would like some input on running a 4:1 on old *** stock axles in my 1990...i figure the lower the crawl speed i can get, the easier it might be on my clutch and stuff. any input from someone with experience would be gravy. thanks!

 

You are right, the lower the gears, the easier the clutching, the more power transfered to the tires, but the worse gas mileage. 4.10's are a good ratio for 33's.... remember to do both axles together! its a common upgrade, so parts are relatively inexpensive....

 

A 4:1 low range or 4.10:1 axle ratio?

 

Think about it in terms of mechanical advantage.

Your 1990 4.0L was spec'd at 224lb-ft of torque when new. Let's figure it's original and tired, so an even 200lb-ft for ease of calculations.

200 lb-ft of torque goes into your AX-15. Your first gear is 3.83:1, meaning your torque is multiplied by 3.83, or 766 lb-ft. Again for ease of calc, let's say there's a 10% parasitic reduction in each component, so 689 lb-ft comes out.

689 lb-ft goes into your NP231 transfer case with a low range of 2.72:1. You'll lose 10% again, so 1,688 lb-ft comes out.

For conversation sake, let's not get into the loss due to splitting that torque front-rear, let's just call it 1,688 lb-ft to your axles.

Now with a 4:1 in that NP231, your output torque is now 2,480 lb-ft, nearly 50% more torque.

Now theoretically you could get that same output by increasing the engine torque by 50%. A bigger engine would put the same stress on the drivetrain.

Now breakage only occurs when the mechanical forces from each direction conflict to a point where they overpower an individual component, pushing it beyond its ability to transmit those forces. Typically the engine output will overpower the tires' traction, and wheel spin is the failure point in that system. Bigger tires increase the leverage that the ground has against the output, and in most cases pushes that failure point back into the drivetrain.

That's why most talk about axle strength revolves around tire size. It's an easy variable to measure, the mechanical advantage larger tires have in resisting your engine's torque output. That being said, if your stock drivetrain can handle 33's, simply adding more mechanical advantage from the engine side (either more engine output or more gearing), it will only translate into more easily breaking the assumably weakest link (traction).

Bottom line, if you can break your axles now, it'll be easier to break them with the 4:1.

Yep, I'm in the groove today. Is it snowing yet?

 

That's assuming you meant the TC 4:1, not axle gears. Same basic premisis applies, only add in another multiplication of torque and narrowing the highest point of forces in conflict (axle shafts, r&p, etc.)

 

Thanks man, so basically its an upgrade I'd only want to think about if I had one tons? Or at least ditch the 30/35 for a 44/44,8.8,8.25? And chromos...right?

 

Since my Cherokee is my DD lifted family vehicle I was thinking about getting the 4:1 case upgrade and keeping the 3:55's for Max Mpg's when I'm not crawling. Is this a bad idea and rear is a 87 Dana 44