28 spline or 31 spline axles for 8.8 coyote

[Bayfunktionkev]

Planning to purchase a detroit truetrac for my 8.8 and wondering if my 28 spline axles are fine and will hold the power of a coyote? They are brand new motive 28 spline axles so if I upgrade to 31 spline axles now with a matching 31 spline truetrac I would be wasting the 28 axles I bought a few months ago.


What other rear lsd or posi do you guys recommend?


Thanks

[CraigS]

I had a TrueTrac and can't recommend it. Why? 1-It adds in some driveline slop that you can drive around somewhat. Otherwise every time you go from gas off the gas on = clunk. 2- It wore itself out in about 4 yrs. Admittedly that includes 18-20 autocrosses each year, but I thought it would be great to not need to worry about replacing limslip clutch plates. What happened was I was noticing some noise and investigation showed that the pocket in the iron housing where the side spider gears sit, wore and the rear wheel had, what would be thought of back in the day, as wheel bearing slop. No internal parts are available. So I trashed it and ordered a Ford unit w/ CF clutch material.
https://www.summitracing.com/search?PageSize=100&SortBy=BestKeywordMatch&SortOrder=Ascending&keyword=ford%20M-4204-F318C
Unfortunately, it requires 31 spline axles.

[NAZ]

Not enough info to determine what size axles you need but here's some simple math to help you decide for yourself. Max torque X 1st gear ratio X rear end ratio = torque to rear axles. If you have an auto trans you also have to factor in the torque multiplication from the torque converter which can more than double the engine torque. This formula does not take into account friction loss or traction -- it gives you the most torque your axles would ever see in a perfect world of no friction and 100% traction.

I'll use my car for an example: Torque converter multiplication = 2.28, 1st gear = 1.8:1, rear end ratio = 3.55:1, max torque @ 5100 stall speed (without nitrous) = 541. So we start multiplying 541 X 2.28 X 1.8 X 3.55 = 7882 lb ft of torque shared by the rear axles. I run 31-spline Moser axles which are rated for 7,000 lbs each which will handle 14,000 lb ft combined IF traction allows the torque to be evenly distributed.

[mike223]

In the roadster, on street tires, your axles aren't going to matter (28 spline vs 31 spline) - because you're going to spin tires rather than break axles.

I've been running a TrueTrac for several years now and have not (ever) found it to be clunky or sloppy at all - very satisfied - sorry to hear about Craig's problems.


Craig, was yours sloppy from the beginning?

Mine never has been - I run 18 - 20 autocrosses over the course of 3 - 4 years, I don't think they are particularly hard on equipment but I'm not running Hoosiers either (500hp / 500 tq 393w).

Looks like 31 spline axles give you more 8.8 differential options: https://www.summitracing.com/search/part-type/differential-carriers/axle-location/rear?N=4294945465%2B4294923849%2B4294945508&PageSize=100&SortBy=Default&SortOrder=Ascending&tw=differential%20&sw=Differential%20Carriers

I'd say it depends on what you want to do with the car...

[Bobby Doug]

Can’t go wrong with the stronger axles. They could be a selling point down the road.

[CraigS]

....
Craig, was yours sloppy from the beginning?

Yes it was. At the time I did a google search and found that it is a common characteristic.