77 differential advice

[john1977]

I have a 77 with 350/340hp,350th and 2400 stall conv.with 3:08 rearend. The car just doesn't have enough to break the tires from a stand still although it is quick. Would like any advice on changing the differential to either 3:36 or 3:55 gears and which you feel would be the better choice. Would like to keep my 3:08's so if you know where i might find a good used or rebuilt unit please let me know. Thanks jw

 

[SG4206]

John-
A set of 3:55s would be nice; but I think you might have a problem with the output of your engine combination. You should be able to light the tires with a 2400 stall.
I've owned stock L-48s that could.

 

[79'vette]

I see that you are listing your HP as 340, so I assume that this is either an older engine or a rebuild? The small block Chevy usually moves its torque band up the RPM spectrum as you build more horsepower, and as such will likely want more than a 3.08 rear (likely a minimum of a 3.42), and will likely like closer to a 2800 stall convertor as well. This combination will let the engine move into its powerband more quickly for more powerful launches off the line.


Off course, your top-end speed will suffer, and if you are running soft compound street tires, you may not get that spinning of the rear tires that you are looking for, but you will get neck snapping 0-60 and 1/4 mile times.

 

[john1977]

My engine is rebuilt and hp begins at 2400 rpm,if i line lock it it and bring it up to 2400 to 2500 it will break tires loose for about 5'.

 

[79'vette]

HP beginning at 2400 doesn't really help in a street car. You can have 5000HP at 7000 RPM, but if your torque doesn't begin until 3000 RPM, it doesn't matter.

Big block Chevys will make torque down low because of the cubic inches no matter what the cam. But a 350 SB chevy that starts making power at 2400 and peaks at 5500 to 6000 will likely not make torque until 2400 to 2800. That puts your 2400 stall convertor out of the power band.

I have an uncle that owns a heavily modified 68 Chevy stepside pickup with a 350 (about 350 HP) in it, and it took a 2800 convertor with 3.73 to get it going, and the weight difference between a Vette and his P/U is about 375lbs.