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Looking to get a little quicker off the line, but not trying to win the 1/4 mile. dont want to be wining at 2500-3000 rpms @40mph. What Diff carrier brand do you recommend and also what ratio for 90 305 TPI with factory MD8 auto, 10 bolt. I'm currently running 2.73's. Have never setup a diff before, so if there is one brand easier to set up than others, that would be helpful too.
Torsen or True-Trac carrier. 3.42 gears at the most.
Setup is all about the same. US Gear, Precision, Motive, Dana, Yukon, all should be perfectly OK. Avoid the Richmond drag-race gears although their "stock" style gears are perfectly fine. I STRONGLY recommend using a crush sleeve eliminator (solid spacer) as it allows tightening the pinion nut MUCH tighter, as well as avoiding the problems where the nut is allowed to loosen because the crush sleeve gets crushed some more during operation (shock loading in reverse will do this). All-new bearings, all-new shims, all-new axle bearings. New axles are a good idea at the same time; stock ones are much weenier to begin with, and are usually destroyed where the bearing runs on them. Not a bad idea to have somebody that can (a) verify that the housing is PERFECTLY straight and (b) weld steel to cast iron (requires heating up the cast iron to at least brown hot in an oven) weld the tubes to the pumpkin. Might want to have em weld on LCARBs at the same time. Then maybe take it somewhere and get it powder-coated. Add a girdle type cover such as from T/A.
Thanks Sofa - seems like I've ready some great experiences with the True-Trac, don't recall exactly what it was, but maybe it was really smooth if I recall. If solid spacer makes it easier, I'm down. what axles would you recommend? Why do you say 3.42 at most? Is there a big difference between 3.27 -vs-3.42
seems like I ran across a website showing what the Automatic could be ordered with, but can't seem to relocate it. Was that for gas mileage purposes or did GM have other better reasons?
GM's reasons for such things are/were emissions, fuel economy, emissions, marketability, emissions, warranty, emissions, and emissions... oh I forgot to mention... EMISSIONS
I'm not sure "performance" was in the top 5 concerns.
The 305 TPI/auto cars came with 2.73/2.77 standard, and 3.23/3.27 if they had G92. 2.73 and 3.23 are the ratios in the 10-bolt, the others were in the 9-bolt.
Problem to be avoided in gearing with TPI is, that TPI itself sets an upper limit on useful engine RPM. Its torque has a VERY HIGH peak at around 3600 RPM; which is what it was designed to do, and did well; but it falls off DRAMATICALLY above that. At some RPM the tuning of the runners (the "T" in "TPI") actually starts to work AGAINST cylinder fill.
Thus, if you have too much gear, the engine RPM will climb rapidly to around 4000 or so, and then can't really go any further except maybe a little bit in 1st gear. What you end up with is one of those tractor-motor wonders that will yank your teeth out at the roots from a stop, but takes 45 seconds to pass a semi at 55 mph on a 2-lane road. And can't pass it at all at 65.
3.23 was a pretty good all-around choice IMO. They let the 5-speed cars go to 3.42 or 3.45 with G92. I recall one of those I raced against, then drove, and then let its owner drive my car (L69 w/ 3.73). My car left harder, had less midrange (he caught back up), but then pulled away steadily. Didn't have the giant "peak" in the torque, but the gear gave it a better launch, and the breathing ability of the carb compared to the TPI let it rev USEFULLY.
Gears work best if they keep the engine in the RPM range that it produces the most power. For a TPI motor anywhere near stock, that's been found to be 3.23 or 3.42 pretty much universally. One with a really good flowing exhaust, heads, cam, etc. can go farther (escape the limits of the "T" somewhat) but it still won't compare to many other induction systems when equipped with the same modifications. The old "all else being equal" concept.
The Torsen and True-Trac are both worm-gear systems. No clutches. Nothing to wear, nothing to chatter. Not as good for pure racing maybe, but arguably, better on the street. After all, we've learned a thing or 2, as a species, about how to make cars "better" in the 60 or so years since the word "posi" became part of our vocabulary.
