is with the engine turning about 2050rpm at 65mph. With the engine stock I was able to travel from the Bush/Beeline highway intersection to Payson without downshifting a single time (traffic cooperated on that one), and without dropping below 60mph. Climbing the rim required downshifting on numerous occasions due to traffic, but I was able to pull the grades in top gear if nobody slowed me down.

Added info for those who don't live in AZ: Mesa (where scout4bta and I live) is 1200ft. Big Lake is at 9000ft. Not sure of the round trip distance, but it's about a 4-5 hour trip one way, depending on speed and traffic.

Sounds like my Dodge was setup like yours is now, 4.10s 35s 5 speed. It's avg was 15.2 over 14K miles. 1.877 gallons per hr.
Difference is mine is a VE pump.

Fuel log

 

[Dougal]

Of course you are fine with it. Two very disimilar vehicles which give a nice, wide spread in mileage figures. I also doubt they are driven in the same way. They are NOT the best results we have. Not even close. Your assertions that they are a typical difference between a 4 and 6 cylinder Cummins are downright absurd.

You might need to re-read my comments on Lonno's results. He's showing a ~7mpg spread and I expect the differences in weight and gearbox to cover about 3 of those. The 7MPG isn't a typical spread.
You saw Dieseldudes results here: http://www.4btswaps.com/forum/showt...thread.php?62650-6BT-vs-4BT-fuel-consumption....&p=630666&viewfull=1#post630666

Riddle me this. If there was no significant fuel consumption difference between a 4BT and 6BT, then why did Cummins bother making the 4B? Same question between the 4B and 3B!

I'm fine with mid 20's from a 4BT in certain vehicles. I find most high 20's claims suspect, particularly given the weight, aerodynamics, and tire sizes of many vehicles sporting such claims. I have very good reasons for such doubts. One example was a guy with a K5 that was similar to mine (37" tires, manual trans) claiming 28mpg highway. Having used a G-Tech dyno to measure the overall drag of my truck, I knew how much HP was required to travel 65mph on flat ground. By using the BSFC curves (which, as you pointed out, are best-case) for his 4BT I concluded that 28mpg was physically impossible. Best case, he would get 22.5. The only way to get 28 was to travel continuously downhill. Or with a hurricane tailwind. Or being towed by another vehicle. Take your pick.

Don't forget halfway decent aerodynamics and light weight. Not exactly a valid comparison against a 7k+ lb brick $#!thouse on mudders flying down the highway at 65+mph.

I agree that we have ridiculous fuel economy claims on this site. That's why I mentioned ignoring the out-liers like the example above.

To make 30USMPG (7.84 litres/100km) at 100km/h (~63mph) it's 7.84 litres/hr and you're only producing about 30kw max.
If you get 24kw of those to the ground the vehicle takes less than 88kg of push (~200lb) to maintain speed on a flat road. Aerodynamics are essential, but weight isn't a huge player. My best fuel economy was when loaded to the roof but on a route where braking was seldom required.

Weight matters most when you're braking a lot (burning that hard earned energy off to heat) and having to replace it with more energy from the fuel tank. The extra inertia in larger tyres are exactly the same. I do a lot in the bike industry and many are paranoid about rotating weight vs static weight, but race results prove it's not that big a deal.

Road bike wheel manufacturers have gone for better aerodynamics with fewer spokes and deeper, more aerodynamic, rim sections. More rotating weight, but better aerodynamics and the race results back it up. Conserving energy matters when your engine only puts out 300 watts.
I'm not a road biker though.

Perhaps you shoudl run that engine then? You seem to not have an issue with woefully inadequate power as long as you can wring every last bit of fuel economy from the vehicle.

I do. I own several of these vehicles, one with the 4BD1T, one with the 300tdi and one with the 3.9V8. The V8 is the one I like the least and will be sold eventually. I have some interesting plans for the 300tdi.

