Why Gearing Works - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 18 (permalink) Old 08-21-2007, 01:22 AM Thread Starter
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Why Gearing Works

...to improve MPG.

The short answer is engine friction.

At a constant road speed all the vehicle loads level out, but the internal drag of the engine is a variable if you are willing to play with the gearing.

Engine friction is both mechanical and fluid friction. Obviously, all those rubbing surfaces, even minimized by efficient bearings and lubrication, imposed a drag torque. also the engine is pumping around two incompressible fluids and one compressible fluid. Those too, impose their own friction torque load. In particular, the pumping friction of air goes up with the square of the engine speed.

Power is torque times RPM. The frictional torque goes up at a shallow parabola. Therefore, the frictional HP curve is cubic. Keep the engine RPM down and the frictional HP is minimized. If the "zero torque" HP is minimized then the fuel burn is minimized.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
315,000 miles
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post #2 of 18 (permalink) Old 08-21-2007, 01:48 AM
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Re: Why Gearing Works

Yep higher gears lowers the RPM's which lowers the fuel used.

2007 DODGE 2500 CREW CAB 4X4 5.9 LITER CUMMINS
2002 FORD F-250 CREW CAB 4X4 POWERSTROKE
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post #3 of 18 (permalink) Old 08-21-2007, 03:24 AM
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Re: Why Gearing Works

at what point to you lose the effectiveness though?

if i'm running 70mph at 1200rpm with different gears i'd get great mileage no doubt, but can the engine maintain speed at that low of an rpm if more drag is applied to the vehicle?

where is the sweet spot in power vs. economy?

2003 F-350 Super Duty
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post #4 of 18 (permalink) Old 08-21-2007, 11:20 PM Thread Starter
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Re: Why Gearing Works

It depends on the engine.

The 7.3 is a low-end torque monster. I have been experimenting around the 1000-1300 RPM range and I think it can be quite effective there.

One problem: Below 1300 the engine becomes naturally aspirated - no boost. Completely burning the fuel needs boost, particularly under load.

I think I'd have to go to a different turbo in order to get some boost at low RPM. Of course I'd lose boost at high RPM, but if I were at 2500 RPM with gearing suitable for 70 MPH @1200 RPM, I would be going 145 MPH and probably cruising at 10,000 feet.

At any rate if the engine lugs and rattles, downshift.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
315,000 miles
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post #5 of 18 (permalink) Old 08-21-2007, 11:40 PM
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Re: Why Gearing Works

if you end up changing the turbo or even the housing, with what you've spent on mods and gears to improve mileage already, how long do you have to drive the truck to break even?

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post #6 of 18 (permalink) Old 08-21-2007, 11:41 PM
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Re: Why Gearing Works

That would be something to see a 7.3 doing 70 @ 1200 RPM and not going down hill.

2007 DODGE 2500 CREW CAB 4X4 5.9 LITER CUMMINS
2002 FORD F-250 CREW CAB 4X4 POWERSTROKE
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post #7 of 18 (permalink) Old 08-22-2007, 02:40 AM Thread Starter
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Re: Why Gearing Works

Fever asked:
"if you end up changing the turbo or even the housing, with what you've spent on mods and gears to improve mileage already, how long do you have to drive the truck to break even?"

Dave sez:
Square root of forever.

Actually if I could pull off my wildest dreams I have about half of what some guys have in wheels and tires.

At some point the whole thing becomes "priceless" as the ad says. Just ask the racers, pullers, dyno kings, and Lords of the Show ' Shine. They know.

I just explore a different corner of the perfomance envelope.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
315,000 miles
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post #8 of 18 (permalink) Old 08-22-2007, 03:01 AM
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Re: Why Gearing Works

sorry dave gotta disagree,though i believe what you are saying, there must be a sweet spot,cause with my 3.73 and 35s i run along way at about 2000 rpms empty,,but towing mileage is about 11 mpg....now awile ageo i had 285/75/16 and didnt get the greatest mileage empty maybe 12,but got 13 fully loaded.....if you have some fabulas anser i really need to hear it cause a re-gear is in my future..nioce il loaded but loaded blows...and I WANT both david......

[SIGPIC][/SIGPIC]2001 f250 4x4 cc sb 6 in lift...dp 6 pos..
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post #9 of 18 (permalink) Old 08-22-2007, 06:41 PM
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Re: Why Gearing Works

Quote:
Originally Posted by Dave Whitmer View Post
Fever asked:
"if you end up changing the turbo or even the housing, with what you've spent on mods and gears to improve mileage already, how long do you have to drive the truck to break even?"

Dave sez:
Square root of forever.

Actually if I could pull off my wildest dreams I have about half of what some guys have in wheels and tires.

At some point the whole thing becomes "priceless" as the ad says. Just ask the racers, pullers, dyno kings, and Lords of the Show ' Shine. They know.

I just explore a different corner of the perfomance envelope.

I understand, it's a passion for you and that's cool

2003 F-350 Super Duty
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post #10 of 18 (permalink) Old 08-23-2007, 12:23 AM Thread Starter
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Re: Why Gearing Works

Take what you think you can use and ignore the rest. I do that with racing technology.

MPG is like speed. It ain't for everybody and it comes at a price. How efficient do you want to be?

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
315,000 miles
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