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Discussion Starter · #1 ·
I installed the steeda finned diff cover on my 2014 GT, and I'm trying to figure out how much fluid to add. I know it's to the top of the stock fill plug but the car is only jacked from the back and I want to fill it from the plug in the new cover. Getting mixed results on the capacity. Owners manual says 4 pints. This LMR link says 2.125qt and 2.625qt for boss or track pack:
This link to the shop manual says 2.5L (=2.625 qt):

I'm wondering if the boss is different b/c it uses this cover or maybe b/c of the torsion? If it's the cover then I would need to use the boss spec?

Any thoughts?
 

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maybe call Steeda?

or have a little extra on hand and fill it until it is full; sounds like you need 3 quarts
 
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Discussion Starter · #3 ·
OK I decided to jack up the front and fill it until it started coming out the plug on the front side of the diff. I took 3 quarts minus 1 1/3 cups. The Steeda cover is basically the same thing as the boss / track pack cover.

...also, I had it completely bone dry before I put the new cover on
 

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I have the cover too (with Lube Locker gasket), ready for install. So, I just got a reply from Steeda that said to use 2qts and the friction modifier with that. A little math shows 2qts + 4oz is indeed 2.125 total as listed above.
 

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be aware, if the fluid you are using already has the friction modifier in it, then you don't need to add it . . . but you do need to fill the diff
 

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I can't remember if the factory service manual says fill it to the top of the fill plug or a measurement below it.

And, I agree not to add modifier if the Amsoil, RP, or whatever already says it's in there. You can always add some later if there's an issue. (Pretty sure the Motorcraft stuff does not have it, hence the note to add the 4oz bottle.)
 

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I guess I have to ask why? The oem's have spent a crapload of time/money to design those covers to circulate the oil, cool it and lubricate the diff gears to the extent well beyond what just about any of the aftermarket companies have done. This is an excellent analysis of typical aftermarket design analysis that has been done.....

 

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why? because fins + easy drain hole at the bottom and fill hole at the top . . . plus it looks cool too

fins are good for road track, they actually do make a difference in keeping the diff cooler and reducing fluid expansion and overflow

fill hole and drain hole are not needed for a regular street car that might have its diff fluid changed once or twice in its lifetime, but come in real handy for a track car where you change it every year or every other year

and yeah I'm pretty sure the Steeda cover is the GT500/Boss 302/FRPP cover with "Steeda" stamped on it
 

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Discussion Starter · #10 ·
I used the redline stuff that includes the friction modifier. Like I said, with the car level on jackstands and the rear suspension hanging it took 3 quarts minus 1&1/3 cup to where it started dripping from the stock fill plug on the front of the diff.
 

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as I understand it, to determine if you need the friction modifier (in addition to what's already in the Redline and other synthetic diff fluids that say so on the label):

do some tight figure 8's in a parking lot; if the rear wheels skip or bind, or the diff "chatters" then you might need to add more friction modifier; or maybe a few more figure 8's to work it in to the clutch packs
 
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