Everytime I read about the 3.92's, someone speculates that they would be good for X tenths in the 1/4. But after a fairly detailed search, I've not been able to find anyone that has timeslips for before/after the gear swap.

Does anyone know of any real-world numbers for this mod by itself?

 

I had the same question but after doing the math these Zs really don't need them, first gear is really steep and in general already pretty aggressivly geared. Unless you have a raised limiter it also forces another shift in the 1/4.

The fastest cars on here are still running the stock ratio.

 

I've got an 05 g35 6mt sedan, so my redline is 7k already. Fuel cut in 1st is a registered 7100, and fuel cut in 2nd and 3rd both is a registered 7300, so I'm not worried about having to shift to 5th. I don't think I'll be trapping high enough.

The reason I ask about the 3.92's is because I'm considering getting a set of shorty drag radials(205/40/17's), which for my car, is about 10% shorter than stock, giving me the effect of the 3.92's. I just want to see some timeslips to help me manage my expectations.

 

I was one of the first one's with the 3.9 and I did a write up here with before and after. After many runs, my conclusion was that I trapped about 1.5mph faster under almost all circumstances, but my e.t. was not reduced. Basically it makes it a little harder to get out of the hole. I had 295/35 street tires on and still couldn't get better than a 2.0- 60'.

 

Thanks man. Just read your review and that's some good info.

 

Trey,

I'm telling you, 205/40R17 DRs are just too short to do you any good. Get a set of light 16" rims (300ZX 16X8s 16lbs) and mount some 245/50 DRs. Forget the gears. The car is already optimally geared for natural aspiration unless you go to a signifcantly taller tire. Too many of the G/Z guys read about the gains Mustangs make from gear swaps (.2-.3 seconds 2mph) and think the same appiles to G/Zs, but they don't realize that Mustangs come from the factory woefully undergeared. A stock 97-01 Cobra with the stock 3.27 gears does about 52mph in 1st, 78mph in 2nd, and 115mph in 3rd. That's undergeared considering the car weighs about 3,400lbs and only makes about 265wtq. By stepping up to 4.11s, the NA Cobra will shed about .2 seconds and going with 4.33s will drop .3. By the way, take note at how much more torque multiplication it takes to get that Cobra to drop .2-.3. You're talking about increasing the ratio by .83 to 1.06. The Stang crowd agrees that going from a 3.27 to a 3.73 (.46) is a waste of time because there's hardly any gain. It "feels" a lot faster, but the ET/MPH will suggest otherwise. IMO, the same applies for a G35 goes from a 3.5 to a 3.9 gear. It's hardly an increase in torque multiplication to make much difference in ET/MPH. Sure it will feel a lot faster, but is it really faster under the lights?


And before some Mustang diehard calls me out on it, yes, there is no such thing as a 2000 Cobra

 

Yup pretty much sums it up... Also add to the fact that the cost of the gears and the install for the Z or G is soo high for the marginal gains its really not worth it... You would be better of just running some short slicks or DR's. I remember when i put 4.10's in my 93' coupe from the stock 3.23's i was freaking out how much better the car felt. Plus untill i changed the speedo gear i was doing like 120mph on the freeway LOL... I miss the cheap mods of the 5.0 mustang--- FRPP Gears plus install where $275

 

So the threads I had seen that said the 3.917 gears drop your 1/4 mile time upwards of .4-.5 seconds is a farce? They are still useful for tracking the car though, no?

 

Dave,
I'm not gonna sit here and say I don't agree with you, but I got some other opinions from some local drag racers(all fbody and mustang types). I specifically asked about going with the shorties, and the concern for sidewall flex(or not) during launch. Other than what you've said, everyone locally agreed(unanimously) that sidewall flex shouldn't be a concern in drag radials. Paraphrasing: "Drag radials are still radials afterall, just a different compound. And there's only so much flex to be induced in a radial." Everyone pretty much agreed that for the price, go with the shorty DR's. At the worst, I'll still match my current 60 foots.

