620 5 Speed Swap and Differential Gears

[fxrsrider]

My truck: 1973 620 L16 SWB A/T

My Trans: Jatco 3N71B

My Rear end: 4.625

 

Been researching the "620 5 speed swap."  datzenmike has been a godsend in helping to understand this whole thing.  Here's where I'm stumped.

 

What is the "ideal" 5-speed to differential gearing for daily street performance.  By "daily street performance" I mean, quick take off when the light turns green, quick through gears, and a comfortable drive in the HOV lane for an hour+ drive.  Something that's fun to drive.

 

Please assume that the L16  engine is just fine...built and tuned for street performance. (it's not a 1.8 or a 2.0 so please save the "you should have this xyz engine instead" comments for a different thread)

 

I've heard that the '80-'83 280zx non turbo close-ratio gearboxes paired with a 3.54 rear differential is "what everyone wants."  Is that true? If so, what is the benefit of this pairing as opposed to others?  Are there comparable box-to-rear end parings that are good for our little trucks?

 

datzenmike has generously explained how the mid-ratio 5 speed gearboxes would best suit my current 4.625 rear end.  Is there a better (or rather, ideal) rear-end gear ratio for the mid-ratio boxes for daily street performance?

 

To sum it up:

Which is mo' betta for a fun, zippy ride:  a close-ratio with X.XXX rear end OR a mid-ratio with X.XXX rear end?

 

Thank you, in advance!

 

[datzenmike]

Any transmission will be zippy with the 4.625 in it. With a differential that high you could get away with a mid or even a close ratio transmission

 

A 3.545 is way too low for an L16 and the 80-83 close ratio.  

 

Differentials. The '79 620 with L20B used a wide ratio transmission with a 4.11. The lower first gear helped get it moving with the 'sluggish' 4.11 differential. All other L20B 620s used a 4.375 and a mid ratio transmission. Seems reasonable that a close ratio would be near ideal with a 4.625. Don't forget this is with a 95 HP 2 liter engine. An L16 is 20 HP less.

 

For the heavy 620 I would select the transmissions this way depending on the differential.

 

less acceleration<<<<<<<>>>>>>> more acceleration

lower revs<<<<<<<<<<<<<>>>>>>>>>>>>>>higher revs

3.364,  3.545,  3.70,  3.90,  4.11,  4.375,  4.625,  4.875

............................................Wide... Mid....Close>>>>>>

 

The close ratio has a monster over drive so the shift from 4th to 5th will be a huge RPM drop. (this transmission was used in a similar weight car with a 160 hp six cylinder) Ok for cruise and good mileage but for passing and climbing a hill or driving into the wind you may have to use 4th with an L16.

[2wheel-lee]

Another variable is tire size. Smaller diameter tires will increase the RPM and improve acceleration. If you're running lower profile tires than stock, note that difference as well.

Edited September 20, 2018 by 2wheel-lee