I have a 72 cutlass "S" with the standard rocket 350 in it. Nothing radical by any means, just an aftermarket intake and carb.

Currently it has a 323 open rear in it. I found a 273 posi and was thinking about doing the swap, but was wondering what I would lose "off the line". I'm not looking for a drag car or anything here, just wondering roughly what the difference would be.

 

small block with 3.23 = not quite a turd but close

small block with 2.73 = a fresh steaming pile

 

The only thing is with the 3.23's around 65 or so you when you let off you can feel it holding you back. Is there a chart anywhere that might be useful in the decision?

 

Not sure what you mean by holding you back when you let off ??

For anything I remotely care about performance 3.23 is the highest gear I would go. 3.42 is my favorite for a driver with no overdrive.

There are great calulators on the Richmond Gear website

http://www.richmondgear.com/calculators.html

 

Can't you swap to the posi differential and keep the same ring and pinion?

 

When I let off the gas around 65 or so its almost like I hit the brakes, lightly. The car doesn't coast very well, I guess is what I'm trying to say.

 

I think you're spoiled by modern cars with less rolling resistance, and less "engine brake" than older cars. When you "take your foot off the gas", take it out of gear. Any slowdown you feel is rolling resistance. The difference between this and what you're describing is engine brake. 3.23s will give you more engine brake than 2.73s.
If the housings are the same, I'd keep the 3.23 ring & pinnion, and use the posi carrier from the 2.73 rears.

 

wmachine, you may be right. This is sort of what I am trying to get rid of and I figured a different rear end ratio would do this. I just fear the 2.73's will take too much away from the car. Wanted some opinions.

Gearhead I couldn't imagine what the 3.42's would be like with the stock motor. It would scream at 55 mph, not something I want. I'm not looking for a race car or anything hat takes away from driveability. I don't want to blow a motor trying to do 70mph.

 

Its not that bad really. I have 3.73 with a 27" tire and I don't shift out of 4th (1:1 ratio) until about 60mph with a bone stock motor. I don't have a tach in the car but its not screaming. I keep meaning to temporarily hook up a tach to see where it is at my shift points.

example using the link to calculators I posted above 3.42 with a 26" tall tire puts you at 2873rpm at 65 MPH

 

Mines only a 3 speed automatic. So my car at 65 really screams, I should have mentioned the 3 speed before.

 

You sound like you are kind of like me, hate to rev the engine any more than absolutely necessary. If that is the case run the taller gear. You will lose some off the line punch, but cruising would be more enjoyable. I would look int an od trans at some time in the future as well.

 

I've thought of the transmission idea, that way I could keep the gears I have, and have the driveability I want. What transmissions you guys running? Is there even a such thing as a 4 speed auto for these cars?

 

I have 3.73's with a stock motor, it does rev a little high (3400 at 65 I think) at highway speeds but I can live with it. Much better "get up and go"

 

My 4th is the same as your 3rd 1:1 ratio

 

Gearhead is right on the money on this subject.

 

3.42 really is an ideal gear for these cars. Great for punch off the line and decent cruising speed without over revving the engine. I am currently having fun with my 4.11's from stoplight to stoplight but 55mph is really difficult and I plan to switch them out for a 3.42 gearset in the next month or two.

Not sure if you can use the 3.23 gear with the 2.73 posi, I know there are 3 series carriers and 2 series carriers and I am not sure what is interchangeable. Send a pm to Monzaz, he is the expert on this subject. Good luck.

 

If the 3.42 are a good match, why is my engine working so hard at 65-70 right now with the 3.23's? I hate to sound like an idiot, but I got into it one day and doing about 80 I thought It was going to blow, I've counted the teeth so I know its got the 3.23's in it.

 

I have a set of 3.73s, and they are great for around town. I live in a densely populated area so I don't have to hit the highway to get where I'm going. I really like 'em. I have a 4 speed in the car (manual). Just think of the sixties when people ordered their cars with 4.11s and drove them all the time. Holy Cow! I couldn't live with that, let alone the gas.

 

What size/diameter tire are you running? My '72 has 3.31 and with a 26" tall tire it was working to hard on the highway. I switched to a 28" tall tire and now it will cruise in the fast lane. Jkaz

 

Well this is certainly a missing part of the equation, and needs to be considered!

 

It sounds like you need engine work, or there is another problem!

I built a 66 that had 3.55's, 26 inch tires and I got 17 verified MPG with a 455 in it, just as long as I kept under 60 miles per hour.
ALL day long.
Drove the frickin' thing to Canada, from AZ.
I hear some whining.
Jim

 

It sounds to me like you grew up in the overdrive era. You would have to had to listen to howling V8 engines with 3.73 or 3.90 gears at 65 miles per hour to appreciate a relitively mild 3.23 gear. If your used to 1800 rpm at 70mph you'll need an overdrive. or learn to live with some engine noise.

 

Put a tach in your car and verify you rpm's. Us old timers are used to hearing the engine and exhaust its music to our ears. I don't believe you have a real problem, just need a better understanding of the older technology,

 

X2 An RPM reading will tell us a lot.

 

Its got the stock 14' ss2 wheels and the stock size tires for the wheels. I think they are 235/70/R14, I think. I will double check. When you guys say 24'' or 28'' tires, are you measuring outside diameter to outside diameter across the tires?

 

Yes. On the car, consider it topmost to bottommost . (Or left to right)

 

3.23 gears not turdy with the torquey Rocket 350. I second, take the carrier from the 2.78 set.