[mels80ga]

I will be putting a Ford 9" rear in my car in replace of the 7.5 10 bolt and I was wondering if anyone could give me an idea of ET change to expect. Let's say with all things being equal, other than wt. and parasitic loss of the 9", will there be a noticeable amount of power loss? I would guess about 50 lbs. of wt. change, I will weigh everything when I make the change. The 10 bolt had a 3.73 gear and I would like to get an idea if the Ford rear had a 3.73 also. Here's another point to ponder, the new rear will actually have a 4.56 gear. Anyone have experience with this type of gear change?

[67cruiser]

aLL ABOUT ENGINE parameters, what do you have?? I am always told it takes 12 h.p. more to run a Ford 9 inch than a 12 bolt rear, kinda stuck with me over the yrs. 456 ok, what rpm are you coming thru the traps at now?? Can your rods handle more rpms?? We need more info. please

[mels80ga]

3400 lb. 467 iron d ports, solid cam 264-274 @.050,.575-.595 lift. With the 3.73 and 26" tires I go through at 4600 in the 1/8 at 93-94 m.p.h. With the 4.56 gear I will change to a 28 or 29.5 tire. The rpm change won't be a problem, I shift at 6000 now and I only run 1/8 mile. I was just wondering if anyone had changed from a 10 or 12 bolt to the 9" with no other changes, and if so how much power loss did they see.

[ponjohn]

What is the combo?

A 4.56 gear is A LOT of gear. One thing for sure the converter will not flash as high and be better coupled.

[Skip Fix]

7.5 10 bolt

7.5-3rd gen?

[mels80ga]

Stated 7.5 10 bolt in first post, the car is a 1980 Grand Am.

[Skip Fix]

I saw the 7.5 in the first, and exactly why I was asking what platform as the frequent ones here don't have a 7.5 except 3rd gens.

[mels80ga]

That shouldn't matter, I was just wondering how much power the 9" would rob vs the 10 bolt. Thanks.

[screamingchief]

It's impossible to answer,and it wont be the same car to car either.

The way the rearend affects the torque converter makes it impossible to predict that with any sorta accuracy.

It's entirely possible,that even with the increase in theoretical losses of the 9" rear,that the interaction with the torque converter could completely negate any/all of those potential losses,or that situation could even yeild a very slight improvement despite the additional theoretical parasitic losses.

Even with a stick combo it would be hard to predict,clutches slip some too.

I would'nt even concern myself with this sorta minutia,not for this sorta deal.



Bret P.

[Skip Fix]

Bet there are more comparisons for 8.5 10 bolt to 9" than 7.5. 7.5 has smaller lighter components so has the potential to be slightly more efficient than a 8.5 also.

Like Bret said sometimes more theory than real. Like a TH 400 vs TH 350. P Dude ran the same swapping them.

[mels80ga]

Thanks for the replies, it would be nice if there was no loss of power. What about the 50lbs. or so I'll gain in unsprung wt., will that make a big difference? I have heard 1lb unsprung = 8lbs sprung. If that is true I would be looking at a 400lb wt. gain which should be a .3-.4 loss in E.T.. Is this true? Thanks.
Mel

[Skip Fix]

Mel "generally" weight loss can be that specific but not always. when I went to skinny fronts I picked up more ET than just the weight loss. When I was doing some NMCA stock classes it was close to 0.1/100lbs for a low 12/ high 11s car.

Sounds like a neat project, good luck with it.

[mels80ga]

Thanks Skip,I experienced the same results when changing to lighter wheels and tires along with aluminum brake drums on the rear. This being rotating wt. is probably why. I will gain about 10lbs. rotational wt. at each brake drum and the other rotational wt. of any significance should be the ring gear, so hopefully the remaining wt. difference will not be significant. All I need is axles to complete the 9" and then I can take some wts. from each rear. I guess I'll never know for sure because I will be running a 4.56 gear vs 3.73 with the 7.5. Thanks, this has made for an interesting discussion. Looking forward to Spring!

[Skip Fix]

My 64 Impala will probably get a 9" in place of the "pumpkin" 10 bolt in it. Don't think my stoker will last too long in front of it with it's dinky axle spline and pinion spline count. Not sure if there is any difference in HP in those two.

[grandville455]

I will have some info in the spring for you, I am going from a 12 bolt 9 3/8 ring gear, 3.42 gears to a 9" rear with a spool, 3.89 gears and 35 spline axles. Not the same I know but still will be interesting.

[mels80ga]

Hopefully we will both have positive results to report Hopeflly I will be able to get a better converter sometime next year also. Good luck.

[Half-Inch Stud]

Rear Axle Efficiency is set & determined by Pinion gear position on ring gear.

Similar to having to take the SIN( angle from trans-tail to driveshaft) & SIN(driveshaft to pinion shaft) Use 1-COS(angle) if you're SIN-free.

So a Ring& Pinion that put the Pinion below the Ring gear centerline (same as Axle centerline) will have an effective ANGLE to have trig applied, despite the best-effort in sliding-tooth design. That angle is an EFFICIENCY HIT whereas the TQ applied has a % going into deflection&loading. Somebody post where I've gone wrong. Thanks, HIS
=====================================
I think the math extreme may be a practical extreme : to place the pinion shaft at the bottom of the ring (close to Ground or closest to Trunk Floor) would result in a worm gear of exciting ratio. WHEN a load is applied to such a Worm-ring the deflection is highest & the tooth loading is highest on the Ring teeth.

I think the optimal other extreme having the pinion shaft in-line with the axle centerline shows highest efficiency, yet deflection is again highest (equal to worm gear) & largest tooth loading on the pinion teeth.

Finally; I dare suggest the sliding "Hypoid-Spheroid" gearset is a matter of brilliance. Optimal pinion shaft position for best tooth&deflection values results in the "Strongest" rear axle for the limit of the materials used. Hi-Nickel Steel comes to mind. HIS

[walterd]

My car went about a tenth quicker when I went from a 10 bolt to 9" but I did also change the rear gears from 3.73 to 3.90 and spool

[Ron H]

Is the 10 bolt a posi? Will the 9" be a posi or spool? What size axles in the 9"?
You will find out quick the 4.56 gears are hurting. I suggest getting a set of 4.11's for it.
The tire size increase alone will cost you.
How the converter acts now and acts later as suggested earlier is anyone's guess. It will be tighter with the lower gear so you may kill your launch and have to loosen once you run it. But the extra weight and friction will cause the converter to loosen, so it may wash?

[mels80ga]

The 10 bolt had a powertrax no slip locker. The 9" has a mini spool with 31 spline axles. Can you explain why a 4.56 is too much gear? I've seen Pontiacs running 4.56 gears and running deep in the 6's in the 1/8. I have a friend who has an E headed 474 that runs 6.0's and I believe he runs a 5.13 gear, although that is with a glide he is still at 5.13 when in high gear. I plan on running a taller tire also which is one reason I wanted more gear, I could not get the 467 to hook with the 3.73 and 26" tires. I borrowed some 29.5 tires to try when I had the 406 in the car and with the 3.73 gear I lost .2 in the 1/8. I had already planned on a converter change when the funds are available. I may be wrong(probably am) but it seems like the sooner I can get into the engines power band of 4,000-6,000 R.P.M. the quicker the car should be.