I know I don't have all the info to make a correct response to this post but...

My current 8.5 rear has an open carrier I am looking to put in a posi unit. The car currently has a low HP 400 from 72ish, it has a turbo 400 behind it. The plan is over the next few years to put the correct 400 HO motor with a M21 4sp.

Phew with that said when I crack into the rearend I want to see what gears are in there, when I add the posi I was planning to put the correct 3.90 gears in? This is where my question lies.. 3.90 gears? what do you think?

Pretty steep gears for the street. I had 3:55's installed in my car a few years ago. It's not bad going down the highway at 55-60 but I will not run it at 70 mph. I plan on installing an OD transmission next year.

Seem steep to me as well unless you're drag racing. I have 373s in mine and that's too steep to drive any distance over 30MPH. The reason I put in the TKO 600.
The heavy duty four pinion diff came with 336 to 433, I'd choose the 336 for general use. But that's just me.

I am doing the same.'69 bird with Low hp '72 400, TH350 and 8.2 , 2.56:1 open diff which is great on the hi-way, but hopeless around the city. I swapped out a 3.55:1 and had the opposite problem. Looking to fit a posi 3.08:1.

I have 3.90 in mine. They are fun in town and in strip but in my opinion too steep to road trip. I ended up installing Tremec TKO600 which works excellent on gears 3.55 and numerically higher. The fun remains same from 1st to 4th as it's geared very close to my original M21 4th being same 1:1. 5th is 0.64 so at cruisin speed 70mph my engine revs only around 2200rpm.

I did a 2,000 mile trip over the Rockies last summer, did the Coca Cola highway at 100 mph, the overdrive gear in the TKO 600 made that speed enjoyable. Just glad I didn't get the car impounded for excessive speed. That KPH to MPH gets me in trouble once in awhile. Averaged around 22 MPG as well. Another 500,000 miles and the savings in gas will have the tranmission paid for.

Ok gears have been identified. 40 ring teeth 13 pinion teeth. 3.08 I'm thinking. So going to add the posi. Open to comments on manufacture and while I'm there I could change the gears. Would like little more punch to get the two wheels turning but not crazy. Keep in mind that I believe the 71 400 in there may only have 250 hp max

I've never done this job before looks pretty straight foreward with exception of shimming to get proper gear mesh.

Leave gears alone? Change while I'm in there? Does maker matter or should I buy budget?

If your plan to to put the Muncie 4 spd, then while I was changing to a posi, I would put the 3.36 gear in. I think that was the highest gearing available with the 4 spd. That will put you around 2700-2800 RPM @ 60 with correct tires and wheels and you will have enough gear to pull out with on a hill. My swapped 2.73 could not pull out on a hill at all and had to get the correct 3.36 gear. I think the 3.36 is a happy medium for me.

My PHS said my car was a 4sp with 3.90 so I guess you could go that high.
I do like the idea of the 3.36, was thinking of flirting as high as 3.56

When I said high the lower the number the higher the gear, 3,36 is the highest. 3.90 is the lowest,

3.36 gears were standard for the HO. 3.55 could be had as performance axle option or 3.55 posi. the 3.36 had the FN wide ratio Muncie. 3.90 and 4.33 gears only available with FX close ratio AND Saf-T-Track posi rear.

As far as an opinion...3.36 I have...as well as 3.55...love them both. I get rubber all 4 gears with both and both have M-20's. If you eventually plan on an M-21 AND 3.90 gears...I would say build it right and once. Jim Mino would say...hell with it...I'll build 8 rears. I would stay away from anything higher than a 3.55 with an M-20 and would only use an M-21 with 3.73 and up.

As far as what gear, ask yourself what kind of driving you will be doing most of the time. If a weekend toy, no long trips. go with the 3.90 and M-21 as you plan based on how your car was ordered. If you want a long cruiser, M-20 with 2.78 to 3.08 (different carrier) If a mix, 3.23 to 3.36. If I remember correctly, 3.36 and up, same carrier, 3.32 and down different carrier.

The other issue is the 250 hp with a high gear..it will work, sure, but a little extreme.

