Didn't you say you were going to provide us with some before there genius, or am I mistaken?

 

Thanks for the genius comment. You on the hater bandwagon now too?

Anyhoo, you're mistaken. I offered to generate the plots and even put forth the effort to explain them, and I'll even waive my normal consulting fees! Bonus! The least someone could do is offer some good dyno data for a stock revup G35.

 

Yes - my family is fairly wealthy, i could have dropped out when I was 16 and sat on my *** until the day I died. But I didn't.

 

If I were you I'd be all over the sciences for sure... kick back and learn about the craziness of the world and contribute to it, rather than get caught up in its contemporary BS.

 

facts are still wrong:

1) a 6mt has a 3.54 fd ratio
2) top speed gearing at 6600 rpm is 181 mph (155mph is reached @5700rpms)
a)given the G<310hp a 3.9fd would lose no topspeed advantage to stock fd ....there is not enough power to reach 181 mph. given the rev & electronic limiter are removed...the 3.9 fd will reach a high rpm in 6th gear...the stock fd won't make it to 6600 rpms.

 

I play forza motor sports

I put roughly My g's parts in it

Then make the FD 3.9


And i lost .2 seconds on my 0-60

(5.0 to 5.2)

My 0-100 got faster by .2 seconds (12.3 to 12.1)


Should these gains be about right? Or does someone have some REAL numbers

 

You should gain a couple tenths off both the 0-60 and 0-100 times.....
A great mod, and relatively cheap.
C.

 

Since nobody could give me any decent dyno data, I had to wait until I dyno'd my own car. Using my Mustang dyno data, I generated force to the ground plots in all 6 gears for the stock final drive and the 3.9:



As you can tell from the plots, the ideal shift points in all gears are redline.

I then ran a series of acceleration simulations for both cars (one with the stock and one with the 3.9 final drive) at starting speeds ranging from 15 mph up to 120 mph in 1 mph increments. The assumptions were as follows:

1) Both cars started in the gear with the highest ground force for a given starting speed.
2) Both cars started at the same initial speed.
3) Both cars started at 0 relative distance.
4) Air resistance.
5) Instant shift times (I could change this, but I didn't care that much).
6) Identical cars otherwise (3650 pounds with driver, OEM 19" tire sizes, etc).

I ran the "race simulations" for three different time periods: 5 seconds, 10 seconds, and 20 seconds, at each of the different initial speeds. I then recorded the final distance between the stock final drive car and 3.9 final drive car at the end of the time period. The results are shown below.

Again, these plots show the distance between the stock final drive car and the 3.9 final drive car at the end of the specified race time on the y-axis in feet when starting from the speed indicated on the x-axis in mph. Note that the y-axis is in feet, so these delta distances are basically all fractions of a car length (a car length is just over 15 feet).

Results for 5 second races:


Results for 10 second races:


Results for 20 second races:


So as hypothesized, the 3.9 car generally has the advantage, and the stock final drive car does in fact have advantages at various starting speeds. In virtually all scenarios the net advantage is very small (a fraction of a car length gained by the end of a "race"). So I stand behind my original comments: the 3.9 will make your car feel faster and accelerate faster in a given gear, but at a lesser speed. The winner of a "race" between you and a car with a stock final drive car depends on the starting speed, with the advantage generally going to the 3.9 car, and the difference in the end not being much (well maybe some consider a fraction of a car length a lot).

You can draw your own conclusions.

 

wow you remind me of my brother in law. nice work by the seriously though. just pleeeeeeeeeeeeaaaaaasseee yes or no anwers only over all is worth getting the 3.9

 

On average your car will be faster, but it's not the end-all super awesome mod performance-wise that everybody makes it out to be.

 

This thread is hilarious. Book Smarts vs Street Smarts...I'm in the Street Smart camp.
Regarding diff ratios we always go for a shorter rear end in our cars, S14 and 15 Silvias come out with a 3.69 rear end that is good for economy and not much else - in goes a 4.33 or a 4.38. My JZX100 Chaser - 3.9 to 4.11 - picked up .3 down the quarter and made it punchier everywhere in the rev range.
For my Skyline/G35 I'm probably going for a 4.1 as a 4.33 will be just too short. One reason the 5AT cars are so slow out of the hole is the 3.5 first gear - coupled with the diabolic 3.23 rear end...
And before anyone says that 3.9 is the shortest rear end you can go, in the US maybe, but then you dont have access to the R34 and S15 stuff like we do.
Jash

 

Is a 4.10 gear too short for the G35, any thoughts? I am in the process of installing the 3.91 on my 5AT coupe. We want to see how the car behaves before we have our own custom gears made. If we feel that there is "room" for a 4.10 gear in the rear end we might go that route. We are looking for function just as much as we are looking for performance. Due to manufacturer demand we can only go with one type of gear, otherwise we would try & do a 3.91 & a 4.10. We are still pondering the option between the two...

 

very good discussion!! 2 thumbs up! especially on the graphs!

 

I dont get the 5 10 15 second race graph? can someone explain?>

 

I wanna race MechEE for pink slips to see if my mods didn't make my car faster. Hahaha!!!