[strokin]

So I've completed a 383 in my heavy 65 Pontiac convert, the motor has exceeded what I wanted so far, full roller block, etec 200 heads, 9.7:1 comp, Duration @ .050'' 242° Intake/240° Exhaust, Lift @ Valve .594'' Intake/.594'' Exhaust, and so on..... Now I knew i was gonna have to do my rearend and gears, not sure of existing gears but super high. I was going to go with 4 11's and a posi but I'm a little worried that I won't be able to control this from a stop, basically I want to he able to launch but at present at any speed in 1st and most part in 2nd I just spin, would posi and a low gear or posi and same gearing be a better option? Just any input would help.

[SSedan64]

To determine the Rear gear ratio;
Open Diff= Rotate 1 wheel 2 revolutions and count Driveshaft Revs.
Posi Diff= Rotate both wheels 1 revolution and count DS Revs.

A Posi will definetly help you traction.
What Diff does the 65 have? 12-Bolt, 10-Bolt?
Some of the large car rears have oddball sizes and can be very difficult to find performance parts for.
May apply if you have the Dropout 8.2" 10-bolt> http://www.bruneauperformance.ca/409...rendparts.html
http://www.bruneauperformance.ca/

Also boxing in the rear Control Arms or aftermarket Control Arms will improve traction.

[strokin]

It is a 12 bolt, I had done the rotation test months ago and came up with a number around 2.25 or so, I realize posi will greatly help, but I'm worried that 4.11's are so deep that it will be easily slipping,

[SSedan64]

If you plan on much street use, something around 3.42 or 3.55 would work better to keep your cruising rpm down. For best performance your Cam will probably need 3.73-3.90 gears tho'.
I wouldn't go higher on the street unless you plan on swapping in an OD Trans such as a 2004R, 700R4, 80E etc...
With an OD Trans higher ratios such as 3.73 & higher gears work well.
An OD Tran's 4th gear will tame the higher ratios so you retain you cruise rpm & have good acceleration:
3.73 X .67 = 2.49
3.73 X .70 = 2.61
4.10 X .67 = 2.74
4.10 X .70 = 2.87

Trans Gear Ratios:
Trans--------1st-----2nd-----3rd------4th Gear-------------------------
TH350-------2.52----1.52----1.00--------------------------------------
TH400-------2.48----1.48----1.00--------------------------------------
TH200-4R----2.74----1.57----1.00-----0.67-----------------------------
TH700-R4----3.06----1.62----1.00-----0.70-----------------------------
4L80E--------2.43----1.49----1.00-----0.75-----------------------------

What Trans are you using now?
With a TH350 Trans and 2.25 rear Gears, running with the Trans in 2nd gear would be like having a 3.42 rear gear.

[strokin]

Great info, much appreciated