Differential 3.9 vs 4.1
 

Who change there diff gears from 3.9 to 4.1 and was it worth it. After change was there any diffrence in driving was it faster ?
I'm looking forward into changing my diff because i did 5 speed swap 3 months ago and just was wondering would it be any faster and is it worth it?
thanks

Faster in top speed or Acceleration? The 3.9 gears will have higher top speed but slower acceleration. If you drag race then go with the 4.3 gears, according to RP at 7500rpm in 5th you'll be limited to 173 mph.

considering that our cars are governed at what is it 157 miles per hour I doubt anyone will miss the top speed that you cant even use anyways.That is if youre correct about having a top speed of 178 mph with 4.3 gears.

just jump directly to a 4.33 or 4.77 R&P.

chances are that u'll have larger tires to boot.

what are you talking around? The only thing FDs top speed is limited by is the wind resistance. I would assume that many heavly modified FDs can reach speeds over 157 without to many issues.

Where did you hear that?

Bad info.......

i think the japanese ones are limited while ours are not?

how much of a difference in acceleration would there be with stock vs 4.3? does it affect our already bad gas mileage much?

My question is would the diffrence be noticble ?

already bought 4.1

i believe that supras are governed at somewhere around that, but FDs are not. i think stock top speed is like 160.

Stock FD just runs out of HP to push through the wind around 157mph. There is a video on the 3rd gen forum of a member hitting 300kph (186mph) in a tunnel in Europe. There is also the video of the FD flipping through the air around that speed at something that looks like Talladega.

I would go with a 4.3 gear if you're looking to track the car and a 4.5/4.7 for drag racing or auto-cross (4.5 if you're going for < 11.0 ET as you'll have to shift into 5th at around 116mph/7500rpm in 4th on a 4.7 assuming stock tire size).

OK experts...what gears would you run if you were to swap in a V8 with the T56?

I'm pretty sure those are wind too.

Gotta laugh at the American way of doing things. "Put a V8 in it", that will solve everything. Oh Lord, why am I so cynical..........???

Back to serious discussion............ The 4.3 ratio found in the series-8 (99-02) version is easily the best ratio for any street driven FD RX-7. You guys over in the USA should all consider it as the difference is really noticable (accelleration) in all gears. As we accererate from 0kph more than we run top speed at 260kph+ the trade off in top end speed is a small price to pay. The small amount of extra fuel consumed is worth while too. After all, a sports car like this isn't really a fuel economy star to start with.

I own a series-8 RX-7 & can tell you from experience. Get the 4.3..........

On a side note, am going to upgrade the diff ratio on my JC Cosmo (90-95) from the standard 3.9 to a more streetable 4.1. Just need a series-6 RX-7 (92-95) to raid for its crown wheel & pinion..........

i didnt know the series 8 had different gears. would it be easier to just get a 99+ diff rather than replacing the gear? if so where could i find one?

i guess 4.3 gears would be fairly usefull then.

Are you on stock 16's or ??????. Larger wheels/tires will change the rpm power/acceleration range as well. Kinda like installing "higher" gears.

I thought going with larger wheels changed the gears accelaration for the worse.
Im on the stock wheels for now but I was considering either 17x9's all around or maybe 18's.I just dont want to lose any power if possible.The car is already a slow ass bastard off the line.Auto fd's need all the help they can get.

Going to a bigger wheel/tire combo will change (slow) your acceleration,..but will or should give you higher top-end speed at max power band/redline.

There is a big difference in wind resistance from 186 mph to 212 mph which the Racing Beat RX-7 flipped.

so even though the overall size of the wheel plus tires remains unchanged you lose power if you go from a 16 inch wheel to a 17 inch wheel? I dont get it .
I thought that as long as you dont change the overall size of the wheel and tire you keep the gear ratio the same.For example the stock wheels are 16 with 250/50 i believe. What if you run 17's with 245/45's?

This is very true. It's a physics thing. Even if the 17"s are the same weight, more weight is pushed farthure outward as compared to the 16"s therefore creating more rotational resistance.

On a side note....the Rx8 guys would easily regain some hp if they downgraded to 16" wheels instead of the 18"s they have.

