Best final drive ratio for RB26 Swap

[mtnickel]

So I am running the stock R180 on my 260z with rb25det and it feels like 5th gear is struggling until I get up to 75mph. I don't plan to be running it above 100mph very offten so I am assuming a lower gear ratio would be better suited for daily driving? I am thinking of using one of the Subaru diff's but I don't know what is going work the best with the rb25det transmission. I guess it would help if I knew what my current diff ratio was. Unfortunately I think it was swamped out by the guy who owned the car before me.

Struggling how? Not building boost? You can check what ratio you have with a little work. Put it in neutral, jack up one side, put a reference mark on the tire, put a reference mark on the drive shaft. Spin the tire around exactly twice and count how many times the diff turns. You'll have to be precise to see if it turns 3.5, 3.7, 3.9 times, etc. You happen to take your car to the track yet? Haven't seen a single post of what a stock RB swapped Z-car does in the 1/4 mile.

 

 

• I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.
  • Lecture on "Electrical Units of Measurement" (3 May 1883), published in Popular Lectures Vol. I, p. 73

 

William Thomson (June 26, 1824–December 17, 1907), 1st Baron Kelvin, often referred to simply as Lord Kelvin, was a Scottish physicist.

 

Well said. I'm a physicist at heart. I'm going to go with the 3.90 for anyone who wanted to know followup. The freeway cruising RPM is more than reasonable and I realized it'll rip away a lot better when dropping to 2nd while going 40-60km/h.

[Vandergriff84]

I am referring to the speed to RPM ratio. Its like trying to cruise at 20mph in 3rd or 4th etcetera. In most cars that I have had (mostly Honda's and Toyota's) you can cruise at say 55mph in 5th no problem and with the RB and my current diff it doesn't start to cruise until around 70-75mph. Once I hit 75 to 80 it cruises with no problem and feels like it could just keep climbing. There is also a slight vibration in 5th that is not present or noticeable in any of the lower gears, I don't know what that is about I'm just hoping that its not in the transmission. I'm not really into drag racing and I haven't really done any hard driving to report on what it can do. I am sure with a little tuning it will do pretty well. 

[mtnickel]

I am referring to the speed to RPM ratio. Its like trying to cruise at 20mph in 3rd or 4th etcetera. In most cars that I have had (mostly Honda's and Toyota's) you can cruise at say 55mph in 5th no problem and with the RB and my current diff it doesn't start to cruise until around 70-75mph. Once I hit 75 to 80 it cruises with no problem and feels like it could just keep climbing. There is also a slight vibration in 5th that is not present or noticeable in any of the lower gears, I don't know what that is about I'm just hoping that its not in the transmission. I'm not really into drag racing and I haven't really done any hard driving to report on what it can do. I am sure with a little tuning it will do pretty well. 

Well sounds like you have a really low diff then, perhaps just cruise in 4th then!

[flatrate]

I've had 3.69 4.08 4.10 and 4.63

 

3.69 is WAY to long car would do like 140MPH in 3rd gear!!

 

4.63 was a bit aggresive for the street but i didn't hate it.. (until i broke the pinion)

 

I'm using Richmond 4.10's now, they are good all around for an RB25 geared car.

 

I think i would rather have a 4.30ish gear though IMO that would be perfect.. however i keep breaking R200's 

[NewZed]

Transmission gear ratios are needed.  Without those, the diff ratios don't really mean much.  Except for 4th gear, which is generally direct, 1:1.  Most people gear their cars for 1st gear, for city driving, so they don't have to ride the clutch to get going, or start in 2nd because 1st is useless.  The only reason to focus on 5th gear is if you spend most of your time on the highway.

[mtnickel]

Transmission gear ratios are needed.  Without those, the diff ratios don't really mean much.  Except for 4th gear, which is generally direct, 1:1.  Most people gear their cars for 1st gear, for city driving, so they don't have to ride the clutch to get going, or start in 2nd because 1st is useless.  The only reason to focus on 5th gear is if you spend most of your time on the highway.

Ya, I was using a speed calculator with transmission, tire size, and rear end ratios that tell all. The R33 RB25 had 4.11's stock and 4.30's in some auto's, so going down a step for a much lighter car seems reasonable. Plus I will be spending a fair bit of time on the highway. I think the Z32 ratios are very similar as well and the NA's have 3.90's while the TT has 3.70's, so we are all really in the ballpark. I only really considered the 3.54 for the traction issues and even lower highway cruise RPM/fuel economy.

 

Mark

[flatrate]

Z32 NA/TT and all RB25/RB26 trannies share the same ratios ( except the getrag)

[mtnickel]

To add to this thread, I've had a little more seat time with my 3.90 and rb25 combo. I'd say it's probably a good compromise for those who are looking to do a little track as it'll go through the gears a little more. For a street car though (which mine mostly is), I may have likely preferred a 3.70. Only reason being, my average cruise speed around 70-75 is just a tick over 3000rpm and would have been just perfect I think (2700-2800) with a slightly lower ratio. Performance wise it's nice though. And I'm amazed at how well the 3.90 with Torsen and 205's hooks up. I mean, probably only running around 250hp right now, but plants Great!

[EF Ian]

I'm running a 4.11 and I can sit at 70mph AT 3100RPM, this is good for me and the car is nice on the motorway, can't imagine using a different FD and this is perfect.

[mtnickel]

To add to this thread, I've had a little more seat time with my 3.90 and rb25 combo. I'd say it's probably a good compromise for those who are looking to do a little track as it'll go through the gears a little more. For a street car though (which mine mostly is), I may have likely preferred a 3.70. Only reason being, my average cruise speed around 70-75 is just a tick over 3000rpm and would have been just perfect I think (2700-2800) with a slightly lower ratio. Performance wise it's nice though. And I'm amazed at how well the 3.90 with Torsen and 205's hooks up. I mean, probably only running around 250hp right now, but plants Great!

 

I may eat my words a bit here. My 3.90 blew due to an out of true centre carrier. Anyways, swapped in a 3.54 I had laying around, and it is surely too long. Yes, the highway cruise at 70-75mph is nice (27-2800rpm), but the cruise at 60 is TOO low @ 2300rpm. Sort of in the droney area of the engine. To add to that, the liveliness off the line and abililty to launch has been impacted. Surely puts a much higher load on the clutch as when I went to launch hard it burned the clutch up a bit instead of working over the tires.

So I will have a full recant and say the 3.90 or higher is probably a good ratio for the RB25. 3000rpm 70mph cruise is butter smooth and really not that high for a 2.5L engine at that speed. Even a buddies s4 v8 with a 6 speed cruises around the same RPM in 6th!

[240zdan]

I'm runnign a 4.11 GTR diff with z32 trans. Gearing is absolutley spot on for twisty roads and city. A bit embarrasing on the freeway though as cruising in 5th is barely keeping up with trucks and minivans. But she has zero issue winding out 5th in a hurry if needed.

Edited November 11, 2015 by 240zdan