Rear ratio question for CD009

[mutantZ]

I'm doing an RB25 swap and I had a question about driveline. Does anyone have a CD009 (350z) transmission with their swap? I was comparing ratios from the rb25 transmission to the cd009, and the CD has a much lower 1st/2nd.

 

Factory GTST's came with a 24"-ish tire with a 4.11 rear and a 3.215 first gear. My car currently has a 25" tire with a 3.54 gear, the CD009 first is 3.78. I am thinking this would more or less put me at factory gearing. Finding a 3.9 or 4.11 is pretty tough in a r200 long nose these days, and my diff is essentially brand new. But I'm working on the backend of the car now so now's the time to do it.

 

Anyone with a setup like this (rb engine/cd009)? How do you like the gear spread? Thanks.

[AydinZ71]

2 hours ago, mutantZ said:

I'm doing an RB25 swap and I had a question about driveline. Does anyone have a CD009 (350z) transmission with their swap? I was comparing ratios from the rb25 transmission to the cd009, and the CD has a much lower 1st/2nd.

 

Factory GTST's came with a 24"-ish tire with a 4.11 rear and a 3.215 first gear. My car currently has a 25" tire with a 3.54 gear, the CD009 first is 3.78. I am thinking this would more or less put me at factory gearing. Finding a 3.9 or 4.11 is pretty tough in a r200 long nose these days, and my diff is essentially brand new. But I'm working on the backend of the car now so now's the time to do it.

 

Anyone with a setup like this (rb engine/cd009)? How do you like the gear spread? Thanks.

how much torque are you planning on running, and where in the rpm band is it peaking? Comparing trans and rear ratios with OEM and your own personal experience is a great idea. However, if you are pulling a much higher torque/weight ratio than the car or you may be used to, I would pick a lower number rear ratio. Speaking from personal experience, my 300ft-lb L28et 71’ was almost unusable in first gear. Too much mechanical leverage for the torque. Tires want to break loose, and you don’t stay in gear long enough to make-up for the time lost in shifting to 2nd. I would often short-shift just to get back down into lower rpm’s and run out second gear for longer. This was on a Nissan 5-Spd wide ratio, and a 3.7:1 LN from a Z31. 
 

you might want to collect a few rear ratios and dial-it-in to your liking (unless it’s a track car, which would benefit from bench calculation and specific track avg spd..) 

 

3.9:1’s are still floating around out there, but 4.11:1 in an R200 case are hard to find.

 

if you are running less than 250 ft-lbs and are not “launching”, you still want a 4:11, you can find that ratio in an R180. They came in a bunch of Datsun mini trucks. I have one laying around I bought last year for $40.

also, you can still find R180 3.9 helical LSD from a subie STI. I believe the yrs are 05-07’ 

[mutantZ]

I bought a running engine and the PO told me his mods. I am guessing I am looking at a powerband that would be similar to this dyno I found with similar setup:

 

It's still using the stock turbo however, and I plan on doing a rebuild after I get the whole car sorted (and test driven!). From the previous posts here from people with RB swaps, a lot seemed to say their car seemed pretty lethargic with 3.7 and lower (numerically smaller) gears. Most of the 240sx guys run pretty steep gears out back. Powerband is tough to say as I haven't built the motor but I don't plan on going youtube big, more like 350-400tops. Mainly just want the efficiency of a modern better flowing turbo on it. I'll spec that when I get to it. I was just seeing if I could cheat the diff with the trans I am going to use anyway. Would be one less part to buy and have my LSD swapped into. That OSgiken oil is $60 a can! 😄

[AydinZ71]

@mutantZ ah so you have a clutch LSD? My helical unit uses gear-oil. No special clutch LSD oil for me  . The high-quality clutch units (like OS Giken) can be tuned but I’m not a good enough driver for that to matter. 
 

It may be best to get her on the road and get a feel for when and how aggressive the torque comes on. It would be difficult to take a guess on what the best ratio will be for you without every single variable on the table (torque curve, redline, tire diameter, trans ratios, what RPM you like to shift at during normal driving), and for street cars it often comes down to driver preference. i don’t really want to shift a bunch of times just to get up to 60, so I prefer taller gears and final ratio. If it was a track car, there absolutely is an ideal ratio that would give you the quickest lap time, but for a street car it’s more important to stay “modest” and not go crazy on either extreme. 
 

the lower on the rpm band the torque hits (specifically for turbo, since NA has a much flatter torque curve) the less mechanical leverage you will need. What rpm the torque spikes will be dictated largely by the size of your turbo relative to displacement, and more minor considerations like how much intake volume you have in your IC loop (lag).