How much of a performance difference would I notice with a posi rear? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-03-2011, 12:55 PM
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How much of a performance difference would I notice with a posi rear?

Hey I have an 86 Cutlass with a high performance 383sbc stroker. On paper it makes about 400-450hp. It has a built 350 turbo tranny. And it's had various weight reduction done.

It has the original stock rear end though. It doesn't have posi. I think it's a 7.5, not sure what gear.

It's weak as hell though. Definitely the weak point of the car. The car isn't very fast and I blame the rear. I owned a 78 Camaro w/ a 355sbc that made about 400hp, and it had a posi 10 bolt with a 3.90 gear, and that thing was fast as hell, it would run a 12.90 on slicks. This cutlass should technically be faster, but it's not.

So yeah, my cutlass launches very weak, and just feels like it has no torque. Like none of the power is getting to the ground. I was planning on getting a new complete Ford 9" through Moser but I'm feeling cheap, and I don't think I'll have the car long enough to sink that kinda money into it.

But if I did put a new rear end in it, either a Ford 9" or go the cheaper route and get a used 8.5 out of a Grand National, how much more performance would I notice?

Do you think one of those rearends would really unleash the potential of my car?

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Old 08-03-2011, 03:10 PM
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Quote:
Originally Posted by valtiel43
Hey I have an 86 Cutlass with a high performance 383sbc stroker. On paper it makes about 400-450hp. It has a built 350 turbo tranny. And it's had various weight reduction done.

It has the original stock rear end though. It doesn't have posi. I think it's a 7.5, not sure what gear.

It's weak as hell though. Definitely the weak point of the car. The car isn't very fast and I blame the rear. I owned a 78 Camaro w/ a 355sbc that made about 400hp, and it had a posi 10 bolt with a 3.90 gear, and that thing was fast as hell, it would run a 12.90 on slicks. This cutlass should technically be faster, but it's not.

So yeah, my cutlass launches very weak, and just feels like it has no torque. Like none of the power is getting to the ground. I was planning on getting a new complete Ford 9" through Moser but I'm feeling cheap, and I don't think I'll have the car long enough to sink that kinda money into it.

But if I did put a new rear end in it, either a Ford 9" or go the cheaper route and get a used 8.5 out of a Grand National, how much more performance would I notice?

Do you think one of those rearends would really unleash the potential of my car?

Well !!! Yes it will help the car for sure... You can have 1000hp motor,, But if you can't put it to the ground, You don't have nothing but sound..Me !!! All I ever use in my hotter cars are 9'' fords... Because I'm set up to narrow my own... So it work's out great for me... And I never broke one yet... So if it was mine... I would go with a 9'',,, But there is other very good rear ends.. So.. Put what you want, But for sure make it a posi... Then your heading in the right direction... But you also have to have the back end set up right to handle the hp..
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Old 08-03-2011, 03:28 PM
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Performance is about the whole package, you cant expect a car to perform its best if you saddle it with mpg type gearing and/or a torque converter that doesnt match the engine.

Your Cutlass could very possibly have 2.14 gears in it, even if its got the 2.41's or 2.56s, theyre still doggy gears for street/strip performance.

As far as posi, if youre getting the power to the ground with out spinning, then a posi isnt needed, but if you got the one wheel peel happening, a posi will help direct the power to the tire with the most traction.

Adding a perfromance gear, and a posi, is the all out best way to go, but not with your 7.5. Its just a weak axle. Unless youre just gonna sell the car, in that case, get a 7.5 out of a Monte SS with the 3.73, just make sure its got posi, because it was an option from the factory, so not all SS cars have it.

8.5's come in 85-87 442's and 84 H/O's also. Same thing with the posi, its not installed unless specifically ordered.
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Old 08-03-2011, 03:36 PM
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What rear gear is in the Cutlass now??

Typical GM G-body 7.5" rears are mostly 2.73, 2.56, or 2.41 rear gear(terrible) in the average normal low performance passenger cars, only the Monte SS got 3.42 or 3.73 gears in the 7.5". That right there can make a world of difference in how powerful the car feels.
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Old 08-03-2011, 07:48 PM
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The 3.73's would make a big difference for you. Would be very noticeable.
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Old 08-03-2011, 08:25 PM
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What stall are you running now???

If your going to be running on the street... I would go with a 2800 to 3000 stall, And 3.73 gears.. And a posi rear end.... Don't let anyone tell you that you don't need a posi... A one wheel pulling car is for little old lady's..LOL

Not a car with a 383 in it... MAN !!!!

I done heard it all.. They put posi rear ends behind hp for a reason !!!!

Get yourself a posi in that car.... Unless you like doing a one wheel smoke show...or donuts ....

If so,Take that motor out in send it to me...
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Old 08-04-2011, 12:58 AM
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If you can't afford a posi (not to offend you or anything) take a look at the lockRight locker from powertrax. I have one and I love it. True posi units are not cheap. A 3.73 would def be an improvement. I have 4.10, but it is not a DD.
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Old 08-04-2011, 02:07 AM
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I´ve had a Powertrax Lockright for years, 15 I think, never a problem, works great.
A very easy installation as well.
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Old 08-04-2011, 09:33 AM
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Get a better rear end and look into your cam profile. I used a Comp cam when I rebuilt my 350, don't remember the exact profile, sounded great but had little torque until the rpms hit about 4 grand. changed the cam out, went to a torquier cam, a world of difference on the low end.
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Old 08-04-2011, 01:02 PM
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Positraction

Quote:
Originally Posted by NEW INTERIORS
What stall are you running now???

