I have an 8.8 in the hot rod I'm building and recently had to drain it to pull the axles out. I have no idea what was in it. It seems like Ford changed recommendations from 75w90 to 75w140 somewhere along the way or depending on the vehicle. Also says to use some kind of friction modifier. All other forums are full of arguing and anecdotal evidence about what to use synthetic vs not, etc etc, so I figured I would trust GRM's anecdotal evidence. What would you fill it with?
I put good quality 80w90 in all my 8.8's, and I don't ever add friction modifier. There is no witchcraft or voodoo going on in there.
Friction modifier allows the clutch plates to slip more easily past each other without chattering; it is an NVH thing. Why would I want my clutch plates to slip easier?? Besides, most high end gear lubes have some level of friction modifier component in them anymore.
Kind of depends on what diff you're using. If it's something like a factory Trac-Lok, I'd use either conventional 80W-90 or synthetic 75-140 and add the friction modifier (unless the synthetic already has the additives in it, like Mobil1).
That said, I have an Eaton TrueTrac in my Supercoupe. I followed Eaton's recommendations and use 80W-90 conventional oil with no friction modifier.
gumby said:I put good quality 80w90 in all my 8.8's, and I don't ever add friction modifier. There is no witchcraft or voodoo going on in there.
Friction modifier allows the clutch plates to slip more easily past each other without chattering; it is an NVH thing. Why would I want my clutch plates to slip easier?? Besides, most high end gear lubes have some level of friction modifier component in them anymore.
Depends on the application. I tried using Redline NS with no additive in my 2011 F-150, and the chatter drove me nuts. Added half a bottle of Ford friction modifier, and it shut right up. In a racing application where lockup is more important, I'd be willing to skip it, but not in a daily.
Other than that, any major-brand 75W-90 synthetic gear oil should do the trick.
Thanks guys, I'll probably go with whatever 75w90ish is on sale. If it chatters, I'll add the extra stuff. This place is the best!
IIRC the switch to 75w140 coincided with a switch to synthetic.
You're also supposed to fill no higher than about 9/16" from the fill hole when using synthetic. Absolutely nobody does this. (I assume it is for reduced windage losses and half a tenth of a hogshead per furlong)
Synthetic is kind of a waste in a diff. The big difference with synthetic in an internal combustion engine is that the oil starts as a pure, lab-based bunch of molecules unlike mineral oil which starts as a million dirty things and they filter out just what they want.
It's like the difference between distilled water and spring water. Starting with synthetic means that the engine oil has more "space" to dissolve more junk, but the oil molecules are primarily the same. Synthetic isn't more slippery than regular oil, and in a differential it doesn't have all the chemicals and crap from blowby. I use synthetic in engines I care about, but the synthetic craze is getting out of hand. Do you know they sell synthetic brake fluid and synthetic coolant too? Newsflash... they're synthetic to start with.
I currently have 80w90 mineral oil in both of my 8.8s. They both have Ford's friction modifier in them. In my opinion, the friction modifier is an absolute must. It does not change torque bias. It changes the threshhold at which they begin to slip. Without it, if it starts slipping at X torque, it just bangs and chatters as it slips. With friction modifier, it still starts slipping at X, it just does so smoothly without hitting your bearings, axle splines, and ring gear with a 50 lb sledge hammer.
Besides, the last thing I want in my traction routine is a chattering torque bias between the wheels.
gumby said:I put good quality 80w90 in all my 8.8's, and I don't ever add friction modifier. There is no witchcraft or voodoo going on in there.
Friction modifier allows the clutch plates to slip more easily past each other without chattering; it is an NVH thing. Why would I want my clutch plates to slip easier?? Besides, most high end gear lubes have some level of friction modifier component in them anymore.
They don't slip easier with friction modifier, otherwise it would be called a lubrication enhancer. It modifies the threshold between grip and slip to prevent damaging chatter.
