2JZfan

Ok, call me crazy but I've been doing a lot of thinking (and cussing) about my 700 mile 2005 G35 6MT and its wonderful 1-2 shift grind...

Obviously these cars (Z's and G's) have had a huge number of tranny problems over the last few years. I've owned several other 6-speed equipped vehicles in the last ten years and never had a problem with any of them. In fact, I've never had a tranny problem on any car including my POS 4-speed '88 Hyundai Excel. Really made me wonder how these trannies could be so bad.

One thing that I have hated since I got the car was the artificial feeling of the drive by wire system. If I'm sitting at idle and I blip the throttle, my foot will be completely back off the gas before the rpms even start to go up. There is an obvious "lag" in the real throttle blade moving vs. the gas pedal moving. Also, when you let of the gas and push in the clutch to upshift, the rpms hang at their previous speed for an unusually long period of time. On my car it's about a second. This also contributes to the many people you see saying how hard it is to shift one of these cars smoothly.

I started putting a few of these things together and I just can't help but wonder if the drive by wire sluggishness isn't what is causing all the tranny problems. Here's what I mean: nobody is really having problems with breaking input shafts or grenading gears, it's almost 100% related to synchro issues (e.g. grinding going from one gear to the next. Furthermore, synchro problems seem a bit odd on a tranny where 2nd gear has a triple cone synchro and other gears have a double cone, specifically to allow for smoother shifts.) Now, if the drive by wire system is holding the rpms up on every shift, it's essentially like you didn't let off the gas. This means as you start to push in the clutch, the motor gives the tranny a final little "spin" up to a higher speed, which in turn means that the synchro has to do that much more work to pull the speed back down to where it needs to be for the next gear. It's basically like you are speed shifting every gear, every time. If I think about it in that light, it doesn't sound surprising at all that there are hundreds of tranny problems floating around.

I'd really like to get some other people's take on this theory and see where the discussion goes.

Thanks
Jeff

Act of God

that's definitely interesting, I don't know enough about mechanics to give it any backup credibility but the logic doesn't seem flawed. This would only apply to fast shifting though right? If you pressed in the clutch and shifted lazy it wouldn't be as bad i believe...

I shift fast.

davidv

iTrader: (2)

“Obviously these cars (Z's and G's) have had a huge number of tranny problems over the last few years”

I wonder what kind of numbers were talking about? Who has a definitive answer? 50 percent – 10 percent – 1 percent?

Them Bones

The tranny should shift fine regardless of what the engine is doing. In fact, my G shifts smoothly if I let the RPMs fall for the 1-2 shift. The syncros are doing the most work then. There should be no problem if the speeds of the shafts are close.

Also, I don't notice any lag in the drive by wire. You may not have payed attention in the past, but many cars with a cable throttle have a short lag after blipping the throttle.

Lastly, the RPMs hanging during a shift seems to be an emmissions control function. Shutting off air to the cylinder while fuel is still being dumped in there is probably bad for the air and your cats. I know I've seen a restricter for the idle air tube in a Ford Focus that enhances the RPM response of the engine. The Ford engine is programmed to do the same thing.

I think the syncros are just poorly desinged in the Z/G. Mine is okay most of the time when warm, but it always manages to surprise me with the occasional grind.

tekk

i think its a simple design fault that inssan has yet to fix.. maybe its in the fluids, maybe synchros, who knows.

at least its not like the early e46 M3s which were throwing rods.. while BMW was blaming the owners!

Netko350Z

iTrader: (8)

Maybe recalibrate the accelerator release position?

2JZfan

it's true that we don't have a percentage for how many tranny failures there have been. On the other hand, if you look around at forums for Camaros, Vettes, Vipers, Cobras, etc. I don't see the 400+ post threads about "yeah I had to have my tranny replaced too"...

As far as the tranny being able to do its job "no matter what the engine is doing" that's true to a point... it is the synchro's job to match the speeds, but obviously the greater the speed differential, the more work it has to do and the faster it will wear out... when you shift quickly, you are doing several things at once - letting off the gas, pushing in the clutch and pulling the shifter into the next gear... there will be a period there where you have only partially disengaged the clutch and yet you've already pulled out of the previous gear... at this point, more engine rpm amounts to changing the tranny's neutral speed (same action that occurs during double clutching), then when you continue to pull the shifter into the next gear the synchro has to deal with a larger speed differential than it would if this hadn't occurred...

As far as the delay in revving, I have to believe this is purposely built into the ECU's logic for one reason or another... when I'm cruising at a steady 3000rpms and floor it the acceleration is instant... the tach moves up at the same time that my foot is going down... so mechanically, why would it be any slower to rev doing the same thing at a lower rpm? (although, I don't believe that *this* particular quirk of the drive by wire system effects the tranny in any way). Also, I've owned other drive by wire manual tranny cars (like a 5-speed IS300) and I never once even thought about the car being drive by wire. From my very first drive in this car I've always been conscious of the fact that it had a slightly unnatural feel, so I know the DBW operation is more exaggerated.

