Kevin Crawford

I am thinking of doing a 4.77 gear swap in the gsl-se for acceleration purposes. I do not care about gas mileage, the car isn't my primary vehicle. It is mainly being used for autocross. I more or less love the unique characteristics of the rotary and do not believe in swapping the motor out for a piston engine. The car has a rb header and exhaust besides this and a stg 1 clutch the drive train is stock. what kind of improvement am I looking at in 0-60 times and 1/4 mile times with this gear. Is it worth the upgrade or should I go with a 4.4 rear gear. I will be doing highway driving to events but no more than 30 minutes to an hour.

Richard Miller

You know, I have been trying to find out a before and after times after gearing changes to no avail. Even the muscle car websites who go into such absurdly detailed specs on the build of the month hasn't posted before and afters for gearing.

EDIT: I dusted off the old CARTEST v 4.5 DOS program and played with comparisons between the 12a 3.9:1 the GSLSE 4.01:1 and a 4.44:1 and 4.77:1. According to the calculations there is no benefit in changing gearing. I really do not believe this and am looking further into this.

chuyler1

iTrader: (1)

For autocross, what matters is whether you need to use 3rd gear or not. In most cases, you are faster if you can complete the entire course in 2nd. Grassroots Motorsports magazine goes into lengthy detail about top speeds, redline, and all that jazz in relation to their project Miatas. Generally speaking, the 5 speed Miatas fair better because of a longer 2nd gear that revs out to 60-62 mph which is the top achievable speed on most small parking lot courses. With the 6 speed, they are having to shift into 3rd a few times which slows them down. So before you go this route, how often are you redlining 2nd during the events you attend, and how much RPM do you have to play with before you'll start having to shift into 3rd?

For road course, the focus is on close ratios between 50 and 100mph. In this situation the most common approach is a Miata 5 speed gear box and 4.77 final drive. It has nearly identical 1st and 2nd ratios but compresses 3rd, 4th, and 5th into your current span of 3rd and 4th. It'll mean more shifting, but since you spend more time at full throttle on a road course, you can recoup those milliseconds lost shifting with the better acceleration you will get. Unless you have some serious mods, you won't run out of gear in 5th down the longest straight with this setup. If road racing is important, ask yourself how often are you using 5th (if you use it at all) and how much RPM do you have to work with on the courses you frequent.

j_tso

iTrader: (2)

A while back someone uploaded old articles from Rotary Rocket magazine which did a feature on upgrades to a 1983 GS.
One of the upgrades was a 4.44 gear set that decreased 0-60 and 0-80 times by 1 second and 0.3 seconds faster in the quarter mile.

12A cars came with 3.9 final drives and the GSL-SE had 4.1, so the change wouldn't be as dramatic compared to a 4.7.

KYPREO

The actual vehicle gearing is also dependent on tyre rolling diameter. The GSL-SE came with a 4.1 final drive but it also had a taller tyre at 205/60/14, so the ultimate gearing isn't that different to a 1st gen running on the 185/70/13.

I have made a spreadsheet which allows you to play around with the effects of altering the final and tyre size on gearing vs rpm and speed and across gears. It is available here: Wheel Sizing, Gearing, and Speedometer Spreadsheet - AusRotary

It doesn't calculate performance improvements, but it is still helpful to check cruising rpm and how your gearing marries up to speeds you'd see on the track.

Judging by my experience and what others have had success with, 4.777 is better suited to a bridgeport/PP engine that continues to make good power above 8,000rpm. On a street car, 4.444 is a good balance between torque, not running out of gears and acceptable cruising rpm.

mustanghammer

iTrader: (3)

Not to pick nits, but the difference between an RX7 gear set and a Miata gear set is 1st and 2nd with a slightly better 5th gear. For road racing the Miata gear set gives you a usable 2nd gear for the slow corners on track and the ratio difference between 2nd and 3rd is not that great. However, between an RX7 and a Miata, 3rd and 4th are basically the same. The significance of this is really felt on street ported engine.

One more thing on road racing and gearing, final drive selection should be based on two things. First you want to get the race car into 4th gear (direct drive) as soon as possible and keep it there as long as you can because there is less drive line HP loss in 4th than there is in 3rd or 5th. Second, you want the race car to run out of revs in 4th gear at the fastest point on the race track. For me, all of the local tracks dictate a 4.88 and at Indy I was running a 4.44.

