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PostPosted: Mon Jul 30, 2012 11:01 am 
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I have a question regarding changing out one differential to another in terms of gear ratio.

I am looking to shorten the gear ratio (currently I have a 3.7:1). I know that 3.54:1 is available for early z31 turbos and 3.9:1 is available on late model nonturbo.

If I switch out the differential, how do you correct the speedometer to show the correct speed (and doesn't involve a black sharpie pen :P)

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Justin
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PostPosted: Mon Jul 30, 2012 1:52 pm 
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jtang wrote:
I have a question regarding changing out one differential to another in terms of gear ratio.

I am looking to shorten the gear ratio (currently I have a 3.7:1). I know that 3.54:1 is available for early z31 turbos and 3.9:1 is available on late model nonturbo.

If I switch out the differential, how do you correct the speedometer to show the correct speed (and doesn't involve a black sharpie pen :P)


Buy the appropriate/matching tooth # speedometer gear (mounted on sending unit end that plugs into the transmission).

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PostPosted: Mon Jul 30, 2012 2:41 pm 
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Unless they've changed things from when I was a kid (very possible),
"Shortening" a gear ratio means moving to a higher number, "quicker" ratio: e.g. moving from a "fast" 3.7:1 to a 'quicker" 4.11:1

"Lengthening", "lowering", or "stretching" the gear ratio means moving from a high-number "quick" ratio to a lower-number "long" or "fast" ratio: e.g. Changing from a 'quick' 4.11:1 to a 'hi-speed' 3.1:1.

Bryan Little's excellent www.DATSUNZGARAGE.com website covers all the different color and cog speedometer gears, too.

Frank

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PostPosted: Mon Jul 30, 2012 3:16 pm 
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Yup,

I am looking to go from 3.7:1 to 3.9:1

Of course this is all theory until I actually find a 3.9 diff!

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PostPosted: Mon Jul 30, 2012 4:55 pm 
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So you're going to make your car very slightly quicker, and very slightly slower, by installing slightly "longer" gears in it.

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PostPosted: Mon Jul 30, 2012 8:16 pm 
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Yup. Just an idea I am toying around with.

For autoX purposes I am pretty low in my power band for 2nd gear. If I switch the diff I think it might help put me deeper into 2nd as opposed to being "inbetween" 1st and 2nd.

But the REAL reason I am looking for another diff is because I may have found a LSD center that I need to swap into a r200 longnose pumpkin. So if I am grabbing one anyway I might as well look for the one most suitable for my purposes. Will know for sure in a week.

I could re-use my pumpkin but I'd rather keep it together. This reduces the "down time" on my car since it's my only motor vehicle here and also leaves me with a spare... just in case.

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PostPosted: Mon Jul 30, 2012 8:42 pm 
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Gotcha. Makes sense.

Bryan Little runs 4.11:1 in his 240Z and it takes off like a rocket. He can almost use 3d gear in AutoX (AutoZ?) while everyone else uses 2nd. His times are very quick (when he doesn't get lost :roll:).

Depending on which wheel/tire sizes you use, that little bit of a difference might not be very noticeable to you. You might want to consider a slightly larger jump.

Frank

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PostPosted: Mon Jul 30, 2012 8:45 pm 
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Are the larger ones easy to find? Looking for the most economical option with preference to faster accel. I'd settle for a 3.7 (stock) if it's the cheapest option... not much funds left after the LSD (for now...).

Do you happen to know if the 280ZX r200 bolts right in to the 300ZX (or any other r200's for that matter)? Where does the 4.11:1 come from? I think it might be from an S12 but I am not sure.

I wonder if that 300zx you saw at Chick's has the diff still (unless Greg C. got to it already.... :P)

ALSO:

Thanks Phrog! :)

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PostPosted: Wed Aug 01, 2012 6:46 am 
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First thought, Try and find a 3.9 R-200. IF, and I mean IF you do, and pay the 200 bucks or so, then it should bolt in. Since you already have a 3.7, sounds close to me, how about some basic engine mods for that quicker pick up. Just a thought. I finally got my 3.9 after 3 years of looking, and had to buy the whole car.....best $300 I spent. Furthermore the 4:11 is even more rear. I have a 4:08 snub nose if interested, but that of course takes alot of modification. (new rear mount, front mount, and driveshaft) FYI the speedo gears are color coded to the differential. Brian Little's site has the chart.


