8 3/4 gears

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70dart340

70dart340

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Hello, all. I have a spare 8 3/4 complete rear end for my '70 Swinger 340. I'm building a stout 408 for her, and it's time to consider the rear assembly. I have a small yoke 741 with a 3.23 SG rear assembly. I have a couple of questions:
Can I replace it with a 742 or 489 SG, or is this overkill?
All the rear disc conversions seem to require 15" wheels. I know Summit has 15" wheels in the SBP, is this the way to go?
Green bearings, or Timken tapered?
Will my stock axles handle about 480HP at the wheels, or do I need to upgrade?
I'm making this like a FAST car, I want to look stock if you pull up next to me.
Thanks to all ahead of time for the fountain of knowledge about to come. Paul.
 
70dart340 said:
Hello, all. I have a spare 8 3/4 complete rear end for my '70 Swinger 340. I'm building a stout 408 for her, and it's time to consider the rear assembly. I have a small yoke 741 with a 3.23 SG rear assembly. I have a couple of questions:
Can I replace it with a 742 or 489 SG, or is this overkill?
All the rear disc conversions seem to require 15" wheels. I know Summit has 15" wheels in the SBP, is this the way to go?
Green bearings, or Timken tapered?
Will my stock axles handle about 480HP at the wheels, or do I need to upgrade?
I'm making this like a FAST car, I want to look stock if you pull up next to me.
Thanks to all ahead of time for the fountain of knowledge about to come. Paul.
Click to expand...
I have a 408 in my '70 Duster with a similar rear as you. Had a local guru build my third member. I had issues with the axle seals leaking (replaced them and leaked again) and ruining the brakes so ordered axles (BBP) with green bearings from Dr Diff. Those bearings have an O-ring that positively stopped the leaking. Upgraded the rear brakes to the larger size at the same time & upgraded the front disc brakes using the Dr. Diff BBP disc kit. Used the OE non-power master cylinder. No issues whatsoever with running that rear. The wheels are 15x8 on the back and 15x7 on the fronts. Tires are 275/65r15 back and 215/60r15 on the front with BFG.

Edit: Ordered OE style rallye wheels but canceled the order after waiting 9 months with no ETA for them. Went with BBP vision wheels all around.
 
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Your 741 should have the clutch style sure grip. Easy ID is the sure grip bolts are opposite of the ring gear bolts. The cone style the bolts are on the same side. I like using a 742 carrier because they do not use a crush sleeve on the pinion as the 489. The 741 has a smaller pinion diameter

489 741 742
rrdd01.jpg
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If it were I doing this, I would do much like Ricks70duster340 did.

All 8-3/4 center sections will swap out between each other. Only the U joint size max hinder a otherwise quick and easy swap.

I have used the Dr.Diff upgraded axles on green bearings & a currently sport a 742 case. I like the bigger 7290 U joint. Your driveshaft is probably the smaller 7260. Ether a new driveshaft or the cross over U joint that has the big and small sizes on it already.

All the rear disc brake conversion kits I have seen require a 15 wheel.
All the axles upgrades I have seen are on the larger 5X4-1/2 bolt size.
This leaves the front at 5X4.

IMO, there is no overkill here.
 
70dart340 said:
Hello, all. I have a spare 8 3/4 complete rear end for my '70 Swinger 340. I'm building a stout 408 for her, and it's time to consider the rear assembly. I have a small yoke 741 with a 3.23 SG rear assembly. I have a couple of questions:
Can I replace it with a 742 or 489 SG, or is this overkill?
All the rear disc conversions seem to require 15" wheels. I know Summit has 15" wheels in the SBP, is this the way to go?
Green bearings, or Timken tapered?
Will my stock axles handle about 480HP at the wheels, or do I need to upgrade?
I'm making this like a FAST car, I want to look stock if you pull up next to me.
Thanks to all ahead of time for the fountain of knowledge about to come. Paul.
Click to expand...

480hp will pull a 3.23 gear through the 1320' traps in 2nd gear. Just be aware of that.

I'd want new axles with the 5x4.5 pattern.