Good grief! Do you not know anything about physics? Bigger tires may achieve the same rpm results as taller gears, but they add rotating mass which has to be accelerated by the engine. They are also often wider, adding more rolling resistance. Bigger tires almost invariably reduce mileage, not increase it. Thus the OVERALL effect is NOT the same. Not even close. Thanks for playing though...

I do quite well with physics.

The rotating mass isn't the bogey man people think it is. Unless you're always on the brakes it is no different to mass anywhere else on the vehicle. Bigger tyres have lower rolling resistance. Which is why bigger trucks run bigger tyres. Width and tread pattern are big contributors to rolling resistance but are optional extras.

 

[boothybunch]

300tdi :nuke:

 

[MaxPF]

I dont see a truck with a 4b, loaded heavy, and going offroad getting a true 25% advantage in fuel usage.

Indeed, you won't. In a lighter, more aerodynamic vehicle you can see a 25% difference, but we're talking about a chassis where a 6 is likely too long or heavy anyway. THAT is where the 4BT really comes in, and why they are a popular swap for sucj vehicles. The OP's 7000lb van does NOT fall into that category, The 6 may be too long, but it is certainly not too much engine for the chassis. That's why I mentioned that a factory DMax van might be the best choice. The 4BT will fit and work, but it's not going to have any power to spare.

the 18 I get out of my truck, is done with a 4" lift and 35's. sometimes I get 19-20 but 18 is the normal. doesnt matter how I drive it, or who is driving it.

I assume you're talking about your crewcab?

Sounds like my Dodge was setup like yours is now, 4.10s 35s 5 speed. It's avg was 15.2 over 14K miles. 1.877 gallons per hr.
Difference is mine is a VE pump.

Fuel log

That is the setup I have now. At the time of that particular trip I was running an SM465, NP241, 3.42 gears, and 37" tires on a 14bsf rear and 10b front.

One big parasite that is commonly overlooked is the engine driven fan. Dodge trucks have viscous clutch fans that are continuously driven. I have an electric fan clutch that only runs during warm weather in city driving. On the highway, even with the AC on, it doesn't run. Summer in-town driving with the fan engaged makes a noticeable difference in power/acceleration, so I can imagine how much it negatively impacts fuel economy.

BTW, Nick's 6BT suburban also has an electric fan clutch which no doubt contributes to the excellent mileage numbers.

 

[MaxPF]

You might need to re-read my comments on Lonno's results. He's showing a ~7mpg spread and I expect the differences in weight and gearbox to cover about 3 of those. The 7MPG isn't a typical spread.

And you have solid numbers to back up your assertions that weight and gearbox cover 3MPG? What about other sources of parasitic drag? It's a bad comparison, but I don't expect you to ever acknowledge the fact.

You saw Dieseldudes results here: http://www.4btswaps.com/forum/showt...thread.php?62650-6BT-vs-4BT-fuel-consumption....&p=630666&viewfull=1#post630666

Indeed. However, more data on the vehicles would be nice. Tire size and type, specific trans and t-case, etc. Some things that people think are insignificant can seriously skew the results.

Riddle me this. If there was no significant fuel consumption difference between a 4BT and 6BT, then why did Cummins bother making the 4B? Same question between the 4B and 3B!

Fascinating. I never said "there was no significant fuel consumption difference between a 4BT and 6BT". What I said was "Oh there is no doubt the 4 is more efficient. My entire argument is that for a heavy vehicle the relatively small difference in fuel economy is not worth the considerable difference in performance." That is a direct quote from my reply in post #62. Your attempt to misrepresent what I have stated only damages your credibility.

I agree that we have ridiculous fuel economy claims on this site. That's why I mentioned ignoring the out-liers like the example above.

To make 30USMPG (7.84 litres/100km) at 100km/h (~63mph) it's 7.84 litres/hr and you're only producing about 30kw max.
If you get 24kw of those to the ground the vehicle takes less than 88kg of push (~200lb) to maintain speed on a flat road. Aerodynamics are essential, but weight isn't a huge player. My best fuel economy was when loaded to the roof but on a route where braking was seldom required.

