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Optimal Rear End

sir_veza

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Some of you may have seen my post last week when I signed up regarding changing my 74 Challenger from 3.91 to either 3.23 or 3.55 rear end.
I do have the option of buying a 3.23 cheap from a friend, but after thinking it over may decline. I'm thinking more about keeping the 3.91 (489 casting) the way it is as I'm already going to install a Hurst Competition shifter this spring and would like to see how that feels with the 3.91 !!
However would like to read what the technical consensus is on the optimal rear end from you guys.
- 4 Speed A833
- 360 stroked to 408
- 27" Tire diameter
- Want the OPTIMAL best rear end for acceleration AND occasional 70 - 75 MPH cruising
- Is 3.55 the best choice or something else?
- I do understand the logic/math etc around the rear end gearing etc but even in the short time being on this forum have read some crazy-interesting E Body stuff that never occurred to me...
Thanks,
Steve
 
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413

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Your wanting OPTIMAL gearing for acceleration and cruising 70-75 MPH is a problem. They don’t go together, each has its own answer. It’s always a compromise.


optimal for acceleration is 4.56
optimal for cruising at 70-75 is 2.76 or 2.94
 

Chryco Psycho

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Doesn't take long to swap the center section assuming the yolk/ U joint is the same size so you could try both & see what you like best
 

704406

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70 Cuda 3:54 Dana, 260 series 15" tires runs approximately 3200 RPM at 60 MPH
413 is correct with his statement, Chryco Psycho has a good point as well, you may have to try a couple of different ratios to find what works best for you.
3:23 is a common ratio as well used in lots of engine and transmission combinations
 

sir_veza

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Doesn't take long to swap the center section assuming the yolk/ U joint is the same size so you could try both & see what you like best
Thanks - (Not 100% certain) But the yoke / U joint should be compatible if the casting is a 489...just tried to find that "sweet spot" rear end (oops sounds like a dating site comment lol) !!
 

sir_veza

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70 Cuda 3:54 Dana, 260 series 15" tires runs approximately 3200 RPM at 60 MPH
413 is correct with his statement, Chryco Psycho has a good point as well, you may have to try a couple of different ratios to find what works best for you.
3:23 is a common ratio as well used in lots of engine and transmission combinations
Thanks - Actually the 3.91 is around 3200 - 3300 rpm at 60 mph now. Around 4000 rpm at 70 mph and a little loud
 

413

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Yes it’s a compromise. I’d choose 323 if it were mine.

The yokes can be different no matter the case number. They sell conversion joints so you don’t have to swap yokes.
 
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sir_veza

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This would be my first time changing out a rear end - assumed the casing would be the main thing. Thanks for letting me know about the conversion joint if necessary. I can get a 3.23 but it might be a little too tame at the low end (just saying)...
 

70chall440

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You are looking at this a bit incorrectly but the advice you are being given is correct based off your question. If your 408 is making the power, a 3.23 will be more than enough to get you more tickets in town than you want. It might not smoke the tires through 4th but will certainly get it done if need be. Personally I am a fan of 3.55's as I feel it is a good middle ground between in acceleration and cruising with something producing around 350-400 HP.

You need to think of the car like a system; everything works with everything else. To achieve what you want you have to approach the system. If you like the 3.91's, then look at a Gear Vendors overdrive unit or maybe change to a 5 or 6 speed, best of both worlds.

I have 3.23 SG in my Cuda which is a stroked 340 making near 500 and it will spin the tires through 3rd, its a monster and thats with 315 Nittos on the back.
 

NoCar340

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With a 4" stroke, theoretically you should have a wider, flatter torque curve than the 360's already-good one unless it's some hairball peaky W-series-headed beast, which I don't think you have. So, you would probably be OK with the wider gear spread of the A833OD. A lot of guys will tell you it's weaker--which it ultimately is--than a standard A833, but I've never blasted one. Drag slicks? 4-link? Clutch dumps? Yeah, you might well damage one by virtue of the aluminum case or floating countershaft, but swapping the guts into a standard iron A833 case gives you both the stronger material and eliminates the floating countershaft (you simply use a standard one). You'd need the truck transmission, which has the long tailshaft like the B/E-body transmission, a 307-bearing main case, and a non-OD countershaft. This arrangement also conveniently eliminates any fussing with your existing bellhousing or clutch fork. Because the overdrive gear is actually in the 3rd-gear spot in the gear stackup, the 3-4 lever on the side cover needs to be flipped; making it work is not rocket surgery.

With a 3.23, your first-gear acceleration is very close to the same as what you have now, since your original transmission should be a 2.47:1 ('71-up) and the overdrive is 3.09:1. Some simple math bears this out, using either a 3.23 or a 3.55 gear:

3.09 x 3.23 = 9.9807, your total final drive in first. 9.9807 / 2.47 = 4.04:1 rear axle equivalence to your existing first gear.