I'm unsure what will break with your 8.75 setup at 480hp. When I had one at that power level it started making diff noise/howl, never broke. I ran Moser axles with green bearings.
 
If u truely have 480 hp to the wheels I would do the big bolt pattern swap. Or maybe Doctor Diff has upgraded small bolt axles. Green bearings for sure. Kim
 
Many thanks to all who answered. I think I'll try Rick's approach. I'm planning on going to 3.73 or 3.91 gears. Summit Racing has steelies and Ralleys in 15" SBP, if I can use those and put some discs on the rear, I'll be happy. FYI, the motor will be a 408" Trick Flow heads and intake, a Racer Brown cam around .685@256, and 10.5 to 1. The carb I have is a QF 750 mechanical secondary. TTI step headers, and MSD E-Curve distributor. A Pat Blaise 727is done, and I'm told a big 3500-4000 converter will work for street and light strip duty.
 
70dart340 said:
Many thanks to all who answered. I think I'll try Rick's approach. I'm planning on going to 3.73 or 3.91 gears. Summit Racing has steelies and Ralleys in 15" SBP, if I can use those and put some discs on the rear, I'll be happy. FYI, the motor will be a 408" Trick Flow heads and intake, a Racer Brown cam around .685@256, and 10.5 to 1. The carb I have is a QF 750 mechanical secondary. TTI step headers, and MSD E-Curve distributor. A Pat Blaise 727is done, and I'm told a big 3500-4000 converter will work for street and light strip duty.
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That cam combo is gonna need a good converter! I am using a PTC 9.5" 3500 unit and it really helps the mild 3.23 gearing. I stayed with drum rear brakes for simplicity, just upgraded to the large shoe/drum size.
 
70dart340 said:
Many thanks to all who answered. I think I'll try Rick's approach. I'm planning on going to 3.73 or 3.91 gears. Summit Racing has steelies and Ralleys in 15" SBP, if I can use those and put some discs on the rear, I'll be happy. FYI, the motor will be a 408" Trick Flow heads and intake, a Racer Brown cam around .685@256, and 10.5 to 1. The carb I have is a QF 750 mechanical secondary. TTI step headers, and MSD E-Curve distributor. A Pat Blaise 727is done, and I'm told a big 3500-4000 converter will work for street and light strip duty.
Click to expand...

Definitely go 3.91 if those are the choices. You don't make enough HP to pull a 3.73 gear through the traps.

You should send your dyno sheet (along with all the other car specs they'll ask for) to whomever is making the convertor for you if you really want what's best for your combination.
 
roccodart440 said:
Definitely go 3.91 if those are the choices. You don't make enough HP to pull a 3.73 gear through the traps.

You should send your dyno sheet (along with all the other car specs they'll ask for) to whomever is making the convertor for you if you really want what's best for your combination.
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I'll take your advice, and thank you for it. The search resumes.
 
I'll go with the 3.91's according to the wise members here. I'll use the green bearings. I'll look at Moser and others for axles. I'll try to put my brake package together, too. If I have to switch to BBP, then so be it. Any suggestions for brakes? Besides the master Dr. Diff. I think I'll be placing an order with him soon. Thanks to all, I appreciate your experience.
 
I would go with the late B body discs. There the biggest the factory offered. On the rear, you could do whatever you wanted from the normal 10 inch or disc.

Most of the stopping power is up front, so concentrate there first.
 
70dart340 said:
I'll go with the 3.91's according to the wise members here. I'll use the green bearings. I'll look at Moser and others for axles. I'll try to put my brake package together, too. If I have to switch to BBP, then so be it. Any suggestions for brakes? Besides the master Dr. Diff. I think I'll be placing an order with him soon. Thanks to all, I appreciate your experience.
Click to expand...

I have run TSM and Wilwood. Both are fine. The one nice thing about TSM is they use GM, rotors and calipers.
 