I would keep the 3.08 with the 250 hp engine for now. Build the engine you want, keeping the 3.08 and see how you like it, since you already have it..If you like it, add a posi carrier. I do not believe 3.08 was available with an M-20 though...if you don't like it..build a 3.36 or 3.55 rear.

I would go with Richmond gears and an Eaton posi..you can find complete rear swap packages including the races, pinion seal, bearings, gear and posi for around $725

Just FYI: With a TH 400 tranny and 3.55 rear, I'm turning 3000 rpm at 60 mph and I don't like pushing it more than that. 2 lane US and State Highways for me only. No cruising the Interstates at 80 mph.

Ok I'll jump in...my 69 came stock with M20 and 3.23 rear. I do find myself looking for 5th gear when I get on the freeway but I think part of it is because we're not used to hearing so much sound coming from the motor in any car. I run 3100-3200 RPMs at 70mph. My daily driver runs almost as high but nowhere near the audible feedback. I run pretty short tires too( 235/45/17). 3100 isn't really that fast but it will wear on you after a couple hundred miles. We won't talk about gas mileage.

Sorry I feel stupid but I am having hard time understanding the rear gears

Lower gear is higher number? 3.08 is high and 3.90 is low?

also just how much change is there in each gear change? I realize transmission and HP affect this but is there a chart to show how much change in performance will occur. For example in my case 400 auto with 250hp motor, with each gear change from the 3.08 now I realize will increase 0-60 time but reduce top end (which is what I am looking for now) SO the move to 3.36 how big a change is that a lot, crazy a lot? How about to 3.55? is the change drastic or minimal? I don't want to change the gears and be like well that doesn't throw my head back?

I'm leaning towards the 3.55 but if someone told me to do 3.73 I would because someday down the road there will me a m21 back in this car, because it is sitting rebuilt in my garage! but if 3.73 wont work for now I can change again!

thanks sorry for my confusion

It can be confusing. And on top of that, changing rear gears isn't something that is easily done unless you have a quick change rear. In your case changing from 3.08 to 3.36 would not make a real noticeable difference. Also, the rear you would want behind an automatic would probably not be the gear you would want behind your 4-speed. I would get the tranny issue finalized before doing anything with the rear.

"... How about to 3.55? is the change drastic or minimal?..."


I'd call a change from 3.08 to 3.55 drastic. But, 3.55's work good with a 4-speed. The 3.73's will give you more acceleration, but will increase your freeway cruising RPM up to well passed 3000 rpm.

A 3.23 would probably be about the best compromise, if you do a lot of 70mph cruising. I suppose. The late 2nd gens used the 3.23's and 3.42's, with the 4-speed.

If you are sure you have a m21 with a 2.20 first gear, you will need at least a 3.73 to get it moving and be happy with the result. I would not change a thing on your car until you ready to do it all.

When I got my car someone had switch the rear gear to a 2.73 it still had the m20 in it. The car was not drivable when I got it. So during the restoration I left the 2.73 in until I was able to find the correct 4 pinion posi and correct 3.36 gears.

During the two summers driving it, I had a very hard time pulling out of even the slightest hill. It was even hard getting it moving with people in the back. Not happy with it at all.

I finally found the correct posi and gears and had Jim from JD Race rebuild the complete rear end he rebuild the posi unit and added the 3.36 gear. I got those in spring 15 and had them in all last summer. They made a world of difference. It still is not a performance rear end taking off. I would probable like even going lower to maybe a 3.55 but I have to try to get a happy medium with my upper rpm's. In 4th @ 60 I'm turning around 2700-2800 rpm. With the 3.55 I would be around 3000 rpm. There are rear gear RPM calculators on the web to show you these differences.

Because I left the 2.73 gears in for the two summers. I am now lookinng at replacing my flywheel and clutch again because I burned it several times trying to get the car to move will the 2.73 are in.

So it may be better to just not change anything till you are ready to do it all. You have a great rear gear for you auto now. If you go higher then the 3.73 with that m21 such as 3.36, I don't think you will be apply using the posi anyway.. You be much happier playing with the posi with the 3.90 gear.