Weight matters most when you're braking a lot (burning that hard earned energy off to heat) and having to replace it with more energy from the fuel tank. The extra inertia in larger tyres are exactly the same.

You are absolutely wrong. Increased weight increases tire rolling resistance, even with the correct inflation pressure for the weight. Furthermore, increased weight makes a massive impact when you have to travel in mountainous terrain. Where I live the terrain is basically flat, but I cannot travel north or east without climbing significant elevation. Weight DOES matter. A lot! If it didn't, manufacturers wouldn't spend considerable engineering resources trying to lighten the vehicles they design.

I do. I own several of these vehicles, one with the 4BD1T, one with the 300tdi and one with the 3.9V8. The V8 is the one I like the least and will be sold eventually. I have some interesting plans for the 300tdi.

Again, good for you. Don't expect most folks on here to share your fascination with low powered engines in heavy rigs.

I do quite well with physics.

Then you should know better.

The rotating mass isn't the bogey man people think it is. Unless you're always on the brakes it is no different to mass anywhere else on the vehicle.

Have you heard the saying in aviation "Take-offs are optional, landings are mandatory."? In automotive circles it can likewise be said "Acceleration is optional, braking is mandatory." The reality is that most of us have to do stop-and-go driving on a regular basis. Whether in-town, off-road, braking for the slow poke in the fast lane, or reducing speed on a descent.

Bigger tyres have lower rolling resistance. Which is why bigger trucks run bigger tyres.

Assuming the same tread and section width, a bigger tire may or may not have lower rolling resistance. Big trucks run bigger tires because you can load more weight on a larger tire. They are also less susceptible to damage from road debris, potholes, etc. and have longer life. Larger tires on trucks are a practical tradeoff which makes sense. Large truck tires are designed with section widths and tread designs optimized for good wear and economy. This is not necessarily the case for the larger light truck tires (33+ inches in diameter) which are typically designed for good off-road attributes.

Width and tread pattern are big contributors to rolling resistance but are optional extras.

In the world of light truck tires extra width almost invariably accompanies increased diameter, and that width increases rolling resistance. It also increases tire mass even further. These off-road oriented tires generally have heavier, tougher construction which increases mass still further. For those of us who need tires with such capabilities it's a necessary trade-off. Whether you like it or not, Dodge trucks on the Fuelly site with larger tires are going to burn more fuel, regardless of whether it improves overall gearing.

 

[Lonno]

FWIW, I originally had a mechanical fan but after running way too cool decided to put an electrical fan on a switch which I found I never used even during 100*heat. I have since removed the fan and never run hotter than 175* after the thermostat opens. So remove that from your equations about parasitic losses due to clutch fans.

 

[Dougal]

Indeed, you won't. In a lighter, more aerodynamic vehicle you can see a 25% difference, but we're talking about a chassis where a 6 is likely too long or heavy anyway. THAT is where the 4BT really comes in, and why they are a popular swap for sucj vehicles. The OP's 7000lb van does NOT fall into that category, The 6 may be too long, but it is certainly not too much engine for the chassis. That's why I mentioned that a factory DMax van might be the best choice. The 4BT will fit and work, but it's not going to have any power to spare.

The slower you go, the bigger the fuel economy difference between big and small engines. What's the idling fuel consumption of a 6BT?

Just how much power do you think you can use offroad? It's a fraction of what you can use on-road unless you plan to play "mud-rail".

100kw at the wheels will pull a 3.5T vehicle (7,700lb) up a 30 degree slope at 20km/h.

One big parasite that is commonly overlooked is the engine driven fan. Dodge trucks have viscous clutch fans that are continuously driven. I have an electric fan clutch that only runs during warm weather in city driving. On the highway, even with the AC on, it doesn't run. Summer in-town driving with the fan engaged makes a noticeable difference in power/acceleration, so I can imagine how much it negatively impacts fuel economy.

