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65 rear axle question

Posted: Sat Apr 15, 2017 8:27 pm
by Classics65
I'm building a street strip 65 Olds starfire and my question is how much power can the stock Pontiac 8.875" take. Has the 12 bolt ring gear 3.42 posi. Should be in the neighborhood of 550hp 600ft-lbs torque. Going to be running the amsoil racing gear fluid and lsd additive if that helps

Re: 65 rear axle question

Posted: Sun Apr 16, 2017 10:35 am
by monzaz
Ok well oil and additive does not make any rear end stronger. ONLY lack of oil can hurt the rear. OR not changing the oil in 10 years. :)

NOW we get this question so often If i had a serious NICKEL for every time people ask about HP and how much can a rear handle question I WOULD BE RICH AND RETIRED. :)

HP really is NOT a factor in rear end pain. IT IS ALL TORQUE period. How fast the torque is delivered and how it is delivered

meaning how fast delivered. Smaller cams in bigger cube motors deliver torque much quick in a stock application. So 500 ft lbs. delivered at 2000 rpm will be much more destructive than a higher revving 500 foot lbs at 4000 rpm motor.

Next is delivery - HOW does your torque get delivered.... Via Automatic transmission or Stick shift or trans brake etc. - So if you have a stick shift the destructive torque can be delivered anywhere in the RPM range which can be very worrisome to us rear differential guys. Drivers have total control of the delivery and how hard it is delivered. Trans brake again is usually at the track so that is for the next paragraph of pain...Auto trans with highstalls above 4000 rpm can also be destructive...but still having a pad of hit as it is a fluid delivery.

Finally last factor is the place your laying down the torque. Street strip off road etc. Sticky tracks with tons of prep are a main pain for the rear.... there is not a release for the torque to go but the drive line components them selves. So combine any and all three of these factors and you have upped the % of damage happening to the differential parts.

Your rear differential is only as strong as the parts in it. Spider gears will be the weakest link in any and all the rear end parts... Reason we use Detroit locker and FULL SPOOLS for hard off road and drag race applications. This is the factor of going as big as you can before you have to watch if weight becomes more of a factor than breakage does. Drag racing is a perfect example. If you want the fastest car you must keep weight minimum but you want your parts strong...So either BIGGER parts or better expensive light weight parts are needed. titanium alloy etc.

So your question 8.875 strong enough - That torque will be handled by a properly built rear on the street NO PROBLEM...Track factor you have a bunch more weight in a FULL SIZE car so that will be a factor and your spider gears will be another factor... axles should be fine. You really do not have many other choices as you have a good ratio there...make certain your spider gears/ clutches are shimmed correctly to keep spiders hitting on the meat of the teeth ...loose clutch packs are the biggest demise of the spider gear system in a Limited Slip unit .
Tighten up that posi unit with shim on the clutch pack make sure your ring and pinion backlash is in the 6 7 8 thousand range and hit it...

Good luck.

WE are trying to make our own 14 bolt custom full size car rear here...Just have not had the time to spend on designing the upper control arm ears needed to use the 9.5 14 bolt truck rears we have been collecting.

Pictured is a Dana 60 Riviera 1968 rear we did

Jim Mitschke

Re: 65 rear axle question

Posted: Mon Apr 17, 2017 8:15 am
by Classics65
Thanks for the info! Right now everything is still set to stock in terms of shims and gear mesh. No whining or grinding in it. Turns real smooth. But I'll definitely keep an eye on it. One last question- I've had a few people tell me I cant/shouldn't run synthetic gear oil in it. Is there any truth to that?

Re: 65 rear axle question

Posted: Tue Apr 18, 2017 5:01 pm
by monzaz
You can use any rear differential fluid... Just some posi unit manufactures have issues with Synthetic and warranty issues.

Synthetic fluid does stop friction and cone and clutch posi units work off of friction to give tight action. Sooo you can see why they might have trouble with that.

Regular Conventional hypoid gear fluid any brand or weight will be fine. with Limited slip differential additive - 4oz for small rears and 6-8 oz for larger 8.5 -12 bolt rears.


Re: 65 rear axle question

Posted: Tue Mar 13, 2018 5:23 pm
by Classics65
Forgot to ask, but how much fluid does this rear end hold and how often should it be changed?

Thanks, Zach

Re: 65 rear axle question

Posted: Wed Mar 14, 2018 1:01 pm
by monzaz
2.25 quart give or take...posi unit will suck up some initially axle tubes will have some stay in the tube so first couple runs will be critical to get the correct level. Remember you keep filling till the oil back-washes from the fill plug on a flat level surface.

Changing the fluid.

ALL really depends on the driver... if you are hard on it drag racing doing burn outs etc all year long once a year would be nice... BUT 20k is not out of the question if your just driving.
I have had rears that NEVER had an oil change and they were fine. Just keeping the level up is the most critical.
IF you have a LIMITED SLIP (posi) unit this type of friction based unit will contaminate the fluid quicker and will produce more heat breaking down the oil quicker...REQUIRING more frequent oil changes. SO really changing the oil too much is never an issue.. . oil does go bad while sitting BUT that will take many many years for it to completely be bad.... ANY oil is better than NO oil. :)

You can always follow GM manufacture specs and that will never do you wrong. :)

Jim Mitschke
J D RAce