Diff and Gearing questions

[Kansei240sx]

Ok as a few of you know, i've got an S1 rb25det in my s13.

Here's my question, i've got a 91 and i've got a super hicas VLSD from another s13, now the gearing is really short for the RB's transmission, for instance, my rpms are really high compared to speed used to be with my KA . 3000 used to be just around 68-70 mph and now its more like 3700 for 70.

I've searched but havent gotten an answer, I've got a 300zx n/a VLSD, and the hicas one... is the only difference between these things the output shafts? Becuase i read the gearing is the same so swapping them out wont do anything at all in terms of changing it.

If so... Which diff should i get to help lengthen the gears? The J30 diff?

Also, are the output shafts swappable from the Z32 R200 to the S13 R200? Becuase i've got a open diff and a z diff and my friend wants it and i wanted to know if they're swapple without any problems.

Thanks in return.

[Joe]

*scratches head* thats a great question but why's it in the RB forum?

im gonna leave it here (because like i said, good question) but try posting in one of the other tech forums too

[Kansei240sx]

I posted it here, becuase it refers to the RB and i wanted to know if any other RB guys had the same issue instead of posting it in general becuase not many RB people surf gen tech disscusion.

Thanks for keepin it here though.

[Darius]

I don't know the answer to the question on whether the Z32 VLSD and the super hicas S13 VLSD have the same gearing, but I do know that the J30 VLSD has 4% taller gearing than the stock peg leg differential. I installed mine before my RB swap and it was noticeably less peppy off the line. But with the RB, I'm thinking it will work out well with the higher hp.

[hannibal]

USDM 240s have 4.083 rear ends, while the J30 has a 3.916 rear. I think the USDM S13 VLSD would have the same gearing as all other american 240s, I have heard there was a 4.36 VLSD in some JDM S13s.

If you end up getting a Z32 diff, it can use J30 5 bolt axles to complete the sawp.

See orions page for much more info...http://www.ka24development.com/vlsd.html

[Wulfgang]

Here's my question, i've got a 91 and i've got a super hicas VLSD from another s13, now the gearing is really short for the RB's transmission, for instance, my rpms are really high compared to speed used to be with my KA . 3000 used to be just around 68-70 mph and now its more like 3700 for 70.

I'm not sure what you think the transmission has to do with it... almost all trannys are 1:1 in 4th gear (or 5th in a 6 speed). Trannys do have slightly different gearing, but overall, if you're doing 3000 rpms at 70 mph in 5th on one transmission then you'll likely be really close to 3000 rpms at 70 mph on any other transmission. In 4th gear you'll be doing EXACTLY the same rpms at any given speed. The transmission is there to give you good gear spacing. Spacing is the key. You want 4th to feel about as different from 3rd as 3rd feels from 2nd. Absolute speed is thus irrelevant since you can change all of that with the final drive ratio.... but still the spacing will feel about the same. So forget about the transmission differences.

So I don't think what you said is correct. AFAIK, all 240's came with 4.08 final drives. That means that if you do 70 mph at 3000 rpms in 4th with a non-LSD from an S13, then you'll be doing 70 mph at 3000 rpms in 4th with any other S13/S14 diff (LSD or not), regardless of the transmission and engine that are in the car.

If anything, you'll want a higher ratio now that you have an RB25, not a lower one. This is because the power band occurs at higher revs. So you want your wheels to spin slower... otherwise it will take you longer to get from say 3rd gear to 4th gear. You don't want to be doing 40 mph in 1st before you even climb into the power band, do you?

So what you have is a higher ratio than before, and that's a good thing. For an RB, you want to be revving higher at 70 mph than you did with your KA. Again, that is a good thing.

However, something is not right. Either you didn't have a 4.08 diff before or you don't have one now. It's possible that you got a 4.36 diff from a Skyline. If you find out that it is a 4.36 and you'd rather have a 3.93, let me know and we can swap

[Kansei240sx]

I see what your're saying , and i was finding out that i'd want something with a taller gear like the J30 diff.

And yes, it is 4000 rpms for 75 mph in 5 th gear, i speed tested it with a friend today on the highway.

[Wulfgang]

It's just my opinion, but I think you'd be dissapointed with taller gearing. The way I see it, you're right where you need to be.