BTW, Nick's 6BT suburban also has an electric fan clutch which no doubt contributes to the excellent mileage numbers.

And you have solid numbers to back up your assertions that weight and gearbox cover 3MPG? What about other sources of parasitic drag? It's a bad comparison, but I don't expect you to ever acknowledge the fact.

Viscous fans aren't continually driven. Unless they're seized. They only lockup and drive when they are required.

We're going around in circles again. Your "other sources of parasitic drag" which we covered a few pages back are not engine specific and thus irrelevant.

Indeed. However, more data on the vehicles would be nice. Tire size and type, specific trans and t-case, etc. Some things that people think are insignificant can seriously skew the results.

I think your reponse to that says it all. Clear cut example showing exactly what happens and you won't beleive it. Instead keep going around in circles looking for other things to muddy the waters.

It's extremely simple. Smaller engines use less fuel. The only exceptions to this rule are found by comparing crappy small engines to excellent bigger engines. But that doesn't apply here.

If the OP wants a 6BT then he'll try to fit one. This will happen regardless of you trying to claim a win for the 6BT in categories which it physically cannot win.

Fascinating. I never said "there was no significant fuel consumption difference between a 4BT and 6BT". What I said was "Oh there is no doubt the 4 is more efficient. My entire argument is that for a heavy vehicle the relatively small difference in fuel economy is not worth the considerable difference in performance." That is a direct quote from my reply in post #62. Your attempt to misrepresent what I have stated only damages your credibility.

See the two bits in red. They are exactly the same thing.

 

[MaxPF]

FWIW, I originally had a mechanical fan but after running way too cool decided to put an electrical fan on a switch which I found I never used even during 100*heat. I have since removed the fan and never run hotter than 175* after the thermostat opens. So remove that from your equations about parasitic losses due to clutch fans.

That pretty much matches my experience. In the summer, when Ac becomes a necessity, the fan is always running unless I'm cruising on the freeway or highway. It makes a big difference in available power and fuel consumption. During winter it's never on. Most people complain about their fuel economy dropping in the winter due to the winter fuel blend, but my experience is the opposite: I get MUCH better mileage in the winter when I don't have to run the AC compressor and that fuel-sucking fan.

 

[Dougal]

FWIW, I originally had a mechanical fan but after running way too cool decided to put an electrical fan on a switch which I found I never used even during 100*heat. I have since removed the fan and never run hotter than 175* after the thermostat opens. So remove that from your equations about parasitic losses due to clutch fans.

I've got the same experience, I only run my electric fans for crawling up hills offroad and using the AC.

You don't run AC in your trucks? Last time I was in San Diego it was winter and still pretty warm.

 

[Mario85]

This is definitely turning into a rehash of the Waggy thread. I will say that personally I'd much rather have power on tap than an extra 2-4 mpg. This isn't my semi (even then I prefer power to mpg especially given what I haul). We can argue engineering all day long. However I'd say by the butt dyno, you'll be far happier given what you've said with a 6bt.

 

[Mario85]

To further expand on my semi comment. profit wise more mpg over the year is more profitable. But IMO in a pleasure vehicle, if 2-5 mpg will break you, maybe you should find cheaper hobbies .

 

[MaxPF]

The slower you go, the bigger the fuel economy difference between big and small engines. What's the idling fuel consumption of a 6BT?

Just how much power do you think you can use offroad? It's a fraction of what you can use on-road unless you plan to play "mud-rail".

100kw at the wheels will pull a 3.5T vehicle (7,700lb) up a 30 degree slope at 20km/h.

Good grief! Who said ANYTHING about off-road? His RV will be doing the majority of his traveling on-road. At highway speeds.

Viscous fans aren't continually driven. Unless they're seized. They only lockup and drive when they are required.

Viscous fans never drop to zero. If you think they do, feel free to reach in and stop one on a cold, running engine. Let us all know how that works out.

We're going around in circles again. Your "other sources of parasitic drag" which we covered a few pages back are not engine specific and thus irrelevant.

