TNTUBA

iTrader: (1)

That chart shows just over 60mph. I was just basing my comment of that chart. But yes I see where an 8500 rpm reline would change that chart completely.

bbundy

see updated chart with autocross shift points. note speed range from 5 to 7000 rpm where all the torque and throttle respose is best.

bbundy

FWIW I'm pretty sure the Cusco 1st and second gear swap will fit in the Miata or the S15 box as well. it would fix the stupid low first and second gear problem in the Miata box has but it wouldn't help the weak 4th gear.

kaisersoze

iTrader: (8)

The cusco gears are 2200-2500 all by themselves.

Leafy

iTrader: (1)

I'm still liking the T101a based tranny. Use a 3.63 rear gear with
3.190
2.103
1.510
1.000

You can put the shifter wherever you want because its a slide loader with the linkage already outside the case. It should be easy to make a bracket to connect the mounting points on the tranny to the PPF (or eliminate the ppf), it weighs the samish as the stock 5 speed and you can get a freshly rebuilt one with whatever ratios you want for $2500. I would find those gears non-offensive. Shifting at redline will squarely put you in the power band of all but he most massive turbos, you will be a bit top speed limited at some track since it would top out at 160 with an 8500rpm redline, but if you had to drive it on the street, highway cruise would be more or less the same as a 5 speed with a 4.3. It would be nice to run it with a 3.3ish rear end but you would end up auto-xing in first.
Ratio's I'd pick for the 3.27 rear gear
2.366
1.780
1.392
1.000

That gives you 177mph @8500rpm in 4th. I really dont think that gearing would be too hateful to drive around paddock with, its more or less the same as a 5 speed's 2nd gear. And at least with my clutch its not that hard to creep in 2nd, and the all carbon friction surfaces really dont give a **** about being slipped.

mr_hyde

iTrader: (4)

Bob, since there are car parts on my couch and in the spare bedroom so you couldn't stay here when Kim kicked you out, I suggest we organize your shop and have a massive part-out sale. You easily have $10k worth of parts you don't use anymore and a new gearbox funded exclusively with the proceeds from a garage sale would get wife approval. Of course I could be wrong since I'm currently between future ex wives but I'm doing my best to help you keep your first!

mr_hyde

iTrader: (4)

...and if you cared about how your zero-60 time looked in Road and Track, that 2nd gear would be awesome! Think you could hook up to be under 3.5 seconds on 275s?

bbundy

x

bbundy

I don't rev to 8500 on a track its way past where a turbo Miata motor makes good torque and power doubt you could even get to 8500rpm with a 1:1 gear as most turbo Miata motors even 350 hp ones don't make enough power at 8500 rpm to push past the aero drag. Typical track shifting in higher gears is done 7000 to 7400 range. Driving my car now when I reach ~7200 rpm in 5th it stops accelerating until I shift to 6th. Top gear ideally needs to go about 150mph at 7000-7200 rpm on the high speed road courses I do, for that you need like a 3.1 rear end if you don’t have an overdrive.

Autocrossing however in second gear it is still pulling hard at 8500 rpm and a gear that reaches redline at 8500 at the top speed point on a typical high speed autocross course a little over 70mph typically is right in the power band at other places on the course where it needs to be and you don’t have to shift as a matter of fact often times you can be in second before you pass through the timing start light. Then the course becomes all gas brake and steer and it’s more like you have an A mod car with a CVT.

bbundy

High speed track section example with my current 6 speed 3.636. Lap record pace almost 13 seconds under spec Miata lap record.
Pacific Raceway: flat out between turn 8 and turn 2.
Turn 8 done in with downshift from 5th to 4th
Back to 5th before turn 9. Actually a bit of a short shift to avoid shifting in 9.
Shift to 6th just before turn 10, flat throttle through slight turn.
Leave in 6th through turn 1, flat throttle through slight turn
Downshift to 5th for turn 2. Right at 100 mph at the apex of 180 degree turn 2.

Note: even with 324 HP acceleration past ~140mph is glacially slow. Top out at just over 150mph on that section.

Leafy

iTrader: (1)

Good point bob. To keep under your 330hp limit you'd be under 200ftlbs at high rpms. But the 2nd one I listed tops out at 155 at 7500rpm.

Savington

iTrader: (31)

Let's put this thread back on track. There are a few really important things when picking transmission ratios, and top speed capability is not even in the top 3.

