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Old 05-23-2005, 09:31 PM
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Which Rear is safer, Posi or Not???

I have searched the hot rod archives and have not found info on which kind of rear is safer for my aplication. Any input would be greatly apreactaited im ready to put a rear in withing a day or 2


I have 2 rear ends that i can possibly use, One is a Posi beleive to be recently rebuilt with a ratio of around 3.28 I also have a non-posi with a ratio of 3.55 or so.

The car im puting together Is a 1969 firebird convertible. With Pontiac 400 motor, and a Muncie M20 probably, Right now the car has everything for a manual but its missing a tranny, but i beleive im going to go for a M20
Im also leaning towards using a Ram Air III cam in this motor

Which is safer, Im still a recently new driver doing my second total restore on a 1969 firebird, My feeling is that a Non-posi is safer. Since if your going 40 miles per hour and you switch down to 3rd instead of both wheels spinning and fishtailing, you still have one wheele with traction so the rear of the car wont break loose. I also understand in rain and such you'd have more traction with a posi. But is it a worth the risk. What is a better choice for me. Im just really afraid with a posi switching gears at 50 both back wheels brake free car fish tails, flips over and since its a convertible, my head gets choped off. Im really worried about safety in this car. Im also 2 young get classic insurance so this is going to be a summer car, including rain and such. With the rear ratio's im not sure which is better for the manual


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Old 05-23-2005, 09:40 PM
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safety

it is the piece behind the wheel..if you are really concerned about getting on your head find the local SCCA guys and sign up for some track time to learn how to drive...

Up to you if you are inside the limits and not overdriving the car and over your head in that car you will be fine..

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Old 05-24-2005, 01:14 AM
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More torque than you're used to & posi that you also aren't used to can cause you a big surprise, especially in the rain. If you use the posi, just be careful, you'll get used to it soon enough (sooner if you scare yourself once or twice). If there's an empty parking lot somewhere, try wet-road driving there (if you can) until you get a feel for the car. Don't forget, different road surfaces can have significantly different traction characteristics, especially when wet.

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Old 05-24-2005, 05:22 AM
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The 2 wheel spin factor is the price you pay for having the posi rear. Overall though, I would go for the posi. Looks like you are building for performance, so why not.
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Old 05-24-2005, 04:34 PM
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I am very happy to hear that as a young driver you are concerned about safety and not one of those kids that puts huge engines in a junk car and drives 150 mph down the highway on unknown rated junkyard tires and feels that brake pads are a waste of money...oh, that sounds like someone I know...

Since you are being cautious I suggest you go with the posi, take it to an empty parking lot as suggested and get the feel for it, or better yet take it to a local strip and run it. Since you are obviously building for performance you will be dissapointed if you don't and will wish you had.
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Old 05-24-2005, 04:50 PM
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Quote:
Originally Posted by stereodud
I have searched the hot rod archives and have not found info on which kind of rear is safer for my aplication. Any input would be greatly apreactaited im ready to put a rear in withing a day or 2


I have 2 rear ends that i can possibly use, One is a Posi beleive to be recently rebuilt with a ratio of around 3.28 I also have a non-posi with a ratio of 3.55 or so.

The car im puting together Is a 1969 firebird convertible. With Pontiac 400 motor, and a Muncie M20 probably, Right now the car has everything for a manual but its missing a tranny, but i beleive im going to go for a M20
Im also leaning towards using a Ram Air III cam in this motor

Which is safer, Im still a recently new driver doing my second total restore on a 1969 firebird, My feeling is that a Non-posi is safer. Since if your going 40 miles per hour and you switch down to 3rd instead of both wheels spinning and fishtailing, you still have one wheele with traction so the rear of the car wont break loose. I also understand in rain and such you'd have more traction with a posi. But is it a worth the risk. What is a better choice for me. Im just really afraid with a posi switching gears at 50 both back wheels brake free car fish tails, flips over and since its a convertible, my head gets choped off. Im really worried about safety in this car. Im also 2 young get classic insurance so this is going to be a summer car, including rain and such. With the rear ratio's im not sure which is better for the manual

I don't think you are correct, in that, with non-posi, you have no wheels with traction with your scenario. Posi is designed to put power to the wheel with the most traction rather than to the wheel with the least traction.