Last time I checked the parasitic drag burns engine power, and thus fuel. That is power which never makes it to the drivetrain, let alone the wheels, so it is relevant.

I think your reponse to that says it all. Clear cut example showing exactly what happens and you won't beleive it. Instead keep going around in circles looking for other things to muddy the waters.

It appears that only you would define a request for more (accurate) information as "muddying the waters".

It's extremely simple. Smaller engines use less fuel. The only exceptions to this rule are found by comparing crappy small engines to excellent bigger engines. But that doesn't apply here.

Again, I never claimed otherwise. The question is how MUCH difference. It's clear that the 50% larger engine doesn't use 50% more fuel. Your cherry-picked examples yield 25%. Vehicles that are actually closely matched yield a much lower percentage.

If the OP wants a 6BT then he'll try to fit one. This will happen regardless of you trying to claim a win for the 6BT in categories which it physically cannot win.

You're the only one trying to make it a win-lose contest by focusing solely on fuel economy. The rest of us understand that there is a balance between performance and fuel economy, and that the ratio between them puts the 4 at a severe disadvantage. Particularly in a heavy vehicle, which is what the OP has.

See the two bits in red. They are exactly the same thing.

You're being disingenuous. A difference of 22.19mpg and 22.20mpg, which is well within the error of calculation, would be "no significant fuel consumption difference". The difference between 20mpg and 22mpg is a "relatively small difference". They are far from being "exactly the same thing". Again, your credibility suffers.

 

[MaxPF]

To further expand on my semi comment. profit wise more mpg over the year is more profitable. But IMO in a pleasure vehicle, if 2-5 mpg will break you, maybe you should find cheaper hobbies .

I think we have the winning post of the thread

 

[Dougal]

I think we have the winning post of the thread

Shame it's fiction. See it's not 2-5mpg and it's not whether it breaks you or not.

We're seeing on pickups the difference between 20 and 25mpg. That's a 25% improvement or a 20% reduction depending on your starting point.

Good grief! Who said ANYTHING about off-road? His RV will be doing the majority of his traveling on-road. At highway speeds.

Viscous fans never drop to zero. If you think they do, feel free to reach in and stop one on a cold, running engine. Let us all know how that works out.

Roachie and You did. You keep mentioning offroad tyres and stuff, then switch back to highway when it doesn't suit. Remember it's a 4x4 van which suggests it won't be always on the tarmac.

I have stopped viscous fans, it's a standard test to see if they are still working and not seized. But smart people use something like a paint-brush. Never, ever use your fingers kids. Stopping them is about the inertia of the fan and that's when you can lose fingers. Keeping them stopped takes very little effort, about 1Nm of torque.
1Nm of torque at 2000rpm means the fan is using 220 watts (about 1/3 of a hp).

Again, I never claimed otherwise. The question is how MUCH difference. It's clear that the 50% larger engine doesn't use 50% more fuel. Your cherry-picked examples yield 25%. Vehicles that are actually closely matched yield a much lower percentage.

You're the only one trying to make it a win-lose contest by focusing solely on fuel economy. The rest of us understand that there is a balance between performance and fuel economy, and that the ratio between them puts the 4 at a severe disadvantage. Particularly in a heavy vehicle, which is what the OP has.

You may need to scroll back to page 1 and start reading. No-one has yet claimed they use 50% more fuel. I have claimed there is 50% more internal friction and 50% more internal heat-loss. This transposes to ~2.5 litres/hour of diesel more to run a 6BT than a 4BT. This fits very well with the real world examples we have.
There are no good examples which backup claims of much closer fuel economy.

Why would I argue on performance, when there is no argument on performance? If you want big power, you find a way to fit the 6BT or some other, much bigger, engine.

The 4 cylinders however, push far bigger RV's around without any problems:

Truck specs here: http://www.isuzu.com.au/Isuzu_Files...u_Files/Spec_Sheets/Historical_Spec_Sheets/2009-2011/NPS250_NPS300-4X4-Crew.pdf