Car was chattering a bit on right turns on slight upward incline. About 112K on the Odometer, seems likely to be accurate (bought from prior owner w 92K about 4 years ago, with Goodyear Polysteel tires on it - which I changed immediately and only mention as tending to support that 92K was probably the correct mileage). No indication of side yolks wearing, at least not yet. My last 78SA had that problem, complete with gouges into the differential case from wearing yokes - not seeing that on current SA.

So, anyway, changed fluid myself. Jacked up car, removed spare and carrier, used fill port and sucked out the old fluid, dark and dirty, filled with 80 90 GL5. From reading posts on here, decided not to use additive. Many seemed to indicate it isn't needed in 2017.
Now the car seems to make more noise from the differential, and also seems to require a very slight bit more pressure on the accelerator, and slows down pretty quickly if foot is lifted off gas.

So...does anyone KNOW what exactly the friction modifying additive for positraction actually does? Please, no guesses or opinions, I am asking for anyone who actually KNOWS what this additive does to reply. I am wondering if it reduces the viscosity of the gear oil a bit. I can't imagine that it increases it.

Or maybe the additive does something else. If it 'modifies friction' does it increase or decease it, and how. Anyone who KNOWS what the additive does, I'd love to have you share your knowledge...

Before I got through the hassle and mess of going back and adding in the additive, I'd like to know, really know, what it does, and by extension if the increased noise and possible increase in need for accelerator pressure might be corrected.

Thanks in advance for anyone who can help.

 

Oh, and from this website, tho nothing on what the additive does
https://www.ringpinion.com/Technical...ini-spools.inc

Eaton (EAT)
Clutch type, similar to Trac-Lok in design, yet far stronger and more aggressive. These units came as an original equipment option in 65-72 12T (12 bolt GM truck), 12P (12 bolt GM passenger), 8.2" GM pass, 63-79 Cast Iron Corvettes. The original OEM 12T case was very weak, but the new design replacement from Eaton is very strong. Four preload springs and two steel preload spring plates. Latest design uses HD nodular iron case, forged side and pinion gears, no-chatter, race-bred, carbon fiber clutches for extreme strength and longevity.

Old design, all steel clutches which are still available in 18 or 22 clutch designs for those who need more lock-up and are not bothered by clutch chatter. For even more lockup customization, there are also four levels of spring pressure available (3 from Eaton and a fourth from GM) that can be used to vary the clutch preload from 200 lbs. to 800 lbs. Originally used in mid-sixties GM muscle cars and trucks. This unit is now available for Ford applications and soon will be available for Dana Spicer, AMC, and Chrysler applications. They have a high tolerance for abuse in high horsepower vehicles. Recommended for everyday driving and severe off-road use (factory GM).

 

I have used the GM Posi additive every time . I never found a reason not to.

And since I have never heard the noises you describe I will continue to use it each time.

I have changed the fluid at least 3 times in the past 5 years, since I have changed the side yokes a few time due to wear or breakage.

I also use the same GM Gear oil / 2017 year replacement.

 

I don't claim to know exactly what it is about the additive that works, but it is there to lubricate the posi clutches. Without the additive, the clutches will stick, resulting in a chatter when turning. It won't have any effect on the differential when travelling in a straight line. And yes you need it in 2017.

 

Don't have an answer for you on the GM additive but it sounds like with 112K on the diff you are experiencing a mechanical issue relative to the diff if the sound is different. As far as the car slowing down with your foot off the gas and acceleration takes more throttle, I doubt changing to a GL5 would cause your issues, but who knows. Is the GL5 synthetic or conventional oil? If conventional oil you would definitely need the GM additive.

Have you tried getting the car up to speed and coasting in neutral to access if the problem is from the diff or the engine braking/tranny?

I have used Mobil 1 Synthetic 75W-90 for 27 years in my diff with zero issues and zero chatter with no additive. The differential is quiet, with smooth posi traction clutch engagement, and no problems....

I have been using Synthetic Redline and Mobil 1 75W-90 for over 30 years in the 4 speed tranny as well but I suspect your 78 is an auto since most were back then