Diff fluid change - ZR-1 Net Registry Forums

garyhugo · 04-29-2016

I'm going to change the fluid in my diff. I have a Dana 44 4.09 gear set. I will use Mobile 1 75W90 fluid. My question is, do I use an additive? If so which one? How much?

Gary

Racinfan83 · 04-29-2016

No additive with Mobil One. I just did mine while I had it up on stands with trans and clutch out. If you don't have a differential drain plug already - I suggest you do it when you change it.

http://www.zr1.net/forum/showthread.php?p=239541

Having just done this - I bought the 1/8 NPT tap on EBay for like $6 and two of the 1/8 NPT plugs on EBay for like $5. I already had the correct 11/32 drill bit. I drilled the thing with it full, using a cordless right angle drill and a drain pan under it. Suggest using a smaller bit to do a pilot dent before using the 11/32 as mine walked a bit...

It took about 1 1/2 liters. FYI it is a Major pain in the

to fill it. Maybe if you have a clean suction gun you can suck new fluid out the bottle and shoot it in there. I didn't have one, tried a couple different hoses attached to the bottle and still made a big mess. So good luck...

Dynomite · 04-30-2016

Quote:

Originally Posted by Racinfan83

No additive with Mobil One. I just did mine while I had it up on stands with trans and clutch out. If you don't have a differential drain plug already - I suggest you do it when you change it.

http://www.zr1.net/forum/showthread.php?p=239541

Having just done this - I bought the 1/8 NPT tap on EBay for like $6 and two of the 1/8 NPT plugs on EBay for like $5. I already had the correct 11/32 drill bit. I drilled the thing with it full, using a cordless right angle drill and a drain pan under it. Suggest using a smaller bit to do a pilot dent before using the 11/32 as mine walked a bit...

It took about 1 1/2 liters. FYI it is a Major pain in the

to fill it. Maybe if you have a clean suction gun you can suck new fluid out the bottle and shoot it in there. I didn't have one, tried a couple different hoses attached to the bottle and still made a big mess. So good luck...

You really need one of those Fluid PUMPs to pump Fluids into the Transmission and Differential housings.

See Also Item #4 LT5 Added Systems
Post 147 - LT5 Added Systems

The Mobile 1 75W-90 has LS on the bottle (Limited Slip).

ENGINE OIL (LT5) ................. TRANSMISSION OIL (ZF S6-40)..........DIFFERENTIAL OIL Limited slip

8.6 quarts (with filter change) .....4.4 pints - Castrol TWS 10W-60 ............3.2 pints - Mobil 1 - 75W-90 Gear Lube
Amsoil 10W-40 AMO Motor Oil .................................................. ........or (Since you are ordering Amzoil anyway)
Bosch 3401 Filter.................................................. .........................3.2 pints - Amsoil - Severe Gear 75W-90
FRAM TG 3985 Filter.................................................. ......................1 Bottle - Amsoil - Slip Lock Additive
Mobile 1 M1-207 Filter

........

.

Post 3 - LT5/ZR-1 Fluids

Hib Halverson · 04-30-2016

Here's the reality of rear axle lube "additives".

All of them are "friction modifiers".

Limited slip differentials in Corvettes use a wet clutch under pressure to limit the slip rate between the two rear wheels under certain condition. The clutch uses flat clutch plates. The slip limiting action depends on friction between those clutch plates. It follows that the level of friction determines the level of slip limiting. To much friction and the plates may not slip enough or they might chatter, which actually is their tendency to repeatedly lock then slip a little which happens rapidly and which may be heard as a squealing noise or, worse, feel like the rear end is "chattering". Not enough friction and the limited slip unit does not function properly.

Because these clutch plates tend to chatter under certain conditions so, way back in the dark ages, GM and other makers of clutch-type limited slips, confronted with customers coming into dealers crying about rear ends which make noise chatter in turns wanting the rear ends fixed under warranty, developed "friction modifying" additives which they put in the rear end lube.

While these additives do modify friction such that, in a rear axle which is in good condition, chatter and noise are eliminated, you have to remember that they also degrade limited slip action, so you only want the amount additive needed to stop the chatter. To much additive is often the case because users of these product are instructed to put the whole bottle in. That instruction is based on these products being created to solve warranty problems.

