How important is a Posi rear? - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 26 (permalink) Old May 25th, 20, 1:51 PM Thread Starter
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How important is a Posi rear?

I had posted yesterday about having an open 10 bolt and looking at different rears And what what fit my 67 Chevelle.

My next question is...how important is a posi rear? I’m doing a frame off restomod and already have about 7k in parts in it and still have 25k or so of parts priced out and left to buy on my build. I know some of you will think we’ll 30k, what does another 2-3k matter?

I just want to make sure it will be absolutely worth it before I add another 2-3k to my build.

I figured I’d add that I’m building my car to cruise around and occasionally get on it but not a drag or racing car by any means. Newer engine, TR6060 6 speed...around 500hp at the motor. I want to be able to efficiently cruise at 60mph but also if the urge arises I would like to be able to get up and go. Want to make sure I have the right rear.

So with that being said, is not having a posi really going to be a big deal if I’m not out ripping it and trying to do burnouts? Is it something I could get by without having right away?

Also if a posi is a MUST have, which gears should I get for what I want to use the car? Doing research I was thinking more like 4.10 or 3.73

I don’t know much about rears and gears which is why I’m asking. Thanks for the help! You guys are the best.
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post #2 of 26 (permalink) Old May 25th, 20, 2:02 PM
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Re: How important is a Posi rear?

Give a real good look at the Detroit Truetrac differential...
I swear by mine...Makes driving very special...
Mine came open and after a good look around at alternatives...This just stood out ...

https://www.eaton.com/us/en-us/catal...dex.htm#tabs-1
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post #3 of 26 (permalink) Old May 25th, 20, 2:05 PM
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Re: How important is a Posi rear?

With 500 hp you are going to want posi traction for sure. I like the gear my car has so I installed a Powertrax lock rite. They don't require removal of the ring and pinion so no set up with shims and bearings. It is a bit harsh on the street though. I think they are about 400.
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post #4 of 26 (permalink) Old May 25th, 20, 4:37 PM
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Freddie
 
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Re: How important is a Posi rear?

Why build an engine with 500 HP and not build a rear end to handle it? You should have built a 250 HP engine if you were not going to build the rear end too.

If you do one burnout while spinning one tire it can damage the differential. One wheel burnouts are very damaging to the rear end. If you think you might want to show off even one time then do the rear end right. As for me, I've NEVER been impress by a one wheel burnout. I had a 4 cylinder Ford Ranger that would do that.


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post #5 of 26 (permalink) Old May 26th, 20, 12:10 PM
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pete
 
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Re: How important is a Posi rear?

Quote:
Originally Posted by USMC93 View Post
I had posted yesterday about having an open 10 bolt and looking at different rears And what what fit my 67 Chevelle.

My next question is...how important is a posi rear? I’m doing a frame off restomod and already have about 7k in parts in it and still have 25k or so of parts priced out and left to buy on my build. I know some of you will think we’ll 30k, what does another 2-3k matter?

I just want to make sure it will be absolutely worth it before I add another 2-3k to my build.

I figured I’d add that I’m building my car to cruise around and occasionally get on it but not a drag or racing car by any means. Newer engine, TR6060 6 speed...around 500hp at the motor. I want to be able to efficiently cruise at 60mph but also if the urge arises I would like to be able to get up and go. Want to make sure I have the right rear.

So with that being said, is not having a posi really going to be a big deal if I’m not out ripping it and trying to do burnouts? Is it something I could get by without having right away?

Also if a posi is a MUST have, which gears should I get for what I want to use the car? Doing research I was thinking more like 4.10 or 3.73

I don’t know much about rears and gears which is why I’m asking. Thanks for the help! You guys are the best.

It depends on how fast you intend to let out the clutch when you "get up and go". If you don't intend to smoke the tire(s), and just step on the gas after the car is moving (mostly in 2nd gear, or higher?), posi is optional. AND, it can always be added later if you feel the need. You state "...I'm not out ripping it and trying to do burnouts..." .

A nice mid range rear (around 3.55?) is where I would focus as I understand your needs.

Wanna save a few bucks? Again, as I understand your needs, I'd back off the 500 hp. I'd focus on low end torque; perhaps a sbc that produced a minimum of 420 lb-ft of torque (at approx 3500 rpm). I would not care what the hp rating was. It would 'fall out' from the following equation:

hp = (torque x rpm)/5252

(ever wonder why ALL hp/torque curves 'cross' at 5252? That's why)

So, at 3500 rpm and 420 lb-ft of torque, hp (at that rpm only) = 280 hp.

As rpm rose above 3500 the torque would drop off some from the max. How much, depends on the 'build' of the engine. I (just me) would not care what hp was at a typical red line of about 5200 rpm. IF the torque dropped to 380 lb-ft at 5200 rpm; hp = 376.

That should be enough for your stated needs (as I understand them). It's a simple, easy, reliable build. A sbc 383 perhaps?

Just some ideas for your consideration.

Pete
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post #6 of 26 (permalink) Old May 26th, 20, 4:43 PM
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Re: How important is a Posi rear?

ahh yesss, the old one-wheel peel. loser city.

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post #7 of 26 (permalink) Old May 26th, 20, 5:41 PM
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Tom
 
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Re: How important is a Posi rear?

In my book, a posi is like a pistol-------------------I never leave home without it.

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post #8 of 26 (permalink) Old May 26th, 20, 5:52 PM
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Re: How important is a Posi rear?

Some folks probably don't necessarily need a posi but if not they wouldn't be able to brag and say "it's got a posi"

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post #9 of 26 (permalink) Old May 26th, 20, 6:12 PM
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Eric
 
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Re: How important is a Posi rear?

