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+--- Thread: 2015 cm2350 tuning?? (/showthread.php?tid=5412)
RE: 2015 cm2350 tuning?? - Explosia - 12-07-2019
(12-06-2019 )Magard Wrote:(12-06-2019 )Magard Wrote: 17th gear is overdrive and under drive at the same time. Not a efficient gear. What tire size.Main box is overdrive. Splitter is underdrive to make 17th gear. A lot of people don’t realize that.
Sorry if this is a stupid question, but can you explain why that matters? If he’s at say, 63@1450 in 17th, how does that differ from 18th, or another gear? The top side is more efficient than the bottom?
RE: 2015 cm2350 tuning?? - Magard - 12-07-2019
It’s a minor thing but it all adds up. So in a constant mesh transmission all the gears turn all the time. That’s 5 gear sets turning. A main shaft and two counter shafts. The main shaft has gears that are floating on the shaft and locked on to the main shaft with sliding collars when you select a gear. The shifting forks move theses collars that lock the gear to the shaft. So in direct drive and lower the input shaft is moving the same speed in direct as the gears. In lower gears the input shaft is moving slower than the gears.
Here is where the parasitic lose comes in. In overdrive the main shaft is spinning faster than the rest of the gears that are floating on it. So 4 gears engaged to the countershafts turning all the time but spinning slower than the main shaft when in overdrive. How much lose is this? I don’t know. But that is the reason mega fleets like Walmart or who ever spec trucks with direct drive 10 speeds and like 2.88 for rear ratio. I had a Walmart cab over with 60 series set up like that. I couldn’t tell if it mattered one way or the other. It pulled a hill fine. Fuel mileage was similar to any other truck so I don’t know.
https://youtu.be/bjJwC211HgU
Next video shows the auxiliary box. See how in theory looking at it 17th gear is overdrive in main box and under drive in auxiliary. Most efficient in theory is direct straight through drive which is 16th gear in your 18 spd. Next most efficient is 18th gear because auxiliary box is in direct. Remember I don’t know if this matters enough to realize this mpg wise. I’m just explaining the theory.
https://youtu.be/3JwlPFvAZ2g
If specking a new truck damn sure don’t spec it to run in 17th gear. Sales men that don’t know this about transmissions will spec trucks to run 18th gear in high speed states and 17th in 55mph states. It is kinda a bastard setup in my mind. The new setup with all this downspeeding of engine rpm. The theory is run overdrive in high speed states and run direct in low speed states. It takes something like 2.5 rears ratio to make that work. So yeah I don’t know. That’s a crazy high ratio. A truck with a Cummins has no business geared like that. Other manufacturers have different liner designs to handle that low rpm.
RE: 2015 cm2350 tuning?? - Mattman - 12-07-2019
What kind of tank are you pulling?
I can't fuel mileage pulling an ISO tank. They sit to far back of cab and they catch wind like a parachute.