KiyoKix

It's been a while now since I've really posted anything here (of course still read a lot), but I need some help. I'm about to put my FC back on the road ('86 GXL N/A) and I've been really wanting to change out the stock 4.10 ring and pinion to a 4.875. Does anyone know where I can get this done or who will sell me an already assembled unit. I know that you can't just bolt this up like the 4.30 and you'd have to have some welding work done. I can't remember where I was looking that offered the service (had to format my computers so I lost quite a bit).

It'd be great if the place had the ring and pinions and did the work as opposed to buying it and shipping it off to someone and then getting it done. Either way I want it and I'd like it soon so that I can drive the car (don't want to drive it again before I get this done). I know that Racing Beat sells the ring and pinion for $415 before shipping, but who will do the work. It bolts up fine in an MX-5 or FB but not the FC...

Any help would be GREATLY appreciated as I'd love to drive my car as soon as possible (please DON'T say just drive it anyway as I'm not looking to do that).

Thanks,
K

msn: Kamui (whatshouldidonow@hotmail.com)
aim: EBXKIX

cool_as_crap

you'd have to have somthing custom made. the 4.875 pinion is too short (made for an fb). you could try to work somthing out with a miata diff, but idk....
i'd recomend looking for a rear-end shop in your area locally

KiyoKix

I'm aware that it will have to be welded up, I've been looking to do this for sometime now. I had a site/shop a while back that did the work due to the shorter 4.875. They were using these in competition vehicles (FC's) and I believe that they took the stock part and welded the two together after cutting so that the length would work. I just don't have the information that I used to have after having to format my computers (it's been a long while now that I haven't looked at it and I'm having a mental fart trying to remember).

Thanks though,
K

Icemark

why so high??? You'll be at 4000 rpm at 60 MPH in 5th.

Maybe a 4.44 or something, but a 4.8... wow extream!

j9fd3s

iTrader: (3)

www.iscracing.net

KiyoKix

I haven't looked at the specifics in a while now, but I was pretty sure it was a tad under 4000 rpm at 60 (I wanna say somewhere around 3300-3500). I'll do the numbers up again (using 205/55 R16's all the way around...possibly 225/50R16 on the rear later on). Do you care to show me the formula that you used to get it, I really don't remember it being quite that high when I did it using the super fuel saver 5th gear the '86 FC was "blessed" with.

I'm not concerned with top speed in anyway (I believe I figured it would be around 247km/h or 154 mph at around 7500 in 5th. That's MORE than enough for me as I never see those speeds anyway and have no plans to really go above 200 km/h. I've considered using a 4.44 before, but I didn't think it would be enough for what I wanted. Unless of course you can prove me wrong

Thanks Mark,
K

KiyoKix

Btw, THANK YOU j9fd3s that's the site I used to have on my computer.

anewconvert

iTrader: (1)

Forget 5th gear. what is the redline (8000) speed in first gear? I can imagine you would tear through 1st gear almost so fast you wouldnt know what to do with yourself.


Someone, not me, should take the time and figure out wht the speeds are for all 5 gears with taht rear end.

BC

Turblown

iTrader: (12)

Cool...

KiyoKix

Okay I found a very nice calculator online and put in the numbers and they're just like I remember then when I did them long ago.


Original Gearing 4.100

--------1st 3.483----------2nd 2.015----------3rd 1.391----------4th 1.000----------5th .762

7500----------39----------67----------97----------135----------178



New 4.875 Ratio

--------1st 3.483----------2nd 2.015----------3rd 1.391----------4th 1.000----------5th .762

7500----------33----------57----------82----------114----------149


4.100 in 5th gear @60mph=2600 (actually it's 62)
--------------------@100mph=4300 (actually 102)


4.875 in 5th gear@60mph=3000
-------------------@100mph=5000


I think these speed will work out very well with the way I'd like my car geared (accel over top speed by a far margin). It will let me stay N/A like I want to w/o having to add a lot of power or do anything too drastic mod wise. If I do wake up crazy and go outside and put on the turbo stuff (not likely anytime soon if ever) then it will be a true monster with very very very very little boost necessary. I may add a turbo at a later date (only low pressure the way I like it...5-7lbs at max boost) and this will be more to my liking than a high boost turbo setup.

Either way I believe this gearing setup will "add" a relatively HUGE boost in torque that I'm looking for while staying N/A and keeping noise lower than what I'd need to do if I was looking for the same boost in torque w/o changing gearing.

K

KiyoKix

Btw, if you'd like to know the numbers for Rpm Drops here they are. This is assuming that you shift at 7500 (fuel cut is 7700, redline is 7000 in an S4 FC like mine).

4.875 Gearing (w/205/55/R16's)
------------------------------------
1st top speed 33mph
2nd gear------4500 (34)
3rd gear------5300 (58)
4th gear------5500 (84)
5th gear------5800 (116)
------------------------------------

4.100 (w/205/60/R15's)
------------------------------------
1st top speed 39
2nd gear------4500 (40)
3rd gear------5300 (68)
4th gear------5500 (99)
5th gear------5800 (136)

Considering the "modest" engine power from a lightly modified S4 N/A this gearing would help a lot more with top speed as well. The original gearing is definately higher, but it would take you out of the power band that you'd need to achieve those speeds whereas with the new gearing you'd be right in the meat of the powerband at those speeds.

