Ford 8.8" Oil Fill Volume

Discussion in 'Corner Carver Racing Tech Discussion' started by GlassTop09, Sep 23, 2022.

  1. GlassTop09

    GlassTop09 Senior Member

    Hi All,
    I'm curious as to how much diff oil volume y'all run in your rear axles to adequately ensure that there is sufficient oil present to cover pinion\ring gearing & bearing lubrication along w\ sufficient cooling while running on track. Do most just do the fill until you spill thru the fill plug, or do you fill to a specific oil volume amount? If you do use a specific oil volume amount, what is it (OEM is 4.25 pts or 2 qts 4 oz which should equal the fill\spill IF you're running a Trac-Lok diff), especially if you're running a diff other than a Trac-Lok (creating different displacement area)?

    I ran across this YouTube video from Richmond Gear which at the 9:30 thru 11:00 time section, it mentions a recommendation for circle track racers to intentionally overfill the rear axle to offset the centrifugal forces that deposit diff fluids into axle tubes (probably has a lot to do w\ oil getting spit out of the axle vents mounted on the axle tubes) to ensure that ring\pinion gears & bearings (LSD friction clutches as well) don't get starved of oil, but doesn't say as to how much:

    Richmond Gear - YouTube

    Any insight given here is greatly appreciated as I've never heard of doing this before but from viewing this video, this is a thing & from doing a search here, no info turns up so I'm inquiring. I can see this being common on long freeway runs at higher speeds as well as on long sweeping straights at the track is why I'm posting.
  2. Greg D

    Greg D Junior Member

    Dino has given you a great resource for total volumes. If you're checking current fill volume, here's the method: 1) Open the fill port; 2) kink your index finger into an "L" at the knuckle; 3) Fill a small amount at a time until your fingertip just dips into the oil.

    The 8.8 is not like the 9" that requires filling until it comes out of the port. I did that one time and I had oil blow out all over the rear of the car after a day at the track. What a mess!
  3. Juice

    Juice forum member

    Two bottles, plus however much makes it weep out of the hole.
    Midlife Crises likes this.
  4. GlassTop09

    GlassTop09 Senior Member

    Ah Ok, fair enough.

    Since none of y'all directly addressed my 2nd paragraph concerning the attached video's info on the advent of adding extra oil volume above the fill plug level (in my 1st paragraph I alluded to the OEM capacity fill volume\fill & spill thus I already am aware of the OEM fluid capacities but thanks for the info anyway) recommendation for circle track racers (why I put this thread in the Corner Carver's section) for any insight (as I also posted, my 1st time reading of this), I'm in the process of testing this out myself since I have the FoMoCo 8.8" Track Pack Finned diff cover installed along w\ my Eaton Detroit Tru-Trac LSD, which removes the main deterrence from doing this (the axle vent location as I have moved mine to the top of the diff cover's baffled axle vent port & plugged the unbaffled vent port in axle tube; also this cover has the upper fill port at top of cover thus making the axle housing's fill port moot thus can just go by oil volume alone).

    I have in times past witnessed my axle puke oil out while on the dyno w\ the OEM recommended 4.25 pts of 75W-140 synthetic oil w\ 4 oz of modifier (running the OEM Trac-Lok diff then) & axle vent mounted in axle tube housing when I was running my FoMoCo 8.8" Girdle so I had installed a JLT axle catch can to contain the pukage until I discovered the FoMoCo 8.8" Track Pack Finned diff cover's top baffled axle vent port......
    The advent of Ford redesigning this diff cover to add this baffled axle vent port IMHO is Ford's way of admitting the existing vent port location in the 8.8" axle tube is a mistake.....should've been located in the top of the diff housing from the jump........

