If you wish to tow a trailer, etc., the 6 would probably be more useful. If you have the 4, and have to work it hard all the time, you'll probably burn more fuel. Other considerations are D-60 front axle for the 6, 4 could probably get by with a D-44, if one is gentle with it, engine room, trans selection, etc.

 

Cummins have a spec sheet for 4BT CPL2303 which states how much power is required to turn the engine at 1500 and 2500rpm.

If you can find a similar spec sheet for a 6BT then you can estimate how much more fuel the 6BT will use at the same load.

 

Find it and post it up.


As for dan's post

It's all down to what you plan to use the vehicle for. If you plan to drive it and not tow or tow small loads, the 4bt would be the better option. But if you plan to work the truck and haul medium to heavy loads, you will need the extra power of the 6bt. Also because you would not be working the 6bt as hard, it would get better mileage than the 4bt in said situation.

it's swings and roundabouts.

 

heres a couple charts that i found real quick. to address the original question, id say a 4bt will do what you want as long your not towing much behind it. at that point the 6bt is going to be more reliable and offer the additional power wanted.

on a side note, i used to work for a company that had several 1 ton chevy vans with 5.7L v8s. they wanted to reduce their fuel bill so they special ordered a 3/4 ton van with a 4.3L v6 as an experiment. its the same bore and stroke as the v8 with 2 cyl chopped off. I dont know the gearing in either van but after the V6 was loaded full of construction tools it obviously had less power and actually consumed more fuel then the v8 vans did. I never drove that van so i have no first hand experience and im sure there are many variables involved but i found it to be interesting enough that i remembered it.

this sounds like a cool RV. best of luck with your build

 

heres a couple charts that i found real quick. to address the original question, id say a 4bt will do what you want as long your not towing much behind it. at that point the 6bt is going to be more reliable and offer the additional power wanted.

Those charts are have the BSFC (efficiency) curves, but they don't have the test data. The CPL2303 chart has a second page which includes really useful data like "motoring friction" which is how much power it takes to turn the engine over at rated torque and rated power.
The BSFC curves are only good for full load and at best efficiency point at full load on a 4BT you are burning ~20 litres/ hour. Best efficiency point on the 6BT and you're burning ~31 litres/hour.

Even in a 7000lb RV you're looking to use a lot less than that.

on a side note, i used to work for a company that had several 1 ton chevy vans with 5.7L v8s. they wanted to reduce their fuel bill so they special ordered a 3/4 ton van with a 4.3L v6 as an experiment. its the same bore and stroke as the v8 with 2 cyl chopped off. I dont know the gearing in either van but after the V6 was loaded full of construction tools it obviously had less power and actually consumed more fuel then the v8 vans did. I never drove that van so i have no first hand experience and im sure there are many variables involved but i found it to be interesting enough that i remembered it.

this sounds like a cool RV. best of luck with your build

Petrol engines hit enrichment near full throttle (full load) and this wastes a lot of fuel to protect the engine from detonation. The more time you spend at full throttle in a petrol the more your fuel economy suffers. Which is likely what you're seeing in those vans.

But diesels don't need or use enrichment. They are most efficient at full load (provided they aren't smoking) when geared to hit max torque. For the 4BT chart you linked up the best operating full load operating point is 1700rpm. It's producing ~85kw at that point

 

my dads 2wd 98 chevy gets 16-17 with a 4bt and 5speed, my buddy has a 97 ford van, converted to 4x4, 4bt 5speed, he gets 17-18. I get 18 out of my 88 crewcab with a 6bt 5speed. guess who has more fun? 4bt's work best in smaller vehicles. my crewcab is 6900lbs.

 

Geese , this is encouraging ..........

After reading all the answers yet , seems like the 4BT woulndt be that much of a difference on fuel consumption ......And power Wise the 6BT is more in my taste )
That`s great because I already own a 90 Ram with a 6BT with NV4500 and Dana 60 with posi ......( I dint mention it , not to jeopardize the answers)

I asked , mostly because I might take a Savana/Express as a base for my 4x4 RV .......I found a member`s thread about converting a savana with a 5.9 L 6BT .....tight but feasible..... ( not allowed to put the link yet ;-( )

it will be either that or a small cube ( wider body ) ......
keep the advices coming ....really apreciated !
love this site !!!!

