diff assembly went right in with no clearancing.

 

Got some proper rear upper hats from FIGS, replaced the trunk vents while I was at it, and painted the ML amp brackets

 

I was thinking about this and realized the RC F and GS F use a 275/35/19 where the IS F uses a 255/40/19. I normally run a 275/30/19 in the back for the summer, and recently had to change the tires, so I put on the PS4S from the GS F (swapped it to A/S3+ a while ago) which are 275/35. That size is slightly taller than the IS F spec tire where the 275/30 is slightly shorter. I'm noticing the difference off the line - it's just a tad slower to gather speed. I bet they went to the 3.03 to get that back and make the off the line performance a little closer to IS F spec.

 

That makes sense. There is really no better explanation for such a small change in gearing. I still want to drive one with a GS350 gear set, knowing well that it would only feel faster.

So did you put an RCF ring and pinion in your ISF diff? If so, where did you get it? I thought Lexus would only sell the entire diff assembly.

 

41201-59285 ring and pinion part number for 3.133 ratio. No need to buy the entire pumpkin, but you'll have to have it set up so you'll need washers/shims etc. for the install.

Oddly enough, you can't buy the IS F housing by itself, but you can buy the RC F housing by itself for a little under $800 brand new. If the Giken spool fits in this housing without clearancing, it might be worth it. Or it might be worth getting a junkyard RC F pumpkin with 3.133 gears and putting a Giken unit in it, just like we see above.

 

What do you think of the GS350 ring set in an ISF? I don’t remember the exact ratio, but it’s around 3.7 to 1. Too much gear?

I had an Fbody trans am with an LS1 and a 6 speed. I went from a 3.5 gear ratio to a 4.10. That car felt like a monster.

 

so the 3.13 combined with the isf factory dia. tire size should give me what I’m hoping for without having too ‘violent’ like I heard you get with a much shorter final gear ratio?

 

I think 3.7 is too much, at least for me and for what I’m wanting to achieve. Plus it might be too much of a difference that it could mess up how gears change say when in auto mode? which I’m sure can be eventually remedied by re-programming the TCU. I’m sure it’ll be fun on the streets but not so much on the freeway. I know someone has or had a 4.10 but he mainly drifts his ISF.

 

Ring and pinion set is 700 bucks. I got an open diff off of a 2015 is350 for 150, since I’m putting an OS unit anyway. The only difference is, the case has no fins for heat sink. The cover does though. Good to know I can get a bare RCF housing, I might end up doing that in the future!

 

I have given up hope on a true, complete tuning solution for this platform. Fortunately, I don’t mind the stock tune, but a transmission tune would be nice.

 

Have you looked at Off the line performance? Loi Tran has tuned many ISF’s and RCF’s with excellent results. He recently tuned an ISF with nitrous and made over 700hp, which btw is running a 3.7 diff.

 

They were able to crack the transmission ECU to tune it?....If so that is good news! Can it be done remotely?

 

I believe just engine ecu for now but transmission tune is in the works. I can ask him. He does remote tune for a selected number of States.

https://www.clublexus.com/forums/is-...d-wtq-isf.html

 

Got the new rear wheels after a 9 mo. wait ,
18x10 +39. So the 9.5 +45 are now in the front.

 

Did you roll your rears?