New guy with rear end question

[dodgecharger72]

      Hey guys this is my first post and I just want to say that since I found this site about two months ago i haven't got one good night's sleep and I have been late to work almost every day because I just cant get myself off of this forum You guys are really knowledgable
      Anyways, I have a 72 dodge charger with a new 360 and a soon to be 727( I have a 904 right now). I do not really know anthing about differentials in general but I do know that my car has the stock rear end that came with the original 318 and 904 and I am looking to upgrade. I am looking for something pretty beefy that could keep up with the 440 I am going to put in it in a few years( I have the block but a 440 doesn't really fit in my plans right now). I am not going to race it or anything but I would definitely like something i could hot rod around town. And what gears should i use?
      Thank you so much for your time

[TylerCharger69]

I'd say a 3:23 Suregrip would be a nice choice....very streetable and decent highway r.p.m.s

[tan top]

hello & welcome 

I'd say a 3:23 Suregrip would be a nice choice....very streetable and decent highway r.p.m.s

good choice  , what rear end you got under it at the moment ?

[dodgecharger72]

I am assuming it is an 8 1/4 inch since it was originally a 318/904 car but I am not sure. How much is a suregrip conversion going to cost me?

[mally69]

if you have an 8 1/4   you will want at least an 8 3/4

[dodgecharger72]

So I will need a whole new rear end or just the differential?

[mally69]

yes if your plans are to install a 440 you'll have to have at least an 8 3/4 and thats an entire different housing,

[TylerCharger69]

Yes...if you go with 8 3/4....you will need the entire deal.....There are three different popular 3rd member casings (popular meaning the most discussed here)  theres a 741 case, a 742 case, and a 489 case.  There are mixed opinions on which is best to use.  Personally...I like a 489 case   These three digit numbers are the last three numbers in the center section casting.  The "489" case being a 69...not sure of later years....and the 742 case being of 68.....the 741  I think is earlier...but im not sure on that one.    Now  one more option is   to just find a nice Dana 60 rear end.....but finding someone to part with that is very unlikely.....not to mention   expensive!!  But as far as ratio goes....since you described you want to keep the streetability of the car......I'd say 3:23....

[green69rt]

      Hey guys this is my first post and I just want to say that since I found this site about two months ago i haven't got one good night's sleep and I have been late to work almost every day because I just cant get myself off of this forum You guys are really knowledgable
      Anyways, I have a 72 dodge charger with a new 360 and a soon to be 727( I have a 904 right now). I do not really know anthing about differentials in general but I do know that my car has the stock rear end that came with the original 318 and 904 and I am looking to upgrade. I am looking for something pretty beefy that could keep up with the 440 I am going to put in it in a few years( I have the block but a 440 doesn't really fit in my plans right now). I am not going to race it or anything but I would definitely like something i could hot rod around town. And what gears should i use?
      Thank you so much for your time

I'm going to make a few guesses and give a couple of opinions so FWIW.  I'd guess you may have a 8 1/4 rear with 2.72 gears or there abouts.  That was a common gear ratio for production cars (meaning not high performance.)  ALso the rear brakes are 2" drums and you may have front discs.

So what would I do?  A cheap step would be to get rid of the 2.72 gears and install some 3.23 or 3.55 gears, these would really up the performance without killing your crusing ability and not over-reving the engine at hiway speeds.  Don't forget to swap out the speedometer pinion gear.  Going much higher than this will limit your ability to take the car on the hiway.

The only thing you can do after that is to go up to the 8 3/4 rear.  Do a suregrip unit with the gearing that suits you. The 8 3/4 will handle lots of HP (<600HP) set up correctly. You also might go the four wheel disc brake route because you want to stop as well as you go.

[dodgecharger72]

Yeah I am looking to do this whole thing in one shot since I am going to have the money soon. I know i can by an aftermarket diff from detroit locker or a few others, but who makes the housing?

[dodgecharger72]

[/quote]
I'm going to make a few guesses and give a couple of opinions so FWIW.  I'd guess you may have a 8 1/4 rear with 2.72 gears or there abouts.  That was a common gear ratio for production cars (meaning not high performance.)  ALso the rear brakes are 2" drums and you may have front discs.