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PostPosted: Wed Aug 01, 2012 9:23 am 
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For the sake of any reader who doesn't yet understand what these ratios mean and what effect they have on the car, here are a few thumbnail rules:

1. The ratio we're describing refers to the relationship btwn the driving gears inside the differential. There are two basic gears in there we're talking about; the smaller (driving) pinion gear which is attached to the driveshaft, and the much larger (driven) ring gear which is attached to the rear wheels. The numbers we're throwing around are the numerical relationship btwn how many revolutions each one makes. For example, if Justin has a 3.9:1 rear ratio, that means his pinion gear turns 3.9 times to make his ring gear (and wheels) turn once. In 4th gear, the transmission ratio is 1:1 (meaning each engine revolution results in one driveshaft revolution). You can determine the differential ratio with the car on a lift and the gearbox in neutral. Marking the driveshaft and the rear tire and turning the driveshaft by hand, count how many times the driveshaft turns until the rear wheel makes one revolution. (I just use the air valve stem as a marker, and sometimes you can get lucky and find a balance weight welded to the driveshaft).

2. Because in 4th gear (direct drive, or 1:1 ratio ~ one engine rev equalling one driveshaft rev), our engine turns 3.9 revolutions to make our rear wheels turn once, we can compute the car's theoretical top speed using basic high school math. (FINALLY! We actually get to use high school math for something useful!).

Knowing the diameter of your rear wheels and tires, you can compute their circumference, which is actually how much distance they roll during a single revolution. (Use C= Pi x Diameter) You can then figure how many engine revolutions it takes to make your rear wheels travel a certain distance (like a 1,320ft quarter mile or a 5,280ft mile). Knowing your engine redline (usually about 7,000 rpm), you can figure how many revolutions per minute your rear wheels will turn in direct-drive 4th gear. From there you can multiply the distance they roll by the number of times they roll per minute (wheel rpm) and that's the theoretical top speed of your car with those gears. Things get complicated if you have an overdrive gear (usually 5th or 6th gear), which lets the gearbox's final drive turn more than once for each engine revolution. But if you're just trying to get a ballpark figure, this will do.

3. The number relationship btwn the driving pinion gear and the driven ring gear (the relationship btwn the driveshaft and the wheels) also indicates what you have more of, speed or torque. In differential ratios, they are almost mutually exclusive ~ you can't have both. You either select a "long" (numerically low) ratio for speed, or a "short" (numerically high) ratio for torque. Selecting an extreme in one will make the other suffer.

The greater number of times the pinion gear (driveshaft) turns to make the ring gear (wheels) turn once, the more TORQUE you develop and the quicker the car overcomes inertia to take off from a standing start. Drag cars commonly use 5.10:1 ratios or greater, meaning the driveshaft spins 5 times to make the wheels turn once. That's advantageous for drag cars, but if you're driving on the street, that ratio would keep your engine screaming up around redline just to get around town at the normal speed limit. That kills fuel economy. You really need a compromise, which will give you great torque and still let you drive the car in normal traffic and at highway speeds without over-revving the engine. Most people opt for a ratio of 'about' 4:1 (3.9:1 or 4.11:1).

The fewer number of times the driveshaft turns to make the rear wheels turn once, the more TOP SPEED you develop, but the slower your car will get up to that speed. Bonnyville speed trial racers have very low overdrive gears because they don't care how long it takes them to build up to 300mph ~ they are only interested in making the car go as fast as that engine is capable of.

In Z car worlds, if you're racing your Z on a fast track and want top speed from it, you chooZe the "lowest" or "longest" gear ratio you can find. If you could buy a 2.9:1 gear ratio, your Z might be capable of a sustained 170mph, but it would be really sluggish building up speed out of slow corners or from a dead stop.

