Is there anybody out there with a 2.56 posi for a 68 Skylark. Is there one from Jegs that will replace the open differential one or is it ....unobtanium?
I got lucky and picked up one locally. I am running it in our '68 skylark. They are out there just not common.
A 2.73/2.78 posi would be much more common. Could you get the posi unit (carrier) from a one of those and put the 2.56 gears on it? Do you really need to worry about breaking the tires loose with 2.56 gears?
Traction? I have a 69 Pontiac LeMans with 350 2bbl rated at 265hp, Turbo 400 and 2.56 gears. That is a great road gear and would not run anything else if driven on the highway. On smooth pavement full throttle launch, it would break loose the right rear and smoke with bias ply tires, not so much with radials but still hampered 60 ft times. I swapped in a 3.60 posi from a GTO and cured that problem but learned how dangerous they can be after being used to an open differential. Power drift when street racing and nearly smacking a curb was scary, not to mention cornering in the rain! I soon put my 2.56 back in because it was my daily driver. Reduced fuel mileage overrode need for performance. We are now building a 455 and the posi may be a necessity, But an overdrive lock up trans will offset 3.60 gears. I warned my son about posi differential characteristics when he built his '67 F100 short bed with a 429 with a C6 and posi. He found out the hard way, Driving home in a light rain on a long sweeping curve on the highway, he rolled into the throttle running 60 mph and swapped ends. Luckily he slid off the roadway into the grassy median and looked out his back window to steer until he got it stopped, amazingly hitting nothing. No harm done but a lesson well learned. And this was without downshift, he was still in drive.
RAtio depends on the camshaft profile your engine uses. IF you have 230 + duration in any size motor you will be required to have steeper gears 3.73 up ish and a stall usually to have any kind of good low end torque out of the hole. This is because the power band of the motor is now 2500-6000 ish and if your stock stall is 1350 you can see it will just drag before it gets to the 2500 area where the torque numbers are starting produce the torque needed to move 3500 # quick out of the hole... So a stock cam has IE 190 HP BT 270-300 FT LBS. torque in the 2000 rpm range.. That same torque is produced from the longer duration cam in the 3000 RPM range So you can see the stall convertor andf gears will be needed to quickly zip past the crappy low RPM range to get thto the same Torque numbers. NOW the longer the duration cam with more lift OF course has the capability to produce HIGHER torque output rating and HO ratings...BUT this all comes with MUCH more RPMS. So you really need to pick a cam where you spend most of your time and or use. DRAG RACE -well we spend all our time FULL THROTTLE and 1/4 mile to 1/8 runs. You can see the Stock cams always produce VERY GOOD low rpm torque and can burn out any ratio as long as you have the cars motor working correctly. IF you replace a carb on a small cam you can sometime kill the off line performance also. MATCH your parts and you will have a fun ride. DO NOT be fooled by HP numbers that camshaft companies advertise. LOOK where the torque comes in at and how much it gives you. 260 - 268 cam shafts are BRUTE force torque with some 3.73 gears will tear the road apart with about 200-2500 stall and still give you a decent low 14 - high 13, 1/4 mile with flat tops regular gas and well maintained tuned ignition .... 327-400 cube motors 3500# cars. 68-70 Buick 2.56 gears Might have a set and also we make NEW posi units for fit these rears 2.56 2.73 2.93 3.23 3.42 We can cover all these ratios for posi set-ups in the 68-70 Buick rear. Thanks, Jim JD
Thanks Jim, that is what I needed to know. I've got a couple projects for you - a 3.64 and maybe the 2.56. I'm not running a long duration cam, just want a cruiser gear.
I have a 69 Firebird with the same engine (350/265) and just put a 200R4in it with 3.08 open rear. I don't drive in the snow or drag race so I'm happy with an open rear. I've heard other stories about cars going straight on clover leafs in light rain with posi rears. The only car I own with a posi rear is my 66 GS which is not running yet. I recently had to move it in my shop around some sharp corners and it took four guys to push it. It didn't like going anything but straight. That has to hurt the handling. I guess that's why NASCAR runs Detroit lockers with ratchets.
