i have an 86-z28
i have recently put in a new engine:
350-.040 over= 356
aluminum heads, port, polished and milled.
10:1 forged piston, forged rods
crane cams- 510/512 lift

when i had this motor built, i slected the cam to make up for the 2:73 rear that i have. and i am very pleased with the performance i now have.

however the rear is not a posi and it is a 2:73.
if i switch to a 3:73 posi rear, will that be too much for the car(crane cams said not to change the rear, i wouldnt need to)
also if i chage it, how will the speedo be effected and how do i fix it.

Depends on the cam.

The lift numbers aren't the most important; the ones to look at are the duration specs. Duration determines where the cam's optimum operating RPM is.

If you have TPI, don't put 3.73 gears in it, they're too much. That much gear moves the RPM that the engine needs to run in, up put of the range where TPI makes its best power. 3.23 or 3.42 give the best results.

You would change the speedo gears in the trans to make the speedo read right.

my engine is a carb motor, it has the origional quadrajet, it has been rebuilt. i wanna say the duration is in the area of 284 i cant remember i do remember it is a cam used in 4x4 and the rpm range was 2000-5700. i do remember that. it is a cam meant for a 5speed manual. hope that helps. so should i go with 3:73 or not?

Not knowing what's in the motor for sure, it's hard to say for sure... but with a typical carb motor's power vs RPM curve, probably so, especially if it has a good exhaust (not one single piece left of a LG4 or L03 exhaust, or anything that will bolt up to them) and good heads. But there's nowhere near enough information to accurately predict the optimum gearing for the rest of your setup, since there's no way given what you've told us to predict the motor's torque curve (i.e. the RPM range where it will produce the most power).

these are the specs for my car
86 z28

engine:
350 bored .040 = 356
10:1 forged pistions
aluminum heads off a 90 vette, port and polished
forged rods
crane cams- 284 duration? 502/512 lift 2000-6200rpm
edelbrock rpm performer intake
quadrajet carb-rebuilt

trany: stock t-5 manual

exhaust: headman headers- 1 5/8 primaries 3in collectors

3in downpipe (meant for the headers and car) highflow catco catylitic converter
3in dynomax exhaust w/ 2 1/2 in tails

anyother info needed just reply asking for specific info, i will try and find the cam card for the cam.

Sounds like the 3.73s would most likely be OK.

You just don't want to be in one of those situations where the motor runs out of breath at 4200 RPM but the gears force it to run at 5500 RPM. Your combo should have enough ooomph at high enough RPM to be happy with those gears.