The way it works is by broadening the range of torque that will cause the clutches to START to slip, not the overall torque bias. Without it, you're potentially losing traction and causing mild damage to differential parts. You reach a certain torque bias, the clutches "jump" at which point you have zero bias until you continue adding more. With the friction modifier, they don't jump. They consistently apply the proper bias the whole time. With as many diffs as I've pulled apart, I can tell instantly if the owner used additive with their clutch-type limited slip. Without it, the clutches look like an abused brake disc with cracks and fissures in the friction material.
If I want chatter, I'll get a toothed locker which I will never do again after rolling a friend's Jeep.
In reply to Curtis73 (Forum Supporter) :
The synthetic's higher lubricity keeps temps down, which is why they say you can use less oil in the diff. Less thermal sink needed.
I don't mind a chattery diff, to me that tells me that it is working. If it chatters that just means I am not on the power enough to be accelerating through both tires. I have used tight limited slips that barked a lot, and welded diffs, and now a Detroit Locker. The Locker makes it apparent how much the slip ratio is apparent when driving, as any significant throttle in any radius corner will make the diff shut up, proof that the rearend is "locked up" and both rear tires are being driven, meanign the inside tire is slipping a lot.
If it is a problem on the street, then that is just a sign that you should be using an open diff and not something that compromises drivability. Most peoples' egos cannot handle this.
Here. I made a quick chart.
Let's assume you're moving forward in a turn. The blue line is with friction modifier, the red is without it. Used random numbers, but this is pretty visually spot on based on the tech
Friction modifier doesn't reduce the overall torque bias, in fact (on the average given the area under the curve) it improves it.
The red (without) reaches a the torque at which it would slip and skips/chatters leaving you with no bias. Then it ramps up again as you continue the curve. The blue (with) reaches the torque at which it would slip and slowly and smoothly gives way and maintains that torque bias.
The friction modifier may reduce peak bias by a tiny bit (as shown by the green arrow), but it adds all of the area lost under the curve (shown by the purple arrow) You can also guess by the shape of the red line, that friction modifier also eliminates all the NVH shown by the purple arrow.
USE FRICTION MODIFIER.
Pete. (l33t FS) said:In reply to Curtis73 (Forum Supporter) :
The synthetic's higher lubricity keeps temps down, which is why they say you can use less oil in the diff. Less thermal sink needed.
I don't mind a chattery diff, to me that tells me that it is working. If it chatters that just means I am not on the power enough to be accelerating through both tires. I have used tight limited slips that barked a lot, and welded diffs, and now a Detroit Locker. The Locker makes it apparent how much the slip ratio is apparent when driving, as any significant throttle in any radius corner will make the diff shut up, proof that the rearend is "locked up" and both rear tires are being driven, meanign the inside tire is slipping a lot.
If it is a problem on the street, then that is just a sign that you should be using an open diff and not something that compromises drivability. Most peoples' egos cannot handle this.
That's like saying "I don't use novacaine for minor surgery because the pain means the doctor is actually cuttin' sumthin." Or "I'm going to dump cow dung on my open wound so the infection reminds me it's there." 
It's not an ego thing, it's math and physics. I don't mind chatter either. I had a Detroit Locker for years. I didn't mind the chatter because I knew that it was supposed to chatter at the expense of other driveline parts. But intentionally giving driveline parts incorrect lube so you can feel the chatter to know it's working is not a logical step in my opinion. You're talking the difference between 30 lbs of hardened tool steel vs 1/8" of organic friction material.
Not trying to be a jerk, just pointing out that your logic doesn't make sense to me as someone who has worked in the driveline industry for nearly 30 years.
In reply to Curtis73 (Forum Supporter) :
I have a Dwtoit Locker in a tool designed to put power down on dirt, as a priority over everything else.
The vehicle I use to drive on the street, as opposed to "I drive it on the street to get me to races", has an open diff. Well, two open diffs, and a computer controlled clutch between the front and the rear, and computer controlled brake activation to force a torque bias to the ground at every corner.
You engineers what you needs and you accepts the compromises.
Use the friction modifier, which coincidentally is some of the stinkiest stuff I know of.
Also, I have a Cobra IRS 8.8 center section I'd trade of a Camaro Getrag or sell. It's 31 spline with 3.55 gears.
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