I've been trying to get my head around how this works. From what I understand so far:
- Every engine has a point of highest efficiency. I remember reading somewhere (but can't find it back) that for diesels that is 10% above peak TQ.
- Since we plan to do road trips, we want the peak efficiency RPM to coincide with enough tire rotations to move the bus at ~65mph
- Since at highway speed, the transmission should be in it's highest gear, we take that gear ratio and combine it with the gear ratio from the back gears
You stick all that in a handy calculator
that somebody linked
Our bus has the A175 configuration of the VT 365, from the manual that engine produces 460TQ @ 1400 RPM (yes, I know TQ is not the unit
and max 175BHP @ 2600 RPM. If I got the 10% right, it would the most efficient RPM at 1500 or so.
Our transmission is an Allison 2500 from likely 2004, from what I read that means it's likely a 5 speed. With if I read the spec sheet correctly a 0.74 overdrive gear.
Our tires are 11r22.5, the calculator needs tire height and Google suggests that is around 40" (not at the bus location to check that)
Dealer ran the VIN and we have a 5.29 rear gear.
If I stick the current config in the calculator I get 2143rpm
Reversing that to find the desired read gear I land at a 3.77 gear.
Do I have any mistakes in the above logic?
Second question, from what I read the VT 365 can be bought in different power configurations. All internals seems to be the same, it's the ECM that determines final output. Other configs are:
A200: 200HP @ 2600 RPM / 520TQ @ 1400 RPM
A215: 215HP @ 2600 RPM / 540TQ @ 1400 RPM
A230: 230HP @ 2600 RPM / 620TQ @ 1500 RPM
The Allison spec sheet
seems to indicate (but not so sure) the 2500 in school bus design takes max 200 HP and 705 Nm (519 Ft-Lb). Would they spec different transmission to go with the different engine configs?
If the transmission can handle more power then the current A175 config, is there any efficiency gains in getting the engine moved to higher power config. And is more always better? I've asked the dealer if they can change the ECU, and what sort of cost that would be.
All insights welcome!