You can read this article to help. http://www.5speeds.com/ratios.html

Oldskool
He has a m21 he is wanting to use. It has a 2.20 first gear unlike the m20 2.56. Pulling out with 3.23 will be a dog. GM would not use gearing higher then 3.70 with a close ratio Muncie. Must options were only allow 3.73 through 4.11

I would not recommend anything anything higher the 3.73 .

One of the 67's I built has a M20 with a 3.55 rear and 26 inch tall tires. To me the gearing is perfect but the owner wants 3.08 or 3.23. It will definitely not drive as nice as it does with 3.55.
Everyone has gotten used to driving 75mph I guess.

Lower gear is higher number?

If you're talking speed yes, if you're talking torque, no. If you're talking ratio, no.

The number is the ratio of the input to the output. High gear in your transmission will be a ratio of 1 to 1. Every time your engine makes one revolution the output shaft will turn one time. If you drive at 3,000 rpm your driveshaft will turn at 3,000 rpm in high gear.
If you put it in first gear with a ratio of say 2.56 to one, the engine will turn 2.56 times for every one revolution of the output shaft. At 3,000 rpm the driveshaft will only be turning 1,171 rpm. 2.56 is a higher number than 1 but the speed is lower. Torque out is torque in times gear ratio. My engine makes 525 pounds feet of torque at 3,000 rpm. Ignoring friction, clutch slippage and other losses we have 525 X 2.56=1,344. In high gear we have 525 in and 525 out.

Same thing with the differential, the ratio is the comparison of the number of teeth in the ring gear to the number of teeth in the pinion. In a 4.11 rear the ring gear has 37 teeth and the pinion has 9 37/9=4.1111 thus every time the driveshaft turns one revolution the axle turns 0.243 times or for every one turn of the axle the driveshaft will turn 4.11 times. At 3,000 rpm engine speed in high gear the axles are only turning 729.9 rpm. With a 3.55 rear the axles turn at 845 rpm. 4.11 is larger than 3.55 but it has less speed at the same rpm

Higher number less speed but higher number higher torque. You move slower in first gear than you do in high gear and slower with a 411 than with a 355, but the torque is much higher in first than high gear and much higher with a 411 than a 355. Engines with higher low-rpm torque can perform better with a lower gear ratio than engines with less low-rpm torque. Our Pontiac engines traditionally made pretty good torque at low rpm and could tolerate a lower gear ratio than some other engines. Chev 327s made good horsepower at high rpm but less low-rpm torque.

The confusion comes from mistaking the speed ratio with the gear ratio. A 4.11:1 rear gear has a higher ratio than a 3.55:1 rear gear, higher number higher ratio.

Differential people refer to higher ratio, higher number gears as low gears. They provide more torque and higher rpm for a given road speed. They call lower ratio, lower numbered gears as high gears. They provide less torque and lower rpm for the same road speed. Don't ask me why, maybe thinking as with transmission first gear is lower than third but first has a higher ratio than third. I always refer to the ratio can't go wrong that way.

RPM formula:
rpm=mphXtransmission ratioXaxle ratioX336/tire diameter

lower gear-redlight gear
higher gear-highway gear

Muddle things up even more?

"Oldskool
He has a m21 he is wanting to use. It has a 2.20 first gear unlike the m20 2.56. Pulling out with 3.23 will be a dog. GM would not use gearing higher then 3.70 with a close ratio Muncie. Must options were only allow 3.73 through 4.11

I would not recommend anything anything higher the 3.73 ...."



"...top end (which is what I am looking for now...)

This is why I mentioned the above 3000 rpm at highway cruise speed. From what his statement says, he is, at least for the present, wanting to keep decent high speed cruise gears. Don't think 3.73's or even 3.55's fit that description. But, now, for me personally, I ran 3.90 gears with a close ratio trans for 60,000 + street miles and loved 'em. But I didn't do hardly any 70mph driving.

