Datsun 620 differential

[Jc125]

Hi, I have a question about a swap I'm doing. I'm installing an L20B with a 280Z close ratio transmission on a Datsun 620 that originally has an A15 engine.
My question here would be if the original diff from an A15 engine will work well with the 280Z close ratio transmission.

If this isn't a good combination of transmission and differential for a daily car what other diff do you recommend swapping out?

I haven't finished the swap yet, but I'd like to get this question cleared up.

Thanks guys 

[Crashtd420]

What year is your 620, also are you sure it's an a15, not a j15.....

 

Also if you have the h190 axles, which i think were in all the trucks from the 520 to the 720, your fine.... 

Not sure what ratio that would be but it will be strong enough if that's your concern...

[Jc125]

Yes , sorry, it's a J15.

1973 

[Jc125]

The doubt I have is that I don't know what ratios it has and if when putting the close ratio transmission, it will be that when driving it, the rpm will be too high, or lose power between gears.

[wayno]

Just drive it first and then decide if you want to change anything.

[Crashtd420]

1 hour ago, Jc125 said:

The doubt I have is that I don't know what ratios it has and if when putting the close ratio transmission, it will be that when driving it, the rpm will be too high, or lose power between gears.

I agree with Wayno, you'll have a better idea once you drive it..... 

Personally I think you'll be fine,  datzenmike can probably give you some real world numbers.... 

 

There is a way to figure out your rear end ratio if you wanna know,  by rotating the wheel while counting your drive shaft rotations... 

Again Mike or a google search can get you that information....

The close ratio might only effect the first 3 gears.... 4th is 1:1 with the engine  .... not sure if the 5th gear is different with a close ratio vs a normal 5spd... 

 

[datzenmike]

First lets identify that transmission, as the 280z did not have the close ratio only the '80-'83 non turbo 280zx had them.

Place the transmission in first gear.

Mark the input and the output splines.

Turn the input 20 times while someone counts the output turns.

 

Output turns:

5.5............ wide ratio

6.0............ mid ratio

6.5............ close ratio

 

So what do your really have?

 

There seems to be some confusion about the RPMs being too high. If anything I would be worried about the RPMs being too LOW!!! All 4th gears are the same one input turn for one output turn. However the close ratio (if you have one) has an extremely low over drive 5th gear which lowers highway cruise RPMs.

 

Lowers the RPM by:

wide... 13.6%..... 3,000 RPMs in 4th becomes..... 2,592 RPMs in 5th

mid..... 18.7%..... 3,000 RPMs in 4th becomes..... 2,439 RPMs in 5th

close.. 25.5%.... 3,000 RPMs in 4th becomes..... 2,235 RPMs in 5th.... here if you don't have enough power at 2,235 RPMs to maintain that 55 MPH the truck will begin to slow OR if driving into a head wind or up an incline you will need to constantly down shift into 4th

 

J15 has to be Mexican 620. No idea what differential it would have. Better post a picture of it. 

 

 

Differential ratio procedure.

 

Jack ONE rear tire up off the ground.

Mark the driveshaft and the tire.

Turn the tire 2 full turns while having the drive shaft turns counted. A little over 4 1/4 driveshaft turns would be a 4.375. If not accurate enough turn the tire 10 full turns and divide the drive shaft turns by 5. That should easily differentiate a 4:11 from a 4:375 from a 4:625