MOBEONER

iTrader: (18)

yeah guys i looked around in the forum and randomly searched Google which lead me to mustang and corvette sites.

how does the 3.90 gearing affects the FD in the street and in the 1/4 mile

how does the 4.10 earing affects the FD in the street and in the 1/4 mile

reason i am asking is because i just don't know much about gearing as i am doing an auto to manual swap and i will leave the auto 3.90 on there.

TURSTY

I have a 3.8 rear diff and this is what I can tell you. You loose (sorry for the metric measures) 20km/h at 100km/h. 100km/h is the same revs as the standard diff at 120Km/h so fuel efficiency suffers.

It lowers top speed by quite a bit, but is good for drag racing and tighter tracks.

3.9 would be even worse and 4.10 would be too much for me.

proz07

iTrader: (6)

^huh???

the 3.9 will give you better gas mileage and higher top speed but suffer down low as it will take more power to turn the wheels same as a fulcrum.

the 4.1 will give you more of that "down low grunt" but your top speed will suffer because of it. But stock fd gearing will put you at something like 194 in fifth redline.

no sure if there is different gears for the manual trans to correct your speed for the different final drive but i would like to find out if anyone knows i would love to put a 3.9 ring/pinion in my fd

it seems you dont know anything about gearing though so try this on for size.
final drive gearing is kinda like a fulcrum the higher the ratio the more lever you have to acelerate the wheels the lower the ratio the smaller the lever making it harder to acelerate the wheels

this is why drag racers use high ratio to get to speed as fast as possible and its also why salt flat racers us super low ratio to get max speed but they need a push to get going.

z

MOBEONER

iTrader: (18)

ok so 3.90 is better long high speed racing.
and 4.10 is better for drag racing.

is this right?

TpCpLaYa

iTrader: (10)

sure...go with the 4.10 and do the job right

zonblitz

iTrader: (9)

or do like me and get a series 8 (1999-2002) FD 4.30 rear end and tranny with shorter 5th gear!

Yes it revs high and I like it! I have a BNR non-seq setup and it keeps me in my powerband for road racing. It isn't great on gas though if you go fast but at 70 staying off the turbos I can get 22-23 mpg. Last time I checked was my trips to deals gap. I can thank Barry Bordes for the nice leaned out cruising cells!

MOBEONER

iTrader: (18)

whats would a "series 8 (1999-2002) FD 4.30 rear end and tranny with shorter 5th gear" do?
faster acceleration or higher MPH?

LAXitives288

iTrader: (1)

better accel

GoodfellaFD3S

iTrader: (213)

Scroll down to the bottom of the page:

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

I would stay the hell away from the 3.90s, and definitely run either 4.10s or 4.30s.

I'm running the RZ 4.30s, partially b/c I have larger than stock diameter rear wheels (275/40-17 tires, 25.7 inch rolling diameter) which effectively lowers your final drive.

Some good reading here:

https://www.rx7club.com/showthread.p...ighlight=gears

and here:

https://www.rx7club.com/showthread.p...ighlight=gears

Aeka GSR

iTrader: (4)

Coming from a v8 standpoint for drag racing it is all about getting large power to the ground, a 3.8 you would end up roasting the tires off the line because the wheels were spinning too fast. A 4.10 or higher it gives you a chance to put the power to the ground since the wheels are now moving slower, but also give you more torque at the same time. Highway driving with 4.10s in my friends 69 Camaro was not fun since the reves were too the moon.

A battle was fought with low power cars too, a 3.8 setup or so would end up bogging a poor little honda so they went with 4.10's or so to get more torque and get the wheels going without the engine fighting about it.

edit: the stock ratio (i think its 4.10) is fine since the trans gearing lets you run those RPMs low when cursing around the street. I think the stock setup is a perfect balance for street and track. If you are a serious drag guy you could do a 4.30 or so but I think that is a bit too much for street.

Aeka GSR

iTrader: (4)

With huge power a 3.8 or so would whip those tires loose because the tires would be spinning faster than a 4.1. A v8 guy will go with a 4.11 not for the quest of torque, but to slow the tires down out of the hole. This is a common problem with v8 drag racing.

Edit: people here are used to turbo lag so the idea of blasting off tires with highway gears sounds backwards, but with instant torque and power coming from a v8 just shock the crap out of the tires and it becomes a problem with 3.8's. I watched friends battle this problem. I grew up around older Chevy cars.

Aeka GSR

iTrader: (4)

Lets make sure we are on the same page, we are talking about wheel speed, right?

The engine itself will spin at a faster rate with 4.10's than with 3.8's relatively speaking to the wheels, thus, your wheels will spin faster with 3.8's than with 4.10's.

You'll slip the **** out of your tires with 3.8's vs 4.10's. I even confirmed it with my friend who has done a ton of drag racing back in the day just to make sure I wasn't losing my mind.

FD3S2005

iTrader: (11)

i was just going to post that link, but i was seeing if anyone else posted it before me.... you got to it first

RLaoFD

iTrader: (6)

Aeka, you have it mixed around.

the gearing ratio means how many times the ring in the diff will turn (axle/tires) to the number of times the eccentric shaft/flywheel will turn.

In other words a 4.10:1 ratio will make the differential ring (your final gear) turn 4.1 times for every 1 revoution of the eccentric shaft/flywheel.

