V8KILLR

To make a long story short.... i have a 3.90 rear gear and i have a manual tranny. I am considering going to the 4.33 or even the 4.77 gear, but want to hear some opinions on what people think are effective. It will be a weekend/street/strip/track car, and i dont care about doing 180 top end.

Thanks!

RX7Elmo

if you don't care too much about top end or revving too high when crusing on the freeway, then get the 4.77 That will give you mad accel in all the gears etc. I think 4.33 would be best for the more street driven daily car.

Danny

jspecracer7

4.77 gearing all the way if you're going to be a weekend warrior.

ronarndt

I had three 2nd gen- two with 4.1 rear and one with 4.3 rear end. It is amazing how much difference the lower gear ratio affects the acceleration. If you don't mind the even worse gas mileage, go with the really low gears for stump pulling acceleration.

BTW- How did you get a 3.9 rear in your manual shift FD? Just going back to the 4.1 it should have will make a difference.
Ron A.

V8KILLR

It use to be an auto, and i did a tranny swap in like 97 or so. I left the rear gear. If i do a 4.77 ... will 1rst, 2nd etc be useless? I am hoping that it will be much better than a 3.90

jimlab

Read this thread...

https://www.rx7club.com/forum/showth...hreadid=107395

V8KILLR

Ahhh... Jim has spoken Thanks Jim... im going to check this out now

V8KILLR

Damn.. good post Jim... so would YOU say the 4.77 or 4.33.... as always i value your opinion.

Thanks again!

jimlab

Normally, I'd say that 4.33s were a waste of time and money. Going to the trouble of opening up the differential and spending the money to go from a 4.10 to only a 4.33 doesn't make sense to me, personally, because the difference in performance is so small.

But since you don't have 4.10s and really should have more gear for a manual transmission car, the 4.33s do make sense, especially since they're cheaper than either the 4.10s or the 4.77s, going by Rotary Performance's pricing.

You'd have another $270 in your pocket that you wouldn't have if you bought the 4.77s, and you'd have a significant improvement over the 3.90s you have now. The 4.33s would probably be the best choice for a daily driver. The 4.77s might be a little too extreme for a street car.

GoRacer

iTrader: (2)

Hmm, rereading Jim's reply on that other post it seams like the 4.5's are the best street choice. I was thinking of changing when I do my synchros but maybe now I will just change 5th gear instead since it looks like the only advantage is at highway speeds.

xstacy7

I'm sorry if I break up the conversation, but I want to learn more about this. Could someone give me a summary of what these numbers you guys are using (4.10, 4.5, 4.77, etc.) and what the "gears" and "rear" is. Sorry if I got in the way, but I figured this is where to ask it, so why not.

Thanks

V8KILLR

Hey Jim.... even for a weekend beast you think the 4.33 would be the better choice? I dont really care about the money being saved vs. performance point of view. If you can add to this... that would be "excellent"

Wargasm

Hello, I made a spreadsheet to play with some "what ifs". If you have Excel, this should let you play with:

Gear ratios in the transmission
Final drive ratio of the ring and pinion
Tire size

Give it a try and have fun!

Brian

V8KILLR

Pretty cool sheet

jimlab

I don't think you'd want 4.77 gears for a car that saw mostly street use. If you spend every weekend at the track, then that might be different, but for the casual racer, it's not something I'd recommend.

jimlab

Bet you didn't count on getting a novel-length answer, though...

The numbers are ratios. "4.10" really means 4.1:1, and refers to the number of times the pinion gear (the gear connected to the car's driveshaft) turns for each revolution of the ring gear. When someone says that the differential of a car is 4.10 or 4.10:1, what they're saying is that the pinion gear turns 4.1 times for each revolution of the ring gear.

This number is determined by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. In the case of the manual transmission 3rd gen., the ring gear has 41 teeth and the pinion gear has 10. Divide 41 by 10 and you get 4.1.

The picture below shows the ring gear inside the differential housing, bolted to the differential. The ring gear converts the motion of the pinion gear 90 degrees to drive the axles. The design of the differential determines how much power is applied to each axle, but we won't get into that here.



The second picture shows the pinion gear at the front of the differential housing. The pinion gear is driven directly by the driveshaft and mates with the ring gear. As you can see, the pinion gear has 10 teeth. You'll just have to trust me that the ring gear above has 41.



The relationship of the ring gear to the wheels is 1:1. For each rotation of the ring gear, the axles make one rotation, and therefore so do the tires. The circumference of the tire dictates how far the car moves during that single rotation. A P275/40-17 tire has a nominal circumference (no tire is perfectly round with the weight of the car settled on it) of 6.72 feet, so for each rotation of the tire, the car moves 6.72 feet.

