I was wondering about gearing. The 3 rotor puts out so much more tourqe than the 13b. So will a 4.33 gear be to much. I know when I was racing mustangs the turbo cars liked a higher gear instead. Any one have any experience with this? I wondered if I should instal 3.90's in the car instead.

 

Um, do you understand what you're proposing?

Shorter rear end will lower top speed.
I dunno what car (i.e. trans) you're looking at, but a Zenki FC turbo trans with a 4.30 would top out around 150mph?
Is that good enough for you?

As it is, the increased torque (and power) makes both the FC turbo and FD 1st gear in the transmission useless - and you want to shorten the rear end gearing?

I dunno where the 3.90 gear set is coming from, but in FC's it's only limited to automatics - this makes this gear set not compatible with the turbo FC rear diff.
You want to run a non-turbo FC rear end now?


-Ted

 

His profile states:

Car:
93 rx7

Therefore, I believe he is looking at a 3rd gen automatic 3.90 vs an aftermarket 4.33 gear set.

Ultimately, it comes down to what you want to use the car for and the power it makes.

 

so what would the car approximatley top out at w/ a 4.33 gear set up w/ a 3 rotor and single turbo?????

 

The same as any other FD with that gearing...assuming the power is there.

 

so are u saying that a 13b fd with the identical gearing as a 20b fd will top out at the same speed??

 

^ YES! The gearing limits you to your top gear at redline. In theory, a stock FD tops out at ~185mph IIRC at redline in 5th. Putting in a 4.33 rear end would lower that substantially to ~160-165mph in 5th at redline. It's all in calculating how fast the rear wheels will be spinning in your top gear at redline, not how much power you'd make.

 

Correct I do have a fd and I was refeering to the auto gearing. In street trim I feel the the torque of the 20b may be to much and maybe with a higher 390 maybe it would make it a little more manageable. Im asking because I dont know and no one has ever talked about this subject before and I always thought people ran the 4.33 gear because the 13b doesnt create much tourqe. Please correct me if im wrong. I do know for a fact my turbo mustang responds better with a 3.55 than say a 3.73.

 

Looking at the posts again I know gearing has nothing to do with power output. But I will say when I had 4.11 gears in my old mustang it would break the tires loose alot easier than my higher gear set. Maybe its all in my head???? If so free feel free to flame me of my incompetence. here is what im getting at do I have more of a chance of keeping traction with a higher gear?

 

Ted
Here is the deal I have 4.33 gears in the car. With the 13b that was just fine but now that im switching to the 3 rotor. I was thinking the 4.33 would be to much gear. So I thought about switching it to the 3.90 gear found in the auto fd cars.

That was the intention of the post. Sorry I didnt make myself clear on the question.

 

i understand now. Thanx fellas.

 

Well, it comes down to how much power you're looking to make out of the 20B?

I dunno about you, but I find anything less than 600 a bit of a waste for a 20B.

Assuming this, this makes 1st gear virtually unusable!

With the 4.33 rear, it just makes that fact worse.

Ideally, a 3.5x would work, but how practical is that without swapping to a Ford IRS or something.

So, let's look at what *is* available out there right now...

I dunno if you know about the OS Giken gear set that drops into the stock trans case.
It lengthens 1st gear, and this might be the ticket.
This OS Giken gear set used to be one of those secret-secret deals, but I'm sure most of the experts know about this by know.
Supposedly, this gear set is what is supposed to help make the FC turbo / FD trans stronger, especially with 20B applications.
Now, this is prior to the Guru Motorsports option - people have posted positive results with the Guru gears, but $5k+ price tag hurts!


-Ted

 

Thanks Ted that is what I was looking for. 600 rwhp is my goal. I will do some research into this gear set.

 

Hmm, if you did the Guru dog gear set for the ultra close ratio and short 5th gear and the tall 3.90 rear end it would have a nice TALL 1st gear and such so you don't waste as much time burning off the tires but still have a reasonable top speed.

Thats how I set my cars up in Grand Turismo <-- ultra poser

Seriously, a ~400hp turbo wastes the tires through 1st and 2nd with 4:10 rear don't go for more low end torque at the expense of top end.

