I have read several posts & threads on how and the benifits of turning up the IP, but no threads I have read addresses how this affects the MPG. Diesel up here in B.C. is now $1.04 a litre that works out to about $3.47 US a gallon. And I dont see it comming down any time soon, 10 years ago when I bought my 86 IDI, diesel was $1.37 US a gallon up here in BC.

 

Bottom line is this.

Any mod you can do to your truck to increase the power will increase the mileage as long as you do not use the power you gain.

In other words if you had a high altitude 6.9 that was putting out lets say 153 HP with 300 foot pounds of torque and drove it for a couple of years and were getting 12 MPG.

Now you go out and get a turbo system that you tweak a bit as you install it, ram air, big easy flowing exhaust, new IP set up for the turbo plus a little and a tweaked set of turbo injectors. (I made this an extreme upgrade for the benefit of clarity)

OK, it is now all installed and tuned just right. You are now putting out 285 HP and 485 foot pounds of torque, so you take it out on the highway and drive it just exactly like it ran before. Same slow starts from the stop light, same slow climb up the mountain. Your mileage will increase probably several miles per gallon to 17 or more.

But here is the catch, if you drive the truck like it will now run your mileage will be the same or less. I don't know anybody that can drive it like it did run after they feel all the power you now have at your disposal. This is made even harder by all the dollars you dropped to make it run like it should.

The power mods make the engine more efficient at light loads.
But they also make it very easy to dump a lot of fuel in the cylinder to make more power.

Hope this makes sense to you.

 

I agree 100 percent with Daves post also I not sure that just turning the fuel screw on the ip will add any mileage at all,on my 84 it just blows alot of black smoke, I can't honestly say I noticed any increase in power, but on my 91 with a banks turbo the mileage is better and the power is there. Keep in mind this is like comparing apples to watermelons

 

Farmfly, my question is not if you would gain any increase in milage rather lose MPG by turnig up the IP, increasing the amount of the fuel. I have no dout as Dave say an effiecent motor runs better, I was curious about everyday commuter driving?

 

I would have to say by just turning the screw you would probably loose a little, here's why. Just for an example, say you put a brick on the gas pedal on a standard trany diesel and had it set so the truck went 60 mph at say 2000 rpm. Inside the ip there is a mechanical governor (springs and counter wieghts that are attached to the fuel metering rod), as the truck approaches a hill the engine slows slightly, this tells the governor to increase the fuel delivery untill it reaches maxium fuel (ie screw setting). Then the engine will loose rpm untill the hill is cleared or require down shifting whatever.
From the factory most non turbo diesels are set at their most effiecient fuel to power ratio.
In college we put a non turbo diesel on a dyno and started turning the fuel screw, the first couple of runs the power went up with small increases in smoke and fuel burn.
Then the power went down, smoke was pitch black and fuel burn was high.
Bottom line is that after all calculations were done the most effiecient fuel screw setting was at the factory. (This was only on a non turbo factory engine with no other modifications, just turning the fuel screw) Hope this helps

 

I have to add a bit to this.
A NA motor will see a very small benefit from increasing the fuel if it sees any at all. There is not enough air to burn the fuel. However if you do exhaust and intake mods to increase the air flow to the engine, it can indead use and burn more fuel which makes more power. The efficency is still there as long as you are not seeing more black smoke than you were.

One of the biggest problems the stock trucks have is a very restrictive exhaust and air intake systems. Diesel engines need large amounts of air that is does not have to work to get, that is why the turbo is such a big improvement.