big dave

...or should I say inefficiency. I'm still trying to figure out why my '07 max falls on it's face after 70 mph at full throttle. If I'm not mistaken, the initial gear ratio for the cvt is somewhere up in the 500s which really helps out at launch.But, what happens after that? Around 70 mph it seems to hit a brick wall. What gives?

When the shifter is in the manual mode, the car seems to accelerate the way you would expect after 70 mph. If there aren't any gears in the tranny, how can "programmed" shifts actually have an affect on performance?

Nietzsche

iTrader: (7)

It's all in the programming. My guess is they are being very conservative at high speeds.

SilverMax_04

The title of this post is "CVT efficiency." Given all of the heat this trannie produces, I manitain that it is not very efficient. (Heat production = lost energy). The CVT has (according to earlier posts) two trannie coolers and a special very high temperature lubricant.

About your question of "hitting a wall" at 70 MPH, I suspect that earlier posts on the CVT are correct. The control system needs to "learn" the way you drive and has not done that, yet. What happens when you floor the accelerator at 70 MPH? Does the CVT go to a lower ratio that allows the engine speed (and vehicle speed) to increase? (It certainly should -- if not take it back to the dealer and complain.)

big dave

I really haven't floored it from 70 mph but maintain that the problem does occur from a standing start. This is a problem that I just don't know how you could explain to the dealership. I guess I just need to wait it out until there are more '07 max owners out there. It just may be the "nature of the beast".

4MySwee

iTrader: (2)

What about 'break in'? Has that happened yet? Is it overrated? Perhaps this is why your car is acting a little fickle?
Swee

DarkSlate

when you floor the gas u will feel a little click on your foot. This will push a button that will down shift and then your off. This is also done when flooring in manual mode.

SilverMax_04

Be certain to watch engine RPM until the VQ is fully broken-in. Not over-reving the VQ until it is ready will add many miles to its life.

DarkSlate

yea i know im at 11,000 i drive 100 miles a day to work.

SilverMax_04

I was speaking more to Dave. He is likely to be newer.

big dave

I am very easy on the car on the street especially since it gets such bad gas mileage . I took it to the track to see how it would run. My intention was to only go once but after a very disturbing first time out, I went back until it ran the way I thought is should. The car still only has just over 3k miles in the 4-5 months I've had it.

I have a car I race on weekends so this is by no means meant to see any more duty at the strip.

SilverMax_04

Dave,
The fuel economy of you Max will improve once the VQ has been fully broken-in (up to about 5 K miles or so).

There are experts on this site (SteVtec) who believe that you get better highway mileage burning regular than burning premium. Remember, the Owners' manual says (page 9-4 of 04 Manual): "However, now and then you may notice light spark knock for a short time while accelerating or driving in the hills. This is not a cause for concern, because you get the greatest fuel benefit when there is light spark knock for a short time under heavy engine load."

This statement is consistent with Mechanical Engineering Engine Lab 101 that says "the best fuel economy in an engine occurs when the engine is operating just at the edge of experiencing light engine knock." Operating just at the edge means that from time to time you will get short-term engine knock at heavy engine loads. Here in the foothills of the Rocky Mountains, I burn 85 octane all of the time and get light engine knock when climbing the hill to my house when the outside temperature is above about 85 degrees. I simply downshift to a lower gear and the knock goes away. I need to keep the RPMs above 2 K when climbing hills in warmer weather.

jcalabria

Nietzche's got it... At any given combination of speed / pedal position / load / etc., the rate at which you accelerate is completely controlled by the maps in the ECU/TCU that select throttle opening, ignition timing, injection pulse timing, etc. With a CVT, the TCU selects not only the ratio at a given point but also the rate of ratio change. With the CVT, when you call for full acceleration the computer runs the engine to near peak torque and then your acceleration is almost completely controlled by the rate of ratio advance that the computer allows the tranny to perform.

Your "experiments" at the track seem to indicate that Nissan was fairly aggressive with this programming at lower speeds but got conservative at higher speeds. Your hybrid technique (auto launch then manual at higher speeds) takes advantage of the CVTs capabilities and Nissan's low speed agressiveness, then overrides their conservatism at higher speeds. By putting it in manual you eliminate the ratio variable from the mapping and you accelerate like any other car - by the engine rpm increasing.

