Quote:
Originally Posted by RustyLugNut
Emissions aside, what you are doing with your truck to reach your goal of 24 mpg is laudable and important as pickups make up a large percentage of some communities. Once you have exhausted the easy engine modifications, you can peruse the aerodynamic possibilities. That opens up a world of gains as our trucks come factory equipped with horrid coefficients of drag.
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Aerodynamic mods are pretty much out of the question since the vehicle has the frontal profile of a barn door to begin with and it frequently gets used off-road.
The next step will to replace the stock ECM with an ECM from a 1996 Bronco or F150 that has the same drivetrain package and California emissions equipment. Two reasons for this; Even though my F350 is a 1997 model, it isn't OBDII. OBDII didn't apply to vehicles over 7,000GVW until 1998. Switching the ECU to an OBDII compliant one will allow me to view live data. The California specific ECU is also a mass-air unit and will allow me to convert from the stock speed-density fuel injection strategy to mass-air. There are kits available with the associated piggyback wiring, ECU, MAF meter, etc. Kit cost is about $600. This change alone should net around 1-2 mpg improvements because of the improved accuracy in fuel metering.
After this, the changes will revolve around building a new engine. The stock 351w powerplant is pretty reliable and there is a HUGE aftermarket for parts. Of course, many of the parts are for improved performance/horsepower but a lot of the same mods that increase specific output can improve efficiency/FE at the same time. Here's a list of what will be changed in the rebuilt engine:
1. Swap the heads for a set of TFS Twisted Wedge heads with 190cc intake runners and 62cc combustion chambers. These are arguably the best head for the 351w from a performance standpoint but they also have a very efficient 'fast burn' combustion chamber design. They also have reduced combustion chamber volume from the stock head and will allow for a 10.5:1 compression ratio with flat-top pistons. Obviously, the impact on fuel economy will be from the ability to tune for lean-burn operation and increase efficiency from the raised compression. This head-piston combination should run just fine on 87 octane fuel and possibly be ok with 85 octane at the 4,700 ft elevation I live at. Heads will also get a set of full roller rocker arms (reduce frictional losses in the valvetrain). The heads alone will not get me to where I want without some careful prep work to the bock. The block/head combination will need to be set up with a quench clearance (sometimes called 'squish') between .040" and .060". The quench area is the space between the flat part of the cylinder head (deck) and the top of the piston when it's at TDC. The .040-.060 clearance is important to promote turbulence in the combustion chamber and to increase the engines resistance to detonation. Most stock engines have a quench clearance from .080 to .100+. Quench clearance this large has the opposite effect by making detonation more likely and allowing a portion of the flame front to be extinguished while leaving some unburned fuel in the large quench area.
2. Regrind the crank to use 351c main bearings instead of the stock 351w bearings. The 351w main bearings are 3.000 inches in diameter. 351c mains are 2.750. Clevite makes swap bearings for this change. This is an old trick used by engine builders to pick up some free power and also has the benefit of reducing the oil flow necessary to lube the bottom end. Several benefits here: Reduced frictional loss, lower burden on the oil pump allowing the use of very light weigh lubricating oil, and less windage loss inside the crankcase because of the reduced oil flow. A windage tray will also be used to further reduce windage loss.
3. Use gapless compression rings, low tension oil rings, and flat-top hypereutectic pistons. The gapless compression rings can add from 1-3% in efficiency alone. The low tension oil rings can reduce rotational torque by 40%. The hypereutectic pistons have a lower coefficient of expansion than forged pistons and are stronger that stock cast pistons.
Still doing a lot of research on cam profiles and I haven't made a decision if I'm going to use something 'off the shelf' or go with a custom grind.
The transmission is still up in the air too. I'm not too fond of the ZF 5-speed manual tranny that Ford used and there are some expensive swap issues if I want to go to a NVG 4500 (5-speed) or NVG 5600 (6-speed). I may stick with the E4OD 4-speed automatic that's in it and use an improved torque converter. There is also the possibility that I can figure out if I can build the E4OD to use the newest specification Mercon LV transmission fluid. Mercon LV spec tranny fluid is very light viscosity fluid and should be good for some significant improvements in the efficiency of the transmission.
Once the engine/transmission assembly is ready to swap, I can begin the process of refining the ECU programming for the new combination. That will be the biggest mental challenge and also the most fun part of this project!