In a nutshell, the time that it takes the car to reach the first 60' foot mark is THE best indicator of how far you're pushing the 8" pinion. Then you refine the available safety margin based on traction/launch shock loads, vehicle weight, and how short the rear gear gets(much thinner pinion gear). Guys that specifically understand and work within the shortcomings can go well over 500 horsepower on street tires. Have also seen a few fast running street cars pushing over 600 horse on occasion too but they aren't pulling any wheelies either. Still see some old style full-tubbed rods and t-buckets running these rears behind some pretty hellacious big-blocks but those cars often weigh WELL under 3,000 lbs. So, it's all highly subjective and relative to the entire combo of parts.
See a lot of that here too, those cars usually only come out to go to shows and cruises. The way they're driven they could use a Pinto rear end and live forever!
The reason the 9 inch takes more power to turn is because of the low pinion Angle. This causes a bad angle on the gears Because the 9" is the only car style Rear Diff with Three Pinion bearings Two in front and one behind it will not distort at high torque levels. It has been said that at 400 hp the Dana 60 takes less power but at 600 hp the Dana 60 will distort the pinion causing it to turn harder than the 9" ford! The 8" ford also has 3 pinion Bearings but it is to weak to consider as a performance Diff.
That's what I am running its a 9-inch 389 gear. I was told in a 8.8 turns easier than 9 and is just as strong
The 8.8 does turn easier than a 9' but as for the being as strong depends on what 8.8 and what 9 inch! A 9" rear can be built to handle a top fuel dragster. a 9' with a single cross shaft and 2 spider gears is no stronger than a 8.8" but a 9" with 4 spider gears and two cross shafts is lots stronger than any 8.8". As Far as Axles go You can get as big of axles for the 8.8 as the 9". The standard axle in both are 28 spline and 31 spline is optional, Aftermarket Axles can be had for both up to 41 spline.
I don't see a 9" holding up to 10,000 HP. TF stuff. Not even 5000hp old school TF stuff. 1000? Doubt it. I've broken them at 500hp or sometimes less, withOUT pinion shafts and gears (we run full spools). The weak link is the pinion support, then the case itself., both of which are not aftermarket in my stuff (aluminum). BTW top fuel stuff uses mostlyStrange and Christman rear end assemblies. The Christman is a little stronger as I am told. The ring gear is about 12" in diameter, and the pinion gear is about 3 1/8" diameter at the spline. There is not much 9" about it, not even close-not even the original design of the 9" is used. It's all aftermarket. The Christman uses a "tail bearing" kind of similar to the 9" but that's about it. Strange does not use a "tail bearing". I can't remember, but I think Christman's also use a pump to force oil to the pinion bearings, otherwise acceleration G-forces would force all the oil to the rear of the case, starving the pinion bearings. JFR typically replaces R&P's about every 30 runs or so. They were doing 50-ish as I recall, but the teeth were cracking, so they went back to 30 runs before servicing. Inspection every pass of course which is easy through a removable top cover. The difference between an 8.8 and a 9" is that when an 8.8 breaks, it spits the gears and carrier out the back of the case. When a 9" breaks, it spits the pinion out of the front and shoves the drive shaft into the transmission. Seen a PG that was ruined once, from a 9" rear end failure. The 8.8 is a WHOLE lot stronger than people give them credit for. The 1st weak link is the axle bearings-which don't support the same loads as the 9" axle bearings do. Then the bearing caps, but even then the caps will hold everything in for a long time even with a stick shift on slicks. Then the axle tubes, they aren't welded all the way around and they can twist/break loose from the center section. Heck I ran a bone stock 8.8 in my F100 for a long time, with a 514" at about 650hp. And "they" said it was weak. 8.8 vs 9". In stock form the 9" is a little stronger. Both are a lot stronger than an 8", and the 8" can take a lot of punishment! The going trend I have seen is to use an 8.8" housing and caps, with a girdled cover, 9" housing ends (so you use 9" axles , brakes, etc and no C-clip eliminators) and these are supporting 12-1500 hp if done correctly (pro gears and a full spool). Supposedly they suck up a little less HP than a 9" too. One racer told me he was .2 quicker and 4 mph faster with an 8.8 compared to the 9" that came out. That surprised me and I still don't believe it until I actually prove it myself.