Liquid, with your 305 power level, a stock, factory "Posi" should be fine. I have one behind my 350 and have not broke it yet. Try searching the classifieds on here. That's where I got mine. Use factory GM gears with the factory GM differential. All I did was swap the pinion gear shim over to the new/used pinion and used the same differential shims. Make sure to keep the left shim on the left side and the right shim on the right side....they are different thickness. I used a pinion spacer, available at Summit, instead of the crush sleeve. It makes installation much easier. Never had a problem with the 6 rear ends that I have swapped gears in. Most V6 engine rear ends came with 3.42 gears.
A stock factory "posi" from a 10-bolt in these cars is one of the biggest POSs EVAH. An Auburn with metal clutches that disintegrate into METAL DUST in the fluid. Doesn't really matter about 305 or power level or any of that. They don't "break"; they just wear out, and become useless, and fill the fluid with metal dust and chips. It's a POS.
At some level, a POS is a POS. No matter what.
Of course, the other "posi" that came in rears of this model, is EVEN WORSE; the good old Grenade-Lock. Let's not even go anywhere close to there though.
GM gears are (or, that is, were at the time these cars were built) what is now called Motive. American Axle & Machine (AAM). They're perfectly good, as far as that goes; only, I would NOT UNDER ANY CIRCUMSTANCES send somebody to the junkyard for gears while they're doing a $1000 and SUPER LABOR INTENSIVE job for the FIRST TIME IN THEIR LIFE, when NEW gears cost less than $200. Yes, I might do it myself; yes, you might do it yourself, and you (or I) might very well be successful, I have no doubt; but SEND A N00B? What you or I would do is NOT THE SAME AS what we should be advising someone with no experience. 2 totally different things. He wouldn't be asking these questions if he knew how to do that, select good ones and so forth, and dodge all the bullets that it brings.
Factory carrier shims are CAST IRON. Totally wear-vulnerable. I have NEVER taken a set out of a rear that were in ANY kind of shape I would consider "useable". (less than .003" of wear... we all try to set rears up AT LEAST that close, no??) Invariably they are HEAVILY worn where the bearing inner race contacts them, ESPECIALLY on the driver's side. In fact I'm pretty sure that some of the WASTED rear ends I've torn down, were wasted SPECIFICALLY because the driver's side shim had worn, allowing the backlash (gear alignment) to open up and the gears to EAT themselves. Under NO CIRCUMSTANCES WHATSOEVER would I EVER put a rear back together with factory shims. Not ANY rear. Not if the shims were as pristine as a 14-yr-old in white cotton panties. They're a POS no matter what. One use only, and even that, only enough to get through the warranty period.
My recommendation remains, a NEW carrier of the worm-gear type NOT clutch type, a NEW set of gears in (yes) some of the same ratios that 6-cyl cars came with although NOT junk 6-cyl gears, a NEW "gear install kit", NEW axle bearings & seals, and NEW axles.
I would also recommend, DO NOT spend this kind of money effort and attention on ANYTHING LESS THAN the best core you can possibly find. Start out with the best, most desirable core you can lay your hands on. If yours has disc brakes then you have the right core. If not, find a PBR equipped rear to do this to, or find the PBR setup and put it on yours. DO NOT spend all this money and still put up with drums (dumbs) or the old cast-iron discs (which your car wouldn't have come with anyway).
I acquired a nice 10 bolt PBR setup a few years back and this project has been on the back burner. Recently I started to nudge myself towards it. funny thing is, when I was trying to remove the factory ring gear the bolt was on tight. so tight, that my impact broke it. that was the moment I decided I'm going new internals Did I do something wrong? Like, is the bolt reverse threaded?
Yes some of those parts would be fine. The Richmond rebuild kits are what I'd choose, from among those. Any of the gears should be OK. If you're getting a carrier, get a 3 series (easier to get one of those new, anyway) so you don't have to use the special "thick" gears. Since those are all stock replacement type gears, you shouldn't even have to futz around too much with gear setup; you should be able to just put in a .035" shim and run it. Just the same, there are ways of checking that easily, we'll get there.
