"83 280ZX close ratio box and 3:90 differential

Engine, Transmission and related drivetrain.
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abisel
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"83 280ZX close ratio box and 3:90 differential

Post by abisel » 16 Aug 2018 09:17

Will this combination cause issues?
Hard to get around in town?
Should the differential be upgraded to a 4:11?
The 280ZX had a 3:90 differential, so....

My engine is an L20B with a WebCam 91 mild street cam and dual Weber 40DCOE. Soon to have a header and 2.25" exhaust.

Please advise.

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funwithmonkeys
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Re: "83 280ZX close ratio box and 3:90 differential

Post by funwithmonkeys » 16 Aug 2018 20:13

I have the ZX close ratio box, the biggest difference is the .74 5th gear. It shouldn't cause you any issues around town but you will find that it will feel gutless in 5th. I have 4.44's and it still falls on it's face in 5th....not that I need it to accelerate hard at that kind of speed, if I wind out 4th I am nearing 190kph when I need to shift.
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Re: "83 280ZX close ratio box and 3:90 differential

Post by datzenmike » 17 Aug 2018 20:00

To confirm you do have the close ratio box, mark the input and output splines, place in first and turn the input spline 20 full turns while someone counts the output turns. (I use an old clutch disc) 20 divided by 3.062 (1st gear ratio) should be 6.5 output turns. A mid ratio is 6.0 turns, a wide ratio transmissions is 5.5 turns.

Not saying you don't have a zx close ratio but basically you can't really tell from the outside by looking at it. Sellers may embellish what they are selling or honestly are only repeating what they were told. I have a zx 5 speed and it's a mid ratio, they did make them for the ZX. I'll bet there are a few people out there that think they have a zx 5 speed but do not.

I have a close ratio box in my stash but hesitate to try it. Right now I have a mid ratio with a 13.6% overdrive and a 4:11 differential. I love it! Fifth is usable in town at 50k and acceleration is fabulous. I don't really have to down shift to pass on the highway either or when climbing hills. I'm sure that doubling the over drive ratio would make 5th gear useless except out on the highway. Now that said, this is a 710 that is 400-500 pounds heavier than the 510.

4th to 5th shift at 5,000RPMs the revs drop to 3,725 with the close ratio. With the mid ratio 5 speed it only drops to 4,320 RPMs. Or 1,3000 RPMs vs . just under 700. 25.5% is a hell of a drop for an over drive and a staggering amount for a 3.90 differential.
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abisel
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Re: "83 280ZX close ratio box and 3:90 differential

Post by abisel » 04 Sep 2018 18:06

Mike,
Sorry for a late reply.
When I rebuilt the transmission, I counted the teeth on the gears. So it is a close ratio. Can't find the sheet with the numbers now.
But if the 280zx had a 3:90 differential with a close ratio box, I'm guessing it did ok on the highway, but then again the engine had a bit more power than the L20B.

So we shall see once I get the transmission installed. I may have to go with a 4:11 differential.

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Re: "83 280ZX close ratio box and 3:90 differential

Post by bertvorgon » 06 Sep 2018 09:17

For what it is worth, just an FYI....

I have a true Nissan Comp close ratio transmission with the .865 overdrive. P/N 32010-N3130
1st gear: 2.906
2nd gear: 1.902
3rd gear: 1.308
4th gear: 1.000
5th gear: .865

This was originally chosen for a variety of reasons, tall 1st gear relatively so it was kinda useful at the slaloms, and the shorter 5th gear so I could pull some top speed, due to the fact that the 5th gear would actually let the motor pull high RPM.

I lived the .7 overdrive tranny and did not like it, as I was just too lame for ME and my needs. One has to ask oneself, do I want a slower engine RPM at freeway speeds? Fine if you do, I get that. Now, I run a 4:6 rear gear, again chosen for racing and acceleration so coupled with a short tire, 20", it was stupidly fast.

Now, with a 23" tall tire and that 5th gear, I do turn about 4,000 RPM on the highway at 100 - 105 KMH. I'm ok with that as when I hit the throttle in 5th, it just bugs off in a hurry.

I think you will be happy with your 4;11 gear set, as that would give you the best of both worlds and some here run that setup.

I am about acceleration first so being a bit buzzy on the highway has never been an issue for me..I wear ear plugs!
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1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
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Re: "83 280ZX close ratio box and 3:90 differential

Post by Byron510 » 06 Sep 2018 10:03

The "close ratio" ZX box - close ratio being a standard misnomer as it really isn't - but it's slightly closer ratio than the early box. However the ZX 'close ratio' nothing like what Bert posted above, which is a true close ratio box. There were three close ration comp boxes from what I understand in the 71B's, wide, middle and ultra close. The ZX "close ratio" box isn't any of the above, but has been widely called a "close ratio" box by people who obviously don't know better and it's now become main stream in the Datsun circles.

