I am in the middle of an S4 swap into my GSL. In my 3 years of RX-ownership, I have collected a 12a trans, a GSL-SE Trans, and an S4 trans. I am wondering which would be best to put in.

Since this is a street driven, back road blasting, long-haul road trip kind of car, I look for 3 things in a transmission: Good off-the-line performance, close ratios in the first 4 gears, and a tall 5th gear for comfortable highway cruising.

Looking at the trans' outright, the GSL-SE trans is the one to have. gears 1-4 are within 1.5% of the 12a trans, and 5th gear is 8% taller than the 12a.

However, going 1 step further, I started to look at the "closeness" of the boxes. To calculate "closeness" I used the following formula:

Closeness (%) = (gear n - gear n+1)/gear n *100

This gives the relative decrease in mechanical advantage between gears.

Well, to my surprise, the FC box is actually substantially "closer" in gears 2-4 than either the 12a or the 13b box! Table-wise, we have:

GEAR CHANGE...12a...-SE...S4
1-2.................. 39.7...39.6..42.4
2-3.................. 35.4...35.1..31.8
3-4.................. 30.2...29.5..26.8
4-5.................. 17.5...24.2..30.3

Which means that all else equal, you'll be quicker through 2-3-4 with an FC trans than with a 12a or 13b trans. On top of that, the FC trans has a super tall (0.697) 5th gear, for wicked highway cruising!

Which brings us to the "All Else". What I have neglected to mention here is the rear end. My car came with the 3.9 rear end. What accelerates a car is torque at the wheels. So, for the same engine power, the multiplication of gear ratio and rear end ratio is a predictor of torque at the wheels, and thus acceleration.

So, for a the 12a trans and the 3.9 rear end (remember, bigger # is quicker):

Gear.....Overall Ratio
1...........14.36
2...........8.67
3...........5.60
4...........3.909
5............3.22

For the S4 trans and the 3.9 rear

Gear.....Overall Ratio... % Slower
1...........13.58...............5.42
2...........7.82.................9.70
3...........5.33.................4.61
4...........3.909...............0
5............3.22................15.52

BUT WAIT! If I were to change the rear end to match the 1st gear ratios, I would need a 4.13 rear end, and that would get me:

Gear.....Overall Ratio... % Slower
1...........14.36...............0.00
2...........8.27.................4.53
3...........5.64.................-0.85 (0.85% quicker!)
4...........4.13.................-5.73 (5.73% quicker!)
5............2.88................10.68

So, I would match the 12a acceleration through 1st gear, be 4.5% slower in second gear, be quicker in 3 and 4, and get a 10% increase in fuel mileage in the highway!

BUT WAIT, THERE'S MORE!!! There is no 4.13 gear... There is a 4.11 from the -se's, but that'll slow me down... What I need is something shorter, like...oh... say.... The 4.33 from the miata. I'll skip the chart, but this combo would be 4.77% quicker in 1, 8.66% quicker in 2, 5.91% quicker in 3, and 4.33% quicker in 4, AND give 6.4% better mileage on the highway! That's like having your cake and eating it too!

I know none of this is news, but math always gets me excited, and to me, this looks like a hot setup!

 

Nice to see someone who gets into the nitty gritty of thier projects. Gearing aside, the S4 tranny will hold up better, but they seem to have aproblem with the 2nd gear synchros going out. I think the longer 1st gear is the cause of this generally. The 4.33 rear won't really get you better cuising mileage due to the higher rpm you will be running. It should get you there quicker though. Funny thing is my FC seems to run better and get better mileage at 80. That seems to be the sweet spot.

 

the thing you're forgetting when you compute the %slower is the engine! yes the gears are taller, but the engine is also going to be turning more rpms, because theres less drop between gears. the less rpm drop between gears means the engine can spend more time in the power band (or have a higher, narrower one), which will usually make the car faster.

so basically you're close, you just need a dyno chart....

 

Honestly, for street use there is precious little to be gained by this, if you want economy go with the longest 5th gear. For street and cruising the "close ratio" gears will have very little to do with your cars performance, hell buy some better tires and get more grip...

Gear ratios are selected for optimum power range whilst cornering at a race track, this is so you dont have to over-rev or change gears whilst cornering. The acceleration advantages are negligable. Some fast tracks need wider ratios, whilst some tighter tracks need closer, most choices are a compromise anyway, this is so that most corners in a lap are at a better rpm range whilst being taken at speed, almost impossible for every corner to be at the optimum as all corners are different.

This wont help your 1/4 mile times at all, unless your car is going off the power band which no RX7 should if driven right. 1/4 mile is grip, acceleration, weight.

I guess I'm not seeing the point, I'm not critisizing just trying to see why.