I have a 93 Chevy P3 Parcel van. It has a 5 speed OD manual trans. and a 4.56 rear. It is about 21'long, 10' tall, and 7 1/2'wide. The origional 6.2 diesel was replaced by a previous owner with a Cummins 3.9 BTA, 105hp unit. I've read NASA's report on modifying. In the future I will be inclosing the rear wheels, blocking part of the front grill, inclosing 3/4 of the under body, adding a front air dam, and a 30" Kamback.
The truck is currently used to transport stuff to craft shows 4-6 times a year. It averages 13.5 mpg at 55-60mph. Adding a 1000-2000# load makes little change in the mpg. Seems a perfect canidate for aero modding.
My other tranportation is a 03' Focus (29mpg) and a Yamaha TW200 (72mpg). But the van makes every drive interesting...
Don't know anything about the GM 6.2 except the early units were comparable hp/torque wise to the Cummins 3.9bt and bta. From what I've read, most of the fleets using stepvans in the 90's tended to pull the GM units sooner or later and install the Cummins unit. In general I just like the 3.9/6bt as they are all mechanical. Execpt for the 10amp's needed to hold open the fuel valve, all the engine needs to run is fuel (ok, plus the 5-6k oil & 10k fuel filter changes).
Power wise I've read most of the yahoo forum on the BT3.9. The Cummins marine version is rated at 250 hp @ 2800 rpm for 800 hours a year for a specified number of years. The BTA unit is suppost to be rated at 130/135 hp (my BTA unit nameplate says 105hp). the BT unit is rated at 105 hp. The basic unit also weighs 700+ pounds installed; comparable to a large block V8.
After a couple of years reading posts and and looking at the Cummins HP charts posted (on one chart in small print) was the altitude rating for the 105hp rating, 10,000 ft.
I live a flat to modest rolling plans area of TX (Lagrange). To maintain 50-60 requires running at full throttle about 1/2 the time. Around Austin, forget 60, I just use 3-4th and try to stay out of the way at 40-50mph. 105 hp is not enough to move a 6000 (empty)# to 8000# brick in Hiway trallic.
Gearing wise the 5speed (NV4500) is miniable. With the 4.56 rear, I top out at 70 in OD (5th) with the governer (2400rpm). At 55 the calculated rpm is around 1900. The rated continious operating range is 1700-2350 rpm. With (260ft.lb torque at 1700rpm) 1(low,nonsyn. never used but would starting up a hill with a heavy load), 2(used as 1st), and 3rd gears are up to 30 with most loads. Then the govenerator kicks in, I have to shift to 4(1/1), and the engine bogs down. Another gear or two in there would sure be nice. Different governor springs could help by raising the max rpm from 2400 to 2600 rpm. My concern changing gov. springs is the van runs thru 2 and 3 easly, and i would keep over revving the engine.
I've thought about getting a 3.75-4.10 rear; I shift into 5(OD) at 50 but then with any headwind, crosswind or incline make it impossable to maintain 50. With the increased Gov. speed I could hit 55 before shifting to 5.
The UP SIDE is that the 105 hp is there at sea level (Houston) and 10,000' (Silverton/Uray CO, where I'd love to take the TW, an ATV, and still have half of a 16'x7x7' box left for THE other vacation stuff). It will fire up after sitting for 6 months (4yr. old battery) with just a wiff of smoke. Empty or loaded the exhaust stays almost clear and the mpg always hovers around 13.5-14. And once the van hits (moissies would be a good description) 60 on an flat Interstate, it can maintain it (ie,no long hills or strong winds). Most of the engines are said to last 300-400k if you don't mess with them. As my engine rebuilding/electrical are nill, the 105hp seems a pefect fit for me.
In that the load makes almost no impact on the mpg (atleast in TX) and the topend gearing (1900rpm/55mph) looks good implies I got one huge aero load to play and chip away at.