As noted, I know little about electric so they very well could be a "must have" for the Flux.
They aren't really a "must have" on a nitro savage. Since this thread is named "Savage HP", and the OP clearly states he's asking about a flux hp, so rc4me.us isn't wrong.
I've been running nitro savage's in one form or another for 14 years. Got my first one as a .21 roller off a member at rcnitrotalk.com back in 2004. Up until the BP diffs came out, diffs were a constant problem for me and many others. Especially in the 2-spider gear years... I wasn't even running a hot/high power engine, just a tame OS21RG, which by today's standards is a total weakling of an engine. When the machined non-BP diffs came out, that was a big help and the alloy cup helped even more. But many still got both of those and went one step further with alloy diff case halves (me included) so there was enough free room to shim properly. Still, with that, diffs were a chore to maintain since you had to take the entire front/rear off the truck and break it down completely to get at them. Even then, I went through my fair share of machined ring gears/pinions. Probably partly due to the old plastic diff cups and ring gears used 10x16 bearings on both sides. Those just didn't hold up well.
When I got my X which had the new style diff case and alloy cup back in 2012, I had been without a savage for a year or so. After my first few bash sessions after throwing in a trusty LRP28S3, I shelled the rear diff, which was machined but not BP. I got the truck used and it looked shiny new, so I don't know how much time was on the diffs. When I replaced them, I did so with BP sintered ring gear diffs and alloy cups. I have yet to damage either the ring or pinions in the X and it's had an LRP30 in it half the time I owned it. I have changed the diff bearings a couple times, which then I tear the diff clear down, check the shims, clean out the 50K oil, reshim, put new bearings on and re-oil/re-grease, then run it until I see the bearings getting wobbly again. It at least had one 8x16 on the ring gear, but still had the 10x16 on the cup side.
I'm really glad they redesigned the diff case. That helped a lot with keeping good mesh as it's more stout than the old 21/25 style case halves and they don't need replaced with alloy. Also, since you can pull the diffs out pretty quickly/easily, it makes maintenance not as much of a chore when the stupid 10x16 bearing goes out.
With my flux HP, I've shattered 3 or 4 pinions in the first 10 months I've had it. It has the machined BP ring gear/alloy cups/super HD outdrives/bones. I've replaced diff bearings twice as well. My latest big change with it was putting in a super 5SC transmission from rc4me.us that has a center diff inside and I put 750K oil in it. I put that in about 5 months ago and so far, haven't busted a pinion again. However, I got an outcast a month after that upgrade, so I haven't taken the flux out nearly as much. But, in the month or so I did, I took it out every weekend and ran it for probably 3 hours each weekend and ran very hard. Lots of hard landings that should have blown a diff.
If you ran 20 gallons through that thing with stock sintered or machined non-BP diffs with those high power engines without frying a single diff, my hat is off to you and you are a much better driver than me with a far better maintenance schedule. Before I started using BP's, I'd be lucky if I could get 2-3 gallons through a savage before shelling out one of the diffs. Especially if I didn't catch the wobbly 10x16 bearing in time.
As for the diffs holding up on the other trucks you have, it's probably a combination of things that makes them last so long. Having a center diff really helps and having less weight overall helps a bunch too. That center diff though does so much better than any slipper since it will tend to transfer the initial bad landing force to the diff with least grip. I'm amazed at how well the outcast diffs hold up considering the torque and weight of the thing. The diff gears are so small compared to the savage. The pinion is a tiny little thing. I did fry one diff though in 5 months, but I'm 99% sure that was due to the diff cover that holds the diff in place and 3 screws with stripped threads.