Gear Ratios:

First things first, we need to have four usable track ratios. 2nd gear in a 5-speed is too short (more on that below), but 3rd gear in the 6-speed is usable. With the 6-speed, you end up with less RPM drop between ratios and the ability to carry a lower gear should the need arise. This isn't doable with a 5-speed - there's one correct gear and that's just how it is (and sometimes you're caught between gears). Less rev drop means more net power output over time and never being caught between gears. The end result is that 6-speed cars are faster on track than 5-speed cars by 0.75-1.0sec on a 2-minute course. IMO, 4 ratios are required.

The placement of those four ratios is just as important as having them. Place one too low and it may as well not exist. Place one too high and you compromise the split between the other three, or end up with a high gear that you can't pull. The lowest of the four ratios needs to be something in the 6.0:1 range, so with a 3.63 rear end, that's a 1.65:1. A high-horsepower Miata can't use 2nd gear in a 5-speed (1.888:1) but 3rd in a 6-speed (1.645) is just tall enough to be usable exiting tight corners, so that's the benchmark. This is less a speed-in-gear issue and more of a pure traction issue, so raising the redline doesn't work. Any gear numerically higher than 6.0:1 can be considered unusable/nonexistent on track.

Once you've placed the first ratio, the goal is to aim for no more than a 25% difference per ratio, and less than that as you go faster. That's far enough apart to make each gear worth having, but close enough to provide an option for the occasional corner that's "between" gears. If you're spinning 7k+ through a corner, it's almost always better to drop into the next gear, and you want the revs in that next gear to be 5600rpm (20%), not 4900rpm (30%).

The factory 6-speed pretty much follows this. It has gear drops are ~24%, 20%, 16%, a usable low ratio, and decent straight-line speed for most tracks (although to be honest, that problem can be sorted with more valve spring and more RPM for the few times it's needed). Any aftermarket option needs to maintain those characteristics, IMO - I'm not keen spending $4-5k on a gearbox option only to go slower on track.

kaisersoze

iTrader: (8)

Well if the other options are too expensive it seems that it is back to what I think was mentioned earlier in the thread. Has anyone tried cryo/shot peening/WPC treatment of the stock gears? Seems like that might result in similar 10-15% increase in fatigue strength and might be as good as the NISMO unit. You would only have to do 3rd-5th probably. even if you did the shift forks and synchros it still shouldnt be 4K.

sixshooter

iTrader: (12)

I offered to help with a local shop that does cryo cycling . It is better than standard cryo treating. But that was a few years ago and I frankly don't care to involve myself in that at this point.

bbundy

I don't think the stock box will be as strong as the NISMO no matter how you treat the gears. the NISMO has reduced helix angle and the gears are selected with tooth counts that make them beefier. It's similar concept to the Quafe gears but for the 6 speed instead of the 5. Based on tooth counts I believe the S15 box is probably around 10 to 15% stronger than the Miata 6 speed and cost about the same assuming it can be made into a Miata box.

codrus

Gear-based boost control?

Racelogic traction control? Or does that break class rules?

--Ian

Savington

iTrader: (31)

That's treating the symptom, not the cause. You also have to shorten the next ratio to manage the gap, which leaves you with larger gaps between gears and/or less top speed.

Dunning Kruger Affect

Only one option: A race CVT. Hold the RPM at 5252 forever.

Leafy

iTrader: (1)

Do you really need the 25% rule with a modern turbo powerband? I know that if I ran ALLOFIT (peak and taper) with the 6758, the 3.27 rear based gearbox I proposed would actually put the car slightly past peak torque if you shifted at a 7500 rpm redline. I was looking at it and trying to figure out how to get MORE space between gears (which was hard being constrained with a 1:1 final ratio). It gives you 3 gears that will work between ~42mph to 152 with a 7500 rpm limit with a 6758 type power band, and 1st gear also gets you down to just below 35mph which pretty much covers even the slowest of auto-x corners. Its more or less a 5 speed with 1st gear left off. Do you really drop under 45mph much on the track? I could see the issue with this gearbox setup if you were running a turbo setup to make more than 450hp.

JasonC SBB

Isn't the solution 3.6 with a 5-speed then? Doesn't that give you 4 ratios (2-5) in the correct ratios and correct RPM drops?