I would not worry about classic insurance, from your post, I would worry about LIFE insurance. Big motor + inexperienced driver worrying about fishtailing in the rain = Accident Waiting to Happen, in my opinion.
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Old 05-24-2005, 08:03 PM
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Don't think that an open rear will prevent a fishtail!!!!!!
One time i was doing about 45 in OverDrive on a country road in my Monte with an open rear and AUTO trans. I approached a left turn and downshifted to DRIVE, then the auto finally dropped a gear (thanks for the delay, GM) the back end got squirlly, but it was no big deal.
Moral of the story; learn to downshift a stick properly (in the rain, ect. without barking the tires) and you should be able to master a POSI.
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Old 05-24-2005, 08:16 PM
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400CID motor and a 3.55 open rear? I don't know why any of these guys would recommend you do that, it sounds ridiculous. You'll rip the rubber right off that tire, use a posi.

Dunno why folks are being dramatic. Just be careful giving it gas around turns.

K
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Old 05-24-2005, 09:22 PM
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Thank you all for the input,

Do you guys think the 3.28 would be a good ratio though, i always thought the 3.55 would be better with the stick. My first 69 bird had a small block chevy in it, with a 200r4 and a 2.73 open rear. That scared me once when i was acelerating thrue a turn once a little with sand on the road,

This is my first car when i first got it [IMG]HPIM0329.JPG[/IMG]
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Old 05-24-2005, 09:29 PM
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Quote:
Originally Posted by stereodud
Thank you all for the input,

Do you guys think the 3.28 would be a good ratio though, i always thought the 3.55 would be better with the stick. My first 69 bird had a small block chevy in it, with a 200r4 and a 2.73 open rear. That scared me once when i was acelerating thrue a turn once a little with sand on the road,

This is my first car when i first got it [IMG]HPIM0329.JPG[/IMG]
I would go with the 3.55 myself.
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Old 05-26-2005, 11:38 AM
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At this point, i think im going to go with the 3.55 and in the future ill still have the option of puting the posi in. Also my driving habits, I dont beat on my cars or speed, My first firebird i dont think i ever went above 70 in it.
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Old 05-27-2005, 04:58 AM
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Quote:
Originally Posted by killerformula
Dunno why folks are being dramatic. Just be careful giving it gas around turns.

K
Not just on the turns Killer............

If his 400 Pontiac is in good condition and is at least a stock spec '69 4 bbl. version he'll need to be careful giving it the gas in a straight line too. A late 60's Pontiac 400 4 bbl. in a Firebird with a 4 speed and 3.55 gears is more than adequate to get away from an inexperienced driver on dry pavement on a straight road.

The thing with a posi to remember is that when it does break loose the rear end of the car may want to swing to the left so be ready to steer to the left just like they taught you to do to correct a skid in Driver's Ed.
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Old 05-27-2005, 05:57 AM
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Have to thought about using your brakes to slow down? You'll have 4 wheels controling you rather then 2. Much safer IMO. Learn to drive with more finese.
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Old 05-27-2005, 09:04 AM
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Put it this way, I got my 'Bird crossed up going in to 3rd one time...and this was before I installed the new rear with posi.
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Old 05-27-2005, 09:29 AM
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i ve got a 400 in my lemans and i will go to the posi because in the rain the slightest touch will break the tire loose and lack of control is the killer you live in new york a parking lot with snow on it will learn you how to handle any loss of traction issues don't get real crazy its real easy to get bent out of shape small car big motor takes alot of skill that can only be acquired with experience if you want a real wake up call go walk around any bone yard ask them where they keep the death wrecks that will give you some perspective
but i take my hat off to you for being mature enough to ask! thanks its refreshing
and hey a parking lot full of snow is a real blast i would call in sick to work every time plus its too dangerous to drive in
1 other piece of advice be aware of what the idiot around you are doing nothing scares me more than being t boned in a intersection by some non driving going no where left hand lane hanging booger eating SOB

good luck and thanks again
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