70dart340 said:
I'll go with the 3.91's according to the wise members here. I'll use the green bearings. I'll look at Moser and others for axles. I'll try to put my brake package together, too. If I have to switch to BBP, then so be it. Any suggestions for brakes? Besides the master Dr. Diff. I think I'll be placing an order with him soon. Thanks to all, I appreciate your experience.
Click to expand...
If this will be primarily a street car, then 3.91's will be pretty steep & you'll have a lot of R's at highway speed. In my case, I chose a higher stall converter & 3.23's. For me it is the best of both worlds. The TC gets the engine up to the power band quickly at low speeds but at highway speeds I'm able to have reasonable economy. Biggest issue I have is traction at lower speeds when I nail it. Steeper gears would only exacerbate that!
 
If your plan includes very sticky tires at a well prepared drag strip .. start keeping your eyes out for a D60...
If I could do it over Ida spent the $1200 building the D60 first instead of $1200 on the 8 3/4 blowing it up in 20 passes and THEN another $1200 on the D60...
But that's my story, hopefully yours is different...
 
Also after my drag days were mostly over (410 gears) I've went back to the nice compromise 355's..
In hindsight the 355's would have been fine on the drag strip... But I have a four speed..
 
3.91 gear is 2,[email protected]. That isn't anything extreme, and a taller tire would cut that down even more. If he takes it to the track it will still be way off peak/optimal RPM, but it will still function.
 
roccodart440 said:
3.91 gear is 2,[email protected]. That isn't anything extreme, and a taller tire would cut that down even more. If he takes it to the track it will still be way off peak/optimal RPM, but it will still function.
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You are certainly welcome to disagree, but the OP asked for opinions and I gave mine. Some of us drive more than 58mph, so at 70 or 75 he would be turning a lot more R's. I've run my 440 with 3.91's and will say that it was certainly torquey! It also drank a lot more fuel, ran hotter, and was much less enjoyable to drive. My opinions & experience!
 
I appreciate any and all opinions. I'm getting conflicting opinions, which is to be expected. Would 3.73's be a reasonable compromise? I'm trying to keep the car factory appearing.
 
70dart340 said:
I appreciate any and all opinions. I'm getting conflicting opinions, which is to be expected. Would 3.73's be a reasonable compromise? I'm trying to keep the car factory appearing.
Click to expand...
Google RPM calculator and figure out your one-to-one gear ratio which is your final gear 3rd gear with an automatic and your tire size and diameter and the gear and the RPM you'll be running and the Cameo have in it and it's optimal rpm. And what you feel you'd be comfortable with. Note in your daily driver for the RPM is at which speeds...
 
70dart340 said:
Many thanks to all who answered. I think I'll try Rick's approach. I'm planning on going to 3.73 or 3.91 gears. Summit Racing has steelies and Ralleys in 15" SBP, if I can use those and put some discs on the rear, I'll be happy. FYI, the motor will be a 408" Trick Flow heads and intake, a Racer Brown cam around .685@256, and 10.5 to 1. The carb I have is a QF 750 mechanical secondary. TTI step headers, and MSD E-Curve distributor. A Pat Blaise 727is done, and I'm told a big 3500-4000 converter will work for street and light strip duty.
Click to expand...
Paul, are your cam numbers right? .685 lift and .256 duration. If their right it won’t be very street friendly. Your gonna need at least 4.30 gears. Kim
 
oldkimmer said:
Paul, are your cam numbers right? .685 lift and .256 duration. If their right it won’t be very street friendly. Your gonna need at least 4.30 gears. Kim
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Those are my thoughts, too. I'll call Jim Brown this week and ask him if wee can tone it down. He wants to get full benefit of the TF heads, but this is primarily a street car. I'd like to run a solid roller lifter. I greatly respect your knowledge and opinion. Could you please give me your thoughts when you get a moment? Respect sent.
 
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70dart340 said:
Those are my thoughts, too. I'll call Jim Brown this week and ask him if wee can tone it down. He want's to get full benefit of the TF heads, but this is primarily a street car. I'd like to run a solid lifter. I greatly respect your knowledge and opinion. Could you please give me your thoughts when you get a moment? Resect sent.
Click to expand...
256 @.050 (if that's correct) is not that big of a cam in a stroked small block, sure it will make some noise at idle but it will be plenty streetable. Tell Jim what you want (and be sure you know what you want) and you will get the right cam.
 
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