Heh, I had just about convinced myself I should just go Cusco RS, then you post this pro-WaveTrac link :P I guess, at the end of the day, I'm not really competitive at this point anyways. Best case maybe I start doing NASA TT late this year or early next year, but even then I'm not planning on actually winning anything anytime soon. So maybe I'll do the WaveTrac for now, and if it's really inferior on laptimes it's not the end of the world. If a year or two from now I actually think it's holding me back, I could always upgrade at that point. Lower-maintenance is worth something. I already spend an inordinate amount of time on car prep stuff.

Posting this here for posterity: since I'll probably have to buy a 6MT pumpkin for whatever diff anyways, I've been researching my rear-end ratio options and trying to rationalize the differences on-track between the options. (Keep in mind the 7AT's transmission ratios are substantially different than 6MT...).

Stock 7AT is 3.357, Stock 6MT is 3.692, and there's also a 4.08 available that people have done. I'm estimating my tires at 789 revs-per-mile for 275/35R18 (although it will vary by tire options of course), and the 7AT's first 5 gears are 4.924, 3.194, 2.043, 1.412, 1.000 - I hope to rarely ever use 6, and never 7, as they get weaker up there. I guess we can pretty much ignore first gear too, I can't imagine needing it on any track.

Redline speeds in gears 2-5:
3.357: 52, 82, 118, 167 (current)
3.692: 47, 74, 107, 152
4.080: 43, 67, 97, 137

Thinking about straight lengths and speeds at my most-typical local tracks: generally I'm in 3 and 4 almost exclusively right now with only the big straight at TWS requiring a move to 5th. My end-speed on that straight is ~135 now on a good lap, but that might get higher as I learn to exit onto it faster, and it's almost certainly a bit higher in the less-common CW direction, so probably plan on 150 as a soft "top speed" that I'll rarely exceed anywhere (and if I do, it won't be for long or matter much). On most of the more-typical straights (e.g. back straight of MSR-H, back straight of TWS), on a good lap I'm sometimes bumping the redline in 4th right at the end, so ~115-120, but it's not worth upshifting before turn-in currently (but I might have to if I get faster eventually). Only COTA has really required 2nd gear, and I won't hit that often.

4.08 seems like it's not a good option right off the bat. I'd be using up a substantial portion of 5th gear on a regular basis (multiple times per lap typically), and I'd be shifting to 6th halfway down the big straight at TWS going CW eventually. I'm not sure I trust the strength of 6 or 7 on this trans to be in them hard every lap on my closest course :)

3.692 is a little bit of a wash with stock on shift points, maybe slightly better. I'd definitely be able to carry through some corners staying in 4th where I currently have to drop to 3rd to keep revs up, and that's worth a lot. However, straights that I can currently stay in 4th through (TWS back, MSR back) would now definitely require a 4->5 upshift partway down the straight. 5th should last me all the way to the end of the big TWS straight.

I used to think the universal "Lower is better" argument for rear-end ratios only really applied to drag racing, but I finally managed to convince myself it's true in the general case for road-racing as well (ignoring shift-points above). What it boils down to is this: the lower the rear end ratio, the shorter each transmission gear's speed range gets. The shorter the transmission gears get, the more time you spend higher in the power band (given equivalent shifting strategies using both rear ends). So, I think I'll take the stock 6MT ratio when I switch diffs.

I just realized there's a 3.917 gear available from Tomei as well, and the 4.08 is actually 4.083 (not that it matters rounded to mph below):

Redline speeds in gears 2-5:
3.357: 52, 82, 118, 167 (current)
3.692: 47, 74, 107, 152
3.917: 44, 70, 101, 143
4.083: 43, 67, 97, 137

I think I may have to do some more analysis, since the gaps between the trans gear ratios aren't linear, either. Maybe somehow graph this all out into a map of overlapping gear powerbands. Then I could figure out, essentially, my ideal shift points (probably all at redline, but it doesn't hurt to check), and then figure out whether the widening gear gaps in the higher gears means I'm losing effective power by pushing some of my "normal" speeds out into that range, and whether the torque gains from the gearing offset that or not at various speeds. Then maybe I could also pull in some notion of average speeds from my track data to find out which ranges are more important as well, if it comes to that.

