RCMAN46 wrote:
I used my 05 Duramax with the Allison 1000 for my example. The 05 has a peak hp at 3000 rpm and peak torque at 1600 rpm. These would be ideal shift points. Max rpm is 3250 for the 05 Duramax.
I assumed 30 inch tires and a 3.08 rear end for one case and same tires and a 4.10 rear end for the second case. I will accelerate from 40 to 65 mph. A typical speed change when getting on a freeway and sometimes passing a slow truck when hauling my 5th wheel.
With the 3.08 I would be in 3rd gear going 40 mph at 1956 rpm and accelerate to 65 mph where I would be turning 3162 rpm.
Now with the 4.10 I would be in 4th gear going 40 mph at 1837 rpm and accelerate to 65 mph where I would be turning 2935 rpm.
Interesting the 3.08 gives me a change of 1216 rpm and the 4.1 gives me a change of 1148 rpm. Almost the same except the 3.08 is greater and Carecraft claims this will result in a shorter acceleration time.
You picked one of the speeds where the 4.10 has just changed to next next higher gear. That's where it will have a disadvantage. The problem with your example is there are many more speeds where the 4.10 has the advantage so
overall it is more favorable. My chart below will show those areas where the 3.73 is still pulling after the 4.10 shifts.
Will the 4.10 still have certain hill/weight combos where it performs more poorly than the 3.73? Yes, when you are between gears (as in just shifting up) you aren't able to get the engine up to its higher RPM HP.
The difference between available HP and HP demand is what provides acceleration. When you look at the chart from an RPM in each gear point of view, notice how for every point on the RPM range (the slopes are each gear), the 4.10 geared truck has a greater HP advantage because it's there when the HP
demand is lower.
The peak of each RPM is 5000 (my choice) and the valleys are ~3000 for the 1-2 shift and ~3350 for the 2-3 shift based upon the 6-speed ratios I used.
Note: I don't expect these actual speeds to match any particular real truck but to simply illustrate the difference. I just used ~35mph for 5k rpm in 1st for the 3.73 truck and 300 ft-lbs flat across 3-5k rpm.
To exaggerate the point I changed the 4.10 to a 10.0 which allows it to use all 6 gears. Notice how much more area there is between the available and demand HP lines. That is "extra" power for accelerating. The other interesting thing is that the higher gears are closer together which would also favor a lower axle ratio working better.
