Final Drive Gearing

[cbh148]

So I should be CA18 powered soon, and something recently crossed my mind; s13's that came with CA's (in Japan at least) came with 4.36 rear ends, whereas most others are 4.08. I had a 4.36 ring and pinion in my n/a KA years back, and it really didn't feel that noticeable. I'm thinking about trying to get a 4.6 or possibly a 4.9 for my CA, however since this is a turbo motor, I was wondering if running a higher final drive will actually makes things better or worse.

The purpose of my CA powered car will be to drift. Maybe 240ish WHP with my s15 ball bearing T28 and I'll be doing local drift events (not really competition).

I'm aware of the hypothesis that turbo cars actually go slower with a higher final drive because they end up spending more time shifting (losing boost) rather than sitting in boost and accelerating, but this theory has always somewhat confused me because it would seem that, regardless of induction method, a car will climb through the revs quicker with a higher final drive, which is in essence accelerating quicker. Perhaps the inherent lag associated with the moment right after changing gears (before boost builds up again) is what causes "multiple gear changes in a turbo car" to be bad?

I'm really just looking for some thoughts on the subject because I really hope I've got it all wrong and someone can convince me not to go rolling around in junkyards looking for 2001-ish XTerra's to get their front diffs. Feel free to share your insight!

[float_6969]

I switched from a 4.08 to the factory designed 4.36. I did this mostly because I do a lot of autoX and needed to get the revs up in 2nd gear because I kept dropping out of boost mid-turn and it was making the car hard to drive.

I don't like it. The VERY first thing I thought when I drove the car with the new gears was that it felt slower. Here is why.

The induction method DOES matter. A lot. With a N/A setup, the air is forced into the combustion chamber only by atmospheric pressure. Because of this, the best way to get MORE air into the combustion chamber is to accelerate the piston as quickly as possible to create a lot of vacuum, so that the air moves faster. NA motor also have less TQ than a turbo motor, given that all other engine parameters are the same. Because of this, NA motors run higher gear ratios because gears are TQ multipliers. So if you run a higher gear, you get more TQ to the wheels, at the sacrifice of revs.

Everything is the opposite for turbo motors. Because the intake tract is pressurized, you want the piston to move slowly in a turbo motor so that time works for you and you can move more air into the combustion chamber. This is why turbo motors tend to make less boost in 1st gear than they do in 5th gear. In 1st gear, the piston accelerates too quickly for the air to fill the combustion chamber completely, so the exhaust energy is lower, the turbo spins slower, and less boost is produced. In 5th gear, the piston moves very slowly, the combustion chamber fills as much as is possibly could, the exhaust energy is VERY high, the turbo spins MUCH faster, and if there were no wastegate, LOTS more boost would be created.

Turbo motors love load. Also, the high gearing KILLS your gas mileage. I'm turning 4K RPMS @ 80mph w/a 4.36 rear diff and the taller-than-what-comes-with-a-CA18DET-transmission, KA/SR 5th gear. (Yea, not only is the diff gear higher in the JDM CA18DET powered S13's, so is the 5th gear) If I were still running the stock CA18DET transmissions 5th gear, I'd be willing to bet I'd be turning 4K RPM at 75mph. Totally unnecessary.

And to make matters worse, it didn't work all that well on the autoX course either. Although I had a bit more TQ coming out of the turns, the motor accelerated too quickly and I was actually SLOWER between turns because the motor revved to fast and I was making less boost.

If it were a NA car, it would have worked great. But it's not and it didn't. I've also changed my focus since then and I'm less worried about my car being a good autoX car. If I make the HP I'm expecting (400HP), I'm actually probably going to go the complete opposite direction and put the gears out of a turbo Z32 in it (3.98 IIRC).

[roast]

If I were still running the stock CA18DET transmissions 5th gear, I'd be willing to bet I'd be turning 4K RPM at 75mph. Totally unnecessary.

Just about... 75mph = 120km/hr and IIRC, it's just under 4k rpm, stock CA transmission and final drive.

[r34 gtr]

Pretty much. Stock CA transmission and 4.08 rear end.

[cbh148]

Thanks for the well constructed reply, float! And I believe z32 TT's were 3.6:1.

This is very good information for me, because I was already giving my credit card that face like this ಠ_ಠ where I knew I was about to drop the ~$300 to get a 4.6 ring & pinion.

For the sake of being helpful (just in case others find this thread looking for relevant information) here's the link to a 4.6 ring & pinion for sale for our R200 diffs. http://www.mazworx.com/index.php?l=prod ... ail&p=2905

[cbh148]

Sorry for the double post.

I suppose that the only use for a 4.6 or greater final drive would be if one was running big rims and wanted to compensate for the effect they have on slowing acceleration (make 18's feel like 15's again!).

[r34 gtr]

Z32 TTs were 3.6. Want one? There is one in my shed. Please someone come get it.

[cbh148]

Hah I'm probably the closest person. Is that you on Weatherly on the north side of the road with the car under a cover? Down closer to the parkway end.

[float_6969]

Z32 TTs were 3.6. Want one? There is one in my shed. Please someone come get it.

I'll trade you for a set of 4.36, LOL! I don't want the diff though, just the ring and pinion.

[cbh148]

what if i come and get it (and meet a local CA guy in the process) from your shed then remove the ring and pinion and mail it to float in exchange for his 4.36??? AND i'll give you $10. and then you can sell me some other ca18 stuff!!

and brad tannehill (another Huntsville CA18 guy) says hey.

[s13drifter88]

hey i wanna come, i like CA ppl lol

[r34 gtr]

Please do. I would love to have it out of my garage. I bought it years ago and never used it. Its still strapped to the pallet!

I can't believe there are so many Huntsville CA people. Makes me sad mine isn't running.

[cbh148]

well between me, kyle, brad, and you, i don't think anybody's CA runs lol. text me your number at 256-679-7788. i'll be in town for the weekend and i can swing by.

isn't there a pulsar guy with a turbo CA18 in huntsville?

[r34 gtr]

Yeah I am not too sure what happened to him. I would swing by and talk to him at work every once in a while. Last I heard he asked me if I wanted to buy his car, but I didn't. Haven't seen him since.

Ill be around today and tomorrow, but I will be in B'Ham on Sunday, so I wont be around. Ill send you a text when I get off work.

[s13drifter88]

u going to world of wheels by any chance?? lol

[cbh148]

ok yeah just text me on saturday (well... today lol). i'm in south huntsville now, up here at my parents' house near weatherly. i'll be around all day.

[ILOVESILVIA(S13)]

If I were still running the stock CA18DET transmissions 5th gear, I'd be willing to bet I'd be turning 4K RPM at 75mph. Totally unnecessary.

Just about... 75mph = 120km/hr and IIRC, it's just under 4k rpm, stock CA transmission and final drive.

Mine runs at about 3100 @ 75mph. I'm not sure what VLSD I have in mine but it came with my clip back in 2005.

[roast]

Just about... 75mph = 120km/hr and IIRC, it's just under 4k rpm, stock CA transmission and final drive.

Mine runs at about 3100 @ 75mph. I'm not sure what VLSD I have in mine but it came with my clip back in 2005.

Right, you have the usdm vlsd? I have a 1988 Silvia K's (stock CA, transmission and 4.36 vlsd)... I honestly can't remember the exact rpm, I only put 1500km on the car last year...