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Discussion Starter · #1 ·
If this has been addressed and I did not find it when searching.......:beer:

say you are driving a truck that weighs 10,000 lbs all the time, which engine would have the better overall mileage.

the 4bt will obviously be run pretty hard while the 6bt wont.

speed and power are not very important.

just for discussion lets compare them both completly stock and slightly moded and ran in their power band.

btw the 6bt would be a early 12v
 

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You didn't post what you were towing - only that the truck weighed 10,000 lbs. I personally wouldn't tow much more than 12k combined GVWR with a 4bt - unless that 4BT was in a John Deere or Case Tractor :happyfinger:
 

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I think he meant a truck w/ a utility body, a boom lift, welding truck, etc that always has equipment on it not a trailer...
Wylie
 

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Discussion Starter · #4 ·
Ok i will be more specific. it would push an expedition/motorhome with 48'' tires and a weight around 10K lbs. And 2500lb trailer a few times a year. aero-drag is huge. speed is governed at 55-60.
 

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I think you'd get equivalent mileage with the two engines, if not worse with the 4bt because of drag and the amount of weight. Stepvans typically got 18ish unloaded. At max GVWR (11k-ish) I'd imagine they'd be pushing 9-12mpg. Add a Trailer and you'll suck down the mileage even more. I'd say for that application Six in a row will be the best bet.
 

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Discussion Starter · #6 ·
a little off topic; does anyone know where I can find bsfc # for the 4bt and the ISB 3.9l engine to see how they compare?
 

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What's a bsfc #, for those of us who are abbreviation challenged?
 

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Well gee Whillikers, who'd a thunk it!? Thanks for the tech info, Average!
 

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a little off topic; does anyone know where I can find bsfc # for the 4bt and the ISB 3.9l engine to see how they compare?
BSFC (Brake Specific Fuel Consumption) curve is on the factory power/torque curves. A 4BT at best point is from memory around 210g/kwH.

Basically there are soo many variables in comparing two engines for the same application. The only way to draw a solid conclusion is to pick the power level you require.
Once you've got the power you require, from each power/torque curve pick the engine speed that's needed to give you that power.
Compare the BSFC at that speed and power and you have your answer.

Basically the 4BT has less internal friction (two less cylinders, 4 less valves etc) and less weight. But if it has to run outside it's best efficiency point to produce the needed power then a 6BT would be better.
 

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Well said, and that points to the other half of the equation: GEARS, GEARS, GEARS...
 

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Discussion Starter · #12 ·
BSFC (Brake Specific Fuel Consumption) curve is on the factory power/torque curves. A 4BT at best point is from memory around 210g/kwH.

Basically there are soo many variables in comparing two engines for the same application. The only way to draw a solid conclusion is to pick the power level you require.
Once you've got the power you require, from each power/torque curve pick the engine speed that's needed to give you that power.
Compare the BSFC at that speed and power and you have your answer.

Basically the 4BT has less internal friction (two less cylinders, 4 less valves etc) and less weight. But if it has to run outside it's best efficiency point to produce the needed power then a 6BT would be better.



:grinpimp: This is great info, except unless I experiment with both engines or can find a somewhat accurate formula for needed hp, It doesn't answer to much.

I have read multiple post of people with 5-7K lb trucks towing a few k lbs with out much trouble? since this truck will only be going around 55 mph and a 10-11klb max weight........ I don't know:confused:
 

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:grinpimp: This is great info, except unless I experiment with both engines or can find a somewhat accurate formula for needed hp, It doesn't answer to much.

I have read multiple post of people with 5-7K lb trucks towing a few k lbs with out much trouble? since this truck will only be going around 55 mph and a 10-11klb max weight........ I don't know:confused:

What I think Dougal's trying to explain (IMO) is that both engines have the capability to move the requested load, however, the 4BT will be operating out of it's "efficient" powerband for moving the amount of weight (10-11k combined weight) you specified. Most trucks, say a 1 ton dually (around 11-12k GVWR) would have a 6cyl/V8 diesel. Your mileage gain would be nil with a 4BT (not to mention costs being nearly equal for a 6BT engine) and IMHO you'd likely suffer mileage due to a lack of power.
 

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ill say it again.

a 6bt swap is CHEAPER than a 4bt.
a 6bt makes more power for less money (it is cheaper than a 4bt)
a 6bt can tow more weight than a 4bt (it is cheaper than a 4bt)

a 4bt is smaller, and fits easier in alot of the older vehicles people use here.
a 4bt gets good milage in smaller vehicle.

gvw on a 4bt is high enough, but i wouldnt run 10,000lbs with one on a constant basis, irregardless of speed and power.
 

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The 4b is going to be over worked with that kind load!The 6b would be the only way to go.

That's just me ,if you want to test the limits of the 4b than do so and lets us know how it turns out.

CrewCab59
 

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ill say it again.

a 6bt swap is CHEAPER than a 4bt.
a 6bt makes more power for less money (it is cheaper than a 4bt)
a 6bt can tow more weight than a 4bt (it is cheaper than a 4bt)

a 4bt is smaller, and fits easier in alot of the older vehicles people use here.
a 4bt gets good milage in smaller vehicle.

gvw on a 4bt is high enough, but i wouldnt run 10,000lbs with one on a constant basis, irregardless of speed and power.
So based on the above, if you can fit the 6BT in, then do it.;)
 
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