My advice on limited slip additives is to 1) only use them if you have noticeable noise or limited slip chatter, 2) use the additive recommended by your lubricant supplier, ie: if you use a petroleum based rear axle lube, the GM additive will work but if you use a synthetic, such as Red Line, which makes its own additive, use Red Line's additive and 3) Use only enough additive to stop the chatter and no more. Done right this means, rather than adding a whole bottle, adding an ounce of the stuff then road testing and adding more only if the chatter is still present.

The best way to road test for limited slip noise or chatter is to first drive the car for ten miles or so to bring the lube up to operating temperature. Then, find a big parking lot with no obstructions and drive the car in slow tight circles in both directions. If you hear noise or feel chatter, use the additive. If you get beyond a full bottle of the stuff, you likely have a high level of wear in the rear axle and need an overhaul.

My rear axle lube of choice in Corvettes is Red Line Heavy Shockproof gear lubricant. I put enough additive in my rear axles such that I am left with just a faint limited slip noise in very tight turns. That tells me I've got the friction level between the clutches just right. Note that I only had to use Red Line Friction Modifier in C4 axles. I have never needed it in either my C5 or my C6.

garyhugo · 04-30-2016

Thanks, racinfan83 and Hib. I think you covered the whole subject. Dynomite one of those fluid pumps is in my future.

Gary

Young1 · 05-20-2021

I am bringing this post back to discuss intervals for rear diff fluid changes. Interval for street and mountain driving? Interval for tracking? Interval for drag racing? My 08 F150 owners manual says no change needed, fluid is good for lifetime. Hmmm. What is best interval for our Z limited slips?

efnfast · 05-20-2021

I do mine annually. It's just the rear diff.
Nice write up Hib, thanks.

Marc Haibeck · 05-22-2021

The C4 Corvette service manual does not specify a lubricant change interval for the differential.

Debris gets shed into the lubricant during the break-in period. I like to replace the lubricant after the break-in is complete. I figure that the break-in is complete after 1000 miles. If you do that, you will find the lubricant is colored dark black. Install new lubricant. After that you will find that the lubricant gets dark very slowly. I run the second fill for 100k miles or 20 years.

I learned to do it this way from a '95 Mercedes 320E service manual.

Young1 · 05-23-2021

Thanks Marc. It was black when I changed it out at 35,000 miles from original owner. She has 65,000 now so good to go for a while yet.

04-29-2016	#1
garyhugo Join Date: Feb 2010 Location: Jacksonville,FL Posts: 242	Diff fluid change I'm going to change the fluid in my diff. I have a Dana 44 4.09 gear set. I will use Mobile 1 75W90 fluid. My question is, do I use an additive? If so which one? How much? Gary __________________ Gary 90-1572

04-29-2016	#2
Racinfan83 Join Date: Apr 2013 Location: Eastern Missouri Posts: 773	Re: Diff fluid change No additive with Mobil One. I just did mine while I had it up on stands with trans and clutch out. If you don't have a differential drain plug already - I suggest you do it when you change it. http://www.zr1.net/forum/showthread.php?p=239541 Having just done this - I bought the 1/8 NPT tap on EBay for like $6 and two of the 1/8 NPT plugs on EBay for like $5. I already had the correct 11/32 drill bit. I drilled the thing with it full, using a cordless right angle drill and a drain pan under it. Suggest using a smaller bit to do a pilot dent before using the 11/32 as mine walked a bit... It took about 1 1/2 liters. FYI it is a Major pain in the to fill it. Maybe if you have a clean suction gun you can suck new fluid out the bottle and shoot it in there. I didn't have one, tried a couple different hoses attached to the bottle and still made a big mess. So good luck... __________________ [LEFT][SIGPIC][/SIGPIC][/LEFT] [CENTER][COLOR=Indigo][SIZE=4][I][B] Now Sold 1990 ZR-1 #2093, Black on Black..[/B][/I][/SIZE][/COLOR][/CENTER]

04-30-2016	#5
garyhugo Join Date: Feb 2010 Location: Jacksonville,FL Posts: 242	Re: Diff fluid change Thanks, racinfan83 and Hib. I think you covered the whole subject. Dynomite one of those fluid pumps is in my future. Gary __________________ Gary 90-1572

05-20-2021	#6
Young1 Join Date: Sep 2016 Location: Ocala, FL Posts: 899	Re: Diff fluid change I am bringing this post back to discuss intervals for rear diff fluid changes. Interval for street and mountain driving? Interval for tracking? Interval for drag racing? My 08 F150 owners manual says no change needed, fluid is good for lifetime. Hmmm. What is best interval for our Z limited slips? __________________ Steve Young 1991 ZR-1 Quasar Blue Metallic/Black #729 Second Owner 1991 ZR-1 Dark Red Metallic/Black #2040 SOLD 1990 ZR-1 Dark Red Metallic/Saddle #1670 SOLD Double your happiness.....complain half as much.

