741 differential.

[Fred69Runner]

So it looks like every one else missed saying the 489 has a bigger pinion shaft than the 741 so you need a new yoke on the differential. They make like drop top say's a staggered U joint so you can match to your drive shaft. If needed you can go to a drive shaft shop and have that end changed so you can use the same size bearing through out. That is a better idea but the front bearing will still be small. At some point or another once I do put the 489 in my car will change from transmission to differential all the bearing's to the same size. Doctor Diff. Has a lot of the part's. Good luck my friend. Russ.

Hi guys, so I've been talking with some of the old school racers around here and being told that unless I'm going to be putting racing slicks on my bird and doing some serious street or strip time on her, that the 741 (with the smaller pinion and u-joint) is more than strong enough to put in a after market sure grip with around 3.55 gears and match up to a mild build 383 through a 4 speed. Any thoughts out there on going that route instead of trying to find and rebuild a 489?
Thanks

 

[RKNRLR]

I had a 489 in my 440 car with a sure grip and 4:10's. added some slicks and BOOM she went.

Have ordered parts from Doctor Diff before, real easy to deal with, parts are quality, and the delivery time is pretty quick, even all the way down here.

 

[Russ69Runner]

The week link will be the size of the pinion. And the smaller bearing's on the drive shaft. Now if you don't do any crazy stuff with the motor and increase the horse power over the top. Or do a lot of burn out's. Don't do any drag racing. You just want a driver it will do just fine. A burn out every now and then should not hurt it.

 

[Fred69Runner]

The week link will be the size of the pinion. And the smaller bearing's on the drive shaft. Now if you don't do any crazy stuff with the motor and increase the horse power over the top. Or do a lot of burn out's. Don't do any drag racing. You just want a driver it will do just fine. A burn out every now and then should not hurt it.

That's exactly the impression I got and what I was thinking. Good luck with your build Russ.

 

[Russ69Runner]

Thanks. Wish I was driving it but that will come. You know like I do you can brake any thing if you try hard enough. The 741 had a grannie gear ration so it left like a lead sled but on the other end you could do 120 mph easy. I put a 3.55 gearing in mine now their saying a 3.23. So will drive it and can change it any time. You can put new gears and pinion in it in a day. Have fun and keep us up on how its going. Russ.

 

[RKNRLR]

The week link will be the size of the pinion. And the smaller bearing's on the drive shaft. Now if you don't do any crazy stuff with the motor and increase the horse power over the top. Or do a lot of burn out's. Don't do any drag racing. You just want a driver it will do just fine. A burn out every now and then should not hurt it.

No extra power? No burnouts? No drag racing?

 

[Russ69Runner]

STOP it. Just a little then we will go and get the 489 differential and brake an axel.

 

[droptop]

Actually the weakest link with a 489 is the axle housing. Under hard acceleration, the housing will flex and pull away from the pinion gear until it strips the ring gear. Ask me how I know this. It takes a lot of torque to do this, but it will happen.

 

[Russ69Runner]

Yep had an tire and axel pass me one night when racing. Said who's tire is that. Then it happed the back end dropped to the ground on the driver's side.

 

[RKNRLR]

I have also learnt this

Actually the weakest link with a 489 is the axle housing. Under hard acceleration, the housing will flex and pull away from the pinion gear until it strips the ring gear. Ask me how I know this. It takes a lot of torque to do this, but it will happen.

lesson. had a cap go bang, luckily not both of them though, so didnt open the car up like a can of SPAM

 

[roadrunninMark]

Fred, unless you are running a hot engine with a 4 speed, or a big block with a lot of torque, and plan on doing a lot of high rpm launches, I would agree the 741 should be good. The 489 you might be looking at might be a cone unit (I think most of them are), which are not rebuild friendly. The units without the cone can be rebuilt over and over, the cones cannot be. I would rather have a clutch 741 over a cone 489.

 

[Russ69Runner]

The cone type has a spacer to put into the unit when rebuilding. Have read on the subject and talked to Doctor Diff. Yes would be better to have the clutch type if you can find one.

 

[roadrunninMark]

Russ, the formula to get you in the ballpark (for choosing your gear ratio), for a nice acceleration is a 10 to 1 ratio. This means you want the first gear, multiplied by the rear to be about "10". So if you have a transmission with a 2.45 first gear (727, older 904, etc. for example) a 4.08 ish gear will get you there. Now, that stinks for highway use but that is why overdrive is nice. For my other car, I am going with the 42RH. A 2.74 first gear and a 3.55 rear gets me to "10", then the overdrive at .69 makes the rear equate to a 2.45. A win-win. I believe CRT does offer a 2.74 first gear setup for a 727 and that can allow you to run the 3.55 and get a better highway experience.

 

[Russ69Runner]

That sound's good but I have a four speed manual transmission. So will have to check into the ratio on the gearing in it. Was told fourth gear is one to one. Will see when it is finished and drive the car. Putting a tach in it so will be able to check my RPM's when driving. Thanks for the info.

 

[roadrunninMark]

You have either a 2.66 or 2.65 (coarse spline) first gear. So a 3.73 gets you about "10". You only use the 1st gear for the multiplication.

 

[Russ69Runner]

Thanks for the info roadrunninMark. I do have a tach going in the car. Like we always said driving is the proof. Have you got your car finished and driving. That is one bad ass looking runner. Russ.

 

[roadrunninMark]

You are welcome. No, the car isn't finished yet, still waiting for the engine to be finished before the shop can finish the car. I am hoping by July it will be done. Thanks, it has been a long road.

 

[Russ69Runner]

Yes don't we know it. Hope to get this one finished this year. But thing's keep getting in the way of progress. Take care and looking forward to seeing more result's of where the car is in the rebuild. Russ.

 

[RKNRLR]

Fred, unless you are running a hot engine with a 4 speed, or a big block with a lot of torque, and plan on doing a lot of high rpm launches, I would agree the 741 should be good.

Ahh, the 3 basic princples of life

 

[Russ69Runner]

In the end if you got a muscle car you can not resist the temptation to do a burn out. Or get on it. So I know you will end up getting a different chunk to put into the rear end. At least a limited slip like the 489. I tried to find the clutch type but the cone type will be just fine for me. As long as both tires grab at the same time.