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Originally Posted by Big Dave
First of all, I have finally solved the electrical problem. I put avionics type sealed connectors on the part under the truck and grommetted all the wire penetrations. So far , so good.
There are two other issues. Do try your best to find a used GV. These things new are pricey beyond belief. $3,000+ pricey. Manual adapters are rare and even saltier. I drive 25,000 miles per year and the fuel economy justifies that, but still took three years to pay off.
A bigger problem is the effect of reduced engine RPM on automatic transmission life. Most automotive automatics use the torque converter to pump fluid to the transmission cooler. Lower engine RPM = lower TC RPM = lower fluid flow to the cooler. Less fluid flow means less heat rejection. Transmission temperatures rise up and spectacular transmissions failures (parts and ATF scattered down the road) result. Ask me how I know this.
I used to have a GM pickup with a weenie 6.2 liter diesel and a TH400. I never thought I'd live to see something destroy a TH400. A GV could. Raking the parts up and investigating found ferrous parts (gears, clutch baskets, shafts, and bearings) all turned blue. This is the unmistakable evidence of excessive heat.
If you have a respected automatic transmission builder, you may want to talk to him. Maybe he'll tell you different.
This is the reason you rarely see diesel pickups with less than 3.73 gears (some Fords and Dodges offer 3.55s). Not the engines. My old 444 International does just fine with a GV AND (no longer available) 3.08s but then I have a stick shift. Heat buildup is not an issue on sticks.
Bigger (Model 1000 and up) Allisons can live with a GV because they have auxiliary fluid cooler pumps that maintain big fluid flow at relatively low engine speeds. That's why Allisons are preferred on commercial vehicles and also why Allisons are so darned pricey. You get what you pay for.
All that said, my experience is that the GV (by itself) is worth an additional 2-3 MPG.
For those who like to improve the aerodynamics of their pickup, lowering engine speed at a given road speed allows even greater MPG improvement. I'd love to see what aerohead's truck would do with a righteous diesel and drive train offering him a 1350 RPM (@70 MPH) cruise.
Say a M-B OW617 turbodiesel engine with a Tremec T-56 six speed (0.5:1 top overdrive) and 3.55 gears. The OW617 turbo would give him all day long 125 HP, and is legendary for durability. With aerohead's super-low CdA, 125 HP would be the greatest of plenty, even in Texas.
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You have given me a lot to mull over considering transmission temps. We have also destroyed multiple TH400's on even non turbo diesel engines. Auxiliary oil coolers help, but that amount of torque at the relatively low RPM seem to thrash them sooner or later. The local transmission guy loved my dad...
I'll talk to my guy and see if he thinks the low RPM will cause the tranny to run hot, and if there is an easy solution that will work to cure this. A cooler would help, but without proper flow I guess it would heat up eventually.
How does the Gear Vendor work towing? I assume most of the miles in your beast are loaded, and I'd be interested to hear how often you are able to use the double overdrive in those situations.
I'm kind of limited on aero, simply because of where I live/work and what I need the truck to do. A soft tonneau would work (too bad my 8 footer won't fit without surgery), but I hook up to a gooseneck and fill the box too often for a topper. Hate to sacrifice ride height because my wife would have to get out of the truck in the winter to see over feed bunks. Anything lower than the axles would get damaged driving through a field. It's still going to be a brick at the end of the day.
I really like the fastback you built. I could cobble/weld something up that would approximate what you have and still let me pull a gooseneck.
Thanks for your suggestions, and I'll try to see what I can dig up regarding possible transmission damage.