??? posi will not bind up like a spool. You must have something wrong with that rear. FULL spooled rear diff cars rear will act like that. My little race car is 2600# but if you turn the front wheels and push if feels like a semi truck.
Say What ?? ... I've driven NOTHING BUT Posi rear wheel drive cars as my daily drivers (here in NY where it rains and snows) since I got my license back in 1979 logging well over 1 million miles and have never had a problem 1979 - 1987 = '69 Sky with Posi, 200,00+ miles driven 1987 - 1989 = '84 ThunderBird TurboCoupe = 175,000+ miles driven 1989 - 1993 = '88 ThunderBird TurboCoupe = 140,000+ miles driven 1993 - 1999 = '91 Lincoln Mark VII = 220,000+ miles driven 1999 - 2005 = '94 RoadMaster Wagon = 180,000+ miles driven 2005 - 2009 = '96 RoadMaster Wagon = 85,000+ miles driven 2009 - present = GMC Envoy XUV - 55,000+ miles driven ... add to that, owning BigRed for 20 years averaging 5-7K per year
I know, Ive been around racing since the 60s modifieds, midgets, sprints, legends, late models. This 66 GS has been sitting for probably 20+ years. I guess something could be stuck. Maybe it will break loose once it's driven. I don't know if it's a cone or clutch type posi unit. I know a guy who has a business rebuilding rears and he leaves the springs out of the cone type rears so it's more like an open rear under light acceleration. When you get on it the spider gears push out on the cones and it becomes a posi.
Limited slip (posi) differentials I am familiar with require a friction modifier be added to the GL5 lube or they will get tight in a corner without adding power, sometimes to the point of squeaking the tires on slick pavement. (Not to be confused with a locking differential) They require a speed differential to spin a flyweight and latch the clutch pack. TexasJohn ---------- Post added at 08:16 PM ---------- Previous post was at 08:00 PM ---------- Power drifting is the nature of the beast with a limited slip. By design, reduced torque to one wheel engages the clutches and tries to lock the differential to balance the torque. I fail to see how a posi will not drift under power, especially with limited traction like wet pavement in a curve. An open differential will break loose one wheel leaving the other side to maintain traction and prevent side slip in a curve. A posi on the other hand, when enough power is applied to break one tire loose automatically results in the other tire breaking loose also and the tail end comes around quicker than you can lift the throttle on wet streets. With an open differential, it will simply spin one tire and less likely to side slip. Someone who has driven nothing but a posi car may not have realized this or doesn't drive like a teenager on wet streets.
DID you ever find your 2.56 gears? What year car and rear do you have? I have a few sets here just need to know the car rear. Jim
Hi Jim, I tried to call but have not connected yet. I have the gears, they are in a '68 housing. The carrier is what I was after...for a 68. I will bring it up to you someday...along with a 69 rear 3.64.
Specialwagon, Do you still want a 2.56:1 posi? I have a 12-bolt 2.56 posi from my '69 that I would like to sell. -Bob C
I have a 700 RWHP 68 Camaro with a detroit locker, 3.55 gears and a 3000 RPM stall and 390x17 drag radials that are pretty worn, and I've put 6000 miles on it with more than 50% in the rain, the only time it's broken traction is when I'm being an idiot and putting my right boot into it. My view is that losing traction has way more to do with driver behaviour than it ever has to do with the equipment. I did not know they do 2.56:1 though. For the Buick I'm looking at a 200R4, fuel injection and a 2.xx rear gear, goal double my fuel economy (daily driver circa 100 miles per day)
A "locker" and a posi and 2 different animals and have different characteristics. "Limited slip (posi) differentials I am familiar with require a friction modifier be added to the GL5 lube or they will get tight in a corner without adding power, sometimes to the point of squeaking the tires on slick pavement. (Not to be confused with a locking differential) They require a speed differential to spin a flyweight and latch the clutch pack. TexasJohn"
