i was doing some research and saw that if you go FI in the future this is probably a bad idea. but wouldnt it really help out on the track or through the canyon. i was watching some of the japanese touge runs and a lot of those guys use like 4.2 + gearing in some FI cars. what are your thoughts? i want to go FI but not for at least one more year. should i go with 3.9 final and what company would you recommend

 

Im planning on going 3.9 especially since i might go supercharged and the gear is really gonna wake up the low end. I really think you'll enjoy it. I think even with a turbo it the gear would really help reduce lag and get you into boost quicker.

 

with FI and 3.9 FD you will be shifting alot more, there are people on here with boost that actually look into lowering their gears so each gear runs longer

 

You've onbviously never driven a turbo Z, the lag is minimal to non existant. Id do 3.9FD on a S/C's set-up, but not on a turbo kit, the TQ and shorter gearing would overpower most 275/285 tire combos and thats with less than 400whp, basically making your 1st 2 gears worthless, a problem that presents itself on really high powered cars-trust me you dont want that.

 

I've read some books on theory behind using turbo's and the thinking is that it helps to have a lower rear end. It helps the turbo spool up faster. I'm not quite clear on all the details but for an example. The 300zx turbo as far as I understand it had a different rear end than the non turbo.

 

shorter gearing just gets u from beginning to end gear faster, but if u go w/ such a high gear (as in les say a 4.2, etc) itll get u throught the gears really fast, and you'll accellerate fast, but you'll top out significanty lower.

alberto, you seem to kno wut your talking about, care to explain a little more??

 

+1 on that. 3.9FD on F.I Zs don't quite work together. 'Power first, traction
later' theory just doesn't apply here. You will have very unplesent moments with this set up as a daily-unless if you wanna run DR everytime

 

In regards to the Touge cars, you're looking at dedicated race cars, not street machines. If you were not worried about tricky launches on the street and low speed drivability, then the 3.9 + turbo could be a potent combination on a road course. Granted, you'd also need to be an experienced track driver to be able to handle the rapidness of shifts and that power. But yeah, on a tight road course where you have rapid acceleration and deceleration and won't exceed the top speed of the vehicle, 3.9 + turbo could be a killer on a fully prepped race car.

The advantage to the 3.9 is it shortens your overall gear ratio so you spend less time in every gear, accelerating more quickly, but shifting more often. A very rough generalization to the 3.9 is that its ~10% shorter than your stock 6MT gearing. So you can accelerate at a rate roughly equivalent to cars with 10% more power, but your top speeds in each gear are ~10% lower too.

As Alberto pointed out, the combination of low end torque from a turbo setup will overpower all but the grippiest tire setups. Its not overly conducive to a street driven car. The advantage going to a CSC setup that has a linear boost delivery, is the car is closer to NA levels at lower RPM's, allowing easier launches.

I've got the 3.917. I'm looking at CSC for a lot of those reasons. Even with the 3.9/Quaife combo on my NA setup that isn't even complete yet; where I might be pushing 240 whp at max, I can still break out my 275 PS2's if I push it really aggressively in the wrong situation.

 

I have a 3.9 with a vortech. Everyone's heard me say this, but I think it's a good combo. The lower gears have more oomph to help launch you better, and then when you get to the higher gears, they're more potent than the regular 3.5 final drive.

with normal street tires and a turbo I wouldn't advise it. The torque hits so early and hard with a turbo that you'll have a handful doing a launch anyway. If you're running a set of wide drag radials or a set of some crazy grippy nitto NT-01 315's in the rear or some slicks on the track then knock yourself out and get the 3.9...as long as you're well under something like 450-500whp

as far as the 1/4 mile goes, the 3.9 will really make that extra shift known. On a road course or autox, it's more of a luck of the draw if you have to do an extra shift on a given straight or not. 2nd gear in autox will be much more potent on small courses.

Otherwise top speed and highway gas mileage are other factors. Some people don't like the shorter gears on certain road courses - depends on the course and length of the straights though - totally luck of the draw. It's still fun though. IMO it's about 15-25% of the thrill of going for a starter level FI

 

Sentry,

I would think that since your rpms are 10% higher with then 3.9, then boost comes on quicker with the Vortech setup. This may be an option for those of us who want more powr, but not at the expense of 3.12 pulley, injectors, belt, retune or in some cases a rebuild. It would be interesting to compare the cost of the 3.12 and accessories and the 3.9+install.

 

the 3.12 and tune is a LOT cheaper option than the 3.9. But yeah, the 3.9 brings you into boost 10% faster with the vortech.

upgrading the pulley to the 3.12 usually never requires bigger injectors - just a retune. In all, it's probably about $300-400 for the pulley, belt, install, and tune for a solid 20-25 more whp