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auto upgrade with shorter diff ratio


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#1 andyk_79

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Posted 09 July 2014 - 07:09 AM

So I'm thinking about what I might do down the track with my z's slush box.............  and I'm genuinely considering keeping it auto so that my wife (who has a very dodgy left ankle) can drive it (the things we do for love!)

One option that has popped to mind is replacing the 3n71b with a 4n71b. Given the ratios on the first 3 gears are the same I'm thinking to couple this swap with a mildly shorter ratio on the diff to improve acceleration slightly while still being able to cruise on the highway with the longer 4th.

Apart from the bell housing change I'm also guessing I would need to have the tailshaft shortened because of the overdrive unit at the back of the 4 speed?

Any other pitfalls or considerations? Funnily enough there's not much info I can find on this with autos being so unpopular.

#2 d3c0y

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Posted 09 July 2014 - 08:38 AM

A standard zed clutch is seriously easy to push in though. Like we are talking Barina easy here.

#3 andyk_79

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Posted 09 July 2014 - 09:53 AM

Yeah it's not just the force required - it is pretty much general movement in the left ankle is non existent.  She could drive a manual in an emergency but would be in a bit of pain for a while afterwards - less than ideal.

#4 d3c0y

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Posted 09 July 2014 - 11:35 AM

Spewing, autos are sucky. Maybe just learn to love your wife a little less? Or get her a really nice daily driver as justification?

#5 andyk_79

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Posted 09 July 2014 - 12:13 PM

Old autos are sucky that's for sure - I can deal with the modern ones (especially in traffic!).  Our family car is auto and she's fine driving that - maybe I'm being over cautious !

Costwise with the performance I would like etc I guess it could end up being a better and cheaper option to just get a full engine and gear box transplant from a GTS-T auto half cut.  I saw one recently for 2 grand (can't remember which importer) that included all the important bits.  Ultimately I'd still like to pursue period appropriate upgrades but if I go down the 4n71b + diff change + headwork/cam/maybe stroke on the L24 I'm starting to think period appropriate might actually be more expensive for less result............

#6 d3c0y

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Posted 09 July 2014 - 12:57 PM

Oh man, in $ vs HP equation its not even close. An RB will walk the L motor ever day of the week in that regard. An RB conversion is so easy too, you get all the parts to bolt one in now and you would go straight for an RB25 NEO.
Stroking an L24 is basically just an L28 anyway. You will spend at least $15k to make an L28 match a stock RB25DET.

#7 dat2kman

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Posted 09 July 2014 - 01:01 PM

3sp and 4 sp Autos both have same top gear ratio, 1:1.
A shorter diff will def improve acceleration in both, but you'll have a slightly higher engine rpm at highway cruise speeds, by a few hundred rpm's.
And use a bit more fuel.
But at least it wont be as sluggish through the gears in the suburban traffic light grand prix!

#8 andyk_79

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Posted 09 July 2014 - 01:12 PM

3sp and 4 sp Autos both have same top gear ratio, 1:1.


I think you'll find they both have the same ratio on 3rd/drive (as well as 1 & 2).  The 4n71b ratio on 4th/OD is 0.686.  I wouldn't bother if the ratio at the top was the same.

#9 andyk_79

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Posted 09 July 2014 - 01:17 PM

Oh man, in $ vs HP equation its not even close. An RB will walk the L motor ever day of the week in that regard. An RB conversion is so easy too, you get all the parts to bolt one in now and you would go straight for an RB25 NEO.
Stroking an L24 is basically just an L28 anyway. You will spend at least $15k to make an L28 match a stock RB25DET.


Yep I was starting to suspect that as I delve more into what I would eventually like to do.  Maybe it's time I start researching the cost of an engine and tranny swap rather than the cost of a rebuild on the L24.  I liked that it was the original engine that came from the factory in the car but common sense has to win out somewhere !!

#10 d3c0y

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Posted 09 July 2014 - 05:03 PM

Just keep the L24 under the bench.


Everything you need to fit it:
http://www.mckinneym... 260 280Z parts





#11 andyk_79

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Posted 29 July 2014 - 08:13 PM

Once again proof that if you can think of it likely someone has done it before:

http://www.classiczc...eed-maxima.html

Still keeping this option in the bottom draw in keeping with the old style build plan.

#12 D.O.G

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Posted 31 July 2014 - 08:24 PM

So I'm thinking about what I might do down the track with my z's slush box.............  and I'm genuinely considering keeping it auto so that my wife (who has a very dodgy left ankle) can drive it (the things we do for love!)

One option that has popped to mind is replacing the 3n71b with a 4n71b. Given the ratios on the first 3 gears are the same I'm thinking to couple this swap with a mildly shorter ratio on the diff to improve acceleration slightly while still being able to cruise on the highway with the longer 4th.

Apart from the bell housing change I'm also guessing I would need to have the tailshaft shortened because of the overdrive unit at the back of the 4 speed?

Any other pitfalls or considerations? Funnily enough there's not much info I can find on this with autos being so unpopular.


The conversion works pretty well in my 280zx.
http://www.viczcar.c...pic,4107.0.html

My car's off the road now, but not due to gearbox problems.
Pete

#13 andyk_79

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Posted 01 August 2014 - 06:46 AM

Thanks mate. I had read your thread before with interest so I'm hoping that doing the same on a 240 is similar to your experience with the 280

#14 andyk_79

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Posted 01 August 2014 - 07:38 AM

Mind if I ask which shop it was that did the work for you - just noticed you're not too far away from me.

#15 D.O.G

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Posted 01 August 2014 - 08:34 AM

Mind if I ask which shop it was that did the work for you - just noticed you're not too far away from me.


Apart from the tail shaft, I did it myself.
The tail shaft was done by a place in Holbeche Road, Arndell Park (near work for me) whose name I've now forgotten. :-[




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