4.875 Differences

40mph=5300 rpm
68mph=6300 rpm
99mph=6600 rpm
136mph=6900rpm

Considering that N/A's really come into their stride above 5000 rpm and peak at around 6500 rpm that should be a serious kick in the pants from the difference in rpm at a given speed. This should also theoretically push the top speed of an N/A vehicle higher than it could originally achieve (stock) with the original gearing w/o changing the engine power. I think this would just use the power better though you'd possibly spend more time shifting (which is fun to me so that's bonus bonus). It would give me the option of leaving it in a higher gear as well and be able to pull out of a corner w/o downshifting. I'm not really concerned with fuel economy even with higher fuel prices as it's not an all year round vehicle and...it's a rotary we don't buy them for fuel economy anyway so anything we get is a bonus .

K

katorx7

I have done this with my FC. I recently sold the complete S4 Diff with 4.88 gears and posi (Mazda Comp parts) to Louis of Rotary Reliability & Racing.

Sylvain Trembly of Speed Source Racing set me up with the custom setup. About $1500.00 including shipping from Florida to Nevada (had to ship my stock differential to them - about 2-3 weeks turn around time).
Very high quality work. Never had any problems with the differential. Had everything powering it from NA steet ported, Camden supercharged to 500 hp V8. Didn't use it too much with the V8 - way too steep ran out of gear at the 900 ft mark.

Project build progress:

https://www.rx7club.com/rx-7-audio-visual-lounge-143/1987-supercharged-fc-457752/


SPEED SOURCE RACING

http://www.speedsourceinc.com/


One of the best performance mods I did for improving acceleration - especially when NA.

owen is fat

do it you'll love it.
when I had a GSR swapped Civic, I took our ther stock ITR tranny's 4.4 ring gear for a 4.9 ring gear from ATS and it totally transformed the car into a torquey beast. it was amazing how much more power the car had thru all the gears, it was one of the best mods ever done to that car.

bump for short gearing n/a!

RX7Tuner.

In SCCA racing the Rx7 coveted ring and pinion is 5.12 which is typically built through ISC. They do good work.

KiyoKix

Yea those are the guys I knew used to do it, but they now ONLY do 5.12's due to turn around time and that is a bit too short for what I'm looking for. I really hope I can find someone that will still do this work. I really really REALLY want this done badly as my two favorite types of mods to do in a car are gearing and suspension (not much on engine power, that is close to last on my list).

Thanks,
K

anewconvert

iTrader: (1)

thanks for the numbers.

On a side note: If you do go turbo the gearing will rpobably hinder more than help. With gearing like that you reduce the load on the engine and increase spool time. You would probably have a hard time getting full boost in 1st gear, possibly second. Now with a blower..... torque monster.


BC

KiyoKix

heh, I forgot about that but yea...a blower would nearly stop the rotation of the earth if I ever got traction

K

KiyoKix

Sweet I finally found a way to get it done. Sylvain just sent me an email saying he'll do it for $375 . I'm waiting to make sure I get the final details so I can get the ball rolling on this. I'm also thinking that I may save a bit more and buy another 4.11 (as long as they sell the stock FC version) from Racing Beat so I retain a stock piece in case I decide to do something different later. He told me it'll take about 4 weeks so I'll probably end up driving before I get it put in since I may go crazy if I have to wait another month or so

Thanks katork7,
K

anewconvert

iTrader: (1)

Just pick up a stock NA diff. You can send out the one you picked up so your car isn't out of commission for 4 weeks. then when the new one comes back you can swap them out in an afternoon.


BC

katorx7

AWESOME !!

Good price too.

Mine got a little pricey since they supplied the 4.88 gears, S4 posi (rebuilt with Mazda Comp parts), stub axles, and setup the differential with all new parts so all I had to do was bolt up the complete diff back in the car.

You'll really feel the difference.
And no worries with Sylvains workmanship.

I've read he has been the dominate force in ITS with their RX7 (1998 & 1999 champs). Also with their Grand Am Cup Porsche

KiyoKix

Haha, I'm looking through the for sale section now to see what I can find. Great minds think alike

K

KiyoKix

Oh yea no worries whatsoEVER with Sylvain I've been a fan of his for a very long time now. He was a monster back in his ITS days and is like a fine wine getting better with age. He's doing excellent work with the RX-8 as well now, I'm glad he could help me out with that busy schedule.

I've always been more willing to do work to the gearing and suspension side of things to gain speed as opposed to engine power. It to me is just more rewarding to see what you can do w/o taking the "easy" way out. Not to mention if I really for some reason want more speed I can always do a few other things and keep it N/A the way I love it.

Thanks again for you help,
K

Mechanic

i havent a clue as to what is in my friends vert, it was a auto > 5 spd swap, with a s4 tranny is the only part i know of.... heres the weird thing, iirc the spedo reads 60mph at 3000+/- 100 rpm in fifth and feels correct as cars fly by me... he has a lsd as tested in the snow, the pickup feels great on his car.... tire size is.... iirc 215/60/15... on my car i got 205/65/15 and 60 mph is about 2600 +/- 100 rpm for me! i have no lsd and the car feels shitty slow. i am about to jack his rear end as i am using his car until he sells it... lol.

KiyoKix

Your friend could have a 4.30 rear end that came in a few FC's, I've driven one or two and they feel very good I just need a bit more oomph and shorter gearing for what I like. The 4.30 is very nice though and does come with an LSD. If you like it they're not that cheap and FAAAAAAR easier to get than what I'm doing (not to mention cheaper) so I'd keep an eye out for one. Enjoy if you get one and now you know his secret...trust me though you would KNOW if it was a 4.88 as it's a ridiculous difference though it's not for everyone.

K

KiyoKix

I've always wanted the same in a vert though to deal with the extra weight and keep the beautiful sound of N/A that I love so much.