Hi guys,

What would be the pros and cons of going to a 3.9 find drive, the car is a stock FD RX7 manual.

Moe Greene

 

It's only a 5% difference, why bother...

con: marginally slower acceleration in every gear
pro: upshift later, acceleration from some speeds will be quicker due to being in a lower transmission gear

You probably wouldn't notice such a small change in a blind/blind test.

 

Going to a lower gear really isn't worth it unless you're making a truckload of power. Some single turbo guys will go down to 3.9's to help with traction.

I went to the RX-8 4.44 ring and pinion, makes the car accelerate quicker and feel more "eager" when you drive.

On a stock or near-stock car, there's no good reason to go to 3.9's

Dale

 

I don't see any reason to go to a lower gear ratio on a anything but an extremely modified car or possibly one with an engine swap that makes significantly more torque than the 13b. I actually prefer the 4.33 gears with the reduced ratio 5th. Not only does it help acceleration, but it helps keep you in the power band. The stock 93-95 5th is just too tall.

 

Only time I'd ever consider it would be with a Magnum Close Ratio swap to hit 60mph in first gear without winging out the engine. Always thought it'd be neat to be able to hang with the 'Vettes (and F40s) that way.

 

3.9 gearing would theoretically improve highway gas mileage due to lower RPMs at cruising speeds, correct? I'm actually planning to go that way with my EFR 7670 upgrade, with the hope of balancing any fuel efficiency losses from the anticipated increased throttle response. Pls let me know if my theory is wrong as I'll be happy to spend that particular portion of the pending mods elsewhere.

 

is this something I can do while having my transmission rebuild?

 

Regardless if your tanny is in or out you can remove your differential and replace the ring and pinion. Just know there are some things you must replace when doing this swap. Some people go cheap and reuse as much as they they can, others like me replace the bearings, crush collar, oil seals, washers etc...pretty much everything. You also might need new spacers as the tolerances will change, so your current spacers might not cut it. Unless you have done this before I would take it to a reputable shop...

All in all swapping the diff R&P the right way is not cheap, and if you don't have the right tools taking the diff out can be a bitch...

There are some threads on here about replacing the diff, rebuilding it, removing it from the car etc...

 

[QUOTE=Spalato;12172106]Regardless if your tanny is in or out you can remove your differential and replace the ring and pinion. Just know there are some things you must replace when doing this swap. Some people go cheap and reuse as much as they they can, others like me replace the bearings, crush collar, oil seals, washers etc...pretty much everything. You also might need new spacers as the tolerances will change, so your current spacers might not cut it. Unless you have done this before I would take it to a reputable shop...

All in all swapping the diff R&P the right way is not cheap, and if you don't have the right tools taking the diff out can be a bitch...

There are some threads on here about replacing the diff, rebuilding it, removing it from the car etc...[/QUOTE

got cha !! thanks. I think I'll let the professionals handle that haha ]

 

I went with the 3.909 diff because I have a JDM transmission with the .806 5th gear and make about 450 whp.

 

I have a JDM trans and swapped out the JDM 5th on it for a US 5th. Not a hard process to do. Just need the main gear and counter gear.

Dale