I'd like to get some lightweight wheels, but I'm just not in a position to spend a bunch of money for wheels AND tires at this point.

As for expected gains, that's the reason I posted this message in the first place. I don't want to expect 3 or 4 tenths, and only see 1 tenth. Now that I know I'll only see a marginal gain, I must reconsider the tire size.

 

you might haunt some of the mustang forums out there... a lot of the mustang guys run a 15x8 5.5bs drag wheel... aka weld draglite or prostar. This just happens to fit the z really well, and you would have no problem picking up a used set of wheels probally with some slicks or Dr's still on them. All you would need is the SS/t (long shank) lug nuts and washers...

I think the weld 15x10 with the 6.5bs will fit too but don't hold me to it, both of these sizes are used alot on the mustang

 

Did you tell them you're running 205/40R17s though? My friend had 205/40R17s on his Contour SVT and they look like rubber bands. 205/40R17s are ~23.5" tall which means the sidewall ~1.5" shorter than the stock G35 215/55R17 sidewall. Your domestic friends probably hear "40 series" and assume everything is good because many of them run 40 and 45 series rubber on their F-Bodies/Stangs/etc. However, they're running 275-315 width rubber. That means a 40 and 45 series tire sidewall is still close to 3". A 285/40R18 is 26.4" tall which means the sidewall is still about 2.7" tall.

Your friends are correct in that DRs don't need a lot of sidewall flex and don't behave like a wrinkle wall slick, but a 1.5" tall sidewall is going to be like a rock and the DRs won't grab like they should. Also, the 3" shorter height will increase your gearing so it's my opinion you'll totally smoke the DRs through 1st because they'll be easily overwhelmed.

Take a look at the cars in the BFG adds that hold world record DR times and you'll see that they're running 15 to 16" rims with very tall sidewalls. DRs are designed to wrinkle somewhat assuming you've got the sidewall height.

 

From what I've researched on here, it's all heresay. I can't find any legitment information and timeslips (much like the UDPs) showing this widely accepted .4-.5 second drop in ET. Now if you were running slicks and could honestly plant of all that new found torque multiplication, then yes, it's possible you could see a .2-.3 second drop in ET with a 1.7 60 foot. However, the guys on this site that run slicks are far and few between. 3.9 gears on street tires aren't going to make any real difference.

 

not sure, but from racing pre and post on street, i have noiced a 2 car improvement... my previous time was 13.9@ 101 with 2.0 60ft... im goin to the track in a month or so when it opens ill post my findings.

 

not sure how much this info will help, but i had a mustang GT 5.0 about 10 years ago, it was an automatic with 3.23 gears. I ran 15.5 at the track, dont remember the trap (average of 5 runs total) switched out the 3.23 gears with 3.73 gears and ran 14.7 (average of 3 to 5 runs total, dont remember exactly), no other mods.

 

You guys are using the 1/4 as a reason to switch out the gears. When they were designed for more track purposes. The true test of the effectiveness of the 3.9 gears is to have 2 Z's, one stock, and one with the gears to do a 0-top speed run. The 3.9 gears still allow you to hit the limited ~155 mph mark. This is where you would see the true gains the 3.9 gears give you.

 

I'm pretty sure no NA Z's on this site are able to hit 155mph on a track straight. I bet most of them are lucky to see 110mph.

 

The point is the car does accelerate faster in every gear with the 3.9, which is the point is was trying to convey in my earlier post. Now this might not be beneficial for the 1/4 because of traction and shifiting points. , but a 0-top speed run between a stock Z and 3.9 Z would show the 3.9's gains

 

So in the end, this is really only a beneficial mod for cars that are tracked.

 

It is also good for street racing from a roll. The Z is a LOT faster with this mod from 40 or 60 to 100mph. This mod alone will put you well ahead of an STI or EVO from a roll on at those speeds.

 

^Well, I don't street race and even if I did, I don't think I'd ever go from a roll.

I do auto-x quite a bit when I can and I would like to track it though.