For axles, I've used ones like these https://www.summitracing.com/parts/aly-12103/ in other vehicles with good results. They come with axle bearings & seals which some of the kits also so, so you'll have an extra set of those.
For checking housing straightness, I'd suggest that it be done with the whole thing COMPLETELY disassembled. The best way I know of is with a known straight rod or pipe of about 1" dia, and 4 cones sort of like what you see for balancing wheels, with holes only acoupla .001"s larger than the rod... with all the bearings seals and everything else gone, put a cone at each end of each axle tube, 2 at the outer ends and 2 in the pumpkin, and press on the housing & tubes until the rod slides through freely. Then once it's straight, weld all the way around the axle tubes, where they go into the housing, and re-fill the factory plug welds. The pumpkin being cast-iron, this has to be done while it's VERY hot; people usually use an oven to heat such things up before trying to weld to them. Odds are, your plug welds leak fluid (take a look and see); if they do, it's a SURE sign that they're cracked, allowing the tubes to move around in the pumpkin. I've had rears where the tubes could rotate in the pumpkin and they made a weird clicking sound sometimes, and ones where I could take my hand and just warp the tubes around in the housing. You don't want ANY of that going on obviously. What the guy shows in the video is useful but not really adequate to the task at hand here.
Of course with the whole thing completely torn down, it's your golden opportunity to take it to the coating shop, and have it de-rusted, cleaned inside & out, and coated. Powder works good, for that matter you could consider ceramic although I've never tried it myself and have no idea how practical it would be to try it on something that large.
And of course a few tools: some you may already have, some you can make, some you'll need to buy.
do you really feel like the rear end cover is better than factory? (Trying to preserve the car as if it rolled off the factory floor, a step back in time) I don't drive the car hard, it gets babied. Have other car for driving like a maniac.
I've sent countless hours 2 years ago cleaning it inside and out. completely removed all rust and then did a por-15 treatment on it and oiled the inside. its been sitting in the garage ever since and no rust has come back. its a thing of beauty, that is constantly in my way POR-15, next best thing to ceramic coating. Tried to emulate the paint so it looks like it just came out of production.
I definitely don't have the equipment or skill to do any welding on this, but I can check for leaks. don't recall seeing any when I was cleaning the inside - that is a process to do by hand. I called around town to get it hot-tanked and no one had a tank big enough.
Notes:Fits 3.23 and numerically higher gear ratios.
Special Attention:Eaton recommends a petroleum/mineral based GL-5 80W90 gear oil for all Truetrac differentials. Do not use synthetic gear oil or friction modifiers.
Yes a girdle type cover is better than factory. What it does is, prevent the opening that the carrier is installed through, from "opening up" under load. When the housing does that, it allows extra backlash as well as gear misalignment, both of which are destructive to gears. The pinion in this model of rear already has a tendency to "climb" up the ring so to speak, because its own mount structure isn't all that substantial; distorting the housing in another axis just adds even more stress on everything.
Not knowing how hard you drive your car, I couldn't begin to guess whether you "need" it or not. It's A Good Idea for any performance-oriented vehicle regardless.
I'm not real big on "preserving" aspects of a car's "originality" that cause failure and premature wear. Especially not when they're not particularly visible, and especially not on a type of car like these, that simply don't have "that kind" of value and never will. You could paint or powder-coat it black to hide it I suppose; it's not real obtrusive regardless, but w/e. If you think that someday someone might care that much about original appearance, store your stock cover in the attic against that possibility.
I also have used POR-15 followed by chassis paint (satin black) on things, but it doesn't come close to the long-term reliability of powder coating. Butt hay, your choice, none of that is really "necessary" anyway, my point mostly is that while it's COMPLETELY torn down, it's the right time.
Last edited by sofakingdom; Dec 19, 2020 at 10:36 AM.
