1973 Automatic 240z, Manual Swap and Diff Questions

[Ownallday]

Hey guys, I recently sold my 1978 Datsun 280z and picked up a 1973 Datsun 240z. I plan on doing a manual swap with a 280z wide ratio 5 speed as I have a rebuilt one. Down the road I plan on doing a NA L28 build and push at least 250hp+ from it with an ITB setup but for now the L24 will have to do for the next two years. I live in California and the car will mostly see city driving, freeway driving but will also see the canyons pretty often in spirited driving and maybe a track day once every year. I am wondering what would be the best differential to pair with the type of driving I will do and my engine goals. I also plan on doing an LSD conversion for whatever diff I get. (Also if anyone can specify if they know what gearing a stock automatic 1973 240z came with would be appreciated).

[Terrapin Z]

Automatics were 3:54 with the 4 speed being 3:36 Most 5 speeds had the 3:90 but in a R200 diff. Other ratios are available as well. 

[Ownallday]

11 minutes ago, Terrapin Z said:

Automatics were 3:54 with the 4 speed being 3:36 Most 5 speeds had the 3:90 but in a R200 diff. Other ratios are available as well. 

Thanks for the response, so a 3.9 would be a good option?

[Yarb]

From past experience with what you are dealing with is the 3:90 ratio the engine will run at 3k rpm on the highway between 60&65 mph. 3:73 is probably optimal if you want to bring it down a bit. Although getting to that speed the 3:90 gets you there quickly.

[Ownallday]

1 hour ago, Yarb said:

From past experience with what you are dealing with is the 3:90 ratio the engine will run at 3k rpm on the highway between 60&65 mph. 3:73 is probably optimal if you want to bring it down a bit. Although getting to that speed the 3:90 gets you there quickly.

Sounds like the 3.9 is the way to go then. my 280z typically ran at 3k rpm at 65 mph and it did not bother me at that rpm.

[dutchzcarguy]

It's ( a ratio ) on automatics 1 : 3,54  on the 5 speed manual it's best 1 : 3,9  for the ratio of the diff.

(I don;t get the numbers up here but i never heard of them 3:90 ???? that's the same as 1:30 ??  must be wrong..)

[Terrapin Z]

9 hours ago, Ownallday said:

Thanks for the response, so a 3.9 would be a good option?

A lot of it depends on what YOU want out of the car. Nissan paired the 5 speeds with 3:90 gears for the most part. it is a good combo. 

I went with 3:70 and a ZX "close" ratio myself but that was my choice. The different 5 speeds have different 5th ratio's too. 

[dutchzcarguy]

21 hours ago, Terrapin Z said:

I went with 3:70 and a ZX "close" ratio myself

As you guys say: WTF.. is 3:70 ? You surely mean 1 on 3,7 or 1:3,7 ratio ????

Could be me as i slept about 10 hours.... this month!!  so.. i'm sorry but wtf..

[dutchzcarguy]

1:3,7 means that the in going axle (On a differential)  turns 3,7 times to turn the outgoing axle only ONES !!!  OR AM I GOING CRAZY ????? 🙃

[Terrapin Z]

2 hours ago, dutchzcarguy said:

As you guys say: WTF.. is 3:70 ? You surely mean 1 on 3,7 or 1:3,7 ratio ????

Could be me as i slept about 10 hours.... this month!!  so.. i'm sorry but wtf..

 

[heyitsrama]

I feel like socal drives fast, you'll probably be doing 70-80 mph on the regular... with a 3.9 the RPMs are going to be up there.