    I have pumped in 5 pts (2.5 qts which is .75 pts above the OEM 4.25 pts capacity) of Lucas HP 85W-140 GL-5 gear oil & to date, all is good w\o issue. The extra oil volume along w\ the 85W-140 vis oil really quieted the Tru-Trac's noise from all the "lash" while diff is unloaded\swapping side loading during TBR operation & should increase heat transfer rate from diff TBR operation\bearings\gear teeth mesh from loading to diff cover\axle housing to improve overall passive cooling performance (2nd purpose of diff oil in addition to lubrication.....can transfer more generated heat w\o increasing heat transfer delta when oil volume is increased) of axle so oil lasts longer w\o breakdown from excessive overheating (which should also lower the amount of internal air thermal expansion thus lower the advent of potential oil puke out from excessive foaming\air exchange as well as excessive internal temp on all axle\pinion seals). I think most serious racers would install active cooling instead (external oil cooler) in addition to also increase total oil volume (to serve the same purpose, more efficient at doing it), but this method is a "poor boy's" way of doing the same thing while ensuring sufficient lubrication at all times\operating conditions to all internal moving parts.

    From my estimation, I can't see using more than 6 pts max (3 qts total) for passive cooling purposes as I think that this volume would achieve what the video recommended for drag racers (fill to level of axles in diff housing to counter G forces incurred from hard launches so bearings....especially pinion bearings.....don't get starved of oil from oil getting pinned to rear of axle diff housing & spread up cover from the high launch G's & subsequent high acceleration rates) but I don't know this as I've never heard of the reasonings for doing this before now but I also know that this is a thing some do\has done so someone has real data on all of this.......which is what I am curious about.

    The 2.5 qts of oil volume may very well already have the static oil level at the axle level in this 8.8" axle housing for all I know.
  5. Juice

    Juice forum member

    I put my 3.73s back in.
    Filled diff through normal fill hole (easier access vs top plug on finned cover.
    Once at level of front fill plug, added a little more for the angle I had the car w/rear jacked up.
    Took a tic over 2 qts of gear oil in all.

    Ps: that vent has been on the axle tubes since the 60s. lol probably longer, but I have not wrenched on anything older than mid 60s Fords.
  6. mattjames

    mattjames Member

    I've done countless rear ends. Fill it til it comes out the fill. Sleep good at night.
  7. Mustang Terlingua

    Mustang Terlingua Junior Member

    I run the Ford Racing rear diff cover, the volume is increased & the vent is moved to the top of the diff cover. It has a drain & you fill it to the fill hole in the front of the diff. I clean & put cut off bolt threads 3/8" 24 & weld up the vent hole in the axle tube & no more leaks or oil on the axle tube. The cover comes with a upper hole but do not fill to this level it is for a diff cooler fitting. This will add volume of oil & cooling fins the cover is aluminum.
  8. GlassTop09

    GlassTop09 Senior Member

    For those who are interested,

    After doing some research, I've figured out what is happening inside a Ford 8.8" S197 integrated SRA axle to cause the oil spit out thru the axle vent when it is mounted in the axle tube & it is a design feature of this axle's housing that supplies oil to the axle tubes to supply oil to the outer axle bearings\seals & Trac-Lok's carrier bearings\LSD clutch packs\ side gears & spider gears (in a Detroit Tru-Trac's case, after carrier bearings the side gears\worm gears then into center of diff where oil drains back into housing sump thru the 4-1\4" holes in the spacer retainer plug from the axle tubes & a small rectangular slot in diff housing at outer edge of diff for air entry to facilitate oil drain out).
    Diff oil is picked up from the sump by the ring gear teeth & is carried over the ring gear & is slung into the pinion bearing oil tray to oil\cool the 2 pinion bearings (the rectangular slot at bottom of housing under inner pinion bearing race is the drain for this tray). When oil is slung into this area, there exists 2 oil drain castings in housing that channel diff oil into both axle tubes. As the ring gear RPM's increase, the amount of oil deposited also increases (oiling rate). This lowers the running oil level in the housing's oil sump (which is the important oil level to maintain.......not so much the static oil fill level).

    The amount of oil deposition rate into the axle tubes can\will at some point exceed the oil drain back rate from both axle tubes back into the diff housing oil sump thru the diff itself thus the "trapped" oil level in both axle tubes can\will rise (also will rise from heat expansion, a result of lowering the running oil sump level due to less oil in sump to passively transfer heat out cover (why OEM diff cover is made of thin stamped steel) so heavily depending on heat transfer thru thicker axle housing tubes.......if the axle vent starts venting air from excessive heat buildup which will cause a pressure drop in the axle tube that it is mounted on & that tube is excessively filling up w\ oil......well I shouldn't have to say what's gonna happen next.