I don't know, I guess I'm still struggling with the idea that after 8 tranny revisions and numerous warranty swap-outs, Nissan just can't spec out a tranny that shifts reliably for a $30k+ car. That seems a bit hard to swallow. I have to think there is something else involved at least as a contributor, if not as the entire cause.

Jeff

jeffw

I think the delay from the throttle is partially the weight of the flywheel and partially the computer. To do stuff like VDC, you have to filter the input from the throttle before sending that information on to the engine. Generally, the fancier the filtering, the more delay there is.
--
Jeff

mavtais

iTrader: (12)

That might not be a good idea because the throttle pedal is very responsive (springy), and if you drive with light shoes (or none at all) you can really feel if you are on the gas or not. My Honda Civic Si pedal was sooo soft that I couldn't feel if I was three pedaling or not. Had to go off the interfaces (tach and sounds) which further delay.

In regards to the tranny wear, I think if you're shifting and the rpm jumps, you should wait until the rpms drop to get a better match. I love the way my Z drives.

albatross83

Yeah.. I noticed the Z has a long delay before the RPMs drop off as well.. My previous cars were all Chevy V8s (T/A, then Vette) so I just figured it was a V6/import thing. At any rate, I just adjusted my shifting accordingly; specifically letting off the throttle a tad earlier before I push in the the clutch. Not specifically to try to preserve my synchros, but mostly because when you hear an engine spin up real quick between shifts, it sounds like an amateur is driving it. My '94 T/A was a 6 speed, and I never had a single problem with the transmission; it was rock solid. I was sort of surprised at how lightweight the Z's transmission seems in comparison, but I just chalked it up to Japanese engineering. The only thing that really bothered me was the nonlinear action of the clutch, but I was willing to overlook that. Now that I'm having my transmission replaced at 7500mi, I'm starting to wish I'd gone with an automatic.

Ztalker

i sold my 2000 VW jetta VR6 as soon as i noticed all VW transmissions grind when shifting from 1st to 2nd...it annoyed the hell out of me. SCREW VWs....
i've been driving my base 03 chrome silver z since 3/03 and i've never had any problem with the tranny...it shifts smooth as silk except a little bit notchy on the 2nd gear and i heard that's the way Nissan made them so as to give the driver a more positive feel.

HarvesterUT

iTrader: (10)

i noticed the "hanging rpms" as well and have commented on it several times here. it is most noticeable in the 1-2 shift. you really have to wait an extra 1/4 sec to engage the next gear else it will be jerky.

narkotic

iTrader: (3)

Change your flywheel, that'll help your throttle response in neutral. The pause is due to the weight of all the engines components and mainly the flywheel/clutch.

Nano

iTrader: (1)

exactly... I hardly have any delay and pauses when revving engine (lightweight pulley and good flowing intake/exhaust helps too....)

Still... I believe that EVEN STOCK, the throttle response is better than 99% of what's on the road.

Ztalker

ditto..i concur

FrankieZZ

ditto to you Ztalker as to the tranny -preorder base Gunmetal 6M no problems whatsoever . Ever .

2JZfan

it's not the flywheel a) i have a supra with a stock (very heavy) dual mass flywheel and it accelerates instantly b) the flywheel doesn't get any lighter at high rpms so why would the car instantly rev from 3000rpms but have a big delay trying to accelerate from 1000? it has to be the control logic in the computer that is processing the throttle sensor and determining what to do with the throttle blade... but again, this lag in accelerating is not what I think is causing the tranny issue, I think that could be related to the lack of natural deceleration when you lift off the gas...

also, i don't know how you can say that most of the people having tranny problems have lightweight flywheels and grabby clutches... that's just not true at all... several people have even mentioned dealers having new, < 20 mile (and obviously stock) cars on their lot that have these problems... if it were only showing up on cars with aftermarket flywheels and/or clutches I seriously doubt that the dealers would have been replacing trannies under warranty...

Ztalker

regarding the light weight flywheel issue........that was just my own personal observation.
i'm sure there are brand new Zs out there with bad trannys on the lot... so buyers beware.

Z BOY

iTrader: (41)

fwiw, die die's tranny was replaced under warranty for what the regional nissan guy said was a factory defect.

Jason@Performance

iTrader: (11)

i think its because the clutch travel is too much...

when the clutch pedal is pusheda ll the way to the ground i think the clutch starts engauging again...

we have this problem with the tiltons so we install a pedal stop and it doesnt hyperextend the clutch...

there have been people who we helped put the clutch pedal stop on their car and their car shifts alot smoother now...