4.10 vs 4.88

itsrx7inAK

itsrx7inAK

New member
In the rear, anyone sporting a 4.88 set on a second gen? Just wondering how the difference was/felt before looking at one
 
A big gear is a necessity on these cars. Most guys run a 4.88 or a 5.12...there might be a few 4.30s too. I guess it really depends on the tracks you plan to race on, but the lack of torque these motors produce means we gotta rev the snot out of 'em and put in a big gear.
 
Used gears are very hard to find. 5.12 is the best all around gear but 4.88 is next in line. They never made any gears except a 3.9, 4.1, and 3.39 that bolt into these cars. All the other ratios must be custom made for them. I have the 4.88 and 5.12 in stock ready to bolt in or can do the modification to the various gear sets that fit. Just giving you some options, not pushing my products.:)
 
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I don't know anyone that is running other than 488's or 512's

We did the calculations and found that one could be better
than another at one track, and visa versa at another, based
only on how many shifts you need to make...

for example, at NJMP Lightning we are on the shift light in 4th
(8300rpm) near start/finish on 488's with 45 tires, we then
keep accelerating on the light to the braking zone. If we
were on 512's with 45's we would need to go to 5th gear.

On the other hand, we could go to 512's with 50 series tires
and end up with almost the same ratio.....and since it is easier
to change tires than diffs, 512's seem like a better choice...

circumference / diff ratio = inches of travel per revolution of the driveshaft

((225*0.45*2/25.4) + 15)*3.14 = 72.1 / 4.88 = 14.78

((225*0.50*2/25.4) + 15)*3.14 = 74.9 / 5.12 = 14.63

But at Pocono, we all need 5th gear, so in that case you
might be able to tweak the performance with 488's to be better
with different tire choices.....

and then again....but in the case of....and conversely...

Bottom line: the 488s were cheaper so we have them in both cars :D

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of course, my calculations make the following simplifying assumptions:

1. That the increase circumference due to increase in tire
pressure during a race is exactly offset by the decrease in
circumference due to tire wear

2. That we discount the both the effect of decreased circumference
at higher altitudes due to the reduced effect of the pressure
differential of inside/outside tires, and the effect of increased
circumference due to the change in gravitational force on the
tire casing based on the distance to the center of mass of the earth...

3. That we ignore any relatavistic foreshortening of the tire
as the speed of the RX7 approaches the speed of light....

oh wait...I think I hear my adopted cousin...

JLawton said:
Shut up and drive!
Click to expand...

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seckerich said:
Used gears are very hard to find. 5.12 is the best all around gear but 4.88 is next in line. They never made any gears except a 3.9, 4.1, and 3.39 that bolt into these cars. All the other ratios must be custom made for them. I have the 4.88 and 5.12 in stock ready to bolt in or can do the modification to the various gear sets that fit. Just giving you some options, not pushing my products.:)
Click to expand...

Steve,
PM sent (a while back) - Do you still have a spare/used 5.12 w| Torsen available?
 
Correct Mark, it is 4.37 I believe. Have to count teeth I guess. I sold the last used 5.12 I had at Barber so all I have are new. Mazda is now over $700 for the 5.12 before we do the modification. They are expensive but it should last forever if you just change the fluid after every other race.
 
steve & i have built gears for a given track , and thats works well at those tracks . but the best overall gears are the 4.88s or 5.12s. we like the 5.12s. racingralph its 86
 
We are running 225/45/15's, with a 4.10 rearend now, and i will look at the 4.88's. Our track is usually an old runway(flat), with a fairly long straight. The top of 4th was held for about 2-3 seconds there. I saw an old thread with 4.88 vendors. Any suggestions on who to use?
 
with the stock 4.10 or 3.9 , you will use 2nd , 3th ,& 4th rarely or never 5th.with the 4.8 or 5.12 you will use 3th , 4th , & 5th and you will get off the corners quicker. these cars/engines don't have any torque , so you have to go with RPMs . i now have a spare 5.12 so i need to will my "old" spare 4.8(ltd) in a housing with axle stubs , ready to put in a 2nd gen rx-7 . racingralph
 
I've been looking for a 4.88 or 5.12 for a while.

What's the going rate for a good used one these days? (I doubt new is in my budget)

Racingralph: What's your location?
 
worth it

I went from 4:30 to the 4:88 and it's really a huge difference on the track.

The factory gearing is so tall it's dumb. I also don't understand the huge gap between 4th and 5th in the regular tranny. (The GTUs 5th helps that.) I think it's an old-fashioned way to do gearing. They'd never make a Miata or RX-8 set up like that these days.
 
I have been recommended to put in 4:88's in my Miata for ITA. Sounds insane, but I used to run ITA, so I'd definitely try it out. :cool:
 
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