Well it is almost that time....

A new rearend is gonna be the next thing to do on my list here pretty soon, and I have narrowed it down to what I want. I will be going with a Currie built 9 inch Ford rear end, Detroit Locker with 3.90 gears and the HD sportsman case. My issue is with the axles. Personally, I'd like to run 33 spline axles, but I don't have that option. The 2 choices that I have now are 31 or 35 spline. What I am looking for are any distinct advantages here on using the 35's over the 31's. Keep in mind that as of right now, I am making 419HP and 430TQ at the rear wheels on the camaro, and that this is a street/strip car. I don't mind going overkill(like I did with pretty much everything lese on the car), but if price becomes a factor, I need to make sure that what I can afford will be adequate for what I want/need. Thanks for any input or suggestions.

 

spool? if so then 35 spline. Less likely to get chewed up on the street.

also, moser was cheaper than currie for me by quite a bit.

www.exoticperformanceplus.com is where I ordered my rear from. Free shipping, no taxes, and 35 spline upgrade was free through his online deal. He's a sponsor on ls1tech.com and Bob was the salesman and was a great guy to work with.

 

unless you see major power upgrades coming in the future, 31 spline is plenty..there are numerous cars out there making north of 600whp with about as much torque and 28-31 spline axels did just fine. My procharged 614whp Z28 was one of them...moser 12 bolt with an eaton posi and 31 spline axels. If the price difference is not much between the two, then bigger is better and stronger also.

 

My car only uses 31 spline axles and I launch hard off the transbrake.

 

and for stephens car to run mid 9's, its gotta be making anywhere between 700-800whp at least.

 

Using the Moser axle calculator, I'm almost to the limit of the 31 spline axles. The main reason I can use them is that I use a powerglide transmission so my torque multiplication factor is low. If I was using a 3 speed, I may need 33 or 35 spline.

Going to 35 spline axles means using a carrier and center section with large bearings. 31 and 33 spline axles can still use factory replacement carrier bearings.

I have a couple of 31 spline spools so I'll keep using 31 spline axles until a break or twist one. When they were new, I painted a line down the length of the axles. Whenever I have to pull an axle, I can see if the line is twisting. No twists yet. Even the axles in my old diff didn't twist.

The Moser axle calculator says that the total amount of torque produced off the line should be less than what one axle can handle. Since we have 2 axles, that's the safety margin.

Maximum engine torque x first gear ratio x rear end gear ratio x 0.9 = calculated torque.

30 spline = 6200 pounds per axle
31 spline = 7000
33 spline = 8200
35 spline = 9600
40 spline = 12,000

 

not trying to steal the thread, but this has been helping me address some of the questions ive had. Im also torn between a 31 spline or a 35 spline and a detroit locker. Is the only differnce between the 31 an 35 just larger bearings and a few more lbs of weight?? Because Mosher lets you have a 31 or a 35 for the same price. Why wouldnt you just go for the 35 for the added strength?? what am i missing? Thanks in advance

 

Hmm well I appreciate all the info. I just get paranoid sometimes, and I really don't want to spend upwards of 4 grand only to keep breaking stuff. With the formula that you gave I can plug in these numbers..

Max engine torque(510)x first gear ratio(2.95)x rear end ratio(3.90)x .9=5280.795

Assuming I were to use any nitrous, the maximum amount that I would spray would be a 200 shot. So, hypthetically speaking the new calculations would be....

Max engine torque of 510+ extra 200 from spray(710) x 1st gear(2.95)x rear end ratio(3.90)x .9=7351.695

So even though I would be a little higher than the torque limit of the 31 spline on the juice, I don't necessarliy think that it would be enough to twist or break it. Looking at it from that perspective, I think the 31 spline will be sufficient unless I am able to get the 35 splines for no or little price difference. Thanks for the insight, it has been most helpful.

 

Basically but larger axles add more weight. Large axles such as 40 spline are gun drilled. The axle is hollow to reduce weight.

Unless you're making stupid amounts of HP, a street car does not need 35 spline axles. 31 is more than enough and if you want a little stronger, you can use 33 instead.

Don't forget, the calculation is for maximum torque off the line. That's where the axles will see the most abuse. Tire slippage reduces the amount of torque getting to the ground. Once a vehicle gets moving there isn't as much strain on the axles such as hitting the NOS after the vehicle is moving.

Moser suggests a 200% safety margin. Putting 7350# to 31 spline axles will still be safe because each axle can handle 7000 pounds by itself for a total of 14000 pounds but 33 spline axles would be safer without added a whole lot of extra weight.

A rough estimate of my engine with dyno 2000 says I produce 770# of torque at 5000 rpm. The powerglide has 1.76 first gear and the diff has 4.86 gears.

770 x 1.76 x 4.86 x .9 = 5927 pounds which is less than many cars. It's getting up there but still below the limit of 31 spline axles. If I kept the same gears in the diff but installed a TH400, it would put 8352# to the wheels and I would need 35 spline axles. Using a TH400 and 4.56 gears I could get by with 33 spline.

It's like everything else. Larger isn't always better. You only need an axle large enough to handle the torque produced. Any larger and it's just a waste or bench racing banter.

 

And I agree with pretty much everything you have said so far. I just like to use the worst case scenario when determining what is best for my application. In this case if I were to go ***** to the wall the 31 spline is still a more logcal choice, and well within the limits of the torque numbers that you had posted earlier.