In a recent post, Kahn noted:

"The reason the 4.0L is only rated to produce 10 more horsepower despite is extra size is one, it is tuned to produce lots of low end torque, and two, its using the same restrictive exhaust the 3.0L uses. The 3.0L is fine with 2", the 4.0L wants 2.25". Just some head modifications will give you more bang for your buck than the same dollar amount in exhaust work."

So lets assume that one wanted to optimize for fuel economy, even if it meant giving up some acceleration or stump pulling ability.

What would you do to a 4.0 to increase the fuel economy?

One may assume that the largest percentage of the miles driven are on open highway mostly flat.

 

Since most cars are designed for quiet operation as a primary goal, they can probably benefit from a less restrictive exhaust system. However, it has to start from the exhaust manifolds, as that's where the flow restrictions begin. I've had a 2.25" cat-back and "performance" muffler on my Aerostar for about 12 years, and I have not seen any noticeable improvements in mileage or power. The cats themselves seem to have 2.25" pipes, but they're probably not "high flow". The exhaust manifolds are designed for the tight packaging of the Aerostar's engine compartment, and not for optimal flow, as are the Y-pipes.

One option that I've considered is aftermarket Ranger parts. I think you can get high performance headers for Rangers with 4.0 engines, but you will have to relocate the oil filter on the Aerostar. Then you need a custom made Y pipe to run to the cats.

 

can't change the laws of physics.
the Aerostar is a 4000 lb square box being pushed through the air.
has the worst aerodynamics of just about any type of passenger/small cargo vehicle on the road.
all the other similar midi vans get worse fuel mileage, pre curvy old Dodge Caravan, the older Toyotas and older Hondas, the old Chev Astro.

a later sequential injection 4.0L will get 20 mpg when driven lt. load below 65 mph on flat freeway in good tune. i get 20 all the time freeway, but then I'm not a left lane driver anymore, stay in the rt. lane with the truckers at 62>64 mph on cruise, match their speed and stay away from the on the throttle camel jockeys in the left hog lane

the 3L gets amazing mileage for it's design age, OHV and old tech. air fuel control, many of these will break 25 mpg.

i've never seen anyone improve enough in the out the factory door fuel mileage on these to pay for the cost of the changes.
now, output horsepower & top ECU cutout rpm a different story.

 

I have done allot to get more mileage out of my 97 aerostar. You should begin buy upgrading your ignition coils. I brought a super ignition coil from summit racing made by accel. I'm able to drive over 60 mph on highway barely taping the gas. It has also increased the horsepower.

It creates a much stronger spark in the combustion chamber allowing a more complete burn of fuel thus requiring less fuel to get going. When you do push the gas down all the way you will feel much more power. Super Ignition coils for the aerostar are almost $100 but I think its well worth the investment. You may also need to replace the ignition coil wires but don't get the cheapest ones. Go with a good brand & quality because of higher voltage and you also might as well replace the spark plugs while everything is apart. Feel free to ask me any questions.

 

That is terrible advise. Accel makes junk, some of the most poorly made coils ever made. If you saw an improvement running those, its because your stock coil was shot. I tried an Accel coil, the posts were crooked, the clips would not stay secured, and one of them cracked within months. The best coil you can run on these is Motorcraft by far. Don't just take my word for it, Accel is known to be garbage in the racing circles too, most of those guys run MSDs instead, or stock coils.

Same with wires, just because its a performance wire doesn't mean its a performance wire, let alone that you will have any gains from it. Ignition does one thing, it ignites the mixture. That means so long as it is working correctly, you will see minimal improvements from a "hotter spark". Performance coils and wires only outperform stock ignition in high compression, high RPM racing applications, where a stock coil may struggle to make enough power to keep up with an engine running 7500 rpm with a 12:1 ratio or a turbocharger.

Mods to improve economy on a 4.0L. I did notice good power results from the Bear River catback system, I use grades and shift points. I can go up inclines without having to drop OD because it takes less throttle to pull the grade than with the stock muffler. The stock muffler is suitable for a 2.3L I4 naturally aspirated, its designed to be quiet and cheap. It is undersized and restrictive for both V6 options in the Aerostar, more so for the 4.0L.

The stock cats are not a performance bottleneck, you will have to spend a LOT of money to converter them to real high flow cats, but the stock ones are more than adequate. All the tru high flow ones will buy you is top end power, they will have almost no effect on low end performance or fuel economy. The stock y-pipe however is a restriction, that is a poorly designed y-pipe. I will be looking into addressing that on my own Aerostars. They just need a freer flowing design, something that merges together more smoothly.

As far as running headers, I have looked into that, the Ranger headers don't fit, and not only require modification to work, they require MAJOR modification to work, as in you might as well just get the flanges you need, cause all the tubing is gonna be custom.

Beyond that, the most important things for economy is the condition of the engine itself. The biggest things that kill economy on these is the Aerodynamic shape (lack thereof), the tires (firmer 6 ply tires have lower rolling resistance, depending on tread), alignment (Aerostars track sideways from the factory, rear alignment may help), old worn out O2 sensors, vacuum and intake leaks (those can kill as much as 5 mpg before they start setting codes). You can also remove the roof rack if you never or rarely use it.

One thing that really helped mine was replacing the the two gaskets that seal the upper and lower gaskets with the fuel rail. These are known to leak, and the Fel Pros don't reliably fix the problem. Ford has "updated" gaskets that do. This may only apply to the plastic manifolds. When I replaced those, the van I have now (1997 AWD) jumped from 14 mpg to 19, and has consistently beaten 17 mpg over the last three tanks. My '94 AWD never got higher than 17, and averaged around this. http://www.fuelly.com/driver/khantyranitar/aerostar. The improvement in economy towards the end there is due to a variety of this, including e-fan conversion, new wheel bearings, new injectors, several new sensors, and such.

 

ACCEL has been the only coil brand I've tried so far. I guess there not the best but since I had noticed a difference in performance I was happy with it. I had also added an oil additive to increase compression which also helped. Upgrading the ignition coils was just one of many things I did. Thanks for the info.

 

The fact you saw a difference only means your stock coil was defective, not that the Accel coil was better. You could have had similar results by replacing the original coil with a new OE one. The Accel coils are cheaper than OE, how can they be better. The answer is they are cheaply made.