[A/S] Gas Vs Diesel

["maurice_riggins"]

Thanks for the comments on the F-250 engines, folks.

Here's something I do know something about and can contribute... for 
those not interested in the details below, they show, that when it 
comes to Ford, the 6.8L gas and 7.3L diesel really have about the 
same pulling power in the midrange where it really counts (pulling up 
the same grade in the same gear) IF the gas engine is running 4.30:1 
gearing and the diesel is 3.73:1.

Yeah the diesel may have more grunt right off the line, and yeah, the 
gas can rev higher, hanging onto each gear a little longer (taking 
advantage of its torque multiplication over the next higher gear) and 
can probably outrun the diesel in a drag race.

The analysis below doesn't consider either that the diesel mileage is 
greater, or that the gas's initial and maintenance costs are lower.

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Tech stuff separator
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Diesel torque comes on at lower rpms, but so does horsepower and max 
engine speed (redline), so the diesel MUST be geared higher (i.e. 
3.73:1 vs 4.30:1 for gas) to put its rpm range in the same mph range 
as the higher rpm gas engine. This gives the gas engine geared at 
4.30:1 has a 1.15 torque multiplication factor over the 3.73:1 
gearing.

A 6.8L gas gets 425 ft-lbs at 3,250 which, with 4.30:1 gearing, is 
comparable to a 7.3L diesel w/3.73:1 that gets 490 ft-lbs at 2800 rpm 
(261 hp) 

We know the 7.3L w/manual transmission gets 275 hp at 2,800 rpm so 
that's 516 ft-lbs. Winner = diesel w/manual over gas

With the auto, the diesel's only 250 hp at 2,600 so at 2,800, it's 
_at best_ 469 ft-lbs. Winner = gas over diesel w/auto

The gas engine, at it's peak hp point (310 hp at 4,250 rpm = 383 ft-
lbs) is running about the same mph as the 3.73:1 diesel does at 3,700 
rpm. This is about the diesel's redline and we don't know what it 
produces here, but we DO know the gas engine can keep revving beyond 
it's peak hp point, taking advantage of the lower gear's torque 
multiplication.

The diesel engine, at it's peak tq point (525 ft-lbs at 1,600 rpm = 
160 hp) is running the same mph as the 4.30:1 gas does at 1,844 rpm. 
We don't know what the torque the gas engine produces at that point, 
but we can say it would have to be at least 455 ft-lbs, which isn't 
likely with a naturally aspirated gas engine, even of that size, at 
that low of an rpm.

I'm still gonna get a diesel though <G> Not only are they cool and 
potentially longer lasting, but when your trailer wheels have sunk in 
the dirt a bit, that low end torque makes it easy to get out. Of 
course you could just put the gas model transfer case in low range 
LOL! Why is it I'm buying a diesel? 

Conversions used above:

HP = (TQ X RPM)/5252 and TQ = (HP X 5252)/RPM

1 HP = 550 ft-lbs/sec = 33,000 ft-lbs/min and one revolution is pi X 
2 X radius = 6.28 divided into 33,000 ft-lbs/min = 5,252 rpm 
conversion factor