That aside, the late ZX box works very well (for me) with the 4:375 and 4:44 gear sets as a compromise for those that are weekend warriors and actually drive the car to events for any distance. I'm in the 150 HP/Torque range with a 2.2 L/Z frankenmotor. The .74 OD is nice on the highway, as my car also runs in the 4K range at 70mph/120KPH.
The lower rear gears help with the tall 1st - which is much taller than the 510 4spd ever had, and the gears do get a bit closer as the 2-3 jump isn't as much of a killer. For auto X, I should have run a lower rear gear if it was available to me, as I often found I needed to be somewhere between 1st and 2nd. Maybe Keith did get it right as I spent more time hunting/shifting which wastes precious time on the course. Keith also had twice the HP/Torque -0 which is obviously the game!
My thoughts anyways, pretty hard to go wrong with the low gear. It equals instant HP under all circumstances and is by far the cheapest way to get 60 ft times and 1/8 mile if that's your thing! Definitely a winner at the stop light GP! And the tall ZX OD helps on the highways side to make the compromise.

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Re: "83 280ZX close ratio box and 3:90 differential

Post by 510rob » 06 Sep 2018 11:16

What about the roadster CR box?

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bertvorgon
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Re: "83 280ZX close ratio box and 3:90 differential

Post by bertvorgon » 06 Sep 2018 13:24

Just as another point of reference, here is a page from the Nissan Competition catalogue:

At least you can really see the difference in a real race transmission, where 1st gear is really tall, baking the clutch when you try to move from the pre-grid to the race track... :(
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1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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Re: "83 280ZX close ratio box and 3:90 differential

Post by icehouse » 06 Sep 2018 17:15

Keith our box is the same as the SR20DE box in my car except I have the 3.3 1st gear, which is useless. 2nd through 5th are the exact same gears. I personally want to sort out a .6 5th gear. As a daily driver .86 SUCKS!!!!! I don't see any of my cars racing future requiring 5th anyways. So if yours blows up in the future you could get a SR box and upgrade to a shifter with a much better feel.
"People don't like it when shit doesn't match their rule of thumb." Sam

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bertvorgon
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Re: "83 280ZX close ratio box and 3:90 differential

Post by bertvorgon » 07 Sep 2018 09:24

That sounds pretty good, does the Sr transmission bolt up to the L-series?

I guess..no..I do know I like to be able to just roll on the throttle in 5th and the car just takes off so 5th gives me a very usable power range when in the mountains, without having to gear down, unless I REALLY want to take off.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

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1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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icehouse
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Re: "83 280ZX close ratio box and 3:90 differential

Post by icehouse » 07 Sep 2018 12:10

The SR trans doesn't bolt on as is. It's easy to make it bolt on though. Just gotta take your bell and drill the shifter fork guide holes bigger and machine the secondary shaft bearing hole in the bell bigger, or run the B size bearing. I've done it. My buddy did his with a delwalt drill! haha.
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Re: "83 280ZX close ratio box and 3:90 differential

Post by bertvorgon » 08 Sep 2018 06:54

All you have to do is........

When the time comes I will use up my spare first.
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1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

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Re: "83 280ZX close ratio box and 3:90 differential

Post by Tedman » 08 Sep 2018 21:49

Something the OP needs to realize- the 280ZX had something like 205/60-15 tires. Much taller 24.7" diameter vs. a nominal 23" tire in a 510. And the L28E had a lot more torque than a four, and a lot more weight to motivate. So gear ratio and diff ratio are not the complete story.

That said, I just went from a 4.38 gear in my 510 to a 3.90 gear with my ZX 5 speed and 23" tires. This because the 4.38 was getting noisy and needs a rebuild. I can say that the car is not as snappy around town, and my slightly breathed upon L20B has just adequate torque to get away from a stop with a light flywheel. I would not run this diff/trans combo with a smaller motor. The relaxed highway cruise RPMs are nice, but require downshifts to 4th on steeper grades where the 4.38 might have pulled it in 5th. I'm a big fan of the 4.38 gear with the ZX trans on the street. You get the benefits of the closer ratios 1-4 and the taller 5th for highway cruising is about the same RPMs as a 0.85:1 5th with a 3.90 rear gear.

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Re: "83 280ZX close ratio box and 3:90 differential

Post by Tedman » 08 Sep 2018 22:23

I guess I should add that the late "close ratio" 280ZX gearboxes with the tall 5th gear can be identified by having the 10mm speedo pinion sleeve retaining nut on the bottom (6 o'clock position). Gear ratios are:

7/70-6/83
3.062 1.858 1.308 1 0.745

Compared with the chart above from the Motorsports catalog you can see that the stock 1-4 ZX ratios are relatively close to the Comp CR box except for 5th gear.

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abisel
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Re: "83 280ZX close ratio box and 3:90 differential

Post by abisel » 09 Sep 2018 04:41

Tedman, thanks for the info.

I do plan to upgrade to 15" wheels/tires which should help a little if I keep the 3:90 diff.

My L20B has been worked on a little too. Head work to install new valve seats, guides, and larger intake valves. A pair of Weber 40DCOE carbs, intake manifold and head ports enlarged to match the manifold gasket and a WebCam 91 mild street cam. Soon to install an Ermish header and 2.25" exhaust system. So the engine has a bit more power than a stock L20B. How it will perform with a close ratio ZX box and a 3:90 diff is still TBD? I do have a line on a 4:11 diff however. So....

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