Theoretically, a CVT should be able to outperform other trannies at virtually all speeds, but its actual performance is completely determined by the programming. Why did Nissan get conservative at the top end? Only they could tell you for sure... could be durability issues, heat issues or they just didn't think it mattered to the majority of buyers. It would be nice to know what the real reasons are, because the CVT ratio mapping is just another parameter that a "chip tuner" could alter if they knew something wouldn't break.

This really isn't unique to the Nissan CVT... in virtually any "drive by wire" car the way it performs is completely under the control of the computer... the position of the gas pedal is only an indication to the computer of your intentions and the computer determines what really happens based on the mappings placed in it by the designers. Limiting performance under certain conditions is part of what goes into those maps. My Volvo S60 T5 severely limits engine torque output in 1st and 2nd gears to avert torque steer and to keep the tranny from exploding. Volvo chip tuners almost universally eliminate this torque limiting.

BTW - my fuel mileage didn't really reach "full maturity" until 10k miles or so. It was truly awful at first, but now is considerably better than my '03 was with the same engine + 4AT, especially around town.

SilverMax_04

Right on -- jc. But I want to make those decisions myself rather than have a computer make them. That's why I will only go for a manual trannie. I love my 6-speed, but wish it had a 7th (top end) gear for cruising the flat interstate highways at lower than 3 K RPM at 82 MPH. A lower RPM would likely improve the highway mileage too, as well as reduce engine wear.

lightonthehill

SilverMax_04 - You are correct that a 7th manual gear, assuming the first six and axle ratio are not changed, would lower RPMs, cabin noise and engine wear, as well as give the MPG a nudge.

For years, I have been disappointed that car manufacturers refer to the highest gear in their vehicles as 'overdrive', when, if fact, it is simply the gear we are most often in from 45 MPH or so to 140 MPH. Totally incorrect nomenclature. What they call 'overdrive' is purely and simply high gear.

A true 'overdrive' should be a gear we use only when cruising around 60MPH or faster on a roadway with no upgrades of any consequence. It should keep the RPMs at least 500 lower than the normal high gear.

I see most manuals in mid to upscale sporty cars moving to 7 speeds in the fairly near future. Unfortunately, I see the 7 speed being mated to an axle ratio set for performance, which will result in faster 0 to 60 (or 0 to 140) times, but not really offering a true overdrive. That is sad.

big dave

Thanks for the info guys. I'm now beginning to understand a bit about how my car works thanks to the replies I've gotten in this post. It sure sounds like you guys know what you're talking about.

I wonder if the cvt in the new altima will work the same way it does in the maxima. From the tests I've seen, the quarter mile mph for the altima is around 95 where I've seen tests that have gotten anywhere from 97-99 from the new camry. Horsepower is about the same and curb weight is within a few pounds.The biggest performance affecting difference I can think of would be the trannys.(camry has a 6 speed manual) Is this yet another indication that cvt is showing it's inefficiency after a certain point?

I've seen folks on this site post 96+ mph for their '03-'06 Maximas...both auto and manuals.

maxhead

If I nail it from a stand still, it dies at about 75ish. Instead of doing the hybrid, (auto until 75, then manual after that), I find that putting in in manual, then right back into auto seems to do something (reset it, give it time to breath, whatever) and the auto starts pulling hard again.

When you're driving 70 and floor it, it will take off like a bat out of hell. Maybe, since the mapping from a standstill and the mapping from 70 are different, the cvt gets stuck in the first map and has a hard time switching to the other mapping variables. Maybe putting it in manual, then back in auto, gives it a chance to start fresh with higher speed mapping.

I'm totally guessing, but that makes sense to me.

VinneousMaximus

Well as scared as some are making me with this whole "efficiency" talk, I am pretty happy with my CVT at this point. Like maxhead just stated, 70+ and punched, my sH#t takes off. And from 30 up she hauls. I just love pulling onto on ramps and opening her up. It feels like I'm hitting 80 in a blink of an eye and must make sure that I slow down or else 90+ is gonna be right there. I do feel a little bit of exhaustion up top at times, but I think it's more of a feeling when I want to pull harder and she just can't. I don't fault the car though, it is what it is and it is only a 255hp sedan, not the G35 coupe. I'm not a fan of the torque steer down low but hey, at least it lets me know she's trying to go, and it does. All in all I'm impressed by the CVT if not for nothing else, the fact the wheneever anyone rides with me it kinda blows their mind that there aren't any gears.

jcalabria

If I slip mine into manual at speed it always drops back into "5th"... yours may be doing this too and you're just forcing that little downshift so that either the tranny can accelerate again or more likely, the downshift just makes it a bit easier for the engine to pull. Either way, with either technique, it just a way to override Nissan's programming, and shows that CVTs just may be the ultimate chip tuning cars.