Now that I've tinkered with the data a bunch, all I've done is further convince myself that lower is always better :) So much better in fact, maybe it's worth the risk of destroying 6th gear on my trans just to put 4.08's in there. It can always be rebuilt :)

Contrary to my intuition, the gears don't get wider as you step up, they get narrower when graphed as Horsepower vs Speed. In other words, the higher you are in the gearbox, the less rpm/power drop there is when you upshift again. This is especially true if I can move some of my shifts out into the 6th gear range, because the 5->6 shift is *very* short. Here it is visually:

http://www.the370z.com/members/wstar...d-vs-power.png

And again with only the outer two datapoints (less clutter):

http://www.the370z.com/members/wstar...ed-clarity.png


Note there are a lot of loose approximations in the data that lead to these graphs (e.g. I didn't have exact-enough dyno figures per-RPM and didn't feel like interpolating, so I stole someone else's dyno graph that had nice grid lines - but the rough shape is all that's important there).

Dude you really inspire me with the things you do and the thought you put in it.

But when and where would you need 6th. Isn't that like over 160? Super long straight away. For the other gears. I'd love to have it. Some tracks it would help. Others I assume it will be just another shift. But it's all a benefit.

Having different rear diff setup does help on different track ;) I used 3.91 and the 3.54 with good success
ive seen somebody with a 4.3 I should start looking for this for next year ! Depending on budget.

I skipped over 1st gear in my graphs, so those peaks are gears 2-6 (the very end of the whole line is the redline in 6th). So, if drop down to 4.083 rear end, my 5->6 upshift will be at 137mph (probably more like 134, because I tend to short-shift a bit by default unless I'm really pushing it). I don't think I'll hit 6th at MSR-Houston with that ratio, but I probably will on TWS-CW and COTA under this setup, given the speed help from the lower gearing earlier in the straight. I almost wish I could go even lower and get to that very short 5->6 shift even more often. It seems there are, in fact, gear sets for 4.364 and 4.636, although I'm not 100% sure they're compatible on our 6MT-R200V rear end. Might be interesting to add that to my data as well...

Here's those even-lower drives vs 4.083:
http://www.the370z.com/members/wstar...nal-drives.png

In that 4.636, the 4->5 upshift would be at ~87mph, 5->6 at ~122mph, and 6->7 at ~142mph

I've now found mention of a 4.90 that came in the 2004 Xterra, which might also fit in our diff :)

New graph of just those lowest-two found so far (4.636 + 4.900), with 7th gear added since max speed limit is (finally) becoming an issue at these ratios. I have feeling there are probably other tradeoffs I'm ignoring if I push this too far, also (like time lost to the 7AT's relatively-fast shifts if I'm shifting much more often), and that aside my max speeds in 7th with those gears are ~160 and ~150. I'm not hitting 150 anywhere yet, but I probably could eventually (esp, again, with the gear change itself helping out).

http://www.the370z.com/members/wstar...-4-636-4-9.png

^- you know at some point you're gonna be spinning the tires because you got so much torque with the low ratios! IMO I wouldn't want to go too low or else you're gonna have to have more throttle control... even worse in rain. maybe i'm just lazy and like to put the pedal down as soon as possible :)

Well, I was planning to re-work my throttle map for better control in the bottom half of the pedal anyways, but, yes :) There are actually a whole lot of reasons not to go extreme on the final drive ratio. More-so than the traction issues I worry about the wear-rate fallout with the 7AT.

According to those who've run FI on their cars, 6 and 7 (and maybe 5, too) aren't as strong as 1-4 in terms of torque-handling and get burnt up when you put too much power through them. I have no idea if moving a larger percentage of my track-time to redlining in those gears will have a similar effect even though I'm NA, over time, and eventually kill those upper gears.

And again - more shifts = more time lost to shifting, and I can't always shift mid-corner if I'm on the edge of traction, so a strategy that requires more shifting may result in more delayed shifts waiting for the car to straighten up and be a net loss.

Still, it's tempting to just slap in that 4.90 and see what happens. It would be a fun experiment, but possibly a costly one if I have to go back and put a different gear in soon afterwards :)

^- that's what gran turismo is for! they need to get that gps overlay stuff in there asap so we can get the texas tracks in the game lol.

Got any thoughts on 3.5 gearing? I have a 3.5 vlsd pumpkin taking up space... I could sell it to you for pretty cheap.

Another thing is a lot of my analysis with regard to upper speed ranges is based on TWS because (a) it's my closest track now and (b) other than *maybe* the back straight of COTA (which I probably end up doing once a year tops), it has the fastest straights around here for my car. If TWS goes away, that probably has significant effect on what speeds I care about on a regular basis :(

I think I have to pick up a used pumpkin no matter what I do, either off someone or off a junkyard. I need to look into that more, but it seems like there's some minor difference in the interior shape of my 7AT pumpkin vs the standard 6MT pumpkin on 350's and 370's, which is why a lot of the LSD mfgs only make it for the 6MT? It could also just be that it's a problem with gear circumference on the gear I'd be swapping out anyways, I don't even know yet.