05-20-2021	#7
efnfast Join Date: Nov 2011 Location: Dunbarton NH Posts: 7,486	Re: Diff fluid change I do mine annually. It's just the rear diff. Nice write up Hib, thanks.

04-30-2016	#4
Hib Halverson Join Date: Jun 2005 Location: CenCoast California Posts: 899	Re: Diff fluid change Here's the reality of rear axle lube "additives". All of them are "friction modifiers". Limited slip differentials in Corvettes use a wet clutch under pressure to limit the slip rate between the two rear wheels under certain condition. The clutch uses flat clutch plates. The slip limiting action depends on friction between those clutch plates. It follows that the level of friction determines the level of slip limiting. To much friction and the plates may not slip enough or they might chatter, which actually is their tendency to repeatedly lock then slip a little which happens rapidly and which may be heard as a squealing noise or, worse, feel like the rear end is "chattering". Not enough friction and the limited slip unit does not function properly. Because these clutch plates tend to chatter under certain conditions so, way back in the dark ages, GM and other makers of clutch-type limited slips, confronted with customers coming into dealers crying about rear ends which make noise chatter in turns wanting the rear ends fixed under warranty, developed "friction modifying" additives which they put in the rear end lube. While these additives do modify friction such that, in a rear axle which is in good condition, chatter and noise are eliminated, you have to remember that they also degrade limited slip action, so you only want the amount additive needed to stop the chatter. To much additive is often the case because users of these product are instructed to put the whole bottle in. That instruction is based on these products being created to solve warranty problems. My advice on limited slip additives is to 1) only use them if you have noticeable noise or limited slip chatter, 2) use the additive recommended by your lubricant supplier, ie: if you use a petroleum based rear axle lube, the GM additive will work but if you use a synthetic, such as Red Line, which makes its own additive, use Red Line's additive and 3) Use only enough additive to stop the chatter and no more. Done right this means, rather than adding a whole bottle, adding an ounce of the stuff then road testing and adding more only if the chatter is still present. The best way to road test for limited slip noise or chatter is to first drive the car for ten miles or so to bring the lube up to operating temperature. Then, find a big parking lot with no obstructions and drive the car in slow tight circles in both directions. If you hear noise or feel chatter, use the additive. If you get beyond a full bottle of the stuff, you likely have a high level of wear in the rear axle and need an overhaul. My rear axle lube of choice in Corvettes is Red Line Heavy Shockproof gear lubricant. I put enough additive in my rear axles such that I am left with just a faint limited slip noise in very tight turns. That tells me I've got the friction level between the clutches just right. Note that I only had to use Red Line Friction Modifier in C4 axles. I have never needed it in either my C5 or my C6. __________________ Hib Halverson Technical Writer former owner 95 VIN 0140 current owner 19 VIN 1878 Last edited by Hib Halverson; 05-01-2016 at 11:48 AM.

05-22-2021	#8
Marc Haibeck Join Date: May 2007 Location: Addison IL Posts: 250	Re: Diff fluid change The C4 Corvette service manual does not specify a lubricant change interval for the differential. Debris gets shed into the lubricant during the break-in period. I like to replace the lubricant after the break-in is complete. I figure that the break-in is complete after 1000 miles. If you do that, you will find the lubricant is colored dark black. Install new lubricant. After that you will find that the lubricant gets dark very slowly. I run the second fill for 100k miles or 20 years. I learned to do it this way from a '95 Mercedes 320E service manual. Last edited by Marc Haibeck; 05-23-2021 at 03:24 AM.

05-23-2021	#9
Young1 Join Date: Sep 2016 Location: Ocala, FL Posts: 899	Re: Diff fluid change Thanks Marc. It was black when I changed it out at 35,000 miles from original owner. She has 65,000 now so good to go for a while yet. __________________ Steve Young 1991 ZR-1 Quasar Blue Metallic/Black #729 Second Owner 1991 ZR-1 Dark Red Metallic/Black #2040 SOLD 1990 ZR-1 Dark Red Metallic/Saddle #1670 SOLD Double your happiness.....complain half as much.

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