So what would I do?  A cheap step would be to get rid of the 2.72 gears and install some 3.23 or 3.55 gears, these would really up the performance without killing your crusing ability and not over-reving the engine at hiway speeds.  Don't forget to swap out the speedometer pinion gear.  Going much higher than this will limit your ability to take the car on the hiway.

The only thing you can do after that is to go up to the 8 3/4 rear.  Do a suregrip unit with the gearing that suits you. The 8 3/4 will handle lots of HP (<600HP) set up correctly. You also might go the four wheel disc brake route because you want to stop as well as you go.
[/quote]
               You're guess was right about my brakes and I'm sure about the gears too. As far as installing 3.23 gears being cheap, does that mean I don't have to change the housing? What exactly is involved?

[b5blue]

Look farther down the road for now, if your certain you want to end up with a big block then you want at least an 8 3/4 axle. So find out what is there now first (post photos if you can't tell) Your wasting money to change gears in the axle if it is not an 8 3/4. I've run a 3.23 sure grip for years and love it, a good all round daily driver ratio. If you do not have an 8 3/4 you want to buy the housing and build from there, you can set up the rear so it's in the car and working before you do any motor/tranny changing. 

[green69rt]

Look farther down the road for now, if your certain you want to end up with a big block then you want at least an 8 3/4 axle. So find out what is there now first (post photos if you can't tell) Your wasting money to change gears in the axle if it is not an 8 3/4. I've run a 3.23 sure grip for years and love it, a good all round daily driver ratio. If you do not have an 8 3/4 you want to buy the housing and build from there, you can set up the rear so it's in the car and working before you do any motor/tranny changing. 

Agree, if you want to do the whole thing in one shot.  The 8 1/4 is a pretty weak unit.  Go directly to the 8 3/4 unit.
I mentioned the gear change in case you want to wait a while (year?) before spending the money for a complete upgrade.
There's a thread around that descibes how to tell which 8 3/4 goes in which car,  they're not all the same even if they have the same part number on the case because the spacing between the backing plates are different, spring locations are different, etc.  Do a search.
Another thing to be aware of is the drive shaft, it may have to be a different length.

[lisiecki1]

i pulled an 8.75 out of a 74 station wagon that was a direct bolt in to my 73....also, in going from 904 to 727 i believe you're going to have to shorten the driveshaft

[GreenMachine]

   Didn't factory big block Chargers in '72 get the 9 1/4? They'll probably be easier to find than an 8 3/4 to fit a 3rd gen.. As far as changing gears in the 9 1/4 housing, it might be harder finding parts for.

[dodgecharger72]

okay I will search for that thread. Where can you buy 8 3/4 inch rear ends from? and yeah 904s are a little shorter so the driveshaft will have to be shortened a few inches

[green69rt]

okay I will search for that thread. Where can you buy 8 3/4 inch rear ends from? and yeah 904s are a little shorter so the driveshaft will have to be shortened a few inches

One place to get rear ends is from Strange engineering.  I don't have any experience with them so maybe others can chime in.

[lisiecki1]

i've never used them, but i see Randy's Ring and Pinion in all the mopar mags too.....

[dodgecharger72]

Ok I will look into that. So I will need a new differential and also the 3:23 gears?

[lisiecki1]

need....no....want...YES!    8.75 + 3.23 suregrip....that's what's going in my 73....then when funds allow it'll be 3.91's and a t56 6 speed

[dodgecharger72]

HAHA I definitely do want it        where are you going to be getting your 8 3/4 rear end from?   And your setup sounds pretty amazing

[gordo1968charger]

i got a strange dana complete,very very happy with it,it will probably outlive the car and myself

[dodgecharger72]

I would LOVE a Dana. Maybe i should hold off and save soome money for one of those...

[dodgecharger72]

I just checked Strange Engineering's website and I can gat a full Dana 60 rear end for my car for $1900. The tech guy said it has everything but the brakes. I think I am going to go this route.  It might take a little longer to get the money but I think it will be worth it.

[dodgecharger72]

So what exactly is involved in bolting in a Dana 60 into a 72 charger with a 727 drum brakes and an 8 1/4 inch rear end?