Hi all,

There's a thing that's been puzzling me for quite some time... maybe some 12-valve experts here could enlighten me?

I used to drive a 1/2-ton Chevy Suburban with a 6.2L diesel. It was an automatic OD and 4x4 with 31in tires, 3.42 rear end. With that truck I got consistent 26 mpg runs, and occasionally up to 29 mpg. The engine had well over 200,000 kms.

I've also owned a 1992 D250 with the lovely 12-valve Cummins, automatic OD, 2x4, extra cab, 3.54 rear end, 200,000 kms.

I bought the Dodge hoping to get even better economy than with the Chevy, but in fact it was worse.
The Dodge never got better than 21 mpg, no matter how I drove it. I did the timing bump, adjusted the valves, checked turbo boost pressure, changed air and fuel filters, etc. I even swapped the 3.54 rear end for a Dana 71 with 3.07 ratio, but to no avail. Aside from fuel economy, the Cummins performed very well, started easily even in cold weather, didn't smoke, ran smooth etc. And I drove it the same way as the Chevy.

Now, 21 mpg isn't too bad. But I just don't understand why the direct-injected 5.9 Cummins, pulling a very similar truck (in terms of weight, aerodynamics and gearing) , wouldn't provide superior fuel economy .
It's an almost universally known fact that direct-injected diesels produce fuel economy around 10-20% better than indirect-injected diesels like GM's 6.2L.
So why would a healthy 5.9 consistently drink 20% more than a 6.2?

 

Maybe the odometer was waaaaaay off on the Chevy...Mark

 

Well first off your 1/2 ton Sub was no where near half the truck a dodge is that v8 was designed to rev to the Moon if you were getting high 20's count your blessing's you had a Gem the motor alone is 1100 lb's then you add the truck to it and your hitting the scales at over 6500 if not more the Dodge was not designed to get high mpg they are Torque Monster's not fuel sipper's

 

GM had a lockup converter.... Dodge didn't do a lockup with the cummins until '94.

 

6.2L GM never put out that much power, the most torque it put down stock was around 300lbs. The 12V puts down a lot more power in the torque curve in the stock setup (450lbs). Two totally different engines to really compare anything two(V against inline).

Another thing to note is the major fuel restriction on the pintle injectors that choked out the GM 6.2L. Because of that, pushing it down the road at 55mph gave it high mpg. Okay for running the truck around but if you wanted that engine to work it got really cranky. The high rev's during a load cause these engines to spin bearings or even turn the crank whacky.

Is high fuel mileage achieved with the 12V? Sure, I've gotten 19-24mpg out of my 12V hauling a 40hp ford diesel tractor. Now if I wanted a vehicle for just cruising around the 4bt would be something to look at for a high mpg engine. If you really want high mpg, VW makes a rabbit that will get you 60-70mpg using a smaller VE pump. You have to have the right tool for the job.

 

The 6.2 was a dog, and didn't have forced induction. No turbo means less air and less fuel burned...and less power.

 

From a design standpoint (i.e. pre-cup vs DI) the 6.2L runs warmer when it is not working hard. That is going to help burn the fuel more completely. This is my other theory besides the torque converter.

 

I can tell you that HMMWV had those 6.2Ls. Did not matter if it was armored or not it always had a range of 300 miles. The tanks are only 20 gallons. We ran JP8. They had a 4L80E or 4L60E, cant remember.

I always found it funny that we had runaways on a regular basis. The solution... dump water on the injection pump lol.

 

My D250's mileage is 15-16 around town 21-22 freeway I let my buddy with a trany shop talk me in to a 47rh tolled me I would get lots better MPG with the lockup converter. I like the way it tows with the 47 rh but I dont think it get enough more MPG's to do all that work and all the $$ to make up for it Ken

 

I will say this, my last 86 6.2 had a Banks turbo kit and was close to running with an early 1st gen on the street, it did not get any better mileage than the Cummins guys were getting, and I was still a half ton truck.

I'm pretty sure if you turned your 6.2 up to what the Cummins makes, you would find your mileage runs about the same.