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My 1971 521


d.p

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On 2/5/2022 at 12:38 PM, datzenmike said:

 

Wayno, DP's smaller tires make the 4.375 more like 4.776.

 

 

 

A 3.545 will be equivalent to a 3.889 with those smaller tires. A 3.70 will feel like a 4.039 and is also a good option.

 

Nothing wrong with 3.70 either.

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I can't tell you what you would like best. Going from 4.375 to 4.038 is a small change about 8.3%. Going from 4.375 down to 3.78? is more like 15.8% almost double.

 

Not suggesting you do this, if I had to drop lower than the 4.375 I would be conservative and go with the 3.70 (feels like 4.038) but only because I have driven a 620 with 4.11s and 33" tires and it was no fun at all. The 33" made the gearing feel like 3.364. I could live with it revving a bit but having power and torque that slow in every gear and if windy having to drive in 4th. On the high way every grade dropped the speed even if you didn't down shift.  

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5 minutes ago, d.p said:

So with the 3.7 and 5th gear at 3300RPMs I would be going 77? 

 

77.85mph, yes, as verified by my own math as well.  That's assuming those transmission ratios and tire sizes are correct for what you have.  I feel like tire sizes are a bit ambiguous.  If you wrapped a tape around one of your tires and got an accurate measurement that would make things more...accurate.  Your tire size gives me a circumference of 74-1/4" using 195/50R15 math...

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That's zx close ratio box numbers. Didn't you say there was virtually no difference shifting into 5th??? If you shift a 25.5% drop you would notice it, in fact the shift from 3rd into 4th happens to be 23% (which is close)..... did it feel the same???????

 

Didn't I suggest you may have a '79 zx mid ratio?

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3.321

2.077

1.308

1.000

0.864... this is only a 13.5% RPM change from 4th 

 

No way to know for sure which transmission it is. '79 was the first year 280zx and they used the mid ratio, I have one.  

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Found this in oilspots thread....

 

Ahh ok, well I did a 5 speed swap and my truck is still spinny on the freeway.  Meaning at 70 its around 3800-4k RPMS which I was it wasn't.   So wouldn't be inline with what @Oilspot is saying about replacing the 3rd member?  Because I swapped from a 4 to a 5 and it didn't make all that much difference at speed on the freeway.  

Edited January 27 by d.p

 

 

Matt what's it with 4.375??????? what he has now. See if it agrees with 3,800-4,000

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6 minutes ago, datzenmike said:

Found this in oilspots thread....

 

Ahh ok, well I did a 5 speed swap and my truck is still spinny on the freeway.  Meaning at 70 its around 3800-4k RPMS which I was it wasn't.   So wouldn't be inline with what @Oilspot is saying about replacing the 3rd member?  Because I swapped from a 4 to a 5 and it didn't make all that much difference at speed on the freeway.  

Edited January 27 by d.p

 

 

Matt what's it with 4.375??????? what he has now. See if it agrees with 3,800-4,000

 

Sounds about right, 3922rpm@70mph with 4.375's. 3.7 would drop him about 600rpm to 3317rpm@70mph.  3.54 would drop 750rpm down to 3173@70mph

 

 

Untitled9.gif

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He actually said the transmission was a '79zx.  OK the math is in the ball park at least.

 

3,173 sounds good for 70. If you go too low RPM the engine won't make enough to push through the air. Windy days are bad and  climbing grades. Naturally you can down shift to 4th and raise the RPMs and you should as 4th is not really a 'gear' it's a direct connection from the clutch to the drive shaft and takes a huge load off all the bearings on the counter shaft.

