Differential Options?

[meet07]

I know that lower geared diff's like the stock diff and the 300zx non turbo diff's help with acceleration. But will that actually make a 5 speed 240sx equiped with a rb25 faster? Ive been scracthing my head wether to keep the j30 diff or swap it out with something that will......get from point A to point B faster....if that makes sense. I know power bands and all play factors but I just want some light to be shed on this...we all want to go faster...right?

I know the muscle car guys throw in lower gear ratios....figured it might do us some good since we have less torque...

what do you guys think?

[johnny butt]

A little vague,

When you say get from point a to point b faster, what does this point a and b consist of exactly?
What I mean is:
Track use ( improve times in 1/8th, 1/4, etc??) or Daily Driving (lose MPG, lol...)?

[Yellow4g63]

I'd say stick with the J30 I don't think ur 25 is peaky like my 20.

[robbie2883]

i just recently swapped to the q45 diff and i absolutely love the gearing. 2nd gear is so long now. 3rd is close to 115. i stay in boost A LOT longer. still haven't dialed in the 35r but i can tell once it's tuned it'll be nice.

[jetdriver]

I am planning on putting the q45 back in my car as well. The sr20 did not have quite enough torque, but that is no problem now. I want to have longer gearing for daily driving.

[meet07]

See now that's kinda what I was looking for. Now wonder if u take for example a modded rb25, (let's say 500-600)one with a q45 diff and other with a s13 led diff, who would win??? Which would b faster in the quarter mile, 80-150mph race?

[gawdzilla]

the Q final drive is pretty tall and it will definitely change your shift points. for drag you basically want to a usable 1st and 2nd, end your run at the very top of a gear. these all vary depending on your turbo/hp, powerband, tire diameter, launch control, etc.

the spreadsheet at the end of this writeup used to be useful, but now the link is busted.
install-guide-q45-differential-swap-int ... 67924.html

with something similar, you can calculate your own ideal diff ratio. i think very high hp drag racers use the Q 3.5/3.6 or the pathfinder 3.1 diff.

http://zilvia.net/f/s-chassis/267957-tu ... assis.html

i would start by figuring out what your trap speed is, then what your max rpm is, then your tire size, and finally figure out which FD is the best for you.

[Darius]

I am in the process of installing a 4.636 front ring and pinion from an xterra with a new 1.5 KAAZ LSD. The J30 gearing is too long. I think the calculation ended up being 240 mph at 8000 rpm in 5th. Retardedly high.

[gawdzilla]

that might be good or it could work against you. rowing through too many gears wastes time, and also a short FD can make your lower gears a bit useless. who knows though maybe it will be better for a low displacement/low torque/high revving motor like the RB

[Darius]

It isn't too crazy because the GTST has a higher final drive ratio than the 240 to begin with. I've done the calcs and I'm still switching into 4th in the 1/4 so it doesn't make any difference in shifts. And it will still cruise 70 mph at a reasonable rpm. I'm confident the gear will work well. Agreed that 1st gear will be mostly useless though. I think it goes to 45 mph.

[BoostFab]

I recommend the q45 rear end with the RB25/26/30. We have installed on several 10secs cars and it takes the beating like no one's business.

[jetdriver]

It is important to note that there are two Q45 ratios: 3.54 and 3.69. I am using the 3.69 ratio from a 1998 Q45.