Diff Ratio.....which one

[cluxford]

OK, all you diff experts...

BBC 632 cubes
makes peak power (torque curve) at 4300 RPM
Turbo 400

I'm thinking a 3.4:1 ratio

Transmission
1st Gear 2.48
2nd Gear 1.48
3rd Gear 1

Diff ratio 3.4

Tire Diametre 28 inches

Final Ratio
1st Gear 8.4 to 1
2nd Gear 5.0 to 1
3rd Gear 3.4 to 1

1st Gear
10 MPH or 16KPH = 1012 RPM
20 MPH or 32KPH = 2024 RPM

2nd Gear
30 MPH or 48KPH = 1812 RPM
40 MPH or 64KPH = 2415 RPM

3rd Gear
50 MPH or 80KPH = 2040 RPM
55 MPH or 88KPH = 2244 RPM
60 MPH or 96KPH = 2448 RPM
70 MPH or 112KPH = 2856 RPM
75 MPH or 120KPH = 3060 RPM


Car will be street driven only....3.4:1 gives good RPM range across reasonable speeds and some up my sleeve if I want to hammer it....

Any views ?

4.10 ratio would see RPMs go up by 200 in first gear and 400 in 3rd gear
a 2.4 ratio would see a drop in RPMs by 200 in 1st gear and 500 in second gear.....way too low...

After people's views, does 3.4:1 look like a good RMP profile....I've never had a big block so want to make sure it will have some manners on the street

[69coupe]

Cluxford
I am going to run 3.7 with 5 speed. Top being a overdrive. Reckon 3.4 would be perfect for the 4 speed

[REDZ-28]

I agree with 69coupe. Should be sweet!!!!!!!!!!

[anychevy]

With a 600+ cube BBC making in excess of 600 ft lbs of torque, around 3.5s would be a nice all round final gear ratio for the street.
I've got 4.10s with 28" tyres and I like it but, 3000 rpm on a freeway ?
I may as well not have a stereo at all !

[cluxford]

I hear ya...that's what was concerning me the most, keeping a nice 1st gear RPM but not having the thing screaming in 3rd

[anychevy]

I hear ya...that's what was concerning me the most, keeping a nice 1st gear RPM but not having the thing screaming in 3rd

But it's a $hit load of fun with a posi and close ratio 4 speed :D