Changing rear gearing

[datsunrides]

Thinking of changing out the stock 521 gearing (4.375?) to something along the lines of a 3.7 or possibly higher. My truck has a KA-E swapped in, so has decent power, but I am somewhat constrained by the 3 speed automatic trans, i.e no 5 speed swap and no room for the 4N71 box. Since I live a ways away from any functions, freeway cruising speed is my main concern, not acceleration or top speed. Are the 3.7 gears a good choice, or can I get away with taller gearing (if I can find it) due to the torque of the KA? If I did the math correctly, swapping from the 4.375 gears to a 3.7 should net an approx 700 rpm drop.

 

Mark

[yello620]

I am still running a L20b in my 620, but i have ran several sets of gears before i decided that the 3.89 gears were the best.

 

I was also stock at 4.375 and the revs were just too high, then i went to 4.11 and those were a waste of my time. Not even close to enough RPM drop.

 

Next i went for the 3.7 gears and thought that they were decent for a while, but realized that the RPM were now too low on the highway, i had my gas pedal into the Weber secondaries to maintain anything over about 70mph.

 

I have now settled on a set of 3.889 gears and they are great. RPM's are good at cruising speeds. Yet the acceleration from stoplights is still completely acceptable, second gear starts are still not a problem.

 

The 3.89 gears can be found very easily in many of the 720 trucks, or in the 510 wagons if you change the spiders gears to truck spiders. So, i would recommend against the 3.7 gears, unless you think the torque of the KA can overcome the extra gear change.

[datzenmike]

There was a Mileage Option 720 that had a 3.364 rear ratio and a small Z20 motor. The Hardbody KA 5 speeds were 3.545s, autos were 3.70 but the Hardbody 2wd is way heavier than a 521. You should easily be able to run 3.545s in a 521if you are not worried about quick take offs.

[datsunrides]

Thanks for the options. I think I will go with either the 3.7 or 3.54 gears. Basically whatever I can find first. So the Hardbody H190 is a direct swap into the 521 H190? Is it like the 720 in that the gear ratio is on the cowl tag?

 

Mark

[Z-train]

I have a set of 3.889's that are going in my LZ-23 engined 620.SHould be about perfect.

[datzenmike]

Thanks for the options. I think I will go with either the 3.7 or 3.54 gears. Basically whatever I can find first. So the Hardbody H190 is a direct swap into the 521 H190? Is it like the 720 in that the gear ratio is on the cowl tag?

 

Mark

 

Check the size of the flange bolt pattern behind the rear U joint but otherwise should. The engine tag is similar to the 720.

[Z-train]

Do the math first:

 

http://www.wallaceracing.com/gear-speed.php

[d510addict]

Old thread I know, but with a fairly stock l20b would I really take a low end power hit by going from stock gears to 3.90's?

[datzenmike]

Old thread I know, but with a fairly stock l20b would I really take a low end power hit by going from stock gears to 3.90's?

 

There is one way to have your cake (performance) and eat it too (economy). Get a 5 speed with over drive. Negligible change to in town pick up and acceleration but has an RPM dropping gas saving 5th gear for the highway.

[d510addict]

Ya I'm looking for one but damn it's not easy to find a local one.

[yello620]

I changed my 76 620 from the stock 4.375 rear gears to 3.89 gears. I am running a L20b that is stock except weber 32/36, And i would have to disagree with the low end power loss, However i am also sure that it depends on how you use your truck. Mine is used for daily driving with the very occasional load or hauling. I would be happy to race any of you 4.375, L20b, weber 32/36 trucks with a 5spd through your third gear. Power loss is not a problem. And then i will shift into fifth and enjoy my low RPM cruising down the highway with way less cabin noise than your 4.375/5spd truck.

 

Enjoy