[Dave Whitmer]

...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.

 

[THEDIESELMAN]

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

 

[Fever]

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?

 

[Dave Whitmer]

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.

 

[Fever]

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?

 

[THEDIESELMAN]

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

 

[Dave Whitmer]

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.

 

[glamis_dave]

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......

 

[Fever]

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

 

[Dave Whitmer]

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?

 

[Fever]

i see you have some added aero equipment in the bed of your truck

have you tried anything else to free up some aero mileage? skidplates to direct air under the vehicle better?, cleaned up front end openings?, etc...

 

[Dave Whitmer]

Fever posted:
“have you tried anything else to free up some aero mileage? skidplates to direct air under the vehicle better?, cleaned up front end openings?, etc...”

Dave sez:
No, but such things are on my to-do list.

I have scored some conveyor belt to make a better front air dam. My Excursion air dam sits 7.5” above grade. To add insult to injury my Ex air dam is beat to a pulp. My plan is to drill 8 or 9 half-inch holes in the bumper and epoxy on some nuts to the back side. There is no getting a wrench on them with the bumper on. Then I punch corresponding holes in a piece of 24” wide conveyor belt and fasten it on with half-inch cap screws and fender washers. I have seen some Class Eights running around with metal bumpers only 2” above grade. With something tough like conveyor belting I can get greedy and go down to Nextel Cup car clearance. What is currently limiting me is that my helper just got useless. I’m not good enough wrench to do this single handed.

I’ve looked at the bottom and it will not be easy to clean up. Keeping the air flow out may work better.

That said, I also have enough belting to put side air curtains where the running boards used to be to further keep air out.

Once I get more hip to laying up fiberglass or find somebody in the Indy area to help me, I have definitely considered fairing over the spare tire lift and fairing it into a roll pan to replace the rear bumper.

I notice some guys talking about junkyard parts for their projects. I want a SD short bed to make up a very slick bed. I look to taper it down both vertically and horizontally. I can reduce the low-pressure area behind the truck by 60-70%. This combined with a good air dam and rear fender skirts should knock my coefficient of drag down by 20%.

Since I have an A.R.E. flat tonneau that I can’t seem to sell, it may get modified to become my lid.

If I could just hit that lottery, I’d chop the top 2”.

As for the front grille, I am looking for a junk yard grille I can merrily chop and glue up a better front end. If I can make it look right, I think it would help some although in this speed range efforts at the tail end are more important than the front end.

There are some Mooneyes in my future.

You should see the trailer idea I have on my autocad…

 

[Fever]

copy the autocad image to a common format and let's see what you've got :rockwoot:

perhaps not the most scientific show, but on the mythbusters episode dealing with mileage in a pickup (tailgate up vs. down) it seemed that in their aero testing the drag created in the rear was massive...looks like you're on the right track there...

 

[Dave Whitmer]

The trailer idea: Imagine a cross between a Hi-Lo, an Airsteam, and a Bowlus Road Chief, but with a refinement I'm seeking a patent on.

Three versions: Cargo trailer, travel trailer, and a toy hauler.

When I get my patent pending, I'll share it.

 

[skylinedan]

How much for your cover? you don't acept PM's so I had to resort to asking you on the thread Just PM me with a answer if you could.

Dan

 

[Dave Whitmer]

Check your PM. It is a nice tonneau and I'd rather sell it than cut it up.

 

[Redfish]

I'm just Throwing a bone out there but doesn't the tire make a big difference?
I mean, numerically you should have better mileage with a larger tire, but in all reality you lose mileage due to the heavier rotational mass as well as a larger foot print, aka friction on the road surface. Thus a smaller, skinnier tire works better correct?

So if you go with a shorter gear and a smaller tire, vs a taller gear and larger tire, and the final drive ratio (rpm/mph) is the same for both, then the shorter gear/smaller tire combo is more efficient?

Is there any validity to this or am I just pissing in the wind?

 

[Dave Whitmer]

“Thus a smaller, skinnier tire works better correct?”

Answer: Yes. In general smaller and skinnier equal less rolling resistance and less rotational inertia. I’m sure somewhere along the line there is a limit to how small works but I haven’t found it yet.


“So if you go with a shorter gear and a smaller tire, vs a taller gear and larger tire, and the final drive ratio (rpm/mph) is the same for both, then the shorter gear/smaller tire combo is more efficient?”

Answer: The shorter (lower numerical ratio) gear and smaller tire is indeed the more efficient combo.

As before, there is always a limit otherwise we’d be running around on lawn mower wheels. I just haven’t found that limit yet.

I run 3.08 gears (with OD that’s 2.21:1 final drive) and 31” OD tires. That gives me 3.67 feet of forward motion per engine revolution. That’s 1700 RPM @ 70 MPH.

If I can pull it off, I am looking at a straight through final drive (1:1) with a 1.5:1 ratio and tires having a diameter of 28.7” (215-75x16) to get 5.0 feet of forward motion per engine revolution. That would have me going down the old superslab at 70 MPH turning 1232 RPM. A 13:20 gear set (1.53) should about do it. That would have me going 1256 RPM at 70 MPH. That would drop my engine frictional HP by another 8 HP.

For such a radical gear/tire setup, I may have to look into a smaller turbo. At 1700 RPM, I am borderline normally aspirated. I need more boost at very low RPM to completely burn the fuel.