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Old 06-19-2017, 07:43 AM
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4.56 posi rear end

Will a 4.56 posi rear end from a 1980 monte carlo fit on a 86 cutlass

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Old 06-19-2017, 10:04 AM
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The wheelbase and track width chart in the wiki shows that these two are both G body cars from the same era, so I would suspect that they would interchange. I would want to eyeball both of them first though.
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Old 06-19-2017, 10:13 AM
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What Tech said.

While the rear end should bolt in with no problem, keep in mind that you may need driveshaft mods depending on what you currently have.

I'll assume you have the stock 7.5. If the rear end you put in is an 8.5, the 8.5 is about 1/2" longer at the pinion and may or may not work with your current drive shaft. If it's a Ford 9" or something else ( like a Chevy 12 bolt) you will definitely need to shorten the driveshaft.
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Old 06-19-2017, 10:33 AM
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Yes it is a 7.5 I am a little concerned about the gears to if i will be fine with 4 56 cause I have a turbo 350 transmission
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Old 06-19-2017, 11:28 AM
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It's going to bolt right in, but you probably won't want to drive it very far with the 4.56.
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Old 06-19-2017, 11:32 AM
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Quote:
Originally Posted by big gear head View Post
It's going to bolt right in, but you probably won't want to drive it very far with the 4.56.
Unless it's for a 90% strip, 10% street vehicle.
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Old 06-19-2017, 12:48 PM
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With 25" diameter rear tires, the engine will run at 4200 RPM at 65 MPH. Screaming !! Certainly not a good street set up.
Here's the formula:

Eng RPM=(MPH X rear ratio X 336 X Trans High Gear Ratio) / (Tire dia inches X Torque converter efficiency)

In your case torque converter efficiency would be 0.90 to 0.95. I used 0.95 for your calculation. Lock up torque converter or a manual clutch is 1.00. Plug in your tire diameter if the 25" is wrong. In any event, it will scream.

Last edited by '48 Austin; 06-19-2017 at 01:10 PM.
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Old 06-19-2017, 02:01 PM
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If you don't have an overdrive automatic, this is going to get annoying quick; unless you have 31" tall rear tires (Think Warren Johnson 80s pro stocker).

Tell us more about what you have going on here?
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Old 06-19-2017, 08:20 PM
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Yes, but with a 700r4 with its 3.06 first gear, it would be insane with 4.56 gears. I would think 4.56 wouldn't work that well with 200 4r. With a good motor, posi,, good tires, and low gears, that 7.5 isn't going to hold up for long unless you don't go all out very often.
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Old 06-19-2017, 09:25 PM
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4.56's n the highway without an OD trans will suck. But in town and at the track will be just fine if your engine handle the rpm's.

Is it a 9" or 8.5"?
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Old 06-26-2017, 08:32 AM
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6000rpm will get you 97mph in 3rd

With a 200r4, 1st gear will get you 36mph at 6000rpm, 2nd gear 62mh, OD(0.67:1) nets you 146mph @ 6000rpm.
2800rpm gets you 68mph.

The 3 speed autos will get you 39mph at 6000rpm in 1st, and 65mph in 2nd again at 6000rpm.

With a 700r4 and its 3.06:1 1st gear, you'd be getting almost 32mph with a 13.95 1st gear final ratio. 2nd gear=60mph, 3rd=98 4th(0.70:1)=140mph.

It'd be fun in the city.

peace
Hog
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