New rear end gears -

ck80

Royal Smart Person
Feb 18, 2014
2,320
113
This is all great feedback, thank you. I tried searching but could not find this type of response. I agree car is not just a bunch of parts slapped together, that's why I'm asking. Ultimately I will upgrade the power, 300ish HP is my goal for a DD. I'd also like some bigger wheels but not going with anything bigger than 18, maybe even 16 or 17. The overdrive trans is for a steeper 1st gear and better highway milage. I might not get there tomorrow when I pick up my car but I can't afford to do all the work at once. So maybe the rear gear is not my next change. Maybe its the wheels/tires. Thanks again.
Here's my order:

Decide on what engine trans you plan to run. Then decide what brakes you need to control it and be happy with where you drive.

Once you know that, then buy your tires and rims. Certain brakes won't fit behind certain rims. Lots of guys prefer thebappearance side of thing before the drivetrain just for affordability purposes and to have different different ride/handling.

Next up I recommend thinking about suspension and exhaust. A heavier engine in a worn out v6 suspension is no good. Likewise, a planned exhaust may be possible to start using before the swap, or at least can be budgeted ahead of time so it's on the shelf for the engine.

Then it's transmission time, because the old stocker often dies behind a new engine.

Knowing what valve body you're running you have a toss up. If you KNOW that your engine will grenade the rear end, you may as well upgrade ahead of the swap because what's the point of dropping tons of cash into something you can't drive or it'll pop? Likewise, no point in installing a brake upgrade when the rear is about to be swapped? If your engine plan is set in place you can run projections on a dyno calculator to have an idea where powerbands roughly lie. Typing in tire size, trans, and different gear ratios will tell you what rpm you will turn at different mph. You can pick which gears you want based on where you want rpm to be for how/where/speed you'll run the motor and your power bands on the motor.

Like the exhaust, think cooling. Maybe get a bigger radiator, maybe electric fans, things like that ahead of time. It's money you'll spend during engine installation anyways, so, do some work ahead, save time on swap weekend.

Then, go for motor last and it'll be a quicker easier install. You've already got rims that will clear bigger brakes if you want them. So on so forth.

Obviously depending what you run and depending what engine you want some steps can be skipped. A 305 car can use the factory exhaust on a 383 engine upgrade. Not the best, but functional to get going. A 350/290hr engine won't kill a 7.5 rear quickly unless you're really abusing stuff, so on so forth. But it's a good order to go through a checklist on.
 

Cuse99

Greasemonkey
Thread starter
Dec 21, 2020
168
28
I found a guy to replace my gears and gave me a few options, including transition to posi. Just changing the gears from 2.73 to ???? what ever (I'm thinking 3.42 but no final decision yet) Parts and labor is about 1500.00 Please don't suggest I do it my self, not gonna happen. Is this too much?
 

ck80

Royal Smart Person
Feb 18, 2014
2,320
113
I found a guy to replace my gears and gave me a few options, including transition to posi. Just changing the gears from 2.73 to ???? what ever (I'm thinking 3.42 but no final decision yet) Parts and labor is about 1500.00 Please don't suggest I do it my self, not gonna happen. Is this too much?
$1500 into a 7.5 is too much.

Scrap it or sell it for $150 to someone. Take that $1600 or so and add it to another grand and have a very nice turn key setup that's built so strong you'll never look at it again.
 

39F

Master Mechanic
Sep 9, 2019
459
63
Pittsburgh PA
I found a guy to replace my gears and gave me a few options, including transition to posi. Just changing the gears from 2.73 to ???? what ever (I'm thinking 3.42 but no final decision yet) Parts and labor is about 1500.00 Please don't suggest I do it my self, not gonna happen. Is this too much?
That's nuts, parts are like 200-250 for gears, seals, shims,ect. is that with locker and brakes also????? Maybe new tires ???
 

ck80

Royal Smart Person
Feb 18, 2014
2,320
113
That's nuts, parts are like 200-250 for gears, seals, shims,ect. is that with locker and brakes also????? Maybe new tires ???
Probably not all that, but hopefully at least some lube before getting bent over and paying. :popcorn:
 
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Reactions: 39F

57 Handyman

Greasemonkey
Feb 6, 2017
158
43
CK80, here's some additional thoughts...search around for an 8.5 in rear end assembly. With some luck, you'll be able to find someone selling a unit from a Buick Grand National which is equipped with 3.42 posi gears. I swapped that rear end assembly into my 87 El Camino with a stock 305 hooked up to a 2004r. Rear tires were 16" IROCs with 245x60 rubber. The change in overall pep and giddy-up was great! Next major modification is dumping all of the smog with ECM stuff and coupling that drivetrain to a semi-modified, balanced 350. Can't wait to punch it!!!
 

83MonteCarloSS

Apprentice
Oct 5, 2015
74
33
OK, but why? Why these gears over 3.08 or 3.23? What's the advantage?
Whatever the trans was calibrated for as far as gears and tire size is what I would use, unless you are planning on making valve body or governor modifications. Small differences in ratio or tire size would be fine if your not recalibrating the trans. Changing the ratio is going to result in different shift points. Ideally I want my transmission to shift between 2,500 and 3,000 rpm unless I'm really babying it or romping on it.
 

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