So, I reckin what is a compromise to some, is totally unacceptable to others. It's not a whole lot different from a cam thread. It all depends on your particular likes, dislikes, and opinions. I suppose that is what a lot of the forum conversations are all about--telling what we like, or what we did, or what we think others should do. Most everybody has an opinion. And, on car forums, most of us give ours, whether others agree or not. Just an observation. I sometimes find it amusing to read what one guy says won't work at all, then 2 guys will come on saying how they did that exact thing and it worked great for them, for years, with no problem.

Hmmm thanks again for all the advice. I can see the view of not changing gears twice. Now and the. Again when I put in the correct trans. But that may be a couple years I don't want to cruise around in a muscle car that can't get both its wheels spinning. For most non car people this is a display of power. That's what I am looking for. I'm not taking it down highway over 65

Expense is not my concern. I am going to do the work myself. My only concern is he ting the gears matched correctly. If I knew u could swap the open carrier out and put in the posi without having to re shim any gears

so much information on this site... its just great.. I'm already looking at this swap.. I just put 3:73 rear end.. and just getting ready to pull the trigger on a 400... I'm gonna have to read thru this thread a few times.. I would have just bought a munci 4 speed... looks like a lot more to it..

"...Expense is not my concern. I am going to do the work myself. My only concern is he ting the gears matched correctly. If I knew u could swap the open carrier out and put in the posi without having to re shim any gears..."


Sounds like this is one area where you should pay an expert to do the job for you. Setting up a rear properly, takes experience. And this is done with exactly the correct shims, placed in exactly the right places. Incorrect set-up can cause excessive noise and premature gear failure.

Gotta learn sometime, but I agree with oldskool more than one rear has been ruined by improper installation. Have someone well trained and practiced at the job do it or guide you through the job. One time where reading the instructions and performing the task just doesn't match up to experience.

I have a good understanding of backlash (setting ring right and left)

and good idea on pinion depth (reading pattern depth and gear mesh depth) seen images of how it should look

I realize this is a pro's job but to me it seems tearing down an engine and rebuilding it is way more complicated, I don't hear that being pushed off on pros?

I always figured back in the day hot rodders didn't just go to a mechanic every time they wanted a mod they just dirtied up their hands and went after it!

Pulling apart an engine would have 100's of parts laying around my garage, but looking inside the differential seems I can count the parts on both hands.

Several years back I tore a 1966 jeep cj5 apart down to the frame(parts everywhere), welded on it (never done before) threw new suspension on it, ran all new wires, built a dash, put all back together and bounced it off rocks and dunes for 2 year before selling it. I believe this is the true spirit of any restoration or project. If every time you need to do something you ran to a mechanic no one would learn anything and no one could afford to rebuild anything.

BAck to engine rebuild, setting pistons wrong. gaskets wrong, timing chain wrong, valves wrong etc would cause catastrophic failure. Setting gears wrong would? make them noisey? make them wear sooner than normal? The worse case in my mind would be running with the gears so poorly set that they were binding and they blew up. But I don't think I would put the cover back on if this was the case?

Bought back lash gauge for couple bucks, got bearing seating set, seal puller, got a garage with a lift and all the wheels off the ground, only specialty tool missing in my mind is a bearing separator and press? I see hillbillies in you tube videos laying in the dirt on a cardboard box changing gear sets, just because they have changed them before are they pros?

If I am oversimplifying it some one please chime in

By the way I appreciate any advice I didn't want to go all rogue, but I don't want to be the type of guy that goes out and buys a muscle car that's fully restored or the person who paid someone to build it, pull out a lawn chair sit in front of it at a show and represent that I actually know something about how it was built. That's not a hot rodder that's a guy with an identity crisis.

I'm going to tell stories of how and where I found parts, how I took advice on what gears to put in it :), how the first gear set was a mistake and I installed it wrong, learned from it , fixed it, then helped other people with the same, and then show people where my blood was spilt on it, where my knuckles were busted. That's who I am! I am only looking for responses from the same type of people, not armchair mechanics spending the weekend waiting for their car to get out of the shop!

So anyway what gear ratio should I use?

Well you have my kinda attitude, go for it.

336:1

Those are my thoughts too & yes go for it.

