Stock Driveshaft Critical Speed?

[CtCarl]

About 20 years ago I swapped out the 3.08s for 3.73s in my otherwise stock '87 LX 5.0. Now I'm wondering, with the stock steel driveshaft spinning that much faster, what its critical speed might be.

I'm thinking about swapping in an aluminum Aerostar driveshaft. Yeah, I know Ford Racing has a better one, but this isn't an excuse to spend money, it's purely a safety thing.

Does anybody have any info on safe top speeds with 3.73s and the stock D/S?

Thanks! Carl

 

[mikestang63]

You are worried about the driveshaft speed because you swapped out 3.08's to 3.73's? It raises the rpms on the motor like maybe 300 at mph. You have nothing to worry about. In fact the lower gears reduce your top speed with all things being equal.

 

[CtCarl]

Yes, I am worried about the driveshaft speed, since it's spinning 22% faster than it was designed to. I'm pretty sure Ford never intended the original steel shaft to be making turns for 170 MPH, but that's where it would be at 140 MPH indicated with the 3.73s.

 

[toyman]

Well you are not actually changing the drive shaft speed at any matching transmission gear ratio. The drive shaft rotational speed equals the engine rpm when the transmission gear ratio is 1:1. What is increased is the rear axle rotational speed.