I see this mis-information being spread around all the time! This is for the people trying to make their minds up on which one to buy, if pulling power is on their lists of "things I need". Or for the people that think just because the 855d is a diesel, then it automatically has more "low-end grunt" than the 825i. They aren't even close, in performance(pulling power), and I've made several different charts to show you the differences of the two.
First, there is the basic horsepower and torque comparison, based on RPM. As you can see below(red is 825i, green is 855d), the 825i has more TQ and more HP than the 855d at every RPM! Even down at where clutch engagement first occurs(roughly 1500 rpms), the 825i has more TQ than the 855d does at peak tq rpm...
825i power stops at 6000 RPMs, and the 855d stops at 3600.
And here is where things get interesting, and show the true potential of each vehicle, as I have accounted for Primary/Secondary gearing and the Differential ratios in each vehicle. Motor TQ x gear ratio x differential ratio = wheel TQ. And that is what we feel accelerating us. Acceleration = Force divided by mass... so the more force you have, or the lighter you are, the faster you accelerate(The harder you can pull!). I did not account for CVT power losses because I'm not positive what they are on the gator setups... I've heard there are 30% losses from the engine to the rear wheels, but excluding them doesn't change our comparisons below. I will show a couple hypothetical "what if" graphs after the first one, for the people who always say "but but but... what if?!?!"... trying to uphold the lowly diesel. :D This isn't meant to hurt anyone's feelings, just to show the actual numbers behind the performance!
This graph depicts the wheel torque of both vehicles in their stock configurations, being held at wide open throttle. 825i shifting ~5500 rpms stock, and the 855d shifting around ~3400 rpms stock. The most notable difference is around the 10-11 mph mark. You can see that the 855d is making around 850-900 ft-lbs of wheel tq, while the 825i is making nearly 2100! This means that at that mph, the 825i has OVER DOUBLE(233%!) the amount of pulling power(wheel tq) than the 855d does! Up near the top speed of the 855d, it has ~300 ft-lbs of wheel tq, while the 825i has ~700... still a hair over double the amount of pulling force.
Another way to look at the following graphs, is pick a number on the TQ side of the scale.... say 1000 lbs. If you follow it out to the blue line of the 855d, you can see that it intersects with the 9 mph mark on the bottom scale... this means that when the 855d is pulling with 1000 lbs of wheel TQ, it is going 9 mph...in comparison, the 825i is pulling with the same force at roughly 21 mph! There is no comparison of pulling power anywhere throughout the rpm band in their stock configurations.
And now for the hypotheticals(to show the people with excuses and illogical reasoning)... The scenarios that would call for re-clutching of the CVT system, or really bogging the machine down with major loads, to achieve the wide-open throttle RPMs that follow...
This graph shows the differences if each vehicle was re-clutched to operate at their respective peak torque RPMs when at full throttle. 825i @ ~3000 RPMS and the 855d at ~2500-2600. The 825i still wins in this department. A lot of people think peak torque RPM is the place to be for CVTs and other vehicles, but pick a few points on this graph and compare the wheel TQ to the graph above to see how inferior "peak torque shifting" really is. Please don't ever do this(in ANY vehicle!).
This graph retains the same clutching setup as above for the 855d, with a 2600 wide-open throttle shift point. But shows a completely neutered 825i setup and simulates an equivalent shift RPM of 2600... to show a direct comparison for the "but my diesel has more low-end grunt" people! Even with the 825i churning along at these comparative, near-idle speeds, it still has roughly 50% more wheel TQ than the 855d.
To finish off this write-up, I'll show a comparison of the 855d shifting at 3400 RPMs(stock) versus a re-clutched version of the same machine that would shift at the peak torque RPM of ~2600. The peak of the blue line you see is the peak torque in "first gear"... the lowest ratio on the CVT pulley setup(1350 ft-lbs is the absolute max this machine could ever make...but still nowhere near the peak wheel tq of the 825i). As the RPMs increase, the wheel TQ would finally hit the point at the top of the red line. After that, AT NO GIVEN MPH would the wheel tq ever be higher than at the shift RPM of 3400(stock). Looks like a 10-15% difference at all MPHs.
1. If you're needing max pulling power, get an 825i. The 855d isn't even remotely close!
2. If all your machines are diesel, then maybe I'd recommend getting an 855d...but only to simply fuel demands.(And only then!)
3. The last chart shows that, for ANY MPH, maximum wheel torque is ALWAYS made at higher RPM(preferably peak hp RPM).
4. The fastest, AND HARDEST PULLING(because that's what wheel TQ equals) cvt setups are the ones that shift at peak HP.(Only exception is peak TQ rpm in low gear)
5. I'm really surprised the 855d isn't setup to shift at the max rpm, since the wheel TQ is kinda low. (Needs all the help it can get)
If you don't fully understand these charts, or have any questions at all, ask away!