i was wondering if anyone has ever done this swap before and could answer some questions. i know there is a thread about this already but it didn't quite answer exactly what i wanted to know.

i'm going from a 3.08 ratio to 3.70 and i was curious what time difference it's going to make in the quarter? and how big will the AIS difference be? i know it's not going to be a crazy difference like i bolted on a blower and ran some nitrous. but what should i expect?

also, i don't have a lift, what the safest place to jack the car up to remove the rear end? i figured i'd block the wheels, lift it from the rear end with a jack, and put jack stands under my sub-frame connectors and remove the jack. would that work and be somewhat safe? whats the best what to do it without a lift??



I really don't mean to be rude but i don't want COMPLETE guesses. and i'm not interested in hearing about what gears i should be getting, or what rear ends are better than what i have. i have what i have and i'm not going for good mileage.

 

It will make a little over a 20% difference in RPMs at all speeds under all conditions.

No way to know what effect it will have on your 1320' time; that depends heavily on the RPM range whatever motor you have, runs best in. Some cars would feel (and time) like you opened a can of about 150 HP and dumped it in. Others would feel like it's time to shift, ALWAYS; forced completely up out of the RPM range that they make power. Effect on ET will follow the same pattern.

I've changed out plenty of gears over the years; never that exact from-to combination though. A couple I can specifically recall that were similar were 2.73 to 3.42, and 3.23 to 3.73. Results were fairly predictable: engines with a downward-sloping torque curve (LG4) HATED it, ones with upward-sloping curves (solid cam motors with big heads) LOVED it. One quick glance at your dyno curve will tell you what your optimum drag strip gear (or at least, mighty close to it) needs to be.

Yes your proposed swap technique will work fine; jack stands under the point where the "frame" end of the LCA attaches, is the best place. In reality you'll find it quite easy, with the possible exception of dealing with rusty hardware.

 

thanks for the quick reply. i'm having a fact overload. i've had to edit my response three times now.

and i have a crate motor with 390 lb/tq with a flat curve (nothing crazy but enough to be fun). it pulls hardest from 3200rpm to 5500rpm. hopefully that 20% puts it right at the beginning of where it pulls hard. right now when i shift out of first to second it bogs for a second util it builds RPM. i'd really like to get rid of that.. and i hope you're right. I've never done a rear end swap before. and i really, really don't want to be crushed by my car or mess anything up.

 

Having been one of those people who "never did a gear swap before" and having done a couple now, I would make sure you have 1) the right tools (dont improvise) 2) a firm understanding of bearing preload, back lash, and gear patterns 3) a list of installation steps you can check off while your setting up your gears.

Removing the rear end from the car makes things about 50 times easier your first time.
Don't skimp on an install kit.
I have set my all my 9 bolt gears at .010" backlash and never had a problem, I would suggest the same for you.

I went from 2.77 to 3.70 but I had a stock LTR setup and the car ran out of gear fast.

 

thank you, but now I'm thinking about taking it to a shop and getting it done. I've been looking into it and it's just too much for me. it didn't sound difficult at first but if it's shimmed wrong apparently they'll get spit right out. i just don't want to gamble on it. and i just ordered a complete install kit from summit and the 3.70 motive gear ring and pinion set. i'm looking to spend around $500 to $600 for install, do you think that's do able? my friend built truck diffs and he said they charged $1,000. that just seems way high to me. and 2.70's to 3.70's, that's a huge leap!

 

If you roll your car in there and say, "There you go, make it fast." 1000 seems about right.
Take the diff out yourself, buy the parts (which you have) and look around for someone who will work with you. Doing the previous mentioned, i know a guy that will do it for 200 w/ all the parts.
Depends on what you want to do yourself.

 

I fyou got the parts a few hundred labor shouldnt be 1000?!

They may want to put bearings in it might not depends on the shop and what they see when they pull it apart. You are smart for paying someone to set it up right. I know Id somehow screw it up.

Went from 308 to 370 posi on my A body it was noticeable....just not enough for the cam I had though.


As said pick for the torque curve not the cruising rpm or youll be doing it again (like I did 2 wks later lol)

Sounds like a 370 will do you good. Bog could be something else. Check your fuel pressure under a load?

 

that's what i figured. i was reading on an LS forum and they said it's only around $400 to $500 to get them installed if you haven the gears and an install kit when you take it in. plus my install kit has new bearings in it. so they wont have to search for them. i just hope i don't get screwed.

and i have a pretty common cam for torque, its the 350 hp 327 vette cam. the car puts down the torque well and has a really flat curve. it pulls hard even with the 3.08's so i'm guessing it'll be an improvement going to the 3.70's. by bog i just meant it lugs along after it shifts for a second. when i shift at the beginning of each gear it drops the rpm out of the main power range because the gears are too tall and it takes a second to start pulling hard again.

 

Maybe you need a little more converter

 

I'm going to do that soon too but that isn't it. i have an aftermarket converter now but i'd like a better one. the gears are just crappy. I've read about other guys having similar issues with them. the kick down is just too much when it shifts if you don't have the power to pull the tall gear. instead of dropping to the top of the range it drops a little below it. i'm hoping the 3.70's will shorten each gear enough to keep me in the top of my range throughout shifts. based on what sofakingdom said and my speed/ratio calculator it should.

and i'd like to just drive my car in and have them pull the rear end out but i know they'll charge more. i'll probably end up taking the rear end out and hauling it in with our truck. since i have the gears, and complete install kit with bearings, i hope the price is lower. based on what other people have said it should help. on other forums they said anything over $500 is insane if you have the parts.

i just really hope i don't get the gears in there and feel disappointed. I don't have crazy high expectations of it making a 13.7 car run 12's. but i'd like to see a tenth or two.. it's more about the feel for me anyway. I just want it to be fun.

 

hey guys, just thought i'd let everyone know whats goin on so far. the car is at the shop getting the gears installed right now. something i wanted to note for anyone thinking about doing this in the future.

I bought the ratech full install kit. it was supposed to have everything in it to complete the rear end. well, it doesn't. it was missing the axle seals ($50). and the gasket is a joke. I would get a better seal if i used a couple sheets of computer paper.

anyway, aside from that, so far the mechanic has said it will be under $500. i'll post a little more when i get the break in done. talk about the difference it makes.

 

Ratech is the worst kit that you can possibly buy. Every kit that Summit and Jeg's sell are just reboxed Ratech kits. The good kits come from Randy's Ring & Pinion and Tom's Differentials.

 

I 100% agree. It turns out not only were the seals missing, the left side bearings were wrong as well and wouldn't fit. Garbage. Thankfully the shop is across the street from NAPA and Car Quest. They had the right parts on the shelf amazingly. It would have been good to know before i bought the kit though. i ended up paying quite a bit extra for the missing and incorrect parts.

To anyone who is going to do this later, DO NOT BUY ANYTHING FROM RATECH.

 

Anyway, i got it back finished and did the 500 mile break in, the car is way more fun. it's spin happy through first but if you get it to hook, rev it out into second, it feels way faster. i hardly notice the typical 700 R4 drop off. second is a blast, and even third and forth feel more aggressive because the kick down is right to the top of my power band in every gear. at 65 mph its pulling 2400 rpm. its not unbearable. it's the best modification I've done aside from the new engine.