Detroit 12.7L MPG.

I would check the charge air cooler for leaks first. That motor should get 6 easy

 

^^^^^ Agree. Best first test is leaks from the turbo thru the CAC to the intake.

 

Depends what speed you driving if its 70mph..I dont wonder.

 

Would also have the valve and inj adjusted

 

Speed is important, but at what RPM the engine is running at that speed is also critical. The 12.7L seems to do it's best at 1400-1475 RPM. The hp, torque, economy charts from Detroit spell this out, and I have found that they are pretty close to correct. if the engine RPM's aren't at their optimal, then running a slow speed may not give the results one is hoping for. One thing that has been true and still is.... gearing. The gearing has to have the engine RPM and road speed matched to give optimum results. This is why some can get good mpg at 65, and die at 60. While going slower decreases the aerodynamic effect, and can improve mpg, it will only do so if the engine is at it's most efficient RPM. And this is even more critical based on the loads hauled and the terrain one is doing it on. It is one thing to get good mpg on flat ground with a light load, it is another to get good mpg with a heavier load on rolling hills. One can get good mpg even heavy and doing rolling hills. Again, things have to be matched up right.

I average mid 7 mpg's, year round, with average gross of 70,000 lb. I am running a pre-egr Series 60 12.7 500 hp, 18 speed, tied to 2.64 rears. At 1400 RPM in 16th (direct) I am running about 61-62 mph. At 1500, I am at 65-66 mph. I keep the RPM's between 1400 and 1475.

 

Cowpie1 What size tires are you running with that configuration as they make a difference too.

 

Cowpie is correct that Detroit Diesel generally recommends 1400rpm for fuel economy:

View attachment 76174
(this graph is for the DDEC VI)

But that is their general recommendation based on all sorts of equipment. The power demands of a van are generally going to be lower than a flatbed which are going to be lower than a car hauler.

So here is a graph of BSFC for the 1994 12.7 Detroit Diesel Series 60 (DDEC III) from SAE (source: http://www.nap.edu/openbook.php?record_id=12258&page=36 ). It helps explain engine efficiency for various power demands at different rpm:

View attachment 76173

In case you haven't run across BSFC before, basically it is the amount of fuel consumed to make a certain amount of horsepower for 1 hour (the lower the number -- the less fuel consumed).

You can see that when power demands are below 140 horsepower, BSFC is best at lower rpms. Above 140 horsepower, things get interesting and you have a higher efficiency island between 1400 & 1700rpm -- with the island being fatter towards 1400rpm. Pretty cool to see that graph -- my own tests showed the cut-off around 150hp...

Anyway, if you are Detroit Diesel and you're giving generic advice to fleets pulling a variety of equipment with drivers who don't care about mileage -- you're going to tell the fleet to gear it for 1400rpm so their drivers can just set their cruise control and put their feet on the dash... But if you're a fuel-conscious driver pulling efficient equipment (requiring less than 130hp on flat ground) and don't have a problem slowing down & downshifting when going uphill, you will probably be better off cruising at lower rpm.