Thanks ,

Dan

 

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...

 

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.

 

I'd say go with the 6bt if you plan to do any towing at all. I have both, and the horsepower difference, though slight, is noticeable. The 4bt had not been the economy motor some have posted up getting 40mpg. I'm in the 20-25 mpg unloaded and it drops off significantly with the hammer down. I pulled my camper (32' HR) again this week since it was cool and I didn't have to worry about engine heat. It handled it, but I had to keep the rpm's up and boost at about 20-25 psi for long periods in the hilly areas. With the fuel down to keep EGT's in check, it grunted along. I do have an HY30 to put on it and I'm hoping it wakes the lower end up a bit. Both are good motors. It really comes down to what edpruss said, "If you have the 4bt". The 6bt's are going to be easier and cheaper to get for most folks. Enjoy!

 

My F250 weighs 6k and I get 22/24 mpg driving normally. My 98 12v weighs 7k ++ and barely gets 16/17mpg driving fairly easy.

That being said the 6bt will be cheaper motor to start with and will be working much easier to do the same job.

Dave

 

A 7000 lb RV isn't exactly light. If you look at this from an economic stand point, the 6bt will be cheaper, assuming the vehicle will accommodate it. The 4bt will probably get a little better fuel mileage, but it won't be gigantic. Probably like low 20's for the 4bt and upper teens for the 6bt. So much of that depends on where and how you drive. To get the 4bt to the same power level as the 6bt will cost a little money plus the initial price may be as much as $1000 more. There are a lot of 4 cyl diesels in delivery vans but those are just that. Delivery vehicles. It would be rare to see one of those running the interstate at 65-75 mph for thousands of miles on a vacation trip. There are 4 cyl Cummins engines used in trucks and buses, but most are not available in this country. We could only wish. The ISBe 4.5 210 would be a heck of an engine in most any vehicle and you could buy it in most any country except here. Of course most of us couldn't afford the darn thing anyway.

 

I think if the US had access to some of the diesel engines that are over here in the uk/europe, there would be some outstanding conversions. But the damn powers that be have put a stop to that didn't they.

 

the vans the 4bts came in were setup to run at 55mph not towing a trailer. they are fine if you're just going to drive the vehicle. if you are going to pull a trailer, you need the bigger engine. the 4bt could pull it, but its not gonna last as long, and mileage will suffer.
my 6bt has 400k on it and it still runs great. I don't think you'll get 400k out of a 4bt if you expect it to lug a trailer around. Just depends on what your intentions are for the vehicle. rv's are typically maxed out without a trailer by the time you get all loaded up to go somewhere.

 

6bt is the way to go. Cheaper, easier to find, and fuel economy is not going to be much worse than a 4bt. The 4bt will be loaded all the time and the 6bt will not be working as hard. The difference in the two will be around 3 mpg give or take a little, not enough for all the extra hassle of the 4bt swap. Just install a 6bt and smile at all the extra power.

 

RED CARD, GET IT BACK ON TRACK OR I'LL REPORT YOU here comes the jacks

 

I am not allowed to comment

 

heres another way to put it. my 6bt runs between 700-800 on the pyro going down the interstate unloaded. my friends van with the 4bt runs at 1200-1250 at the same speed. he's maxed out just driving at highway speed. can't be good for the engine, much less if he needs to accelerate to pass he can quickly peg the egt's.

 

My 4bt runs 600* @ 70mph with 12/14#'s boost. I don't even break 1000* going uphill @ 70 with 25/30psi on the single Hx30. Is your friends van heavy or does he have a big exhaust housing on the turbo?

