OK finally got back in town from Annual Training, changed the Oil and Filter to Mobil Delvac 15W40 and new filter, changed the Fuel Filter, Removed the Air filter and removed the restrictions in the filter housing, plan on replacing the 2.5" exhaust to 3.5" exhaust. but now i am wondering which would be better, a 3.5" free flow single or dual it out with an H pipe to balance pressures ?
plan on making a ram air to get cooler air into the intake as well... presently the truck is stock, last tank fill up and mileage worked out to be 17.94 mpg, that is mostly short trip, 14.7 miles to work and back, both cities. will the improvements help the mpg's or just make a little more power ?

Any Guidance would be appreciated.

 

Diesels are funny.
Anything you can do to increase the power will increase the fuel mileage if you don't use the extra power.

I know that is a strange statement.

Another way is like this,
Let's say you engine was putting out 100 HP.
Let's also say you drive the same road every day.

Now we add a turbo, new injectors, and a new IP.
This doubles the HP to 200.

If you go out and use the extra 100 HP you gained to blast up the hills and drive faster everywhere you go, the fuel mileage will go down.
But if you keep driving it just like you did when it was 100 HP, the fuel mileage will go up because it is a more efficient engine.

Here where I live we have lots of mountains and hills.
Out on I 79 there are many hills that I used to top at 45 MPH or so with my original 6.9 NA motor getting about 17 MPG.
Now I have a modified 6.9 turbo engine.
I can get on the interstate and cruise at what ever speed I want to.
If I hold 60 to 65 MPH my fuel mileage is 14 MPG.
If I hold 70 to 75 MPH my fuel mileage is 10.5 MPG.
If I wanted to I could run 85 MPH for the entire length of I 79, but the fuel mileage would be down in the single digits.
If I had the time and patients to drive it like it used to run back when it was a NA 6.9 I am sure I could be getting better than 17 MPG even though the fuel is not near as good as it was back then.

By back then I am talking about the days when the sulphur content of the fuel was over 5000 PPM instead of the ULSD <15 PPM that we can buy today.
Every time they reduce the sulphur content, power and MPG take a hit.

You will gain power, but if you choose to use it, the MPG may go down.
If you can drive it like you do now, the MPG will go up.

 

May have come on board a bit late to add to this but if it helps others than all the better.
My truck service shop has installed a custom 4" diameter smooth bore metal cold air intake running from just behind front of grill, installed a twin 3" exhaust system with open straight shot mufflers, electric fuel lift pump with max of 9 psi, now has 2 fuel filters, K & N air filter, recommend I always run Powerkleen additive @ 6 0zs per 15 gal, to increase cetane % and lube IP, and have opened IP by 1/8 turn, and the truck, crew cab dually, can now hold the 85 mph on the cruise control and averaging 15 mpg, when I run between KC, MO, and Scottsdale, AZ, loaded with tools and gear for work. Engine runs cool under load, steady 50 psi oil pressure, and we change oil and filter, Mobil Delvac, every 3000 miles religiously.
It would appears that these engines need to breathe and they were strangled in their original format with a small diameter exhaust and flimsy plastic corrugated air intake located between the hood and grill.

 

Again to help any member considering changes. This 1988, 2wd dually truck now has from front to rear.

1. 1993 front clip
2. twin 100 watts front spotlights and twin 50 watt rear lights for load inspection at night
3. steering stabilizer
4. oil cooler
5. all 6 tires have the synthetic tire balance beads, absolutely amazing effective concept.
6. high amp alternator
7. 4 kilowatt Mitsubishi starter
8. single plate clutch, much better than the twin.
9. electric fuel pump and twin filters
10. blocked off fuel filter return to tank line
11. twin 3" exhaust system with open mufflers
12. K % N air filter
13. gas shock absorbers on all 4 locations
14. 1" lift all round and extra overload leaf springs on rears
15. electric braking module
16. Highway Product's totally enclosed bed tool box, real good design with 4 seperate compartments, ladder racks and all in polished aluminum.