Also, as far as cleaning, you can do a hella good job with engine degreaser, brushes and whatnot, and the quarter car wash. Especially if you also take along some real high-power soap to augment the car wash's stuff. Do it on a hot sunny day and leave the thing in the sun to warm up real good before you start: heat makes everything come cleaner. After you take out the axle bearings you'll DEFINITELY want to run a rag soaked in solvent through the axle tubes: you'll know why I say that after you do it, and see what comes out.
Since those are all stock replacement type gears, you shouldn't even have to futz around too much with gear setup; you should be able to just put in a .035" shim and run it. Just the same, there are ways of checking that easily, we'll get there.
havent studied it recently, but seems like the old way was to get things close, then burn a crush sleeve and check gear pattern with some yellow paste and backlash with dial. rinse and repeat until gear pattern looks good. does this still need done, or are you saying throw some shims in there and probably good enough.
Yeah you can do it that way; lots of people do. Me, I'm not worth a crap at evaluating "patterns" once they get past the obviously defective, so I try to avoid depending on them.
In my limited experience, when replacing stock gears with other stock or high-quality stock replacement gears, the gears are within a .001" or 2 of always the same. The thing that varies is the HOUSING. As I've tried to explain before, imagine a set of gears randomly hanging out in space somewhere, perfectly aligned; and you're trying to drape this completely random housing around it. The shims go with the HOUSING, because that's the thing that has all the variation in it, NOT the gears, which are FAR more consistent.
I've taken apart quite a few GM axles, and in the later-model ones (let's say mid 70s and newer) virtually ALL of them had a ,035" pinion shim. Not just GM: I have acoupla Jeeps in my driveway for example, and their Dana 44 rears and Dana 35 fronts ALSO had .035" in them. Must be a trend. Almost as though, either the factory didn't even bother to check, or they got the machine work on the HOUSING right, to where that was the thickness they needed. I've put a .035" right back in, and haven't had even ONE that needed to come back apart for that.
That DOES NOT mean that yours will NECESSARILY be .035"; that you can put the same back in as what you took out; or any such. LIKELY, absolutely. NECESSARILY, not so much.
One of the tools I have made is a set of pinion bearings with their bores honed out with a brake cyl hone so that they slide on and off of pinions easily. Bearings are even more precise than gears: the variation in them is down in the .0001"s, MUCH better than the .001"s we work with when doing gears. So I use my honed bearings for all the setup kind of stuff, and when I'm happy, put the "real" ones in at the end.
For pinion setup that means, I put the same thickness shim back under the pinion tail bearing that came out of that housing, if I know what it was; use my honed ones to set the crush sleeve eliminator, to attain the desired preload (I aim for the high side of the spec... 24 or 25 in-lbs, and if it goes an in-lb or 2 over because of the "steps" in shims, so be it), WITHOUT the pinion seal installed; once I get that, I put the carrier in, and get the backlash I want (again I aim for the high side of the spec: there will always be "error", and if the backlash is a .001" or 2 too great, I have a tiny bit more slop, but if I have even ONE ,001" too little, the 2 gears, which GROW when they get hot, attempt to occupy the same volume, and I think we all know what happens when 2 moving parts try to occupy the same volume); check the pattern if I feel so inclined; then once I'm happy with all that, take it all back apart, put the "real" pinion bearings in, put the pinon seal in, add AT LEAST .005" to each carrier shim and preferably .010" if I can, to get enough preload on those; and final-assemble it.
If you can find a 9 bolt, and you are ok with 3.7 gears (working fine for me at bolt-on 305 power level - about 185 RWHP, and still working great with my 350 at 295 RWHP), they aren't a bad option. Stronger than the 10 bolt, and can get a 3 series Torsen for them now along with the 3.7 gears and TA girdle cover. I'm pretty hard on mine and am about to install a Procharger at 15+ psi..... I guess we shall see.
Did I do something wrong? Like, is the bolt reverse threaded?
