Every 6.5 turbo I have ever owned has struggled to break the 20 MPG mark.
Usually 17-18 tops with the average at around 16
The huge cloud of BLACK smoke is unburned fuel, whicj equals MPG and $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ out the tail pipe.
The small puff of smoke on the Turbo enignes is not a biggy as long as it clears when the turbo spools up.
The larger precups in the turbo engines (larger port) reduces the swirl effect of the gasses in the cylinder and this was the real key to to great MPG's on the 6.2's of the 80's
As the velocity of the gasses exiting the precup port drops (larger port) the thorough mixing of the air fuel mixture also goes away to some extent.
Retarded timing will cause a marked drop in MPG and an increase in EGT's
If you can get the burn done in the cylinder and all done before it's shoved out the exhaust, the results are a much cooler exhaust flow.
Running the timing a tad more advanced will cool things off some, just dont get crazy with the advance.
As the IP wears out the advance piston (DB2 pumps) wears out and the advance stops working correctly and or fails to advance.
Also the POP pressure of the injectors can really make a bunch of difference in the power levels as well as the smoke.
When the injectors are new and the spray pattern and the POP are right up to snuff the fuel spray is a very fine MIST and this allows for a very clean and efficient burn.
When the POP falls off (read this as worn out injectors) the fuel delivry takes on more of a pee stream instead of a nice cone of mist.
Large droplets of fuel ignite slowly and take far longer to burn completely.
This situation basically results in a lot of black smoke and High EGT's and a loss of power.
As the power fall off the foot presses harder on the pedal to get more seat of the pants and with the seat of the pants feel come even higher EGT'S and more BLACK SMOKE.
Its not always just getting fuel into the thing that makes Power and mileage.
Timing is very important as is the length of time that the fuel is being injected.
If the pop pressure is LOW and the fuel slobbers into the precup the ignition is poor and takes far longer before the "fire" is burning good. When this happens the optimal crank angle is gone before the fuel is all used up and then the exhaust cycle is begining even with unburned or partially burned fuel still in the cylinder.
The "Fuel Map" is a very complex issue for sure. The common rail stuff thats all controlled with the computer and the timing, delivery amounts and such are extremely accurate makes things a completely different game than we have on the older engines with a mechanical pump (even the DS4 is mechaincal as far as the actual injection goes)
The extremely high pressures that are used on the DMAX, Strokes and the cummins makes this a whole different ball game.
The one big disadvantage of the IDI engine is the fact that the precup soaks up a huge amount of heat right in the head, where we really dont want it.
Back in the old days of 160 HP maximum 6.2 engines this was not an issue as these engines were designed to get good mileage and not to make huge amounts of power.
As the power levels rose so did the issues with heat and the tendency to self destruct.
The 6.5 TD, the way it left the factory is a compromise engine, sort of in between the 6.2 N/A and the Dmax
Getting a 6.5 TD up to around 225 HP is about the best you can hope for and have good reliability.
The mileage is going to suffer as the reason for the great mileage (small precups and little fuel) is gone, replaced by larger cups/ports and far more fuel.
The Banks Kits offered a great compromise for the 6.2 in that they put some much needed extra air into the engine and a tad more fuel, all while keeping the precups with small ports.
Using a set of precups with the smaller ports can improve the mileage in a 6.5 TD but with a loss of some power.
When we step over into the land of the Dmax and the others DI engines the game is totally different. The cumbustion all takes place in a little bowl in the top of the piston and the flame propagation is totally different.
The transfer of combustion heat is mostly absorbed by the water jacket around the cylinders and not the head.
The piston on the DI engines does take a bit more heat, but they are designed to shed this heat readily.
The Dmax engine uses aluminum heads which also helps transfer more of the waste heat into the coolant flow.
Just some thoughts on a very complex issue
Missy
