Do rear-ends make a difference on a chassis dyno?

[SML350]

Just a random question that popped into my head. Say you had a Dana 60 and an 8 3/4 (or any other), both with the same gear ratio, would you see any difference in RWHP if you ran them behind the same setup? Does weight of the unit or internals add drag and affect numbers from rear to rear? Like I said, I was just curious, don't even know if this Q makes any sense but hey, y'all will let me know.
Thanks!
Andrew

[dschumers]

I honestly don't think you'd see any difference.

[451boy]

yes they'll make a small difference. For example, look at the money guys spend to put Chevy 12 bolt gears into Ford 9 inch housings. They do that because the Chevy gear set is more efficient. Maybe a 10 hp difference on a serious SS race car.

[vanishPt]

451, I would like to hear more about that. I have read articles where it states that the 8-3/4 is 'several hp" more efficient than the Ford 9 inch.

[dewme5]

hot rod did a study on that. took a hemi dart (i believe) put a dana 60 with 3.55's, and then a quick swap after 3 runs. did it with an 8 3/4 with 3.55's also. there wasnt more than just a couple hundreths differance. I was suprised. and i am also looking for a good dana right now too.!!

sure there is a differance. but not much.

[prostreetchallenger]

I JUST SWAPPED out an 8-3/4 with 4.89 for a dana 60 with 4.87,s and my times did not change at all.

[DusterDave]

With regard to the Hot Rod test, it showed that neither the extra heft of the Dana and its less efficient design didn't slow the Dart down much. If my memory serves me, didn't it 60 ft. slightly better with the Dana?

Because of the robust design of the Dana 60, I stuffed one under my Duster. It has a very Mopar look, although this housing actually came from a 1985 Ford F250 pickup.

[PRO]

The test dewme5 mentioned showed -.008 going from an 8.75 to a Dana with the same gears.Actually the ford 9" is the biggest offender to hp,although its stronger due to the fact that the pinion gear has 3 support bearings although this doesnt require more hp but the pinion gears location in respect to the ring gear does,its at the bottom of the ring gear and this relationship is stronger but akes more effort to turn.Most racers in the 10s or better will sacrifice the hp deficit in order to gain longevity and reliability.Also the o.d. of the ring gear adds strength so a 9" is marginally stronger than an 8.75,thats also why the Dana (9.75) is so strong........PRO.....

[Comp Chassis]

Stock 9" is not really any stronger than an 8 3/4. In fact, the differntial in a 9" leaves a lot to be desired. There is so little room in it that they are notorious for burning up and breaking.

As for the third pinion bearing, it has to be there to keep the thing together. Without it, the bolt in pinion support would snap off and never be able to hold any HP at all.

[PRO]

The 9" is superior to the 8.75,now I dont like to argue comp but settle it this way,call Mark Williams,Moser or Randys ring and Pinion and ,their the experts and have seen and tried everything imaginable,dont get me wrong I bleed Mopar but by design the 9" is stronger for the simple reason that the location of the pinion gear to ring has better geometry..........PRO........

[Comp Chassis]

If the geometry of a 9" is so great why does it take more HP to turn? Most other rear axles turn the ring gear, the 9" attempts to push it.

I repair more broken 9" units than 8 3/4. In stock, and I repeat stock form they are not that much better of a unit. Now, spend the money on an after market carrier, put in a daytona pinion support and throw away the differential for a spool you have something fairly strong.

I do this stuff all day every day. I see what breaks and what doesnt. I build very few cars with 8 3/4 rears anymore because eventually they come back broken. Sometimes the customer will decide he wants to try and and I will build it for them. I also build 9" units for the customers that want them, but, I also highly suggest that they go with all good after market stuff. For the buck, the Dana 60 is my hands down choice. On a typical back half, using a 9" over the 60 costs an additional $1000 if we use a stock housing. If they want a fab housing with the back brace and all the other goodies, the price can be an additional $3000.

As for weight, in race form, the Dana is about 40 lbs heavier, unless you go with aluminum stuff in the 9" Then it is about 50 lbs. But, the 9" takes more hp to turn so the it is a wash. The next obvious argument is ease of gear change. Yep, if you have an extra $1500 pumpkin sitting around. I can change gears in a dana just as fast or faster than a 9".