dorkman52000

I was hoping someone could shed some light on this. On Rotary Performance's website, they have differentials up for sale. My car is the '88 GXL with an LSD rear end, and I know it has a 4.10 to 1 gear ratio. I was wondering if anyone has done the swap to a 3.90 rear end from an automatic, like they offer on their site. Is this a good idea? I know acceleration will be affected, as well as overall gas mileage, but is it recommended? I've always wanted to get better gas mileage in my FC, but I love its performance as well.

I guess my real question is, does it make much of a difference? Also, I know the top speed of the vehicle will be increased provided you have enough power to reach that top speed. But in an N/A, is it possible? Hopefully you guys can give some other info and advice. Thanks!

scathcart

Gas milage will have no noticable change.
Top speed will not change at all.

Falcoms

I'd say go for the 4.33:1 rear end. Better accel., therefore giving better gas mileage (more gear reduction=less resistance=less power needed to get to speed=better gas mileage)

Icemark

No, that goes the other way... better accell, worse highway and cruising gas mileage as the RPMs are considerably higher based on speed in gear.

A lower gear (for example 3.909) will provide better gas mileage at highway and cruising, but slightly worse accell but a higher top speed.

A high gear (for example a 4.3) would provide faster accelleration but at a loss of gas mileage and top speed.

aka_rocket

i know this is about rear differentials.
haven't been a car nut for long so lots i don't quite understand yet.
so this is similar to that of the GTUs tranny's gearing and regular S5/S4 tranny's.

Tofuball

I've got one of each sitting around

They all look the same on the outside! Its annoying.

Some of the RX-7s that come through here, (S4 GXLs espeically) have a little yellow tag on the differential telling you that it is an LSD

The TII diffs look different tho.

jprx7

i have a 3.90 rear end because my car was originally an auto but was switched over. My gas mileage around town sucks but on the interstate it really shows a huge improvment over around town driving. My car isn't running completely right so the mpg numbers won't mean much, but I've been getting around 12 city (mostly sitting at idle between my apartment and campus, i go about 3 miles round trip) but if I drive almost entirely interstate i get close to 20 mpg. It's running around 3000 rpm when the car's doing between 70 and 80 (been awhile since my car's seen interstate or i've been in 5th gear for that matter).
Patrick

twistedriver

I am not sure about gas milage. But isn't top speed is depended on power right? I mean most of us can't even top out our 5th gear.

Valkyrie

It depends on both power and gear ratios, as well as the car's weight, rev limit, and drag coefficient (sp...).

MOST cars won't ever redline their top gear because the top gear is usually designed for better cruising gas mileage (slightly less so on sports cars, but it's still true), and the cars weight and wind resistance just won't let you reach the reline in top gear with the car's stock power... but upgrade it and you can probably reach it. Of course, on most race courses and streets, you'd run out of space before hitting the top as well... lol.

If you have serious power, it's a total waste to get shorter gear ratios, since you can accelerate like a bat out of hell no matter what kind of gear ratio you have, and you also have the power advantage to let you get to a serious top speed if you use higher ratios... so I think stock or higher gears would be better for a car like that (do they even make affordable higher gear sets?).

But on car with near-stock power, lower ratios will definately give you a performance boost (assuming you're not racing in a straight line for like 2 miles, heheh).

CoreyMH

Well either way you look at, shorter gears means quicker acceleration, weather you have the a lot of power or not. Shorter gears means that you will hit your peak horspower quicker and more often, your top speed will not suffer that much, i have a 4.88 and i can probaly hit 140,150 no problem. But you have serious problems if you are trying to hit more than that on the road, and you wont even hit that at a track. If you have an n/a dont go to a longer gear, if anything go shorter. Driving around town i get around 18 mpg, on the highway i really dont know because i dont really have to take it. But at 60 my rps are at like 3k so if you do a lot of highway driving just keep your rearend. Just dont go to the 3.9, it just simply robs you out of everything.

dorkman52000

Thanks for the info. I think I'll just stick with the OEM LSD unit that I have, since my car is pretty much stock for now and it will probably never be putting out enough power to max out the RPMs in top gear.

I'm going to change the gear oil in both the tranny and the rear end this weekend...is there a special additive I have to use for the rear end since it's an LSD? Or can I get away with just using the 80W 90 gear oil that I'm going to put in the transmission?

Green_Streak

now, the question is what should i do? i have the lovely combination of a 5 speed and a 3.909 rear end from the factory. if i were to bother changing the gears out back, what should i go for?

Turbonut

iTrader: (1)

Get a 4.10 lsd, as you'll probably not find a 4.33 used for sale. New they run about $500 plus the cost of someone setting the ring and pinion and new parts. Probably $600/$700. Unless you're in need of all the performance you can get, I wouldn't bother changing to the 4.10 unless you find one very cheap, or your diff is going bad.

Tofuball

It is easier to just grab the whole diff off another RX-7 then to just buy new gears.

The 4.1 opens are very common, and it is not that hard to put a miata torsen or any other carrier in.

rototrix

I have a 88 N/A w/o LSD but I recently acquired a 87 T2 rear end w/ a 99 FD Torsen LSD, will this fit on my car? Do I have to make any modifications?

Icemark

No it will not fit, without replacing the carrier, half shafts, and drive shaft.

rototrix

So the carrier, half shafts, and drive shaft will have to be from a 87 T2?

coldfire

iTrader: (1)

wow, i can't believe nobody has mention the effect of torque multiplication. it is not simply the gear ratios that lead to better acceleration, but also the effect of torque multiplication increase when going to a higher ratio. for example, going from a 3.90 to a 4.33 ratio will give you an 11% increase in torque delivery to the wheels. in drag racing some very fast cars use auto transmissions w/ high performance torque convertors that actually multiply torque by a great deal...


i think the best diff (aside from some aftermarkets) for the FC is the one that was put in the infini 4. not only was it a Torsen type LSD, but it also had a 4.3 ratio. only 4.3 ratio that came here was the GTUs, which was not a LSD and probably not strong enough for a TII...

Crawlin RX-7

I have a 3.90 laying around if your interested