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I'm getting ready to buy this early 70s nova 12 bolt diff and according to the guy its already posi and has some work done to it seems like it might have its got a nice aluminum cover and both wheels spin the same direction I checked the numbers on the diff itself and it says its an open diff with 3.31 is there any way to find out exactly what it is does it have casting numbers?
 

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The ratio should be stamped on the ring gear itself, well the number of pinion and ring teeth are, you'll have to calculate the ratio by dividing the amount of pinion teeth into the amount of ring gear teeth. If it is now a posi and was not originally the number stamped on the passenger side tube will be irrelevant anyway. You can also turn the pinion and count how many revolutions it takes for one full turn of the axle.
http://www.chevellestuff.net/qd/generic_rear_axle_info.htm
 

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Discussion Starter #5
The ratio should be stamped on the ring gear itself, well the number of pinion and ring teeth are, you'll have to calculate the ratio by dividing the amount of pinion teeth into the amount of ring gear teeth. If it is now a posi and was not originally the number stamped on the passenger side tube will be irrelevant anyway. You can also turn the pinion and count how many revolutions it takes for one full turn of the axle.
http://www.chevellestuff.net/qd/generic_rear_axle_info.htm
Ooooooh now I get it thanks! Its not so difficult once its put that way now one more question what would be the best ratio for all around driving with no power loss?
 

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We need to no a bit about the engine and tranny as well as your driving, whether it's highway or around town, or a combo. I ran 331's behind a 435 hp BB and 4 spd it would have bee a great gear with an automatic. I went o a 373 gear and love it, although it's not overly highway friendly, but still reasonable. A 355 is a great choice for a 3 spd auto or 411 with a 4 spd OD tranny.
 

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Discussion Starter #7
We need to no a bit about the engine and tranny as well as your driving, whether it's highway or around town, or a combo. I ran 331's behind a 435 hp BB and 4 spd it would have bee a great gear with an automatic. I went o a 373 gear and love it, although it's not overly highway friendly, but still reasonable. A 355 is a great choice for a 3 spd auto or 411 with a 4 spd OD tranny.
I have a mild performance sbc its a 350 bored 40 with an edelbrock performer RPM cam with a th350 trans and its going to be both highway and around town
 

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I have a mild performance sbc its a 350 bored 40 with an edelbrock performer RPM cam with a th350 trans and its going to be both highway and around town
IMO, i would say a 331 or 355 with a mild SB and 350. The 355 will be better off the line with a bit more RPM on the highway, but more fun for a cruiser.:yes:
 

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IMO, i would say a 331 or 355 with a mild SB and 350. The 355 will be better off the line with a bit more RPM on the highway, but more fun for a cruiser.:yes:
Cool thanks I was thinking probably inclined more towards the 355 though huh better off the line than the 331? While still keeping my highway driva:turn:bility
 

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Cool thanks I was thinking probably inclined more towards the 355 though huh better off the line than the 331? While still keeping my highway driva:turn:bility
The higher the ratio, the more mechanical advantage there is to the engine for acceleration with increased rpm's. It should only be a few more hundred rpm on the highway from 331 to 355. 373 IMO, is were the highway drivability begins to fall off. There are online calculators that will calculate you rpm at 60 mph with your particular tire size.
 

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Discussion Starter #11
The higher the ratio, the more mechanical advantage there is to the engine for acceleration with increased rpm's. It should only be a few more hundred rpm on the highway from 331 to 355. 373 IMO, is were the highway drivability begins to fall off. There are online calculators that will calculate you rpm at 60 mph with your particular tire size.
so is the 3.73 border line ok? Well what would be best for my set up? the 3.55 or the 3.73?
 

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I think you'll enjoy the 355 more, most of my driving is around town and a bit of highway. I think if I was driving 50/50 city, highway, I'd rather have the 355.
 

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I think you'll enjoy the 355 more, most of my driving is around town and a bit of highway. I think if I was driving 50/50 city, highway, I'd rather have the 355.
Mine is going to be like 80/20 with most stop and go in town driving and a little highway so maybe 3.73 is best for me?
 

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72 Frame off, bare metal resto-mod. 383, TH350, Eaton Posi, Complete new suspension, disk brakes
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Be careful

Choosing a gear ratio is picking your fuel economy too. My 383 sbc, th350 has a 3:73 posi and 29" tires. My rpm at 60 is over 3000 which is considered high nowadays. The car does do better mpg than my boat that has twin 4.3 v6's, and gets about 2.5 mpg at 27 mph, and the car is not my daily driver, so fuel economy isn't a goal for me.
If most of what you are going to do is drive it around town and quick acceleration isn't a big deal, go for the higher gears. If you want to be the fastest guy at an intersection go for some really low, like 5:3's, but your top speed may be 40 mph. It's all about how you are going to use the car.:cool:
 

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Discussion Starter #15
Choosing a gear ratio is picking your fuel economy too. My 383 sbc, th350 has a 3:73 posi and 29" tires. My rpm at 60 is over 3000 which is considered high nowadays. The car does do better mpg than my boat that has twin 4.3 v6's, and gets about 2.5 mpg at 27 mph, and the car is not my daily driver, so fuel economy isn't a goal for me.
If most of what you are going to do is drive it around town and quick acceleration isn't a big deal, go for the higher gears. If you want to be the fastest guy at an intersection go for some really low, like 5:3's, but your top speed may be 40 mph. It's all about how you are going to use the car.:cool:
Well im just looking for around street some highway driving and when I wanna get up and go I wanna go with good top end and low end power lol what is best for me?
 

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I stepped out for a bit!:D

As mentioned, 373's will reduce the fuel milage and take a bit away from highway driving, but I figure I only use my car for fun, so I wanted the lower 373 gears over 355. I was going to go with 411's, but I'm glad I chose the 373's now.

If milage isn't a concern and your driving 80/20 more city, the 373's are more fun for spirited driving.
 

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I stepped out for a bit!:D

As mentioned, 373's will reduce the fuel milage and take a bit away from highway driving, but I figure I only use my car for fun, so I wanted the lower 373 gears over 355. I was going to go with 411's, but I'm glad I chose the 373's now.

If milage isn't a concern and your driving 80/20 more city, the 373's are more fun for spirited driving.
Looks like I'm running 3.73! Thanks guys this is my first hot rod build I want it to be fun to drive:devil:
 
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