Opinions on my final drive gear
 

I swapped from an auto to a manual and am still running the 390 gears. With all the mods I am estimating 310-320 whp. Trying to keep on somewhat of a budget, but still do things right. I am pleased with the power output, but can always use more acceleration. Looking for opinions.
leave 390
buy used 410 rear diff and swap?
buy 430 gears and have a shop rebuild mine? It has about 54k on it.
It may not be as bad as oil threads, but there are many, many threads on this. None of which imo really answer the question. Is it worth the $? Worth it to me would start around knocking .5 or more seconds off a 0-60 or 1/4 mile run. The only confirmed thread from 390 to 430 the guy stated .2 sec faster in 1/4 and that was with a penalty of 3mph slower. I find this hard to believe.
I have one pro telling me 390 is where it is at and another telling me 390 to 430 is night and day. I am probably looking at 1k+ for purchase and install of the 430.
Anyone have a bottom line answer? This is more important to me know as I just put my new wheels and tires on which are taller that stock making the 390 even worse for acceleration! Or better depending on how you look at it. I have the basics of gearing so no need to elaborate. Just looking for opinions and if anyone can verify knocking off more than .2 sec on a 1/4 mile time would be nice. .2 sec less and lower trap speed is not my idea of 1k plus mod as this time.

You could probably get a 4.44 from an RX8 for a couple hundred dollars (plus labour to switch the r+p over)

I have thought about that also. Are the Rx8 and TII the same strength? I was under the impression the rx8 gear is not that strong. Also I am not so sure if that would be too tall. Like aggravating for around town and general spirited back road blasting with a lot of slower turns.

First it depends if the car is a track only or street or combination use car.

I installed the 3.909 differential in my street car for two reasons:
(1) I have the .806 close ratio Japanese 5th gear and if used with the standard 4.1 diff your street mileage goes down and the revs go up higher and top speed drops
Thus with the .806 and 3.909 first through fourth gears are longer that work fine with the increased rwhp and fifth is closer with a more reasonable about 186 top speed.
(2) With the 3.909 and 400+ rhp and 285/35-18 Advan AD08R it is difficult to get traction at full power even in second gear. A 4.44 would really reduce top end speed and mileage and even worse
traction problems.

Remember the stock engine had only about 225 rwhp and many of us are over 400. Are low gears are too low.

Thx for responding.
This car is mostly for spirited driving, but it will see the track and drag strip. I definitely would like to have the acceleration equivalent to be in at least high 12s in a 1/4 (with street tires). I think that should be possible with 100 extra hp from stock. I am running 275s on the rear so I dont think traction should be too much of an issue at this stage. At 400 I am sure the 390 works great. My plan was to keep here around this power level and look for other means to increase acceleration. I could up the boost another 2 psi probably with current tune and be safe, but would prefer not too. Of course we never have enough power so maybe after water inj. I will raise her up a little. Until then the next/ only other mod I can think of for increasing acceleration would be the rear end. I will not reduce the weight any further. Just not sure if I can justify the cost. Trying to pull max acceleration with keeping her reliable as possible.
Eventually I will get bored and look for the 400+ number, but I need to learn her at this stage first.

The RX-8 R&P is a great way to go. I've been running one for years. Search and read my thread on it.

The 4.44 isn't too aggressive, it's a great setup for a street car and VERY cheap.

Dale

All the searches and I never saw your set up or speak about hearing. I will give it a look.

My mistake. I actually did read that awhile ago. I guess I just haven't found what I am looking for. I think I need a 1/4 mile calculator so I can plug all the gears in with hp, weight, etc, and different rear end ratios to get an idea of what to expect. I am sure a 4 44 would be a whole different vehicle compared to the 390. No so sure about the 410 and 430.

The difference between 4:30 and 4:44 is so small as to not be noticeable. Big difference is the 4:30 R&P is hard to find and very expensive, the 4:44 is easily available with an RX-8 diff, I think I bought a good used diff for like $150 or so.

This thread -

https://www.rx7club.com/3rd-generati...os-you-984896/

documents the different 5th gears, final drives, and what speed they run at.

I went from the 4:10 to the 4:44. The car seems more "eager" with the 4:44 and gets off the line nicely. If you did a 5 speed swap and still have the auto diff it's well worth going to the 4:44.

Dale

I found my 4.3 R&P in a cheap $100 used RX-8 diff casing on craigslist

the 2004-2005 Rx8's are all 4.44

the 2006-2008 Rx8 manuals keep the 4.44, the automatic (6 speed) goes to 4.3

the 2009-2011 Rx8 is 4.77 for the m/t and 4.3 for the at

Those RX8 pumpkins aren't a direct swap I assume... or are they?

Nope, not even close. You have to crack it open and get the ring and pinion out.

If you can find a shop that does a lot of Jeep/4WD stuff they can swap a ring and pinion in their sleep. It's not super hard but there is some precision to it and a little learning curve. I haven't done it personally but I know the principles of it. I've always just had it done by a local shop - typically super cheap and fast turn around.

Dale

Gotcha

Link to a very helpful writeup, should you choose to do the swap yourself:

https://www.rx7club.com/3rd-generati...riteup-897359/

Thanks Primer. Very much appreciated.

Dale if I remember correctly your running very close to stock diameter. I am running a 275/35-18 on the rear. That puts me at almost a 3.80 since I have the 3.90 currently (comparatively speaking to stock diameter). A 4.44 puts me at about a about 4.57 which may be a little to much for me. I think a 4.30 puts me at like 4.18.
I am probably getting way to technical, but I want to make sure I am hitting 60mph+ in 2nd and would like to be over 90 in 3rd.
I may be pushing this mod back further as watching some videos and trying to calculate a true advantage for acceleration doesnt see like it is the biggest bang for the buck. I do think I am way off what I need to be running with a 3.90 with 25.58 tire diameter.
I was accelerating on an on ramp earlier today. I was at 50mph in 2nd the ground was damp not wet and I went wot once I straightened out and the rear broke loose. I guess I am curious of how traction will be with changing these gears out. One way to find out I guess. This may be a major issue I have with my current suspension set up though. I am estimating 320 to the wheels. Again, with this weight it is nothing to sneeze at, but I wouldn't think it is enough power to have it cut loose today. I think I am way to stiff in the rear.

Never mind Dale I see your running a larger diameter than I am I think. I saw your running a 295 18.

I am running a 265/40/17, actually now I think it's a 255/40/17 (swapped out tires a year or two back).

Some of your math is wrong, you really need to play with that gear calculator.

Calculator site -

BMW E46 Gear Ratio / Cruising RPM calculator

Stock FD transmission ratios -

https://www.mazdatrix.com/faq/ratios.htm

Playing around with it a little bit you'll still be able to hit over 60mph at redline in 2nd.

Dale

https://cimg3.ibsrv.net/gimg/www.rx7...8181a13a1a.jpg
https://cimg4.ibsrv.net/gimg/www.rx7...0c17ea24f1.jpg
That's how I was kind of looking at it.

Big thing is if you did an auto-manual conversion you're leaving a lot of acceleration on the table with the 3:90 rear end.

Use the calc I posted, it really lets you see everything, such as what speed you get to at each gear at redline.

Dale

Thanks Dale. I will check it out now.

If you want an even more comprehensive calculator with gear shift charts, overlay charts comparing 2 different gear sets etc, then check this one I made many years ago: Wheel Sizing, Gearing, and Speedometer Spreadsheet - AusRotary

Thanks,
I need to make a decision pretty soon