I've searched and searched and...

So now I ask, has anyone taken their mildly modded Z WITH 3.9 final drive to the track and gotten numbers?

I learned that most of the guys with the FD got it for autoX purposes and that it doesn't help much in the 1/4 cause of extra shifting or whatever.

I would like to know if anyone has improved their 1/4 times as a result of this mod.

Thanks in advance for any useful input.

 

"zillinois" posted a review a while back and the gains were only a couple tenths but i want to say that the track conditions were bad...he seemed happy with the gain if i remember correctly.

edit: After some searching i am wrong, it was more of a review for the tilton clutch but he threw the gears in at the same time. He has commented on the gears in a few threads though.

https://my350z.com/forum/showthread....threadid=98858 <---that is one of them

 

Thanks for the reply kwiker. Very informative.

I guess I will def have to get my rev limiter raised now.

It is unbelievable how some people put what seems like so much time into disecting mods and throwing out very useful info.

Thanks Zillinois!

 

In most NA Z's, a 3.9FD will not require an extra shift into 5th, you will just finish 10% higher up in 4th gear. Nobody has truly tested one out, but Id say an easy .2 with a good driver.

 

Boy do I have good news for you guys...
I just went to the track on wednesday to test this out (sears point for us old timers, infineon raceway for the new school.) Here's what I have:
2003 350z track (a few ticks under 30k mi)
When I ran it with just the test pipes i was hitting about 14.1-14.0
Now I have the plenum spacer (don't really think it did anything)
central 20 3.9 final drive
275 bfg kds in the rear

pulled consistent 13.6s at about 102-103. Big problem in 4th gear though b/c i can't finish in 4th if i get a good launch (1.9 or lower.)

If i pull 2.0s it barely makes it out (hits the "soft" revlimit at the end)

last run of the night i was losing so i just stayed on it and hit fuel cut before the end (OUCH)

best run of the night was 13.6 @ 90mph (i consider this the best run b/c i might have hit 13.5 if i stayed on it but i was well ahead so i hit brakes.)

so now i'm probably gonna go with the technosquare l-spec so i can rev it higher and hopefully hit 13.5ish. at about 350 bucks it seems like its worth it.

so was it worth 1/2 a second? almost.. i'm sure the tires also helped my launches out. but end result is that i don't wanna go back without the raised rev limit!

***EDIT i'm stupid... i wasn't even hitting 102 and i was hitting the rev limiter. highest mph was only 101.8! I don't think i ever saw 103. don't know what i was smoking.

 

A 3.9 final drives sounds like a disadvantage if you cannot cross the finish line at the top of fourth gear. BTW nice numbers. Post a time slip here: https://my350z.com/forum/drag/233840-top-25-1-4-mile-times-for-tt-st-supercharger-nitrous-bolt-ons-stock.html.

 

thanks davidv i posted the slip on the other thread.

well i know it did improve my time... if i go back before the technosquare l-spec reflash then i'll try shifting into 5th.

I think the central 20 3.9 fd is worth it if you don't care about gas mileage-if you thought your z was bad on gas wait till you try this out! I really don't drive the z every day since i have to park in some tight spots every day. Got a really crappy crx for that.

i'm in the left lane (69)

 

i wonder why it makes your gas usuage worse. Never really got into it is it because of the power being avaible all the time earlier?

 

Simple-cuz your cruising at a higher rpm's at all times.

OP-nice times, Im surprised you cant go over 102mph with the 3.9FD, what size rear tires do you run you should be losing 10mph in 4th gear.

 

they're 275/40/18 on the stock track rays 18s.

before i did this i went to http://www.miata.net/garage/tirecalc.html
to see how close the tire diameter would be in relation to stock and it was dead on (well about .5mm smaller)

i remember trying out some other calculator online where you punch in gear ratios + final drive ratio + tire diameter to get top speeds in every gear and it said i could hit 104 or 105. i figured i wouldn't hit that anyways but maybe the calculator i chose was off somehow.

i'm thinking the technosquare ecu will do for now until the 08 g coupe or the gtr comes out

 

It shifts your RPM band. So for example, with the stock 3.5 FD, at 80mph, your cruising in 6th at 3000RPMs. With the 3.9 FD, at the same speed, you're cruising more at 3300RPM's or so.

 

will the 3.9 gears give a significant gain in highway pulls as well?

 

yup its really just changing the ratio that the tranny turns wheels turn at.. kinda like the same effect as putting on smaller diameter wheels