3.09 x 3.55 = 10.9695, your total final drive in first. 10.9695 / 2.47 = 4.44:1 rear axle equivalence to your existing first gear.

So with either choice, you've got better acceleration in first than your existing arrangement. Now let's consider the other end of the scale, the overdrive gear. Depending on who's writing the article, overdrive is either 0.73 or 0.76:1, but we'll go with the higher number as "worst case":

3.23 x 0.76 = 2.45:1 final drive in overdrive.

3.55 x 0.76 = 2.698:1 final drive in overdrive.

Based on your 26.58" rear tire, if memory serves, that puts you at either 2,327 or 2,558RPM @ 75MPH, respectively.

Yes, the RPM drop between 1st and 2nd is significant but it's not unbearable (and if you haven't gotten your point across by the top of first gear, you probably never will 😁). With a torquey stroker, it might not even be all that noticeable. The overdrive gearset's first three ratios are almost exactly the same as those in a light-duty 3-speed manual.

If you want to stick with a close-ratio, there is another option: From 1964-'67, many A-bodies used a close-ratio transmission with a 3.09 first gear, same as the overdrive. These can be easily identified by the half-round groove ahead of the clutch splines. Those gears swap right into your existing case without any strength concerns whatsoever. In conjunction with the 3.23, you still improve first-gear acceleration but are now only around 3,000RPM at 75MPH. Guys at swap meets have nearly begged me to drag away their ball-and-trunion (flanged) output A-body transmissions with the 3.09 gearset! The last one cost me $100--he yelled "Make me an offer!" as I walked away--with the complete factory 1965 Hurst shifter still bolted to it.

I have one of each of the above transmissions for my Challenger. I'll run the 3.09 close-ratio with a 3.55 locally/everyday. However, two of my sisters, my daughter, and my best friend all live well over 1,000 miles from me. If the spirit moves me to visit one of 'em, I can swap in the OD and a 2.94:1 center section for the trip and cruise 85MPH with the tach below 2,400RPM.

The "low first gear" trick was actually used by Chrysler in the A99x transmissions. In the late '70s, Chrysler couldn't afford to tool up for an overdrive transmission, so they switched the A904's first gear from 2.45 to 2.77, then proceeded to install axle gears as low as mid-2.20s to keep the highway RPM reasonable (my '84 Fifth Avenue had a 2.26:1 axle, Sure Grip no less).
 

moparleo

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Now you know why the overdrive after market is so hot. The old saying that you "can't have your cake and eat it too" came about for a reason. The "blend" is what the factory gave you. The "Hemi" cars generally came with a 3.54 Dana to try to give something that would still live in everyday driving.
There are race cars and there are street cars. And then there is all the stuff people spend $$$$ trying to make a dream come true.
No magic snake oil. Just physics. Good bottom end will always come at the cost of top end.
 

sir_veza

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Yup!! But I'm thinking the 3.23 might be the way to go at least for the year...
One of my friends suggests a 5 or 6 speed but to me that's not an option. Want to stay in the 70's lol
 

NoCar340

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Yup!! But I'm thinking the 3.23 might be the way to go at least for the year...
One of my friends suggests a 5 or 6 speed but to me that's not an option. Want to stay in the 70's lol
I have no problem with people using aftermarket manual transmissions, but I like to use factory-style solutions wherever possible. I want my cars to be long-haul capable on a moment's whim. That means no muss, no fuss, no waiting six days for someone to send me an oddball replacement part. If I smoke the clutch in Nebraska, a parts store should be able to get me one the next day for about $125. It won't be gee-whiz magazine-boy parts, but it will get me back on the road. Working in auto parts for almost 35 years has taught me many things. One is that speed-part vendors are never in a hurry to ship. I paid K&N to next-day ship a customer's part, which they did in fact do... after waiting five days to actually get the part to UPS. Good luck getting a 1-wire alternator or MSD Digital 7 the next day when you're broke down in Asswash, Idaho.

It's not a concern if your car never leaves your hometown. Mine will, probably fairly often. I love road trips in old cars. I used to semi-annually run home in my 440 Six Pack-powered '72 Charger when I lived in GA--1,100 miles one way.
 

chargerdon

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I was going to give you the link to the RPM Speed Calculator, but, it uses adobe flashplayer which can no longer be used.

In my case with my 360/408 and Lunati Voodoo cam i ended up installing a 3.23 sure grip along with my 14" (25" tall tires) and the A518 overdrive Torqueflite. I like this combo as it can still easily be driven on the street, produces enough grunt in first gear that my tires can barely handle it, yet flip the overdrive switch and i can cruise at 75 and around 2300 RPM to save gas and my ears !! I do NOT drag race the car, other than to take it to test and tune at the local dragstrip once a year for fun. Last time there i ran a 9.04 1/8 mile that was pretty badly limited to the traction of my 235/60/14 tires.
 

340cudakf

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I would keep the 3.91 and use a Gear Vendor overdrive. I have a 3.55 with the Gear Vendors. Wish I had the 3.91.
 
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