rcdash

iTrader: (18)

Now that high horsepower 5AT 350z/G35 are starting to emerge thanks to offerings from SGP and GTM (600 whp+ capability with raised stall ~ 3200 rpms), I've been wondering if some of the gearing shouldn't be altered to match. Specifically, I've been considering the 2005-2007 2WD Nissan Pathfinder's R200 3.133 drive ratio rear differential to drop a Quaife into and swap into my G35 coupe (which will hopefully have 500whp or near there).

Any opinions on this? What is the predicted impact on performance in road racing and the 1/4?

What about an even lower diff: 2.86 is also a potential ratio in a R200 I believe?

Here are the drive ratios as they stand now.

Specs for stock G35/350z 5AT: 3.54, 2.264, 1.417, 1.00, 0.834 w/ rear diff 3.357

Here are speeds up to 7000 rpm in each gear w/ stock 3.357 rear diff (near stock 26.1" tire height):

Here are speeds up to 7000 rpm in each gear w/ 3.133 rear diff:

Specs for other high performance automatics

2008 Z06 505 hp / 470 tq: 2.97, 2.07, 1.43, 1.00, 0.71, 0.57 w/ rear diff 2.56

2007 Porsche turbo 480 hp / 505 tq: 3.60, 2.19, 1.41, 1.00, 0.83 w/ rear diff 3.05

2006, 2007 Aston Martin 450 hp / 420 tq: rear diff 3.07

2007 SLK Roadster AMG 355 hp / 376 tq: 4.38, 2.86, 1.92, 1.37, 1.00, 0.82 w/ rear diff 3.06

Alberto

iTrader: (14)

If a 3.3FD is awesome for our 6MT's above 500whp, Id think the 3.3 is right on for the auto. Keep it! I couldnt imagine any lower...

rcdash

iTrader: (18)

I've noticed that autos are always geared lower than manuals though - what are your thoughts on the specs for the other performance automatics?

I presume this is by design due to torque multiplication off the line with a torque converter and inherently one fewer gear to work with.

It would mean fewer gear shifts on the auto which might make up for any performance loss and would be less stress on the tranny. Say if you're aiming for a trap in the low 130s, a 2.86 rear axle will take you there at the top of 3rd gear. Not sure if that's the kind of thinking that should go behind the selection process...

booger

iTrader: (6)

Im not sure your speed chart is all that acurate . If remember right on a dyno that my speed at 7000rpm was 143mph in 4th.

Anyway...I had the 3.5 rear with 420whp [Mustang dyno ] and it was to much . 1st and 2nd was useless . Ive gone back to the 3.3 and it seems to be just right so far . If I ever get this car running right and tuned right , the power should be 30 to 50 more whp and I'll see then

EDIT

I do remember in 3rd my speed at 7200rpm in 3rd was 108mph exactly

rcdash

iTrader: (18)

Were those speeds with the 3.5 rear diff? I used the calculator at this site (could be off I suppose):

http://www.et-studios.com/motorsports/gears/gears.html

booger

iTrader: (6)

no..they were with the stock 3.3

rcdash

iTrader: (18)

I recalculated and the numbers come out exactly the same with a 26.1" tire diameter (295/35R18), which is just slightly smaller than stock.

I also tried this site: http://www.f-body.org/gears/ and I get the same numbers also (within 1 mph).

Not sure if I'm repeating a mistake but I think I've got it right.

How does the Mustang dyno hook up to the rear axle (edit: I think it is standard rollers, right) - are your wheels actually running on it - and are your wheels stock diameter?

booger

iTrader: (6)

18 inch tire should be a 40 series tire should it ? That would make it closer to stock . Thats what I run

coachk

iTrader: (21)

In the 1/4 mile I run 295/35/18 DR's with a stock FD and my rev limiter is set at 7200 or 7300(can't remember which). When racing the 1/4 mile I shift manually at 7000rpms for each shift and finish at the top of 4th gear at approx.6800rpms. I'm not sure if thats what your asking about but its a perfect setup for my car.

rcdash

iTrader: (18)

coachk> just trying to figure out if my speed/gear ratio tables above are correct. Can you recall what speeds you were at when you hit the rev limiter in each gear?

booger> the info you gave doesn't confirm tire diameter. The stock spec for rear tires I believe is 245/45/R18, which for a PS2 tire is an overall diameter of 26.7" (= 110.25 mm tire height * 2 + 18 in wheel diameter). The 295/35/18 is only slightly smaller at 26.1" (so will be running at a slightly slower speed) than stock for any given gear and rpm.

booger

iTrader: (6)

I looked at my stock 17's and they are 50 series tire . So 45 series should be right [close to stock] for 18's . Im running 40's with my 18's . But a 35 series would set your speeds and rpm's off even more . How much ? I dont know .

rcdash

iTrader: (18)

booger, the overall height depends on the width and the aspect ratio. So a 295 * .35 = 103.25 mm and 255 * .40 = 102 mm and 255 * .45 = 101.25 would all be almost identical in height (assuming all are 18").

If you give me the full tire spec you ran on the dyno (width, aspect ratio, and wheel diameter), then we can rule that out as the difference observed.

thx man.

booger

iTrader: (6)

275/40 18's

to much math for this old man

but I dont see how wide a tire is , make any diff on how tall it is . And height [ diameter] is what will change rpm and speeds

rcdash

iTrader: (18)

The second number represents an aspect ratio (a factor that is multipled times the width to get the actual height). For your 275/40 R 18, the height is actually calculated from the width * the aspect ratio (this gives the wheel "thickness", but you need to multiply by 2 to account for that part above and below the wheel) + the wheel height = total height.

So 275 * .40 = 110 mm * 2 = 220 mm. That's equal to 8.66" + 18" wheel diameter = total height = should be 26.66 inches. That is nearly identical to the stock height of 26.7 inches (as you said ).

So we can rule that out as a source of error - bummer.

rcdash

iTrader: (18)

To test this theory, I could just try to get a hold of some bigger tires (to mimic a different final drive ratio) and try it out... maybe used/old set and mount them on the spare wheels.

EDIT: Of course, there is a limit to the size of tire that will fit in the wheel well...