Changing Diff Gearing....need an education!

[goingballistic]

OK, as many are no doubt aware by now, I'm stroking out my 81. As the picture becomes more clear ( maybe muddier??) I am concerned about the rear end. I have read many articles and stuff on this forum and others about the rear end ratios but am looking for a little help to clear my head ( not easy at my age!).

P.S. to those of you have supplied input so far on this project MY HAT IS OFF to you for all the help.........picked the new "old" block last night.......the saga begins with disassembly of the block today, the guy I'm working with on this is custom grinding the cam!!!!! SWEEEEEETTTTT.

I understand the rear ratio is approx 2.7:1 and am toying with a new ratio of 3.72:1 or higher. What I'm looking for is some help in selecting the "right" rear end as opposed to "guessing"............(if you're an engineer, you'll understand the guessing part ).

I have an auto 350, looking to change the TQ converter to a 2500+ stall with approx 435HP, I'm worried I'll rip the rear end out ( I'm building for low end torque not max speed). I want the car to jump off the line but also want the mid range acceleration say at 65mph to also jump...........I love those kids faces when you stuff a "stock" car and the car comes off the ground

Does anyone know where to get info on this stuff, Minimum ratio before you rip it out, max ratio and mid range. I'm trying to make a somewhat educated decision about how to do this and what some of the trade off are for the varying ratios, also need to know if the shaft has to be changed.

Any educational links would be appreciated, informed input, reading, experience from others ,etc....thanks.

 

[Chris Kennedy]

I also have an '81 4-spd, and at some point want to uprate power and am concerned about rearend strength. I would go to the Kiesler Automotive site for information on a 5 spd. overdrive they market for our cars. The site also has lots of info on gear ratios. I do a lot of high speed driving, and like the fact that the current rear ratio gives a good cruise (about 30 mph per 1000 rpm). I also run a Richmond gear 4-spd, which gives better performance 1 to 3rd ratios, and the standard 1 to 1 top. My overall recommendation would be to go to the mid-range of what's available---that is, while you can uprate power and go to a more acceleration oriented rear ratio (especially if you get a 5 spd with overdrive), don't go to the extreme, as you might overstress the car/rearend or run things too close to the limit. You don't want the car to be like of Lotus, in which everything is stressed to the limit and there is no safety/strength margin, especially given the fact that our cars are 23 years old.

Support the USA!
--Chris Kennedy

 

[4 webers]

GB I am running an 82 with the alloy rear housing . Last year I installed a 3:7 gear with a tremec 5 speed . The engine is built for good all around power output and is quite strong in the 400 hp range , shock is the main cause of driveline failure as is driver abuse . If your motor is strong you will have no need to resort to the pop the clutch tricks . Rather you will have some traction problems when applying a liberal dose of throttle . If you stay away from slicks or oversized tires breakage will not be a problem .


geo

 

[Glensgages]

...shock is the main cause of driveline failure as is driver abuse...

Gotta agree w/ George here; high torque mated to the aluminum rear is a recipe for disaster, regardless of gear-ratio.
As long as you keep 'hard' rear tires on the car, that' won't allow a 'dead-hook', and an automatic, you should be OK...
I'd think, as far as the rear is concerned, it'd last longer if you'd 'walk' it off the line, then apply 'spray' in the mid/upper RPMs, like the ricers do, which'll cushion the shock to the ring/pinion...
Just my $.02 worth

 

[norvalwilhelm]

Why do you think that you will rip the rearend out with a gear change. I ran alot more horse then 435 for years with slicks, a nitrous bigblock and high stall converter and other then twisting a few axles never had a problem with hard launches.
Our rearends are not that weak. I ran 4.11's for more then 10 years and only recently went back to the 308's because I had too much problems launching on big street tires.
I hate lots of gearing in the back and no overdrive transmission. One of my cars also has a nitrous 355, 456 gearing and .68 overdrive to give a final ratio of about 3.2 and I am always looking for another gear cruising on the highway.
Why buzz the motor so high all the time?? I would hate 3.55 or higher without an overdrive on the expressways.
Engine wear, gas mileage and temperature sore with high numerical gearing.
I have had many cars with 456, 411, 355 and now my main car is 308's and I like them the best.
My 456's are only good for launching off the line with slicks. Street tires just go up in smoke.