Unless you feel like 140ish mph is not fast enough, that is. The point is that you'll be faster in the 1/4 mile, faster from 0-60, etc., all with good gear spacing.

Taller gearing will reduce your acceleration where it most counts (in the higher gears), but you'll get a bit more top speed, assuming your setup can handle that speed.

Like I said, just my opinion... there's no physical law that says 4.36 is better than 3.93. Just a matter of taste and practicality.

[Shocker]

whoa.. is it just me or do your numbers sound wayy to high man.. For instance, my rb25, with stock 4.10 r33 vlsd, 60 mph = 2700 rpm in 5th, i dont understand how yours are soo much higher? you running on 13 inch donuts? 4000 rpms in 5th should be right at + - 100 mph... make sure your rear end isnt like a 4.6 or 4.3 something definitally seems wrong... heres the 25 transmission ratiostaken from skylines down under.

RB25DETGear Ratios1st - 3.2142nd - 1.9253rd - 1.3024th - 1.0005th - 0.752Reverse - 3.369Final - 4.363

[Kansei240sx]

I know for a dead fact 4k isnt 100 mph lol, its really bothering me why its so high... I mean for instance, i've just got a super hicas lsd, and i've got such wierd highway speed in 5th, just to keep up with my friends Z at a 70-75 mph pace, i've got to be around 4 grand.

My tires and rims are 14 inch rim on 215/60R/14

[dark_munchkin]

Your overall wheel+tire height is shorter than stock. Stock is a 15" with a 205/60.

[Shocker]

ever consider your tach here? did you get it re calibrated? if not it will read higher...

[Wulfgang]

Yeah, no kidding. What tach are you using anyway? And why do you have 14" wheels? That's obviously where the problem is. You shouldn't have 14" wheels on anything other than a Yugo.

To drive home my point about the transmission having nothing to do with it, Shocker says RB25DET trannys have a 5th gear ratio of 0.752. By comparison, 240SX trannys have 0.759. Yes, only 1% different. The final drive needs to be matched to the engine and it's power band, not to the transmission.

[Kansei240sx]

The tire dimesions with the rim are the exact same height with the tires as the factory steelies with tires. When i posted the tire size earlier i wasnt to sure if they were 215/60 or 215/65's

Yes the Tach is calibrated and im using the factory one.

[dark_munchkin]

My alloys are 15x6 (or 6.5) and the stock tire is a 205/60. You'd need a 70 sidewall to get the same overall diameter.

[rb20detsilvia]

p215/60r14 are 24.16 inches tall and P205/60r15 are 24.69 inches tall so that wouldnt make that much of a differance

[l0nestar]

whoa.. is it just me or do your numbers sound wayy to high man.. For instance, my rb25, with stock 4.10 r33 vlsd, 60 mph = 2700 rpm in 5th, i dont understand how yours are soo much higher? you running on 13 inch donuts? 4000 rpms in 5th should be right at + - 100 mph... make sure your rear end isnt like a 4.6 or 4.3 something definitally seems wrong... heres the 25 transmission ratiostaken from skylines down under.

RB25DETGear Ratios1st - 3.2142nd - 1.9253rd - 1.3024th - 1.0005th - 0.752Reverse - 3.369Final - 4.363

Woah. I just purchased an RB25 R33 half-cut + K-member with VLSD. What is the gearing in the VLSD? 4.10? Just wondering. Where should my RPM's be in 4th gear + approximate speed? On 17" rims, I can get exact size of my rubber, cant remember off hand

[Shocker]

well ive got 215/65/16's on with the 4.10 rear end, at 1900 in 4th its 35 mph.

[l0nestar]

WTF? Are you sure.. Only 35 MPH? What about at 3000 rpm and 4000 rpm? Good Lord man, that is low, what speed does your car top out at? Your using the RB25 eng + 5sp + R33 VLSD?

[DrifterTom]

most of us couldn't tell you. stock speedos top out before the car does. some people may have a speed cut at 115-120.

[l0nestar]

Well, I actually feel like an idiot. I was in my daily (94 Probe GT (v6+5sp)) and was in 4th @around 1900 and was at about 40 MPH. I would like to sincerely appoplogize for the n00bish ness of my recent posts

l0nestar.