2015 Mustang IRS w/cast iron diff housing

[SlowSVT]

I noted in this months edition of MM&FF the S550 IRS Ford chose to use a cast iron diff housing in the 8.8. I wonder what the motivation for that is. "Heat" and "weight" can't be one of them. The cover looks pretty beefy, is still cast aluminum and has nice widely spaces mounting stanchions. Not sure how thrilled a road racer will be with this as the SN95 all aluminum diff will get hotter than a firecracker after a few laps. Perhaps a diff cooler will almost mandatory as iron moves heat like molasses. Doesn't look like it holds anymore oil either. I think I would prefer this in aluminum just to help wick heat away. Should be interesting to see if the S550 road racers have issues with here.

 

[oldmodman]

Cast iron is incredibly stable dimensionally. It doesn't twist under load, right up to the point where it fractures.

Where an aluminum center section would be lighter it can flex. Which could allow the pinion gear to "walk" up the ring gear with every shift.

Eventually it could lead to a fracture.

Both types can fail but the cast iron will probably last longer. All things being equal.

 

[SlowSVT]

Cast iron is incredibly stable dimensionally. It doesn't twist under load, right up to the point where it fractures.

Where an aluminum center section would be lighter it can flex. Which could allow the pinion gear to "walk" up the ring gear with every shift.

Eventually it could lead to a fracture.

Both types can fail but the cast iron will probably last longer. All things being equal.

True statement

For a drag racer this is a plus, not as much for everyone else. Given the choice I'd op for aluminum mainly for the cooling effect. There is only about a quart and a half of oil in the pumpkin and not enough of it to carry away a lot of heat off the gears. The clutch disc in the Eaton Trac Loc don't like that very much, not sure what posi carrier Ford installed in the S550 it may be a Torson unit which is way more tolerant of the heat. Something tells me a diff cooler on a road racer will be mandatory.

 

[Tractionless1]

Seen Terminator owners use the old T-bird/Couger cast iron IRS in place of our AL only go back to AL after breaking the Iron's.