70special

So I want to change my rear end gears, its a 8.5 "O" type rear end in my 70 Cutlass S. I have a choice between 3.42 and 3.90. I'd like to start going to the track more often but it is a daily driver. I have a 700R4 trans with electronic lock up torque converter. Have not been to a dyno, but it's pretty torquey as is with stock gears, 2.73 or 2.78 not sure at this time.

Aceshigh

Literally 3 posts below yours is a related topic......
Gears and Carrier for the faux 12 bolt 4 year limited production rear end are extortionately priced.

https://classicoldsmobile.com/forums...-rear-end.html

The 3.42 is a fairly medium range gear that's good all around. That's what I would go with personally.
I have it in my 78z28 from the factory still. I have an LS1 / 4L60e in it, and it runs like a cool raped ape.
275/40/17's on that car.

Tire size is IMPORTANT to factor in as well. Bigger gets you lower RPM's http://riverviewtire.com/Tireheightchart.html

The final gear on the 700R4 is the .70 so you can calculate your RPM's at specific speeds here.
http://www.angelfire.com/fl/procrastination/rear.html

I put a 3.73 rear end in my Olds because I'm going with a T56 six speed that has a .50 final gear.
So with a 27" tire , I'll be turning 1500rpm in 6th gear at 65mph. I have 285/40/18's

IF you have small tires from the factory......obviously that kills your RPM's

Rickman48

I'd personally go with the 3:90's as 1st gear will be almost 12 to 1!!
Provided you're not using tiny tires, noone's gonna catch ya!
And the O/D's gonna bring it down to 2:73 - close to where you're at now!

507OLDS

The gears will be almost $300.00 by the time they get to your door,a new posi unit $525.00,and a complete bearing/seal/install kit $150.00. You could get away with 3:90's with your OD trans.You would get better track performance with those,compared to the 3:42's,but the 3:42's are a nice all-around gear as well.

monzaz

Working on bring the posi price down for the 'O' typr 12 bolt...Hang tight. Give me another couple weeks I have a NEW machinist working on it. Jim

BLUE72RAGTOP

700r4 has a pretty low first gear with a bit of a spread going to 2nd. My experience on several street driven vehicles (27" or smaller tires) is that the 3.42's are just about perfect. With the 3.90's and decent horsepower, 1st gear is so low you will light up the tires and have to back out of the throttle to hook-up which will put you in 2nd at too low of an rpm, or you will try to baby it through 1st, and will still shift into 2nd too soon. If you hold each gear manually, you still have to baby it in first. On the track with slicks, the 3.90's will give you an advantage over the 3.42's, but I'd still stay with the 3.42's on a street car

70special

Thanks for all the info from everyone. I'm going with the 3.42's for my gears.

gearheads78

Using a Ford 8.8? I have talked to someone that sais that what they did with minimal machine work.

507OLDS

The new posi carriers that SuperCars sells,are 8.8" Ford units that have been remachined to fit the O-axle.Reider Racing was doing this.DTS bought out Reider Racing,but DTS is still offering the units.Basicly,they are welding-up/machining the ring gear journal,to accept the O-axle ring gear,then redrilling the correct bolt spacing pattern on the flange.The Ford units don't have webs going all the way to the edge of the flange.They have a nice flat,machined surface,so it makes it easy to move over & drill another set of holes.They also place some thin shims behind the carrier bearings,to get the width the same.
I have had this done several times,but recently,I have been using the new Eaton 8.5" 10-bolt empty blanks for the 71-up 8.5" 10-bolt.They are making these with that same outer flange now,so you can drill another set of holes.I have the ring gear flange welded-up/machined just as I have done with the 8.8 Ford units.The Eaton unit gives better options for clutches & side/spider gears,to make a real nice unit,with hand-picked parts.However,a custom carrier like this is not the economy route.The newer Ford units are better than the older ones,& you can adapt the clutches from the Eaton 12-bolt Chevy/8.5" 10-bolt into these,for a real nice unit.The new carbon-fiber/poly garbage is not as good as the original style knurled solid steel clutches that came in the original Eaton units.These clutches are still available,and I use these in ALL Eaton units that I build or rebuild,as well as the newer Ford units.I don't just pull a unit out of a box & throw it in.
I have also machined spools for the O-axle,for those die-hard Olds racers that insist on using their O-axle.Again,not the economy route,but anything can be done.
You'll be happy with the 3:42's & that OD trans.Good luck with your project.

Aceshigh

Yep. It's very steep and alot of fun for holeshots.

I had a CK Performance Stage II 700R4 from Chris back in 2003.
Chris is a pioneer of sorts, and was trying to build a new input drum which
is the weak spot on these transmissions IIRC> I wouldn't put it behind a BB.

I put it in my 2nd gen Z28 and when I finally decided to go the LSX route
I sold it with my SBC 350. I loved that 1st gear though.