Posi Surprise

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Posi Surprise

Post by crice63 on Sun Jan 29, 2017 3:29 am

I've been slowly inventorying the parts car (73 Laguna). Yesterday finally got under it and cleaned off the axle stamp to see what kind of rear it had. Not much visible, started with AC--2.73, so nothing exciting I guessed.



But the more I looked at the photo, I seemed to see some signs etched over the dim 5. I could swear it looked like the word posi plus some other letters. Wishful thinking? So today I went out and polished it some more, and though it was not clear at all, it still seemed like something.



So I went ahead and pulled off the differential cover. Nice surprise.







I'm not sure what the Richmond numbers mean, or exactly what has been written on the gear by the installer, but maybe someone here can decode it. I rotated the wheels to check the gear sizes, watching the inside turn as well. The wheel went around twice to get the ring gear to go around once, and that took just a hair over 4 rotations of the drive shaft, so I'm guessing it's a 4.10. I did not guess that the parts car had this, so it's like a late Christmas gift. cheers (I thought one checked gears by rotating the wheel once--which would give me 2-1. How does the wheel spin twice while the ring gear spins only once?)

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Re: Posi Surprise

Post by Joe73 on Sun Jan 29, 2017 8:35 am

Thats a cool find. The 273 on the housing would not be the gear ratio. Housing stampings are in coded letters. So you would have to break that down. Unless someone else stamped it.
Cool find having some 4.10s in there. The posi unit is a factory gm piece.

I would not run that rear until it is fully disassembled and all the rust removed or it will eat itself alive. That ring gear is going to have some good pitting when done. But after a good cleanup, I'd run it, when it blows it blows.

As for the 4.10 ratio, not very streetable. I still have 4.10s in my car and its really not fun cruising. Even if you put in an overdrive, it will be a bit much. With my 4 speed, I usually take off from a light in 4th gear with no bog. But it sure is great on a launch.

Good luck with it and nice find.
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Re: Posi Surprise

Post by fasrnur on Sun Jan 29, 2017 10:30 am

I wish I could get so lucky. lol Nice find indeed. If it is a 4:10 ratio, it should have stamped somewhere on the ring gear 41 10.
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Re: Posi Surprise

Post by Joe73 on Sun Jan 29, 2017 11:51 am

I agree. Try to get the numbers off the pinion gear OR try to count the pinion teeth. Then you can get an accurate ratio.
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Re: Posi Surprise

Post by crice63 on Mon Jan 30, 2017 12:40 am

Update. Here is a pic of the pinion gear. It has 10 teeth. I counted 41 on the ring gear, so 41/10 = 4.1. Didn't even need a calculator for that.



There were no other stamps on the ring gear that I could see beyond what is in the photo up top, with Richmond and the numbers. No 41 10 anywhere I could see. I did not look between the teeth, just on the outside rim.

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Re: Posi Surprise

Post by brownbomber77 on Mon Jan 30, 2017 5:25 pm

Dang son....that's awesome

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Re: Posi Surprise

Post by BBMALIBU on Fri Feb 03, 2017 2:22 pm

Joe73 wrote: As for the 4.10 ratio, not very streetable.

It is in my car Very Happy
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Re: Posi Surprise

Post by Joe73 on Fri Feb 03, 2017 3:27 pm

Me too. Fun but not good for cruising.
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Re: Posi Surprise

Post by fasrnur on Fri Feb 03, 2017 5:07 pm

Yeah 4.10's aren't fun for cruising, but what the heck. Let the motor sing to ya.  GoFast
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Re: Posi Surprise

Post by thatfnthing on Fri Feb 03, 2017 5:21 pm

fasrnur wrote:Yeah 4.10's aren't fun for cruising, but what the heck.

X2 on that. My cousin has a 72 Camaro with 4.10 gears and a TH350. At 45 mph the motor is over 3000 RPM.
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Re: Posi Surprise

Post by BBMALIBU on Fri Feb 03, 2017 5:50 pm

thatfnthing wrote:X2 on that.  My cousin has a 72 Camaro with 4.10 gears and a TH350. At 45 mph the motor is over 3000 RPM.  

I'm running a 29" tall rear tire and at 45 mph.....my RPM's are about 2000 - 2200.

How tall is his rear tire?
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Re: Posi Surprise

Post by thatfnthing on Fri Feb 03, 2017 6:51 pm

I just texted him to find out. We'll see if he gets back to me before I leave town. I know they're not that tall, though.
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Re: Posi Surprise

Post by Joe73 on Fri Feb 03, 2017 7:28 pm

I was running 26" rears and I was 3000+ on the highway. Sold those tires and I'll be going with 28" this time around. My opinion is that our cars need at least 28" diameter to fill the wells.
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Re: Posi Surprise

Post by fasrnur on Fri Feb 03, 2017 9:10 pm

I agree Joe, the bigger the better it looks.
Maybe this will help others.

http://www.g3gm.com/t4096-tire-gear-ratio-guide?highlight=gear+ratios
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Re: Posi Surprise

Post by crice63 on Sat Feb 04, 2017 12:48 am

I'm running 255x60R15 cobras, about 28" diameter. With my TH350, according to the chart, on 4.11 gears I'd be ~3200rpm @ 65mph, a little bit high. Right now, and foreseeably, this is a daily driver. I just love driving it, don't want to leave it in the garage. My plan was to go 200-4R/3.42, to keep the gas mileage reasonable while still getting some pick-me-up when needed. I have the 700-r4 in my s10 2.8 liter/3.07 rear, and it just feels really nice to drop into overdrive at about 55 and then accelerate up to 70. Considering how much better the Elco feels on the road all around, I can imagine that overdrive would be very satisfying at highway speeds.

But having the 4.10 in hand now is really tempting. I mean, from a 73 laguna to a 75 elco, it's as straight a swap as such a thing can be, right? Why not try it for the summer?

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Re: Posi Surprise

Post by Joe73 on Sat Feb 04, 2017 8:13 am

Definitely try it out. But pull the position and the axles and remove all the rust or it will eat itself alive. And check those outer bearings for rust as well.
When you do go overdrive trans, go 3.73 out back. You'll be very happy with that. Especially with the overdrives low first gear. Best of both worlds.
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Re: Posi Surprise

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