Higher (numerically) gear ratios (4.10, 4.33, 4.77) are referred to as "short" gears. Lower gear ratios (2.73, 2.90, 3.23) are referred to as "tall" gears. A car with short gears will accelerate more quickly, but the top speed in each gear will be lower than it would be in a car with taller gears.

In a 3rd gen. RX-7 with a manual transmission, 1st gear is 3.483:1, another ratio. In this case, it means that the input shaft of the transmission (and therefore the engine) must make 3.48 revolutions for each rotation of the output shaft. The output shaft, of course, turns the driveline, which is turning the pinion gear. Since the engine must make 3.48 revolutions to turn the driveline once while the transmission is in first gear, and the pinion gear must make 4.1 revolutions to turn the ring gear (and tires) once, we multiply the two ratios to determine the "final drive" ratio of 1st gear, and come up with 14.28:1 (3.48 x 4.1). The final drive ratio tells us how many engine revolutions are required for each single rotation of the tires.

Given this ratio, we can easily calculate how far the car moves in a minute at a given engine rpm. At 2,000 rpm in first gear, for example, we can determine that the tires will make approximately 140 (2,000 / 14.28) revolutions per minute. Taking our tire circumference of 6.72 feet, that means the car will move 940.8 (140 x 6.72) feet per minute. Multiply that result by 60 (56,448) and we know how far the car will move in an hour. Divide that number by 5,280 feet (1 mile) and now we know that at 2,000 rpm in 1st gear, that the car is moving at 10.7 (56,448 / 5,280) mph. Voila!

Now you see the effect that gearing has on the speed of the vehicle. If we substitute a higher transmission gear (say 3rd gear, which is 1.39:1), at the same 2,000 engine rpm, the car is now moving at 26.8 mph. Calculate the final drive ratio by multiplying 1.39 x 4.1 (transmission gear ratio x differential gear ratio) and follow the rest of the math.

A lower (taller) gear ratio allows us to move the vehicle faster with the same amount of work (engine rpm), but a higher (short) gear ratio allows us to get the car into motion more easily by giving the engine a greater mechanical advantage (torque multiplication). Because one gear ratio cannot do both effectively, that is why transmissions have multiple gears, and each higher gear has a numerically lower (taller) gear ratio than the one below it. Here are the gear ratios of the 3rd gen. 5-speed...

1st - 3.48:1
2nd - 2.02:1
3rd - 1.39:1
4th - 1.00:1
5th - 0.72:1

Because of its superior mechanical advantage, first gear (3.48:1) gets the car's weight rolling very easily, but has a top speed limited to about 43 mph. Obviously we can't travel at highway speeds in 1st gear, nor would we want to run the engine at redline for long periods of time. Fifth gear, (0.72:1, also known as an "overdrive" gear... or in other words, a gear which has a ratio of less than 1:1) on the other hand, is far better suited to cruising at highway speeds, because 5th gear brings the engine speed down to about 2,700 rpm at 70 mph. But we can't start the car out in 5th gear, because it lacks the torque multiplication to get a (relatively) heavy car rolling from a standstill. Nor can we shift straight from 1st to 5th. Well we can, but we wouldn't want to because it would bog the engine excessively and probably stall the car.

That's where the other three (2nd, 3rd, 4th) gears come into play. As we reach higher rpm in 1st gear, we shift to 2nd, and engine rpm drops while vehicle speed continues to increase. Shift to 3rd and again, engine rpm is lower while vehicle speed is proportionately higher. And so on...

You're welcome.

JimmyJimboJet

great post jim, i've always wondered the same thing.

rotorbrain

jim do you wanna be my dad? why cant my dad feed me full of information like that.


jimlab = my leige

paul

ronarndt

rotorbrain-

So......... for those of us old enough to know how to calculate gear ratios, compression ratios, remember when hi test gas was $0.29/gal (note that it used to be called hi-test instead of high octane).....does that mean we have groupies? If so, I could really use some help washing and waxing several cars, a truck and a really big horse trailer. Also mowing lawn, shovelling out horse barn, etc. etc. In return, I can educate you on the finer aspects of doing repairs on 1950s and 1960s British cars at the side of the road.
Ron A.

BATMAN

I have been thinking about the 4.33 as well.

One reason for moving in this direction is that I have 17" all around.

Is the RZ running 17's and 4.33 rear-end?

After dropping the 17's on my car it feels just a tad slower. So I figure the 4.33 will do the trick.

milkman2k52

I thought the rz had the same outside tire diameter as the us-spec?

BATMAN

it seems to be the case.