So now if you think about it, a wheel spinning at a 3.90:1 ratio will give you less rotation than a 4.10:1 ratio when the engine is going the same rate.

This means that the tire will spin less (cover less circumference) w/ a 3.9 ratio when an engine is moving at the same rate. The wheels will have less tendency to spin.

speed rII

RLaoFD: you are wrong, aeka is right.
The 4.1 is shorter than the 3.9, smaller the number is then faster you go.
And its 4.1 revolutions on flywheel and 1 on wheel.
If you dont believe me, just check those links above and see what gear ratio will give better top speed.

RLaoFD

iTrader: (6)

We aren't arguing about top speed we are arguing about initial wheel spin from a launch. thanks.

Lets reiterate things I've established.

Out of the hole: 3.9 will give less wheel spin

Higher RPM: 3.9 will give higher top end ( this is implied in my explanation due to lower RPM for same speeds of both)

Aeka GSR

iTrader: (4)

only if the engine has a problem with the lower torque output of a 3.9 and ends up lugging it, then yes a 4.10 will have you spinnin your wheels because of the added torque.
I'm talking about big, off the line power that won't struggle with the lower torque output of a 3.9 and just roast it compared to a 4.10. I think this is where people are not comprehending, I'm talking big power and torque off the line which probably should not have been brought into the conversation

ACE0000

It is true the first number is the rotation of the flywheel and the 2nd is of the transmission(ie 3.9:1 vs 4.1:1), so it effectively multiplies torque, the engine is spinning more for each spin of the wheels so it provides more power per rotation. Also because there is less power put into each rotation lower gear ratios make the engine work harder to produce each spin, however you will get more distance covered per rpm since the wheels spin more per rpm so higher mph.

I think I understand what Aeka is saying. Even though conventionally you usually assume that higher gear ratios cause more wheelpsin because the engine is spinning more, and the wheels are spinning more loosely since less power is require to push them. But, when a car has a ton of torque already sometimes just the fact that the wheels are spinning less per revolution can help reduce wheelspin even though more torque is being multiplied. It most likely will give you a little more control over wheelspin with the throttle at lower rpms on cars with massive amounts of torque.

real1st

iTrader: (3)

This is very interesting.

I am actually in the middle of a swap. From automatic to manual.

Compliments of RPM Motorsports.

I am considering this issue as we speak, I have an auto diff in my car, and was considering putting in the LSD 4.1:1 instead.

I was told it is not a huge deal, but I am still considering it. The biggest thing is the LSD part that I am considering.

real1st

ACE0000

I'm not entirely sure as far as any type of actual example or anything but I am thinking a 3.9 to a 4.1 would be a considerable difference especially with the final gear which effects all of the forward gears. But I'm not really sure of anything actual quantitative it just seems like kind of a big difference to me.

ACE0000

I agree for the most part, but he was more talking about cars with massive amounts of torque and low powerbands like large drag racing V8s. Some of them are already getting to their redline at the same amount of rpms a big turbo RX-7 would spool fully. At higher rpms the wheels are spinning so much already that the mechanical advantage provided by higher gearing will be much more likely to cause traction loss even though the wheels are technically spinning less. However at a lower rpm it is technically possible that the amount of times the wheels are spinning is more important than the mechanical advantage provided by gearing as far as traction loss. Since at 1500 or 2000 engine rpms they are only spinning at a few hundred rpms 30 or 40 rpms can be a big difference. While at 4000 or 5000 rpms on a high hp high rpm car since the wheels are already spinning at nearly 1000 rpms 100 rpms either way might not make a big difference. Of course there is no real way to tell unless someone was good enough at physics they could compare the effects on traction of the mechanical advantage and rpms of the tires and see which one had a greater effect at each rpm, which I know I couldn't.

GreatShamanGT

iTrader: (9)

We're talking about RX-7s....... their traction control is their lack of torque.

TURSTY

F$%^ I dont know why I wrote that I have a 4.8 rear. I need to sleep more. I have been doing this alot lately!

4.8 seems to work well with my car mind you Im making over 600atw. I requires a little more "driving" of the car but accel is kick ***. This higher gearing KILLS top end though.

SonicRaT

iTrader: (6)

This is all wrong. Gearing gives is a means of mehanical advantage, the higher the ratio the greater the mechanical advantage of the engine to the wheels.

With the SAME torque applied, the mechanical advantage of a 4.10 will be greater (meaning it's easier for the engine to turn the wheels) causing easier wheel spin. NOTHING will change this.

The fact that you've got enough torque to spin 3.9's or enough torque to not bog doesn't change the fact that if you put 4.1's on the same vehicle they'd spin easier (and that usually implies quicker). If you've got enough torque to roast 3.9's, you'll explode the 4.1's because they'll spin considerably easier. The only area that this Big power and torque or little power and toque doesn't change mechanical advantage, so that's the only reason it shouldn't have been mentioned, not because the FD lacks it as you seem to imply.


The only difference as far as 'roasting' would be the overall top speed that the tire attained while it was spinning, as it'd obviously spin at a higher speed once the 3.9 wound itself out, but that's pretty pointless as far as launching goes if you're spinning yourself all the way through 1st gear to redline.

GreatShamanGT

iTrader: (9)

I think he was talking about cars with a lot of torque. He obviously wasn't talking about the RX-7