Also, there are definitely plans for a turbo around the same time as these driveline modifications. So the next question becomes, what would be a sufficient hp/tq goal to move a bus with this gearing? Did you have any power goals when you laid out your turbo setup?
One method that comes to mind, to get a ballpark, is figuring the percent increase in gearing and shooting for a corresponding percent increase in hp. I'm going to go with 4.33 to get an upper end of how much power I might need. 5.57 / 4.33 = ~1.29. So maybe I should be looking for approximately 30% more power. The 6.9 IDI has about 170 hp stock. Adding 30% to that gives 221 hp. Thewespaul in his
turbo selection thread gives 2.75 hp per cc of fuel, which would mean I would need about 80 cc of fuel to make that power. I've read the stock IP maxes out at 70-75 cc. So maybe not quite there.
The same calculation for 4.56 gears:
5.57 / 4.56 = ~1.22
1.22 x 170 hp = 207 hp
207 hp / 2.75 cc per hp = ~75 cc
That hp goal is potentially within the capabilities of the stock pump, even. Might as well try a turbo install on the stock pump and if it's really not enough, drop a 90 cc pump in there for a little extra headroom. I do like the idea of being able to run stock pumps purchased used, though, should there be any problems, as towing a bus from the side of the road is a situation I absolutely want to avoid. And it would be nice to not have to worry about running WMO etc. through an expensive built IP.
I've been doing a little more turbo reading and the TP38/GTP38 seems like a pretty good turbo for an IDI application, really, especially considering they seem to be pretty plentiful and cheap. R&D analyzes a map for the GTP38 in
this thread. It sounds like some have had success switching to the smaller .87 A/R exhaust housing for a faster spool as well. It looks a bit big for a stock IP though--I wonder if it would be useless with the stock IP, or provide a mild improvement with plenty of room to grow into fueling mods. Were you thinking you would run a 90 cc IP?