Aeromodifications for large motorhome
 

My situation is not one typically discussed here but the phyics of fuel economy are the same. My 2008 Class A diesel motorhome gets 8 mpg at 65 mph. I have a goal of getting a solid 11 mpg. It is 26,000 lbs, 8 by 13 feet for a frontal area of 104 sq. ft. It is shaped like a brick (front and rear) with rounded corners. It has 2 large ac units on top and a couple of vents stickins up also. The engine is a Cummins 360 hp, 8.3 liters, variable blade twin turbo, computer controled fuel injection and computer controled transmission.
My assumption is 70% of energy is used to overcome aerodynamic drag so that should be my focus. I own a body shop have the means to create a front end that looks like a Japanese bullet train with a remote controlled liftable air dam 3 inches from the pavement. I could also modify the rear to look like the rear of the bullet train.
Would this be enough to gain the desired 3 mpg? Or could it even give me more gain? Any other ideas.
Thanks,
Randy

Great fab skills & $$! Should be a great thread...

Great elevation pics of your model would solicit plenty of ideas.

You may have to look into improving things under the hood also. Synthetics, software upgrades, exhaust, intake; all things should be taken into account. Pittsburgh Power is probably the best for the Cummins, type them in and see what things they have for the ISC.

Look at my avatar; they make trucks that are shaped like my old W900 ( Peterbilt 389s ) get 8 mpg and better loaded ( 80K gross ) with near 900hp; and that's with engines in the 12.7 to 16L range.

You would get most of your potential mileage gain by building a boattail on the back end, which, if designed properly, could also add a considerable amount of additional storage space (inside the boattail). Next, would be to do what you could (underpanelling, fairings, wheel well skirts, etc.) to clean up the air flows along the underside, sides, and top of the RV. A bullet nose would help, but would have much less effect than adding the boattail.

One way to quickly test what aero mods could do to improve your RV's mileage would be to note what your mileage is when drafting, as that is the mileage you could get by reducing your Cd. I've aeromodded my vehicle to the point where drafting at legal speeds provides very little improvement to my mileage, which wasn't the case when I started.

Pull all the stick-out parts off the roof and go 55.

A boat tail would be far easier for me to do. Are you saying that the rear of this 26,000 brick is more important than the front? I understand about the partial vacume created by the rear but I never thought it could be more important than the front.

I will check into engine type mods but I thought that the ISC 360 should be tuned pretty well already. The shape of a class A motorhome just seems so....terrible. I am also concerned about changing the computer settings and ruining the $2000 computer module some how. I will check with the Pittsburg power website and see what they have.
Randy

I couldn't find it, but sometime in the last month on a thread here, someone posted a drawing from one of the classic aerodynamics research programs showing a brick shape, then rounding either the front, the back, or both and showing the resultant Cd reduction. Rounding the back produced by far the greatest Cd reduction.

There is a lot of room for tuning the engine, and if you drive it right, the mileage will improve. The computer that Pittsburgh Power has is an add on unit that is multi adjustable and does not affect the stock ecm.

While I'm thinking about it, check with Snow Performance about their water/methanol injection. Keeps the egt's down and does boost the mileage.

Pics or model?

Considering you say the front is flat I would think you'd get a lot of drag reduction from making it look like a bullet train... But can you make a new angled/curved windshield or are you stuck with the existing one?