I have autocrossed with an RX7 gear set and a Miata gear set with a 4.88 rear end. Because the 2nd gear ratio in the Miata gear set is higher I was able to spend more time in 2nd. That is a positive! Also, because the 2nd and 3rd gear ratios in the Miata gear set are closer together the engine stayed in it's power band better. Earlier I autocrossed the same car with an RX7 transmission and the same 4.88 rear end with 22" tall Hoosiers on a Nationals course at Forbes Field. I did allot of shifting between 2nd and 3rd which was allot of work! So for me the magic setup would be a Miata gear set and a 4.88 rear end depending on the max RPMs on the engine and tire diameter.

All of that said, a rotary engine does not have enough torque or a broad enough power band to able to run competitively without shifting gears in an autocross. You need a V8 for that! My old CP Mustang was setup to run 72MPH at 7600RPM in 2nd gear. That car was shifted once per run.

For Autocross max speeds, I used to calculate based on 70MPH for a Nationals course and mid 60's for local stuff.

chuyler1

iTrader: (1)

Not sure if my writing implied, but the Miata gear set needs to coincide with a change to the rear end such as 4.77 or 4.88 as you suggested. I've also seen people run 5.10 on high revving Bridgeports. The majority of the tracks in the Northeast have top speeds around 100-115mph which works well for a modified 12a or mostly stock 13b spending a lot of time in 3rd and 4th with a little bit of 5th at the end of the longest straights.

Kevin Crawford

Did you have to do anything special besides possibly besides a custom trans mount to mount up the miata transmission? I ended up buying a kia sportage front diff. I have a fc 6 port s5 rebuilt motor that I will be putting in the car next year with an aluminum flywheel. I am hoping that I can get alittle more acceleration out of the car matched with the kia gears.

mustanghammer

iTrader: (3)

The Miata gear set installs in an RX7 transmission case so externally there are no differences. So trans mounts, clutch/flywheel, driveshaft, etc are reused. The input shaft on the Miata gear set has to be shortened so it is not a complete bolt in. I have had a couple of these transmission built for me and it generally runs around $1000-$1500 between the purchase of a 1.8L Miata transmission, machine work and reassembly. I believe there is an article on this on the Mazatrix website and there have been threads on the conversion on this site. Also the Mazdatrix web site lists all of the OE Mazda transmission gear ratios for transmissions that can be used with a rotary. The Miata has the closest ratios of all of them, is fairly strong and you can still get parts.

Your car will accelerate quicker with the rear end gear. Traction will be an issue in first no matter what transmission you use.

From an Autox perspective, the main thing to look at is ratio spacing in the transmission because with an RX7 you are going to be shifting gears on course. Going to a 4.78/4.88.4.62, etc allows you to use 2nd and 3rd when you are in the heart of the course instead of more 1st and 2nd with the 3.9/4.1 rear end. The gain is that 2nd and 3rd are close together than 1st and 2nd. The same logic applies to road racing where the goal is to use the gears that are closest together ratio wise in the transmission. Look at the ratio chart that KYPREO linked in his thread and note the speed changes. Here is another one that I like to use:
https://www.racegearbox.com/gear-ratio-calculator/

Kevin Crawford

Is there anything you would recommend me replacing while I have the diff took apart. This will be my first gear swap, however I feel pretty comfortable in myself to do it considering there are a couple how to's on this. I know I will need a dial indicator and magnetc base and a torque wrench. The car has about 130,000 miles on it doesnt really make any abnormal noises from the rear end while driving.

GSLSEforme

You should plan on replacing bearings(and races) carrier,pinion,pinion seal,crush collar. Consider going over clutches,shims,washers in LSD unit. These are all wear components with a finite life and at 130k miles,a good portion of that is used up.

Not likely as a 1st timer getting pinion bearings off and on without damage-even with correct tools. Setting up new gearset with used/loose bearings not best recipe for success.

Boriquaguerrero

iTrader: (18)

Boriquaguerrero

iTrader: (18)

Johnny_Cracker

iTrader: (1)

I did the 4.77 with Al flywheel in my SE, retained the LSD ... Put on the Mariah glass hood too for a substantial weight reduction and it sure feels a lot quicker now! Mine is a non-daily street car. It's quiet, no whine but just a tiny bit more backlash than I'd like.

You said it'll be your first diff setup (like mine was), so my advise is to invest in LOTS of crush tubes and a gallon of Ben Gay to sooth your newly acquired tennis elbow from endlessly checking your gear pattern. ..Did I mention crush tubes?

..Oh, and buy a hydraulic shop press if you don't have one yet.