Sorry guys, but I figured I had to try here.
Mostly spirited back road with track and drag thrown in. 310-320 whp fd. Did an auto to manual swap and retained the 390 rear. Looking for ways to improve acceleration without any added power so makes sense to look at gears. I will not be decreasing the weight any further.
For the money is it worth going for a 430? I would love to be in the 12s in the 1/4 for now. I have read multiple threads on this. None of which had any real solid info imo. One said he knocked off .2 sec in 1/4 while losing 3mph. Another said he knocked off a second in the 1/4.
I find both hard to believe. So before I go spend a grand I want to make sure it is worth swapping out a 390 for a 410 or 430. I cant see a 410 being worth it, but I am sure I could pick up a used one cheap. My diff has about 54k on it. Any help would be appreciated!! If no diff water injection and a couple more psi I guess.

 

Go with Rx-8 4.4
Cheaper to find and build

 

I've heard the metal in the rx8 r&p is soft/weak and has a tendency to wear out (gears getting worn/sharpened) quickly. Has anyone experienced this first hand?

 

I'd say that's what happens if you chuck an rx8 gearset into an rx7 diff without setting the gear contact properly.

 

OEM gearsets tend to have good hardening for long life. You only see wear problems on drag-only gearsets that sacrifice a hardening to gain some flexibility, which is a lot better than cracking.

 

I have a FD with the rx8 4.44 it feels great. If you enjoy more low end response.
I prefer the higher gearing over the stock 4.10. Yes I am aware has a lower top speed now. 158 or 153 or 150 mph or so.
Then my 3rd fc was a GTUs with 4.33 final drive ratio. That's why I am partial towards it.

Also my fd is mildly modified. downpipe, RB presilencer, catback, aftermarket smic, cheapbastard intake, composite driveshaft, etc,
On 255/40/17 tires on 17x8.5 +35 wheels. Yes do some math and calculations before you jump in head first with the 4.44. Final drive ratio calculators are online. I used Forza and Gran Turismo to simulate it. Also a GPS tagged speedometer can help you guage your speed more accurately.

 

Thanks for the additional input!

 

I have a 4.44 diff from an rx8 installed in my street and track car. No problems so far with the ring and pinion. I don't drag race so the launches I do aren't particularly harsh but I run about 450whp on my current setup. I really liked stepping up from 4.1 to 4.44. I would definitely recommend the upgrade.
Remember that your speedo would not give the correct reading, so I adjusted the small screw behind the dashboard. There is a really detailed writeup of that process on here somewhere. Just do a search. Good luck bro.

 

If you have the original USDM transmission with the tall 0.719 5th gear you could go straight to the 4.77 rear end from the later RX-8.

Still will trap you 125mph in 4th gear at 8,000rpm in the 1/4 mile and will provide a more involved experience on back roads as you will finally have to (get to) shift.

If you already replaced the trans with a JDM one and the ultra short 0.806 5th gear you may not want to do this as 65mph cruise would be 3,4000rpm (as opposed to 3,000rpm with USDM 5th and 4.77 rear).

Then again, I would love the short 5th and the 4.77 rear, but I increased redline to 9,000rpm.

 

 

.806 is ultra short? It's barely any different from the .719.

Next time try driving with the 4.78s and a .868 final with 205/50-15 tires...

 

Context-
the stock 5th gears available in FD RX-7 transmissions

Stock 5th gear ratios are 0.719 for all USDM, 0.762 for short JDM standard and 0.806 for ultra short JDM optional 5th.


Story time-
I went from stock 0.762 in my S4 TII to the 0.806 in a JDM S4 trans and it felt crazy how much harder it pulled from 140-170mph, but it couldn't even make 180mph (P in the MPH of the FC 160mph speedo) anymore with the 8,000rpm revlimit.

0.806 5th and 4.77 rear with stock 8,000 rev limit won't even get you to the 150mph + stock top speed of an FD. But it would get you to 150mph quickly!

 

I know I'm late to the party but the 3.9 gears will get you into the 12s no problem with 310-320rwhp.

I ran 12.5@113 (2.1s 60ft) on street tires with a 3.9 diff on my intake/exhaust/tune 5 speed swap at 12psi. I'd guess you could do pretty similar (or better) with the auto gears. Put a tire on it and it's likely in the 11s at that power level with a solid launch.

Video:

 

Thx Copeland and no you are not too late as I am still in debate. I am giving serious thought to the 4.30 and maybe 4.44 since I could get one cheap. I am concerned the 4.44 is a little too low though.
I really appreciate the video. I am figuring I am pushing around 300-320. I could be wrong. I was going to upgrade my injectors, but I may end up doing my diff first. I truely ha e no idea, but it is nice seeing some actual footage and know it is possible with the 390.

 

 

This is what I came up with for my tire size in the rear.