    It is this design limitation in this Ford 8.8" integrated SRA axle housing that Ford engineers finally recognized\admitted & for the 2013 MY, instead of adding costs to redo the entire production process to redesign the axle vent port at top of the 8.8" diff housing section (to permanently "rectify" this spit out issue......also this platform was coming to its end of production run), it was much more cost effective to design this axle vent w\ baffle into this specific finned aluminum diff cover originally designed for the 13-14 GT500\13 Boss 302 LS\R production cars (track oriented) only but was also offered as an option for any 13-14 GT's ordered w\ the Track Pack package, then offer it as an OEM upgrade "aftermarket" part for the other Ford 8.8" S197 SRA axle equipped cars (05-14) thru the Ford Performance line.

    Now also understand, a Ford Trac-Lok diff was designed to run a 75W-140 vis synthetic oil (higher temp operating range to satisfy a "1-time fill" or lifetime of car capacity--no changeout--for warranty purposes) so it's cold\hot flow properties are accounted for when flow back thru the diff from the axle tubes at an optimum operating temp of 250*F-280*F (to properly evaporate any condensed water vapor entry from air exchange off heat cycling) but any oil temps that exceed the 300*F-330*F range will start oil oxidizing (oil breaking down.....thus the stinky smell) so the goal is to keep the running oil temps below 300*F but above 212*F.......which means that the optimal running oil level is critical to prevent over working of the oil causing excessive aeration & excessive heat generation (which reduces oil film strength that is necessary to maintain protection to gears\bearings\LSD clutch discs when running oil level is too high.....has nothing in common w\ the static oil fill level), but enough to maintain effective passive heat transfer rate thru the diff cover while the oil is in contact w\ it. A Tru-Trac diff is designed for a GL-5 mineral-based oil but synthetic oils can also be long as no friction modifier is used (specifically 80W-90 vis to maintain designed oil drain back rate thru the diff so if a 85W-140 vis oil is used, this diff drain back rate can be a little slower due to the slower hot viscosity flow rate vs 80W-90 vis oil--depending on running oil temps) thus can aggravate the axle tube fill level (increase) thus aggravate the axle vent oil spit out issue if axle vent is left mounted on axle tube thus excessively lowering the running oil sump level reducing passive heat transfer you know to be aware of this. Slightly overfilling the diff sump can counter this axle tube overfill effect to some extent w\o incurring excessive oil aeration at speed thus "poor boy method" as long as the axle vent is remounted to top of diff housing......but the best method is to install an external oil cooler system......Ford mentions this in the 07-14 GT500's OM if car is to be used on track (pg 33). I also have the Eaton brochures in hand that have the info on the Tru-Trac diff as well.

    Ford started using this type of integrated SRA axle design in the mid-late 70's w\ the advent of the 7.5"\8.8" axles......not earlier. The old earlier Ford 8"\9" axles is a different design altogether that didn't suffer from this limitation (used a separate diff carrier section design not integrated into the axle housing thus the diff can't affect the oil drain back rate from the axle tubes to the housing's oil sump--this is controlled thru the sump's static fill level) thus the axle tubes would never fill to the axle vent port from any "trapped" oil).

    Provided for informational purposes.
    Last edited: Sep 29, 2022
  9. JC SSP

    JC SSP Member

    I have owned V8 mustangs since the 80's and I re-fill to factory specs with quality lube (i.e., Royal Purple, Valvoline, Redline, etc). It's just part of a good preventive maintenance program and it goes a long way in keeping your car on the road. If you have a stock LSD (clutch-packs) don't forget the friction modifier. Some lubes come with it already, but I always add the little bottle. Its stinks like hole hell but works!

    I run an Aburn Pro in one Stang and standard Tack-lock LSD's on the others, so this is just basic drain and fill procedure, nothing fancy.

    Take care of your rear end and it will take care of you. LOL
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