Quote:
Originally Posted by wolf396 View Post
With 500 hp you are going to want posi traction for sure. I like the gear my car has so I installed a Powertrax lock rite. They don't require removal of the ring and pinion so no set up with shims and bearings. It is a bit harsh on the street though. I think they are about 400.
I went a similar route with a Nitro Gear Lunchbox locker, outside of the clicking noises when turning its pretty smooth. It was in the $375 range. I didn't want to spend big $$$ on a posi rear. Pretty simple install with no special tools or skill required, just follow the instructions.

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post #10 of 26 (permalink) Old May 26th, 20, 6:34 PM
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Chevelle Super Sports need positraction period! Mine had the 4:10 and probably was a bit low but oh the few drag races I did have were well worth it! Even on the highway it was fine but it’ll keep you on your toes so you won’t go to sleep at the wheel! I never was much for burnouts.... not my thing and my clutch was just fine! You drive it right even with 500 HP it won’t let you down. I was probably running about 450 HP with my L78 and M/T Headers!
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post #11 of 26 (permalink) Old May 26th, 20, 7:01 PM
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Re: How important is a Posi rear?

And posi is really good in snow.
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post #12 of 26 (permalink) Old May 26th, 20, 7:27 PM
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Re: How important is a Posi rear?

A wise man once said: Don't piss into the wind, tug on Superman's cape, or build a 500HP Muscle Car without a Posi.

Then again, it could have been a washed up stripper who said that while I was being served cheap whiskey....but the point is still the same.

On a more serious note, a sincere thank you for your service to our Country. It's much appreciated by those with the proper cognitive skills.

'69 Malibu 383/TCI TH350/8.2 3.36 Eaton Posi. Street Edge 10" 2800, E-Tec 170s, Custom Mike Jones HR Cam, RPM Air-Gap, StreetAvenger 770, Pertronix HEI, Hedman 1 5/8" Headers, Moser Axles, PMT 1320R Kit. [email protected] 111 MPH, 1.6865 60 ft, Nitto 555
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post #13 of 26 (permalink) Old May 26th, 20, 7:46 PM
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Re: How important is a Posi rear?

500HP you bet you will need posi. A posi unit is about 500-600 bucks plus the cost to install. I do my own work so I don't know how much a shop will charge to install. Maybe $400. PLus another 300-400 for the gears. It's not a big job but best left to the pro. The TR6060 comes with different gear ratios. The one in my Shelby had a 2.66 but older Shelby's had 2.94 which takes away the need for hot gears but that tranny has two overdrives so if you don't go with gears it will be waste. My Shelby with 662 Hp had a 2.66 first gear with 3.31's and it took a bit to get it rolling. Car did 140mph in 3rd though!!!! To me you are skimping on the rear end (need 12 bolt posi) and over killing the tranny. Your drivetrain is only as strong as its weakest link..... which is the 10 bolt.
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post #14 of 26 (permalink) Old May 26th, 20, 8:45 PM
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Re: How important is a Posi rear?

I would not Rebuild the 10 Bolt with the HP you have

I was convinced into Rebuilding my 50+ yr old Posi 12 Bolt
by a Pro Shop/Pro Racer that had been in Business for 35+ years
a year later it was back to a one peg legger and it had cost me 1/2 of what a New Rearend would have

So I found a new Shop that was run by current Pro Racers and ordered a Posi S60 from Strange Engineering
https://www.strangeengineering.net/p...ing-rear-ends/

https://www.strangeengineering.net/p...ing-rear-ends/

https://www.strangeengineering.net/p...g-mounts.html/
I ordered it with the options/upgrades I thought I needed with the way I use/drive my car including
- Gear Ratio 3.70/3.73
- Helical gear differential…………. OPRS02
- 35 spline Alloy Axle Package
- 1/2" Wheel Stubs
- Chrome moly yoke………………… OPRS18
- LPW aluminum cover…………….. OPRS06
- S-Series pinion yoke – U1600
- Ford Ends with 11" Drum Brakes and Brake Lines Installed

Almost 7 years now and everything is working the way I had hoped, see my Sig

When I am not driving my car on the Street/Hwy on 200 Mile Trips to Relatives etc. in nice weather
I drive it up to the Track 50 Miles and go Racing all day then drive back Home

67 Chevelle Malibu SS Cdn
Born 283 / PG / A51
468/TH400 2,500 stall/ S60 3.70 Gs
Prev Best on M/T ET S/S #3454 street Tires/Dress 2018/05/20
60' = 1.664
1/8 = 7.532 et / 91 mph
1/4 = 11.814 et / 115 mph
NEW Best with 3,500+ Stall TC 2019/09/08
60' = 1.547
1/8 = 7.311 et / 91.76 mph
1/4 = 11.595 et / 114.28 mph


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post #15 of 26 (permalink) Old May 26th, 20, 11:17 PM
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Re: How important is a Posi rear?

I would not recommend putting a posi in an 8.2" 10 bolt. It's absolutely not worth it. In saying this, I'm assuming that you're going to have somebody else do the work, which adds to the expense. There's a reason that 10 bolt rears have a bad reputation. They're weak, particularly in the spider gear department, especially with a manual in front of them.

If you're looking at it in a now vs. later point of view, yes, later is fine. It will allow you to drive the car some with the existing gear set and decide if you like the ratio or not, and what may be better. then it's time to upgrade to a better (stronger) rear diff before you dump a bunch of $$ into one that's just going to blow up.

Last I looked, a good rear was running about $2500 complete, with posi, good axles and brakes. Pick your manufacturer, GM 12 bolt, Ford 9" or Strange (improved) Dana60 (S60).

Devin
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