I am having a cryoed TII diff installed in my FD within the next few weeks and have been looking into swapping in a different final drive (4.3 or 4.77) at the same time. From a standpoint of saving money on labor, it's either now or never for my R1.

Based on past threads I have combed through, it appears that the 4.77 is just too extreme for a street car, leading to much worse gas mileage and an almost unusable 1st gear, along with greatly increased cruising rpms at typical highway speeds of 70-80 mph. So the 4.77s are out for what I am looking for, but the 4.3s are still an option.

To try and gain some insite and make the most informed decision, I ran some #s on an excel spreadsheet that ccarlisi has. Below you will find the mph in each gear at 8000 rpm for a car with stock 4.1 final drive and stock wheels/tires:

1st gear-----41mph
2nd gear-----71mph
3rd gear------104mph
4th gear------144mph
5th gear-------200mph

look what happens when you switch to a larger rolling diameter (in this case my 275/40/17 rears) but keep the same 4.1 final drive.....you get lower numerical effective gearing, basically a loss of acceleration:

1st gear-----43mph
2nd gear-----74mph
3rd gear------107mph
4th gear------149mph
5th gear-------207mph

Not too long ago I had my stock wheels on for a week or so and noticed surprisingly better acceleration in all gears. This is attributable to the lighter stock wheels/tires, but the change in gearing plays a part as well.

Now, look at an FD rolling on 275/40/17s with the 4.3 final drive:

1st gear-----41mph
2nd gear-----70mph
3rd gear------102mph
4th gear------142mph
5th gear-------197mph

You're just about back to the stock numbers, within 1-2 mph in 1st through 4th gears.

For guys running an even larger rolling diameter, the loss of acceleration due to gearing wil be even more pronounced.

Based on all of this (and unless there are other downsides to the 4.3 gears that I am unaware of) I plan to order a set of 4.3 gears and have them installed along with my TII diff.

 

rich,

while youre at it, can you do mine too. i'll provide the beer ....

 

I just ordered my 4.3 gearset from www.rx7.com, they have them in stock.

I'm surprised there hasnt been more interest in this. Lots of guys get a set of big wheels/tires, install a Kaaz diff and then clunk down the road slower than they used to on stock wheels. It's an untapped acceleration gain, people

 

As you may be right about the acceleration I personally running a single turbo go through first gear and second in a matter of a second or 2. I don't think that I would even notice the difference on the gear ratios. It would be a nice upgrade for the guys still running stock twins. BTW I am running a 265/35/18 on the rear.

R.K.

 

Hm, I'm just not sure about that. (<----lol, bill lumberg voice) The top of second gear is at least 70 mph, and a bit higher with big wheels and tires. I don't think many FDs are going 0 to 70 in 2 seconds

I'm making as much as or more power than the guys running single turbos on pump gas and I *definitely* felt that my FD was noticeably faster with stock wheels and tires in 2nd and 3rd gear, and also in first gear when I could get traction.

Regardless, I will post my results when I get the car back. I borrowed a g tech so may be able to make some timed before and after runs.

 

What I meant to say is first gear is a waste. I can't get traction and it hits redline in a matter of seconds. Second gear is a little better. I didn't mean that I hit 70mph in 2 seconds. That would be one hell of a rush. Anyways that would be cool if you can post up some results before and after the mod. Keep us posted.

R.K.

 

I'm running taller than stock tires:

245/40/18
275/35/18

I did this because I wanted wider rubber and I got a sweet deal on these tires.


So, does bigger diameter wheels/tires = better gas milage & slower accelleration?

 

I had the 4.3 gears in my first FD, (the one I raced) combined with 9.5" Forgelines on 255/40/17 tires, all around. The gear change was DEFINITELY felt everywhere, track or street, and I loved it. I wouldn't mind doing it on my present FD, as well.

 

based on all the research I did, it would certainly appear that way. Going with 275/40/17s like I did leads to ~5% change in gearing (effectively from a 4.10 to a 3.90), thus moving to a 4.3 final drive counteracts this. My speedo should be correct now with the 4.3 too

 

That's some good feedback, thanks. Between the thread on removing the fender liners pieces to increase top end acceleration and this thread, you have contributed some valuable insight to the forum already

 

My speedo's been off since I got the 4.33 installed and went with the JDM 99 rims and tire diameter. 17 inch rim with 255/40. Where's the calculator to estimate mine?

Tim

 

I'd say you need better tires then.

 

Tim, 255/40 17 is very close to stock tire diameter, so the chart at the bottom of this page should be pretty damn close.

http://www.rx7.com/store/rx7/fddrivetrain_rearend.html

 

I was gonna search,but since we're on the subject,what revs at 60MPH (100KMH)
if you have 4.77 ratio?

 

Follow the link I posted right above....

 

No 4.77 ratio there,but I suppose comparing to the other ratio differences around
3000+ rpms???

 

Yep, smaller diameter tires accelerate faster.

If you're drag-racing, you may want larger diameter tires if you CANNOT hook-up, but on a road-course trying to stay closest to the original diameter (or smaller) is best. When deciding on some track wheels & tires, I chose to stay with a 16-inch diameter wheel (9-inches wide) because my current favorite tire is a 265/45WR16 which has an overall diameter of 25.2-inches.

The ideal is 25-inches (225/50VR16) diameter for the FD RX7.

BTW: You should have seen the race-prepped original MINI at Roebling Road.

Not only did that baby accelerate faster than some of the Porsches, but it was faster through the turns, and it had tires smaller than some their brake rotors (13-inches)!

:-) neil

 

FYI folks,
I used to have the Bridgestone spec. sheet on the S-01's that came on these cars, and they listed the diameter as 25" even, rather than the 24.85" that the mathematical calculation would tell you it's supposed to be.

That's almost exactly what most 255/40/17's are.

 

Oops, my bad. It does look like it would be around 3000 rpm at 60mph.

 

I just installed a rebuilt tranny with the short .806 JDM 5th gear. The USA is .719.

Currently I run 255/40-17 AVS ES100 tires and even at 10PSI boost with my GT35R, it will spin all the way through 1st. At 15PSI boost, 1st is totally useless if floored, and 2nd gear also spins way too much.

I plan to go 275/40-17 R compound on the rears to help traction in the lower gears.

With the short 5th, it is almost like still being in 4th when driving 70+mph.
Instant spool and ZOOM-ZOOM! Thus instead of having the 207mph 5th top end, it will be 185mph.

So I will gain in 1st, 2nd, and 5th, and lose in 3rd and 4th.

 

I just had my 4.3 gears installed along with my TII lsd, but can't give any impressions because my RE toe links and trailing arms were installed simultaneously and my FD is in *bad* need of an alignment......

 

I used a special method and just matched lengths of the stock ones to the new ones before installing. This is the 'damn close' method. I used it somewhat often. It would have come in handy for your drive to the alignment shop.

I notice you say "installed" as in someone else installing - who are the googly bandits working on your car?

Shoot me a PM about that thing with the doohigky

 

I'd be careful with R compound tires for street, unless they made them much different during the past decade. The biggest mistake I ever made with my FD back when I was racing it, was to try using the race, R compound tires for the street. They were harsh because the super stiff sidewalls, and slippery beyond belief.

Those tires HAD TO BE warmed up before the compound started working, just as if it was a racing slick. On the street you NEVER get to warm them up. I tried making my favorite canyon runs, all out, than check temperatures and never even got close to the kind of temperatures that those compunds required. You simply can not possibly lean on them hard enough and long enough to get them up to temps.

Cold race tires are far worse for stick than the cheapest of all discount street tires.

Albert