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5 speed vs. 3.9 diff - either or both?

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Boink

Boink

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Hi all:
I have an opportunity to acquire a re-built Datsun 210 5 speed with conversion kit (for my 1275) as well as a 3.9 differential (as I'm currently with a rib-case tranny and the standard 4.22 diff).
I'm not a freeway driver, though I must admit that it would be nice to shift to a 5th at 45-50mph... or to have lower revs at those speeds. I'll add that I think my rib-case is getting a tiny bit worn and I think my clutch fork is a little bent. Still, everything is functioning fairly well.
I'll probably generate a lot of opinions, but I'll take 'em.
Would the 5 speed be enough, OR the 3.9 diff, OR both?
If I decide on any of these, and get lazy and pay someone to do the work (as I'm not up for these sorts of projects like I used to be), how many hours would be involved (or, alternatively, what sort of ball-park cost given that I'd have the parts).
Thanks for your thoughts.
 
As far as installation, it can be done in a weekend or even a single day if you have "all your ducks in a row" and if you are focused on the task. I've seen people do it in a day or take several months.

As to your main question...

I just posted this info in another similar thread. Going from a 3.9 to a 3.7 will only drop your RPM by 150 to 175 RPM at either 60 or 70 MPH. Compare the numbers for a 5 speed with any of the other ratios. (3.9/3.7/3.55)

For comparison purposes, based on a fixed tired size of 155/80R13 = diameter of 22.76"

Tire Diameter(inches)------------------ 22.76_____22.76_____ 22.76
Gear Ratio---------------------------------3.90______3.73 ______3.55
Miles Per Hour---------------------------70________70________70
Overdrive %(none=0)-------------------0 _________0 _________0
RPM @ 70 MPH-In Drive-----------4030_____3855 (-175)_____3669 (-186)

Tire Diameter(inches)--------------------22.76_____22.76_______22.76
Gear Ratio--------------------------------- 3.90_______3.73_______3.55
Miles Per Hour-----------------------------60_________60_________60
Overdrive %(none=0)--------------------0___________0_________0
RPM @ 60 MPH-In Drive--------------3454_____3304 (-150)_____3144 (-160)

Datsun 5 speed w/.821 overdrive (60A) (60L = .846)

RPM @ 70 MPH-In overdrive----------------3309 3165 (-144)_____3012(-151)
RPM @ 60 MPH-In overdrive----------------2836 2712 (-124)_____2581 (-131)
 
Thanks for the quick reply Gerard. I'd be going from a 4.22 diff to a 3.9, however, it would appear that I'll get a greater reduction in high end revs from going to the 5 speed, no? Would combining the two compromise acceleration in 1st or 2nd?

As for doing it myself, the major in and out wouldn't be bad. Might be the fettling I'm not so keen on. The conversion kit is for a Morris Minor without hydraulics (and the "kit" is a direct-from-Paul-Asgeirsson one), so I'd have to probably add a slave cylinder. [wish I was in your neighborhood :smile: ]
 
I find the 3.9 with the 5 speed is a good combo, but better if you have more than bone stock performance from the engine. For street driving, I don't think you'll it sluggish in low gears. Some people even go to a 3.7 and like it, but I think 3.9 is plenty. If you are not doing much highway driving, I'm not sure why you'd make the swap to a 3.9 though?

Boink said:
Thanks for the quick reply Gerard. I'd be going from a 4.22 diff to a 3.9, however, it would appear that I'll get a greater reduction in high end revs from going to the 5 speed, no? Would combining the two compromise acceleration in 1st or 2nd?
Click to expand...
 
Thanks again. The 3.9 is only because "it's there." Good advice to just do the 5 speed with Paul's conversion kit.
 
Is that the kit you got in the deal?

Boink said:
Thanks again. The 3.9 is only because "it's there." Good advice to just do the 5 speed with Paul's conversion kit.
Click to expand...
 
Well, sort of. A dear friend of mine knew Paul well (and I'd met Paul on a few occasions - not far from me). This friend is sort of cleaning house... has many goodies. He had originally planned to put the 5 speed and Paul's kit in a Morris Minor (with 1275).

BTW, don't know if you ever saw Paul's place here in Portland, but it was an amazing garage. SO much Morris stuff.
 
I knew Paul for years, and he used to visit the SF Bay Area from time-to-time, so I would see him periodically. I only saw his garage in pictures. That's the kit to have. Hopefully, you already have the machined flywheel and Datsun clutch, but the kit for the Morris won't have the correct rear mount for the Sprite, nor the mount for the slave cylinder (the Morris is a mechanical clutch).

Boink said:
Well, sort of. A dear friend of mine knew Paul well (and I'd met Paul on a few occasions - not far from me). This friend is sort of cleaning house... has many goodies. He had originally planned to put the 5 speed and Paul's kit in a Morris Minor (with 1275).

BTW, don't know if you ever saw Paul's place here in Portland, but it was an amazing garage. SO much Morris stuff.
Click to expand...
 
Figured you knew him. Good guy.
Yes, I think I have everything except to resolve the clutch slave. I need to take stock in it all (before I get too carried away). Maybe this won't work. I'll have to check all that first and then ask for your advice. Any recommendations appreciated.

Paul's garage was one of those lovely large old mounds-of-stuff ones, but he knew where everything was.
 
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Check my 5 speed page. You can see all the components required for the Sprite 5 speed kit listed. There is also a video link at the top of the page which will give you visuals of all the pieces needed. All the rest you have will work; just the transmission mount is different.

https://gerardsgarage.com/Garage/Tech/5speedKit.htm
 
Thanks again. Yes, I was going to visit your page on this.

I see you have the link to Paul's memorial page too.
 
If you have questions, you can email me directly.
 
A difficulty with installing only the 3.9 is that the stock transmission's non-synchro first gear would need to be used a bunch more in city/regular driving with the 3.9 than with the 4.22. Doug
 
twas_brillig said:
A difficulty with installing only the 3.9 is that the stock transmission's non-synchro first gear would need to be used a bunch more in city/regular driving with the 3.9 than with the 4.22. Doug
Click to expand...

Thanks. Good point. Hadn't thought of that.
 
Mark,
I was running a 1275 with the 5 speed and the 4.22 diff. I would definitely be trying the 3.9 with the 5 speed if I had one in hand.
I have Gerard's conversion kit. With my engine failure I'm in the process of dismantling all of it, so if want to compare any pieces side by side let me know.
Gary
 
Hey Gary. I should have looked more closely at your Bugeye at the ABFM.
How long will you have everything out of the car? You are relatively close by and I could easily pop over there. Is this a winter project?
 
Mark,
Yeah this is an unexpected winter project, tearing all down now. Let me know if you want to come take a look.
ps I'm looking for a replacement 1275 if you have one laying around. ;)

Gary
 
gfholl said:
Mark,
Yeah this is an unexpected winter project, tearing all down now. Let me know if you want to come take a look.
ps I'm looking for a replacement 1275 if you have one laying around. ;)
Gary
Click to expand...


I may have a lead! Let me contact my buddy (same as with the spare 5 speed) and see.
I'd sure like to come by. Hopefully not in a hurry (and this weekend is supposed to be dreadful). Being retired now, I'm easy.
 
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