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I went from a 4.37 to a 3.89 and the drop in rpm was fine for the work truck without the trailer, I went from 4.88 to 4.37 in my 1965 Datsun 320 with a A15/dual SUs and I was fine with that, I have never skipped 3 gear ratios myself, but my NL320 I bought from Mike Klotz has 3.89 gears in it, it is alright to get going empty of extra weight, it is not that great above 60mph but that is alright for that truck and that was skipping ahead 4 gear ratios, it had 4.8 gears stock, 4.6, 4.3, 4.1, 3.89, I think the 3.7 is going to work as he has a souped up engine, the only thing that I think about is what cam he has in it, if it is a HP cam(loping), starting out might be sluggish, if the rpm is too low on the freeway it will be slow to rev up there also as high performance cams need rpms to shine and he is moving away from higher rpms, but he can always put the 4.3 gears back in or look for a 3.89 gearset.

I do not know how d.p drives, if he just putts around and wants to putt on the freeway at lower rpms I expect he will be fine, but 3.5 gears I think is pushing it for getting it moving, especially if at a light on an uphill grade, I did not have a big issue when driving just the 521 work truck with 3.89 gears, but with the trailer behind it I had a major issue, I went back to 4.3 gears, then I tried 4.8 gears and they were great but I wore them out in a year, I now have 4.6 gears and am happy, that is great gearing for the work truck/trailer, I wish the other LZ23 engine was in the truck though.

If it had been me, I would have gone with 3.89 gears, but the tire size is a curve ball, I think the 3.7 gears will work because of the tires, but I cannot change what 5th gear ratio he has, but I think it will help with power on the freeway as it is not a large drop in rpms.

Edited by wayno
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When Nissan designed it, it was a truck capable of carrying a payload so it is geared accordingly (for starting from a stop and for climbing grades) but if driven as a car you could drop down. If from 4.375 down to 3.889 that's about 12% so this would seem to be the yardstick to use. 

 

If you have a 4 speed a 13.5% over drive 5 speed swap is the same as a 3.889 when in 5th and you get to keep the acceleration and load carrying of the original gears..

 

 

Myself, I wouldn't care about the sound of revving on the highway, it would strictly be 'does it get better mileage' with the lower numbers and still be comfortable to drive around in (not too sluggish) 

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14 hours ago, datzenmike said:

That's zx close ratio box numbers. Didn't you say there was virtually no difference shifting into 5th??? If you shift a 25.5% drop you would notice it, in fact the shift from 3rd into 4th happens to be 23% (which is close)..... did it feel the same???????

 

Didn't I suggest you may have a '79 zx mid ratio?

 

Yes you suggested this when I got it and again, it had '79 280zx 5 speed' written on it when I got it. At least with an additional rear end I would have options, right now I do not.  I really dont like driving on the freeway because of how high the RPMs are, its just not really fun to me.  I would prefer lower RPMs on the freeway and still be able to ring it out around town.  I do use it as a truck but only to get mulch, flowers and shit like that and I live in a flat part of the country so hills or heavy loads aren't necessarily a concern to me.  

 

 

Edited by d.p
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11 hours ago, thisismatt said:

 

Sounds about right, 3922rpm@70mph with 4.375's. 3.7 would drop him about 600rpm to 3317rpm@70mph.  3.54 would drop 750rpm down to 3173@70mph

 

 

Untitled9.gif

 

Yeah that looks right about when I see at that speed.  Good work my man.  

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I feel like 3.7 is a good compromise. When I had my 510 and paid more attention to that scene it seemed like 3.7 was a sought after ratio and 3.54 was a little too high, at least for the guys doing 4 cylinder swaps, SR20DE/T for instance.

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Agreed, getting the 3.70 from Wayno and will throw it in when I get time.  To swap a rear end do I need to slide the axles out, pull the diff out and put the 3.70 in?  Do I need to take the drums off or can I slide the axles out far enough with them still on? 

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43 minutes ago, d.p said:

Agreed, getting the 3.70 from Wayno and will throw it in when I get time.  To swap a rear end do I need to slide the axles out, pull the diff out and put the 3.70 in?  Do I need to take the drums off or can I slide the axles out far enough with them still on? 

You dont have to remove the drums but your ebrake cable and brake line might not have enough room to move.... 

Also might be worth considering replacing the inner axle seal while you have it apart.... 

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