"So anyway what gear ratio should I use? "
Already been stated. 3.73-3.90 with the m21
and 3.08 - 3.36 with your Turbo

What would be the downside with 3:55 with the turbo. Besides limiting top end for highway. With 3:55 I would still be able to drive 65 which I don't do very often. What about on the bottum end. Will I not be able to hook up or is the problem thy first gear will be two quick? Don't want to change from 3:08 to 3:36 and not like the change or is that change big enough that I'll be blowing my eyeballs out the back of my head

Nothing wrong with the 3.55 it could be ordered with the turbo so it will be fine and fun. If you want to blow eyeball in the back of head. go 3.90. It could be ordered with the turbo and your m21. Change once and your set.

Classic!

The internals of 3.90 geared Th400's were stronger than their PX coded counterparts to handle the torque and horsepower.

I have a 3.55 in the 67...tachs out at 3500 on the freeway..not too bad.

IMO, a quality gearset from Richmond is only $250-300 including races, bearings and shims. Why not try the 3.90 and see how you like it with the M-21, both which, if I recall correctly, were ordered with your 68.

If you don't like the 3.90's..go with something numerically lower.

In my opinion, I would decide what transmission you are going with. From reading, it appears you are on the fence and people are trying to provide suggestions. What would make this easier is to decide which transmission you will go with first. Will the Th400 you have now hold up with 3.90 gears? Probably not for long. Will an M-21 hold up with 3.90? That is what it was designed for if the internals are good.

Either way...Th400 for now with XXX gears you may or may not be happy with..same with an M-21 with your existing gears. As you said, you have the drive to change gearsets out and can't be any harder than rebuilding an engine. Same holds true for swapping out a Th400 to an M-21 as long as you have all the parts. You can swap out the Th400 in 6-8 hours for an M-21. The only way you'd be happy is to actually install it and drive it...Or find someone with XXX gears with X trans...with "X" HP motor.

Either way you go..

Now this is the conversation I wanted. I need some more parts to do m21 install which at earliest would be next year. Also plan to do myself. Car originally was four speed so some of the parts are still in there

As far as running the 3.90 one change would be great if running those gears is an option. As far as the factory 400turbo matched with 3.90 being built stronger keep in mind that would have been I behind 350+ horsepower so with my weaker motor maybe the trans will hold up a year or two with the 3.90 and if I install right I don't have to do it again

What do you guys recommend for the posi if I go 3.90 and do it once I might as well spend more on quality

Also not to brag but my dream mancave garage is almost completed. I thought I would add photo to get your thoughts.

Nice Garage. I say Eaton Posi would be the best choice.

I hear motive and Yukon and pretty quite gears. I hear Richmond roar loud.

I think you're getting such different opinions on what rear gear to use because of what you said you were looking for as far as performance. I first read you wanted "more punch but not crazy", then "top end is what I'm looking for now", then you want to throw your head back. You are not going to get it all with a rear gear change. You will have to have a compromise of something in between. I was advising on retaining top end.

Just my opinion but, I think the 3.08:1 gears you have now are way too low a ratio. A change to a 3.36:1 would give you a bit more acceleration from stop and still give you your wanted top end. A 3.55:1 will give a bit more acceleration again but you'd lose some of the top end you want. 7.73 and up ratios will give you the head snap you want but forget the top end you want. Your last few posts make me think you want more punch from stand-still and you are leaning away from the top end, if so go for the 3:90:1 or mybe 411s. If you still want to drive on the highway without listening to the engine scream and watching the gas gauge needle plummet then compromise somewhere around a 3.55:1.

Most of us make opinions from past experiences. One guy may have a 373 rear and doesn't think it's bad on the highway at all, another guy may think that same 373 was crap on the highway. I'm the latter, I stayed in the right lane and watched everone pass me. Maybe I wouldn't have minded as much if I didn't have the noise from the 14" Flowmasters deafening me, but I really hated those gears on the highway.

Like I said that is just my opinion and I'm not an expert.

Yes I know it's kind of ridiculous talking about gas milage and muscle cars at the same time, maybe because gas is $6+ here.

no Al, he wants to and I quote

"is that change big enough that I'll be blowing my eyeballs out the back of my head"