Re: [A/S] F250 PSD Mileage

["Maurice Riggins <maurice.riggins@xxxxxxxxxxxxxxxx>" <maurice.riggins@xxxxxxxxxxxxxxxx>]

Oh yeah!

Yes, the specs say the 7.3L makes peak torque of 525 ft-lbs at 
1,600 rpm.  And when the torque converter isn't locked, it's 
providing 1.86:1 torque multiplication, and that's 976 ft-lbs into 
the transmission.  The 4R100 automatic is only rated for 1000 ft-lbs 
on the input, so I wouldn't go hot-rodding the engine.  I sure 
wouldn't chip it or use a tuner, because Ford's looking hard these 
days for excuses to deny warranty coverage.

At 2,600 rpm, where hp for the automatic is published to be 250, 
torque is still 505 ft-lbs, or 96% of peak. Torque = (hp X 5252)/rpm 
BTW.  Even if torque drops off linearly 20 ft-lbs over 1,000 rpm (and 
it doesn't, it drops slower at first and faster further out), you're 
only off one ft-lb for every 50 rpm over 1600.  In short, the reality 
is that at 2000 rpm, you have 99% of the peak torque.  VERY little 
difference in engine torque between 58 mph at 1600 rpm and 71 mph at 
2000.  

Major difference in aerodynamic drag, though!  It goes up with the 
square of the increase in speed, so 71 mph has 50% more than 58 mph! 
And horsepower to overcome aerodynamic drag goes up with the CUBE of 
the increase in speed, so you need a 83% more to do 71 mph vs 58!  
When you add in the increased rolling resistance, you can say it 
takes twice as much hp to hold 71 mph as it does 58.  So that, more 
than torque, is the reason to tow at a lower speed.

Don't be afraid to use rpm.  You have to keep in mind that not only 
does OD lower engine input rpm to 70% of driveshaft output rpm, but 
it also lowers output torque to 70% of input torque.  So while you 
have as much as 525 ft-lbs going in (depending on throttle position) 
at 1600 engine rpm with OD, you only have 367 ft-lbs going out at 
2300 driveshaft rpm.

If you need to pull a hill at 58 mph, its MUCH better to punch OD off 
and have maybe 510 ft-lbs at 2,320 rpm engine rpm going through the 
transmission at 1:1 in Drive coming out as 510 ft-lbs at 2,320 
driveshaft rpm.  You have 43% more torque to the tires than in OD.  
BTW, it's better to ease off the throttle a bit before you get to the 
hill, so the torque converter can unlock, the transmission can shift, 
and the torque converter can relock, without being under load.

Hope this helps!
--
Maurice
WBCCI 5446