Only the top level & some limited edition versions of the series-8 (99-02) got the 4.3 ratio. Here are a few examples of what specification of the "series-8" got what. Not all versions are mentioned.

Type-RS............... (production model) = 4.3
Type-RZ..................... (special model) = 4.3
Type-SpiritR............... (special model) = 4.3
Type-R Bathurst... (production model) = 4.1
Type-R................. (production model) = 4.1
Type-RB............... (production model) = 4.1

Worth noting, all the versions with the 4.3 diff ratio got 17" wheels as standard....

so then really accelaration with 17 inch rims and 4.3 gears are equivalent to 16 inch rims and 4.1 gears right? it should even out.
Thanx guys for the explanation.I guess im not going to go with 17 inch rims then.I already have shitty accelaration off the line.I dont need any help.im already screwed with the 3.9 gears.Going with 17 inch rims would be like switching to 3.7 gears.Screw that.
What about width? does it affect anything concerning gear ratios? for example running 16x8 in the front and 16x9's in the back?

Increasing rim diameter won't always hurt acceleration. The thing that will affect acceleration is the rolling diameter of the tire. If you increase the rim diameter, but keep the same rolling diameter (through reduction in aspect ratio of the tire) you can end up with the same overall gearing. Adding unsprung weight with big heavy chrome wheels will degrade performance, but that will be from rotational weight, not gearing differences.

Where exactly did you study your physics???

uh oh........... my spidey senses is telling me that there is going to be some candle-light flamage.......

ok so which one is it? can I run 17 inch rims with thinner tires and keep the rolling diameter the same and not have the rear end gearing change or what.Because ive looked for aftermarket rims in 16 inch and theres very little available.Now 17 inch rims are a totally different story.I was thinking of either going with some gold gram light pros or some 5zigen fno1r-c's in hyper black 17x9's all around.

A proper plus-sizing will not change the gearing. Selecting a good quality light wheel with a light, grippy tire will not hurt your acceleration measurably. Low rpm/speed throttle response MAY be affected very slightly, that's about it. This is assuming you aren't putting cheap-ass 28 lb wheels on the car.

Well this is something I saw on a TV program (Sportscar Revolution).

They dynoed their project Acura in stock form then upgraded the wheels to a larger diameter. The overall diameter of the wheel & tire combo didn't change compared to stock but the car lost 5hp after they dynoed it again. Also I think the wheels were in fact lighter than the stock ones. I know it doesn't make much since at first but the explanation they gave made since after they explained what had happend. This is the way I visualized their explanation.

Take two egualy sized bicycle wheels(the spoke kind) and strap on weights(lets say 10lbs worth evenly distributed). On one wheel we'll place the weights close to the center of rotation and the other farthur away form the center of rotation. If both tires are rotated the wheel with the weight farthure outward will be slightly harder to get moving as compared to the wheel with more of the weight toward the center. Does that make since to anyone?

Another example: go to a playground and have someone spin you around on a merry-go-round. Place youself in the center and have someone push you around. As you are rotating, move towards to outer section and watch how fast the merry-go-round slows downs. In this situation nothing changed except the position of the weight. I'm pretty sure most of use can relate to this situation.

hmm.t von does have a valid point I guess.well considering the fact that im either stuck with the stock wheels or I can have a wide variety of wheels in 17 inch trim I guess 17's will have to do.I just hate to think that I might lose power because of aftermarket wheels.

5 hp may be noticeable on an RXS, but not an FD. That's assuming it really was a 5 hp loss. The Dynojet is inconsistent as hell, you could easily get a 10 hp difference between runs without changing anything.....

I guess youre right about that.Hmm decisions decisions.
I think my first real costly mod is going to be a auto to 5 speed swap.
After that ill worry about rims and bodykits.

hmmm, i hit over 160 just fine :-)

http://64.158.28.140/MPEG/TwinVision...s_with_dad.mpg

Pretty sure your car also has an aftermarket ECU, which is where the speed cutoff programming would be located. ;)

true :-)

i should look into different gearing, I still have some rpm left in 5th at the end of the straight at all the big tracks (BIR, RA, MAM, et) and it would be advantageous to shorten it up (right?) for better acceleration.