So even though the injectors are cycling more times, less fuel is travelling to the cylinders? Because that's what it all comes down to right?

 

all I know is that at 44 mph and od off a could rest a feather on the pedal and maintain speed. With the od on I would have to use 10-15% percent throttle to maintain speed. You tell me wich one saves more gas. Try it, I bet youll be suprised. I average around 380 miles to the tank in my 3/4 ton SO I know it works.

 

Depends on the length of time the injector is open for each cycle. Higher load (heavier throttle at lower RPM) will result in a longer injector pulse.

3ms pulse x 1500 revolutions (x.5) x 8 injectors = 18000 ms
1ms pulse x 2000 revolutions (x.5) x 8 injectors = 8000 ms

Even though the engine is spinning faster, and the injectors are squirting more often, the duration of the squirt is lower, resulting in less fuel burned.

Clearly these are numbers pulled from my butt, but basically that's what we're talking about here. Lugging the motor requires more fuel than not lugging.



I think you're using the wrong fuel though. Even with a 14k pound trailer in tow I didn't have too many hills that slowed me down.

 

Pretty much. When you hit that sweet spot where the throttle plates are open enough that the pumping losses are relatively low but the RPM's are still low enough that friction losses don't yet dominate the equation, everything just cruises along with the least necessary effort.

When RPMs are low and the throttle plates are mostly closed, a considerable bit of effort is required to move all of that air past the throttle plates. Open 'em up some, and the effort required drops at an inverse square rate... so if you had, say, one square inch of opening, then bumped it up to two square inches, the effort required to draw the same volume of air is four times less. That's how pumping losses work.

Imagine you've got a shot of whiskey, and your friend has a shot of whiskey, and you've got a bet about who can finish off his shot first -- you've got a regular drinking straw and he's got a swizzle straw. Same shot, relatively equal suck power available for both of you, but you're going to win the bet every time. Heck, you could finish off a five ounce glass of wine before he could get that shot down. And when it's done, he's likely to be gasping from the exertion.

Same gig when you're running in too high a gear for your road speed. The engine RPM is down and the throttle plates are less open so you've got big pumping losses, AND you're in the shallow part of the torque curve pushing against a shorter lever (lower final drive ratio). Drop a gear, get the throttle plates opened further to reduce pumping losses, and then as long as your RPM's are still low enough that you're not encountering big friction losses, you're in the steeper part of the torque curve AND pushing on a longer lever so you're demanding significantly less energy from each revolution of the engine.

There's more to it, of course, but that's the gist of the matter.

 

Good info. Learn something new every day.

 

Best explanation I have read yet.

 

great explanation....

but.....

o/d STILL sucks in these trucks....

back when I had time to play with it all the time, I would run diagnostic software w/ capture.. I also have a vacuum gauge (though the software does too), and a wide band a/f gauge..

you can learn some things about economy and power ranges that are counter intuitive and surprising while doing such... I find that o/d doesn't help shizen unless I'm running at or above 60mph/1950RPM's (4th gear o/d, t/c lock, w/4.56:1 axle ratio and on 34.5" tires).. I can get about the same economy around 45mph in third unlocked as in fourth locked.. the truck is under it's torque ramp in fourth, but well inside it in third.. the response to throttle is a TON better, and there are no drifting gears.. or vacuum needles fluttering.. or rich a/f while the engine tries to catch up to the demand..

 

I have found that most of the vehicles that I have driven with an OD option behave much better around town with it off. It keeps the transmission from hunting for gears and dropping into and out of OD all of the time. I would think that would put less wear on the trans mission. You can also get away with lighter throttle inputs when you are out of OD around town and not have to floor board it to get any response. I think of it like a manual. I would never drive through town at 40 mph in 5th gear in my truck.

 

The entire time we were in Virginia my truck rarely saw OD On, so many hills OD was almost always Off.

Took a trip to Chapel Hill, NC to pickup a motorcycle and tools, etc.. Anyway, I found that while on the interstate my truck had easier passing power/ability to pickup speed with OD On and above 2k RPM, then compared to passing with OD Off in the same range.

She just naturally gained speed better with OD on then off. That was also with a clogged catalytic converter which has since been removed from the equation. Curious to see if the results are any different....

 

I LOLzed!!