If your going to be running on the street... I would go with a 2800 to 3000 stall, And 3.73 gears.. And a posi rear end.... Don't let anyone tell you that you don't need a posi... A one wheel pulling car is for little old lady's..LOL

Not a car with a 383 in it... MAN !!!!

I done heard it all.. They put posi rear ends behind hp for a reason !!!!

Get yourself a posi in that car.... Unless you like doing a one wheel smoke show...or donuts ....

If so,Take that motor out in send it to me...


The first vehicles equipped with limited slip differentials (LSD) were used for rural mail delivery which often had one tire in the dirt and the other tire on pavement. Off-road and SUV benefit from a LSD or a locking differential when one of the rear tires loses contact with the ground. The advantage I found when using a LSD in comparison to a open differential was that the LSD was stronger than a open differential because it was designed to be used with the GM Performance Axle Option (lower ratio-higher numerically). My '69 Firebird 400 had the Pontiac Performance Axle Option which was a 10-bolt, 4-pinion Safe-T-Track but only because it was equipped with 4.33:1 gears. The later LSD (G80 option) are weak units because the torque of the vehicles using them is low in comparison the the GM vehicles in the 1960s. A GM G80 Gov-Loc is a excellent unit for what is is designed to do but will not take abuse or more than 300 RWHP.

If both rear tires have equal traction, as is usually found on a drag strip and on good pavement, a LSD offers no advantage. However, if you are going to change your "freeway flyer" gears to a higher numerical gear ratio for better acceleration, you must also change the differential to a series that will accept those gear ratios. Therefore you might as well install a LSD or a locking differential for the extra strength or in case you get one of the rear tires off the ground or in the dirt.

Last edited by MouseFink; 08-04-2011 at 01:11 PM.
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Old 08-04-2011, 01:45 PM
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Quote:
Originally Posted by MouseFink
The first vehicles equipped with limited slip differentials (LSD) were used for rural mail delivery which often had one tire in the dirt and the other tire on pavement. Off-road and SUV benefit from a LSD or a locking differential when one of the rear tires loses contact with the ground. The advantage I found when using a LSD in comparison to a open differential was that the LSD was stronger than a open differential because it was designed to be used with the GM Performance Axle Option (lower ratio-higher numerically). My '69 Firebird 400 had the Pontiac Performance Axle Option which was a 10-bolt, 4-pinion Safe-T-Track but only because it was equipped with 4.33:1 gears. The later LSD (G80 option) are weak units because the torque of the vehicles using them is low in comparison the the GM vehicles in the 1960s. A GM G80 Gov-Loc is a excellent unit for what is is designed to do but will not take abuse or more than 300 RWHP.

If both rear tires have equal traction, as is usually found on a drag strip and on good pavement, a LSD offers no advantage. However, if you are going to change your "freeway flyer" gears to a higher numerical gear ratio for better acceleration, you must also change the differential to a series that will accept those gear ratios. Therefore you might as well install a LSD or a locking differential for the extra strength or in case you get one of the rear tires off the ground or in the dirt.


That's all good.....
I know the least I would do would be a Locking Differential..
And as for the dirt,, Well !!! I myself don't do the dirt thing with any of my Hot rods.. When You stick your foot in it,, It should be on the pavement and pointing straight ahead.. If you leave from a dirt road with your foot in it.. WELL !!! I don't think I really need to post what I think about something that stupid....


To the poster here... Do what you want to.... Good luck with what ever you decide... I done said way to much about this..


I really thought we where talking about a ''STREET'' car here... Not 4x4's..
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Old 08-04-2011, 02:04 PM
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4x4 and street

You missed the point...

The point I was trying to make was:

If you are going to drive or race a vehicle on pavement with equal traction on both rear tires...a LSD or locking differential will not be of any advantage.
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Old 08-04-2011, 02:11 PM
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Gears, gears, gears.
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Old 08-07-2011, 02:23 PM
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Well judging from what I've heard a posi rear seems like it will really unleash the beast in this car.

I'd like to stick a new Moser 9" complete rear end in it, but I don't think I'll have this car for more than a year or so. Next summer I plan on selling it and putting the money towards one of the cars I really want.

So I'm thinking just put a posi 7.5 out of a Monte SS into it. I'll just have to keep the slicks off it. It's going to be mostly a street car anyways.

What parts on those 7.5's tend to break with high power? If I could get a 7.5 with posi and a 3.73 gear for like $300 max, then reinforce the weak parts a little bit and keep the total cost under like $700 then I think it would be a good deal if it could handle a few drag runs.

This car has a 3500 stall btw. And I think the current gear is a 2.41.
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Old 08-07-2011, 08:00 PM
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Multipling the torque from 2.76 gears to 3.73 is a nice choice, for those who can take cruising 65 at 3000rpm . That is with one wheel or two spinning.

A posi, sure grip, lincoln locker, or powertrax will take some hp away at all areas of the power curve. The power level makes the traction worth it. All said, posi is a parasitic loss in rwhp.
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