Some Gear Ratio Analysis

[UP2MTNS]

So with the new transmission coming, I'm trying to mentally figure out what the different (lower) gear ratios mean for my driving.

I'm staying with the stock 3.9 final drive ratio, but I'm a little concerned because all I hear about rally transmissions is 4.1 or even 4.44. I want to make sure I don't go through all this work and find out I should have done something different. (at the same time, getting new front/rear ring and pistons and possibly different transfer gears adds a bit more cost and a lot more work (like pulling and rebuilding the rear diff))

So I found a cool little website online and started crunching numbers and relearned how to do cool little graphs in excel. And here is what I came up with:

http://img41.imageshack.us/img41/5918/gearratioanalysisgif.gif

I realize there's a lot of lines here, but its pretty interesting.

GREEN is my current stock WRX gears...plotted in RPMs vs SPEED
RED will be the new Group N ratios.

(Yes, Group-N 1st gear is actually longer than the stock WRX)

POWER BAND starts at the yellow line, 3500 rpms...would ideally like to stay between 3500 and 5500 rpms.

The circles are the speed range for ME for a given turn (dictated by stage notes....so a Right 2, Right 4 and Right 5).

So read this as.....for a Right(or left) 4, I can typically take that around 50mph...give or take a few mph depending on if its 'open' or 'tightens' or what ever. But you can see that I'm already at the top of the power band in STOCK 2nd gear....but if I upshift, i'm right at the lower powerband limit, and if I need to take it slower (closer to 40mph), then 3rd gear will easily bog.


Same with a Right 5....you can see where the green/stock lines are either on the high end of the power band, or on the low end....BUT, the red Group N ratio splits the difference nicely.

This seems to line up with my experience that there's a 'power gap' in the longer WRX gears because I would be pushing redline in some corners, but then in others using a higher gear and end up bogging and having to downshift again.


SO, IMO....this should be a nice setup. Of course, its easy to get the data to agree with what you want it so say, so what do you guys think?

I'm not sure if all the data is right though....if you look at 1st gear (stock) at 6000rpms is says approx 33mph......at that speed, if you upshift to 2nd gear you're then down below 3500 rpms (follow that point horizontally to the left until you hit the other green line)....is that right? A + 2500rpm drop from 1st to 2nd starting at 6000 rpms?

The others look right.....5th gear is correct at 3000rpms at ~80mph, etc.

I did the ratio/speed conversion at this link and just and plugged and chugged. If anyone wants to see my raw data, just send me a PM.
http://pw1.netcom.com/~sgalaba/mph.htm (http://pw1.netcom.com/%7Esgalaba/mph.htm)
tire width: 205
aspect ratio: 65
rim: 15
final drive: 3.9



WRX stock ratios from here:
http://www.cars101.com/subaru/impreza/wrxsti2004.html

Group N ratios from here:
http://www.flatironstuning.com/p-1977-modena-group-n-ratio-dog-box-gearset-for-the-subaru-5mt-transmission.aspx

[hoche]

That looks right to me and jibes with my experience playing with bugeyes at rallycrosses. There's a big gap between first and second where it'll be bouncing off the rev-limiter and shift and it bogs.

It looks to me like the RA gearset is designed for a bigger final drive. With the 3.9 you'll be suffering some on launch, but then if you can spin your tires all the way through first anyway it won't matter to you.

As for the stage note numberings, what you'll find is that you'll recalibrate your own brain by about halfway through the first stage.

[Rally OBXT]

The stock gearing on the WRX from 1st to 2nd sucks balls. If I was running an N/A car, I would NOT like the new 1st gear, but since you are turbo you should be OK.

[williaty]

It's also interesting to compare it to the RS gearing. This is the 2005 2.5RS 5MT gearing compared to the STi-RA 5MT gearset with a 3.900 final drive. The x-axis is engine speed in RPM, the y-axis is vehicle speed in MPH.

http://farm6.static.flickr.com/5248/5317273718_ba28ebe68c_b.jpg

This helps explain why I can beat WRXes to 40MPH or so. My 1st is lower than the RA, then 2nd is equivalent to the RA, and after that my gearing is longer.

[UP2MTNS]

The stock gearing on the WRX from 1st to 2nd sucks balls. If I was running an N/A car, I would NOT like the new 1st gear, but since you are turbo you should be OK.

yeah, 1st gear will be interesting....launching will be fine, at at speed it will be nice on R2's cause I won't hit the revlimiter as early (gorman was always brutal...so many turns higher than my 1st gear, but too slow for 2nd) but if there's a slow hairpin or whatever that will be difficult...I just hope I don't have to clutch kick it:kyle: