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#1054143 - 10/30/17 02:55 PM Rear end suggestions  
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kcaudill Offline
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Blacksburg, Va
Looking to put a posi in my monte carlo. Looking for suggestions on posi to buy and rear end gearing to use. Its going to be mostly a street car with occasional strip use. was thinking 3:73 but would like opinions.

#1054145 - 10/30/17 03:02 PM Re: Rear end suggestions [Re: kcaudill]  
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upflying Offline
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For my winter project, I am expecting delivery of a Quick Performance 9" any day now. Eaton posi, 3.70, disc brakes.
The stock 7.5" (or is 7.65"?) 10 bolt is fine for street use but it won't last with a posi, strip launches and sticky tires. Ideally you want the 8.5 10 bolt out of the GN but it is hard to find.
http://www.quickperformance.com/

Last edited by upflying; 10/30/17 03:18 PM.

86 MCSS Notchback coupe, LS3, 4L65E, column shift, silver with maroon bench interior

#1054146 - 10/30/17 03:21 PM Re: Rear end suggestions [Re: kcaudill]  
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MC96 Offline
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St. Johns MI
I would disagree only with the 8.5" being ideal, I think the 9" is ideal, although a bolt in replacement is pricy, but so is an 8.5" once you go through it (and find one!)


86 SS
400SBC, 4l80e, MSD Atomic injection/trans controller, Tilt glass clip, all tubular arms, corvette brakes, 9" rear
In progress
#1054147 - 10/30/17 03:50 PM Re: Rear end suggestions [Re: kcaudill]  
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kcaudill Offline
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Blacksburg, Va
Thanks for the info. What about gearing? 3:73 be a good choice? Or should i think about 4:11?

#1054154 - 10/30/17 07:26 PM Re: Rear end suggestions [Re: kcaudill]  
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upflying Offline
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Gearing depends on what you are using it for and what transmission you are running., lower gearing (higher numerically) is better for drag racing but hurts mpg and causes higher rpm in street driving. Higher gearing, (lower numerically) is great for highway and mpg but hurts acceleration. If winning and low E.T. is important then go with 4.11.
For most of us, compromise gearing is best, thats why GM chose 3.73 for our cars.

Last edited by upflying; 10/30/17 08:16 PM.

86 MCSS Notchback coupe, LS3, 4L65E, column shift, silver with maroon bench interior

#1054177 - 11/01/17 03:23 AM Re: Rear end suggestions [Re: upflying]  
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Fred SS Offline
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Originally Posted by upflying
Gearing depends on what you are using it for and what transmission you are running., lower gearing (higher numerically) is better for drag racing but hurts mpg and causes higher rpm in street driving. Higher gearing, (lower numerically) is great for highway and mpg but hurts acceleration. If winning and low E.T. is important then go with 4.11.
For most of us, compromise gearing is best, thats why GM chose 3.73 for our cars.


Upflying is correct on all his points. If you are mainly concerned with winning races then get 4.11 gears. But if you mainly cruise with a few drag strip passes, then a 3.73 is a good compromise. How much HP/Tq are you currently running? Do you plan to do major HP upgrades in the future? If so, the a 9" would be a good choice, but there are others; 12 Bolt, Strange S60, etc.

My plans are to get the 7.625 posi rebuilt with a new Eaton 28 spline posi unit and 28 spline axles. I only cruise with no plans for drag strip use. Good luck with your project.

Last edited by Fred SS; 11/01/17 03:24 AM.

87 SS, Harwood cowl induction hood, UMI trailing arms. Edelbrock TES Headers (Jet Hot Coated). GM Goodwrench 350, TPI & Trick Flow 56cc alum heads, SLP runners, ported plenum, Comp Cams 260AH-12 cam, Trick Flow 1.5 roller rockers, Bilsteins, TCI 2400 stall converter, Ramcharger fans, SLP ZR1 rims.
#1054178 - 11/01/17 08:23 AM Re: Rear end suggestions [Re: kcaudill]  
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axld Offline
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lakeland,fl
+1 on the Quick 9". I choose 3:70 gears ,/ I had the factory 3:73/. With my 4 speed autos low first gear I should have went with 3:50's. This is from my years of experience owning monte carlo's , The factory rear ends are sketchy at the drag strip if you are running sticky tires. At least install a c-clip eliminator kit.


86 SS 5.3/4L60E. custom paint-17'' N90 wheels -quick 9'' - custom dyno tune- UMI/BMR suspension- Headman mids with x pipe -and a lot of other $ parts.
#1054179 - 11/01/17 12:32 PM Re: Rear end suggestions [Re: kcaudill]  
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kcaudill Offline
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Blacksburg, Va
Thanks guys,
Planning on mostly street use with occasional strip use.
Setup is 350 with cam and heads, approx 380 -400hp TH400 trans
Current rear end has highway gears in it. not sure what it is but taller than a 3;73 open diff
Cruising highway turning approx 1800rpm at 65mph

#1054180 - 11/01/17 01:39 PM Re: Rear end suggestions [Re: kcaudill]  
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upflying Offline
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Your TH400 doesn't have overdrive, you definitely don't want anything deeper than 3.73 for street and occasional strip.


86 MCSS Notchback coupe, LS3, 4L65E, column shift, silver with maroon bench interior

#1054182 - 11/01/17 03:02 PM Re: Rear end suggestions [Re: kcaudill]  
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mmc427ss Offline
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If you're taching 1800 at 65 now with the stock 200R in the 87 LS the rear gear ratio is probably 3.08.

Question, why going with a TH400, especially if you are staying with the 7 1/2" rear. A 400 consumes a lot of hp to spin it. If you don't want to spend the money to replace the rear, or upgrade the 200R internals I would suggest a TH350 instead. It will hold up to 400+, use less of that 400 hp to spin it, be cheaper to build, and would be a bolt in.

Here's a calculator to play with for rpm/mph.
http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

By the time you buy a posi, R&P, bearings, a girdle, and do the setup you could have a lot of money in a 7 1/2. You would do better to buy a used 7 1/2 with posi and the 3.73s out of a donor SS. They show up for sale often. To do an 8 1/2" rear today is expensive, and hard to come by. A 9" complete is also expensive by the time you do brakes.
Don't forget changing the trans or rear usually will effect driveshaft length. You would like to have one made only one time.
Bob

#1054211 - 11/03/17 12:02 PM Re: Rear end suggestions [Re: kcaudill]  
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kcaudill Offline
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Blacksburg, Va
Wow thanks Bob, lots of good info. With that i did find a good used currie rear for the monte on a craigs list add. it has the 3;73 gear but no posi and it looks to be a 9" with larger shafts, i didnt measure, but looks to the eye and hand bigger. Paid 500 for it + gas and about 3 hrs of driving. Good deal? I will post up pics here soon

#1054212 - 11/03/17 12:56 PM Re: Rear end suggestions [Re: kcaudill]  
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MC96 Offline
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St. Johns MI
Great price for a bolt in 9"

A posi carrier is usually $300+ no matter which rear you have.

Last edited by MC96; 11/03/17 12:56 PM.

86 SS
400SBC, 4l80e, MSD Atomic injection/trans controller, Tilt glass clip, all tubular arms, corvette brakes, 9" rear
In progress

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