I have a 91 formula with a stock auto 700r transmission. I assum this is the 10 bolt everyone talks about, since it has 10 bolts on the cover. I have 3.23 gears stock and I want to buy 3.73. I see designations of 7 1/2" and 8.5" on summit. I think it must be the 7 1/2" right? Also what range of gears can I put in that housing without changing the housing? Also, what would be a good upgrade on the whole rear end for occassional weekend drag racing using MT ET street slicks? Thanks
Larry

 

Yup, you want the 7.5 inch gears, 3.73s for a 3 series carrier. You might be able to go into the 4.xx series gears, but I'm not sure. A posi of some sort would be a good thing for racing.

 

i think you go as high, numerically, as a 4:56. that's what i ahve in my camaro.

 

You can use 7.5" and 7.625" gearsets. I think all of the aftermarket stuff is all 7.5".

 

You have the 7 and whatever rear, not any 8 and anything size. The 2 so-called "different" 7" sizes are identical. That number is the inner diameter of the ring gear, where it is a light press fit over the outer diameter of the carrier. There are not 2 sizes, there is only one which is actually about 7.6" (I forget the exact number), which is why the same gears fit them all from 1978 to 2002. You surely couldn't press a 7.5" thing over a 7.625" thing, and a 7.625" thing wouldn't center itself on a 7.5" thing like it's supposed to. This is one of those myths we should have a "sticky" for.

You already have the 3-series carrier, so any gear from 3.23 on up to at least 4.10 will bolt right in. Unless you have massively modified the motor (specifically, removed the TPI and replaced it with something with RPM potential beyond 5000), anything past 3.73 is definitely overkill. For that matter, 3.73 may even be too much, especially if it's a 305 auto. 3.42 seems to be about the sweet spot for that induction as it puts the motor into the right RPM range through a pass.

 

Thanks for that info. I have a midly modified 350. I have read some stuff by LIngenfelter and also on the tech board, "all about gears" or some such title. I dont remember the specifics but according to these charts and calculations the 3.73 shoul be better for me. I guess you should be at your maximum rpm and power just as you cross the 1/4 mile finish line, as I recall. The 3.23 is OK, I run 2.0 60' and 9.06 in the 1/8 mile, but if I stomp on it I spin a lot, so I want to do the gears and use the 3.73 with slicks at the track. Ive heard that even with 3.73 its not to bad on the highway with the overdrive. Anyway I really don't care about the highway with this car. I only drive it on weekends to cruise or race. I might drive it 1 hour to the other side of Houston for a gathering but thats it.

While Ive got your attention, what rear end would be good as an upgrade for the drag racing? Something strong that can take the wear and tear. I don't know much about rear ends yet (at least on cars). There is a 9 bolt, also something about a Ford rear end that is good (if anything Ford could be good). ???

 

If you run 1/8 mile, the 3.73 would be fine. For 1/4 mile, the car would probably ET lower with 3.42s, and would almost certainly run at a higher mph.

As far as highway driving, my car (83 HO Z28) came with 3.73s from the factory, and it has quite a few highway miles on it. It drove from San Diego to Cincinnati a few months ago, to and from Indy a bunch of times, to and from South Bend in the last couple of weeks, etc. There's nothing wrong with 3.73s on the highway.

The 9-bolt is pretty tough, but hard to find the good ratios just laying around. Plus, nearly all of them have the brakes that suck, but your car has the good rear brakes (if it has disks). Gears are expensive but available.

F*rd 9" rears mostly suck using stock parts, but there are lots of aftermarket parts tha tfit the platform that are as strong as it gets. NASCAR guys for instance use the 9" exclusively, for ease of gear changes, strength, etc. It's REAL expensive to get one made up to fit one of these cars, about $2000 entry level, and it goes up from there for better axles, stronger center sctions, bracing, etc.

But if you're running a 305 TPI auto, the 7½" 10-bolt will be fine for a long time. It will easily handle anything that combination can dish out, unless you're also running a blower or 150 HP of nitrous, and have a really serious torque converter.

 

Thanks for the info. Its a 350 currently slightly modified, future moderately modified.