 

A while back I made the same trip 500 miles Bakersfield CA. to Flagstaff AZ. first trip was at ~11,000# CGVW with a average speed of 68mph and I got 14mpg the second trip was ~16,000lbs CVGW with an average speed 50mph and I got just over 18mpg so I "think" engine speed has more to do with efficiency than the weight you are moving.Another gauge is that my truck will top out at ~80mph weather it is on level ground or pulling a 6% or 7% grade so it would seem that drag trumps weight as far as overall/total engine load.I also agree with Dave in that my 4BT is happier loaded vs empty,on the first trip i mentioned I had the throttle to the floor for ~200 miles straight most of it uphill 2300/2400rpm 25psi with 1150*egt's on the grades and it ran flawlessly, what more can you ask of an engine with less than 4 liters.

 

yes its heavy, its converted to 4x4, with a popup camper top and is like the original poster was asking about running a 4bt in. but its not heavier than my truck.
Also, your automatic is going to suck more fuel around town than a stick in your 98 truck. dodges soggy torque converters suck fuel in town.

 

Yeah, I keep waiting for the tranny to start slipping so I can upgrade it and the t/c but sum bitch keeps hanging on. My 2001 Ram got better mileage with a built trans and triple disc t/c and I had that thing tuned pretty hot.

 

I am almost afraid to coment on this thread.
I have two trucks both standard and 4x4 with 4.10 gears both weigh about the same only 100-200lbs differance one has a ve6bt and one a ve 4bt.
I have compaired mileage and the 4bt gets on average 4 mpg better doing the same thing but is slower when loaded.
If I were building a vehicle that weighted 7000lbs loaded and it saw more secondary roads than interstates, towed under 5000lbs infrequently and fuel costs go back to $4+ a gallon, (as we all know they will) I would consider the 4bt.
If the vehicle is 7000lbs empty and I loaded it heavly or towing frequently or a 4bt is very expensive to aquire I would use a 6bt

 

I am almost afraid to coment on this thread.
I have two trucks both standard and 4x4 with 4.10 gears both weigh about the same only 100-200lbs differance one has a ve6bt and one a ve 4bt.
I have compaired mileage and the 4bt gets on average 4 mpg better doing the same thing but is slower when loaded.

Haha! agreed!

while a cool comparison, unless the two engines were installed in the same truck, and you used a meter to actually measure the amount of fuel going through each engine to do the exact same thing, all we have is an idle comparison. we are not even taking into consideration the set up on each engine. both may have VE pumps, but what of injector size, turbo variables, exhaust size and routing, etc? is one engine set up to be more efficient than the other?

there are so many variables to each individual truck and each person building it that to say so and so has a 6 that does this and such and such has a 4 that does this is all pretty moot as far as factual data goes.

while i appreciate the data being shown, i dont know how the isuzu engine ever came into question. i thought the decision was between 4bts and 6bts? none the less, is the question simply which uses more fuel, or which will provide the driver with the most fulfilling experience? perhaps the two are interchangeable, but they are not for me. i like to know i have power on tap for what i want to do. having driven b series in both 4 and 6, i was more impressed with the 6. it was better to drive and was in a bigger heavier vehicle. to me anyhow. i also like big blocks and gas station coffee, so i dont mind a fuel stop.

i really doubt that the best way to decide what is the best engine for this usage is hammering engineering documents and lab calculations/measurements back and forth. those things very rarely matter when building a custom vehicle. driver temperament and expectations and vehicle use/limitations are a much better source of intel when it comes to making a build plan. so what we need to know, is the absolute deciding factor fuel consumption, and if so, what does the rest of the vehicle set up and actual use look like?

 

it's like i said in the beginning.

Swings and roundabouts.

Build it to SUIT YOUR SITUATION.

 

Yoru right it would have been cheaper/easier to do a 6BT in my truck but it was too damn heavy/long to fit "comfortably" in it and I imagine that I will need to overhaul the 4 sooner than if I had shoehorned a 6 in it,but I think that I ended up with a better balanced package overall with the 4 but every build is a series of compromises.

 

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.

 

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.

 

300tdi :nuke:

 

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.

 

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.

 

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

 

are we still waffling on about this?

 

I must say, impressive.