IRS noise

[Malibu46pt]

I have the IRS from FFR. There is playin the rear wheels. They rotate free about 3-5 degrees before the differential engages. This appears to be play in the differential. It is the 3.55 provided by FFR. This condition causes a thumping sound when shifting or letting off and on the accelerator.
Is this normal?????
Help

[Nigel Allen]

It is normal. Without some clearance (backlash) between the gears in the differential, there would be no room for lubricating oil between the teeth.

It is definitely more noticeable in these cars as we sit very close to the rear end.

[chmhasy]

Which IRS do you have (2015 or newer mustang, Thunderbird IRS)? Do you have a True track, limited slip, locker?
I have a Thunderbird with a true track and I get the noise that you spoke of.

[Malibu46pt]

It is the IRS supplied by FFR 3.55