4. So for normal street use with spirited driving, when you don't care about fuel economy, a gear ratio of 'about' 4:1 is excellent. It will limit your top speed by maybe 10mph, but let you leave a stoplight or zip around an 'AutoZ' course much more quickly than stock gears do. Nissan/Datsun knows their customers want lukewarm performance and not bad fuel economy, so they shoot for slightly 'longer' gears than 4:1 when they sell the cars stock.

Sorry to bore you ~ I know nobody asked! :oops:


Attachments:
File comment: Here's an example of the small PINION gear, driving the larger RING gear. The arrow indicates forward.
Ring and pinion .jpg
Ring and pinion .jpg [ 39.68 KiB | Viewed 5769 times ]

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Last edited by Frank T on Sun Jan 27, 2013 11:10 pm, edited 1 time in total.
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PostPosted: Sun Aug 05, 2012 8:03 pm 
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8) :twisted:


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IMG_0585.JPG [ 132.3 KiB | Viewed 5740 times ]

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PostPosted: Sun Aug 05, 2012 9:31 pm 
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Yummy! :D

For those who might be lost, we're looking at the back end of this differential, facing froward toward the front of the car, with the case cover removed. The smaller pinion gear is hiding out of sight behind the gear carrier in the center of the picture.

You COULD count the number of teeth in either gear (ring and pinion) to determine the gear ratio. But there are marks on the back end of the gear carrier which tell us what the ratio is without counting. They're usually in code, and don't read in plaindress language like "3.90:1". You need a Datsun shop manual to determine the code, otherwise you have to count the teeth or spin the pinion to determine how many times it turns for one turn of the ring gear.

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PostPosted: Sun Aug 05, 2012 9:44 pm 
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Thanks for making the thread less esoteric, Frank!

Unfortunately I won't be able to use this ring/pinion. This is a short-nosed R200.

Buuuut, the interesting thing is the bit in the center. It is a Nismo 2-way limited slip unit that I just got. I will need to swap the center into a long-nosed R200 pumpkin--still on the hunt for that.

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PostPosted: Mon Aug 06, 2012 8:39 am 
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Limited Slip Differential Unit:

Because two wheels (on the same axle) must revolve at different speeds while turning a circle, the rear differential must allow them to do that.

The 'planetary gear set' in the differential gear carrier (in the center of Justin's photo) can allow the inside wheel to hold steady while the outside wheel turns quickly to complete the turn. During turns, engine power is focused (up to 100%) to the outside wheel.

However, in racing or high-performance cars this is neither necessary nor desirable. Drivers of those cars want both drive wheels pushing as hard as possible at all times. So they make a non-slip or limited-slip differential, which causes both rear wheels to turn under full power regardless of the direction the car is headed.


Limited-slip differentials will allow the inside wheel to relinquish power for a few revolutions to allow for 'somewhat' normal turns, then both wheels resume spinning at the same rate. Some racers "lock up" their differential gears by welding them together to prevent any 'slip' at all, thereby making the cheapest 'non-slip' differential available (commonly called "Lockers").


The older Z carz used three types of rear differentials:the lightweight R-180, the beefier and stronger R-200, and long-or short-nosed cases. Limited slip gears were options in the R200 cases.


Attachments:
File comment: Cutaway of a simplified differential gear set
differential_cut_away.jpg
differential_cut_away.jpg [ 20.48 KiB | Viewed 5723 times ]

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PostPosted: Sun Jan 13, 2013 5:41 pm 
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I thought I would post up some updated photos. Thanks a lot to Greg for helping me get a 3.9:1 R200.

Pulling Nismo LSD from the R200 Short nose (S14):

Image

Image

Note the "diamond" shape thing just under the pinion gear means it is a 2-way LSD.

Pulling Open Differential from R200 Long Nose (Z31):

Image

Image

Image

The two centers next to each other:

Image

Next I need to swap the pinion gear from the open differential over to the LSD. Then re-assemble the R200 longnose (sell the R200 short nose on some silvia forums). Then finally swap the R200 longnose for the one that is in my car currently. Keepin' busy!

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