 

I'm following this thread too, and I'm using 4.30 gears in the rear end. Now, it seems that I need to switch them out for FD auto (3.90) gears?

 

I went ahead and got in on Jims 8.8 cradle so looks like im going to have a 3.55
Let you know how it worked out when its done.

 

Same here I joined in on the cradle group buy. I want to see how well the T56 and 3.55's work with the 3 rotor. I might put 3.73, or even 4.10's in.

 

I would go with the 4.10 for better acceleration, You would still be able to take advantage of the tall 6th gear for better highway mileage.

 

If you make a spreadsheet with your dyno curve (even if it's only imaginary at this point) and your ratios and tire sizes, you can figure out the RPM in each gear for every speed.

What you ultimately will find is that given your engine doesn't change and you ONLY mess with the rear end......

higher number (like 4.33) rear end will provide more acceleration in 1st gear but will limit your top speed somewhat.

a lower number rear (like a 3.9) will make your top speed higher, but it will reduce your maximum force at the wheels when you get started.

Now... here's the interesting part... how much power can you put down to the ground in first gear? Let me say that my car (about 400 rwhp and 335 rwtq) can not hold traction in first gear with 255 wide back tires at ALL. I can only use about half throttle or I get terrible wheelspin. If I was just going to drag race, I guess that would be kinda fun, but I only drive on the street and some on road courses... so I want my diff to stay in one piece!

So let's say your 3 rotor puts out... ohhhh say 450 rwtq and you have 275 wide back tires. Think you're going to hold that in 1st? no way.

In my opinion, leave the current diff gears alone until you build the car up and then if you want to, go to a 3.9 or something to make the power more usable. Your top speed would be ridiculous with the stock FD tranny, 3.9 rear, and 600 rwhp or so!

The "ultimate" solution is to have a completely different transmission with a taller first gear to avoid the wheelspin problems in 1st. 2nd would be slightly taller, and 3rd and 4th would be about where they are now.

 

Very Well Put

 

Im trying to reduce my wheel spin. As for the rear gears the 8.8 has 3.55 so Im sticking with that for know. Then I will mess with the trany when it goes.

 

What about the wheelspin in 2nd and 3rd?

I have a T-II rear-end (4.10) and 17" wheels with 275-wide tires. If I ever do the single-turbo conversion, I think I'll put 315s on the back.

 

How would a fd behave if it had the following gearing with the 4.3 final end. would it be any good for drag racing, i dont think it would but i jusy want someone else’s opinions.

1st 2.915
2nd 1.859
3rd 1.349

Thanks

Erdin

 

ok, maybe i can bring in some insight since i have some experience first hand w/ my last setup.

ignor the fact the rear was going to break any second.

I had my miata turbo'd to about 14psi and made about 225hp.
Now w/ the miata tranny (very short gearing) and a 430 rear the 95hp miata had an illussion of "power". Think about this... when i added about 150more horses the 430 rear and stock tranny together didnt work well. I went through the gears way too quick just sucked to drive (topped out at 145mph approx). SO i put in a ford rear 8.8 373, stronger and planning ahead. Then the car was FAR more enjoyable to drive.."felt" a bit slower but first was not lightning fast to go through.

so now think of your rx7 tranny and rear together and how it is now. Image more than doubling the HP's. Also ask... Are you really going for a landspeed record? (forget topping it out)

I plan to try my 373 rear w/ a tII tranny (possible straight cut gearing) and I think i might need to 1) not tap 5th in the 1/4 and 2) not have an issue w/ 1st being too short. Sure top speed will be well over 170mph but i am not looking to find it. I recommend a 390 gearing and if you need to go to a lower number (illussion of longer gear) get a gear made... part of the reason i went ford ,so many options.

make sure you cover the tires and suspension issue too.


hope i help,
bryan

 

The OS Giken trans parts ( http://www.enonvativforce.com/close_gear.htm ) + a 3.9 Ring/Pinion would probably yield something fairly driveable.

You would not be able to floor it in first without a ton of wheelspin, but once you got into 2nd I think it would work fine.

Your gears would run about like this:

1st 0-60
2nd 60-85
3rd 85-120
4th 120-150
5th 150-185 (redline at 8000)