I agree with Vinneous... I find this discussion "academically" interesting but the fact is that in 99% of my driving the CVT has been the best automatic tranny I've experienced (and I've experienced a lot!) - it goes like hell when I need it to, cruises effortlessly at my 80mph comfort zone, and it gives me better gas mileage around town, in a bigger car, than my 5.5 Gen did with a 4AT. Any complaints I have about my '07 - like the sloppy shocks, tinny doors and idiotic TPMS system - don't come anywhere the tranny.

dla

jc, well said.

I would think that Nissan and other CVT manufacturers have designed their profiles to have the vehicle cruise at the most efficient point (meaning friction/heat, wear and fuel efficiency) around 60 to 70 mph. While cruising at this speed the driven pulley and the drive pulley would have to undergo a large percent change the get the perceived torque (forward thrust) that most are accustomed to, like say when passing. In doing so that would require more gas, and hence a lower mpg. In a situation like this, an auto trannie down shifts, and a manual trannie is pick what gear you want. I would think that Nissan was not conservative on the upper end, but more of a limitation of the CVT because they were trying to maximize fuel efficient. If they made the CVT feel like a manual or auto (or more aggressive) at cruising speed, then I would thing the CVT would always “hunt” for a stable rpm especially when not on flat ground. Additionally, if they made a CVT more aggressive on the top end, wouldn’t they have to stealing it for the bottom end!

jcalabria

There may be limitations in the CVT due to slippage, heat, torque capacity at certain pulley positions etc, but its definitely not a limitation in available ratios relative to the other tranny types. Besides the virtually infinite intermediate ratio selections, the Jatco CVT also has a wider ratio spread low-high than most other transmissions, so its not an issue of stealing from one end to get give it to the other. The mapping just needs some tweaking (within whatever durability limitations may exist) to optimize what is selected for a given situation. Whatever ratio you need is there for the asking... just need to figure out how to ask for it.

I think that Nissan has programmed the TCU to satisfy the majority of people the majority of the time, just like they do with ECU programming. Choices/compromises are made. There has definitely been a market for ECU tuners that cater to folks who are willing sacrifice something - economy, durability, etc - to gain something else. If and when somebody ever cracks the TCU's programming, modifying the TCU ratio map(s) will be no different than modifying ignition, throttle or fuel maps in the ECU.

Nietzsche

iTrader: (7)

CVT's really are the ultimate transmission. Unlimited gears, smooth acceleration, no shift shock, fully programmable interface. There is nothing it can't do if you know how to work it.

jcalabria

Absolutely... its like upgrading from a PC to a Cray supercomputer... It can do amazing things but you gotta keep it cool and you gotta know how to program it! If somebody wanted to they could program it to exactly emulate ANY automatic transmission out there, from an old 2-speed Powerglide to a Mercedes 7-speed or Lexus 8-speed. But why limit it to 6 or 7 or 8 ratios when thousands are available?

jcalabria

For the younger among us, the term "overdrive" originally was used to describe a small two speed gearbox that was added to the tailshafts of a manual transmission to create, as Light mentioned, a more relaxed high-speed cruising gear. Normal "direct" gear was straight 1:1, "overdrive" had a gear multiplication rather than the typical reduction ratios of the transmission itself, resulting in the output shaft being "overdriven" - it turned faster than the input shaft. I remember my uncle installed one of these units on his '49 Mercury flathead coupe. Big hand lever under the dash to shift it.

After the energy crisis of the 70's, manufacturers began adding the same essential capability directly into their transmissions - manual & automatic - where a 4th or 5th gear was added that would allow the transmissions output shaft to be overdriven relative to its input shaft. Today, in some 6/7/8 speed transmissions, 2 or 3 of the top gears are all technically overdrives, but in the end all that matters for cruising is the overall gear ratio which includes the final drive ratio as well. A 1:1 direct drive top gear coupled to 3:1 final drive has the exact same overall gear ratio as .75:1 "overdrive" top gear coupled to 4:1 final drive. Its just another choice for the designer to make... use the extra gears to shorten the spacing between gears or tack 'em up top to relax the cruising.

Anyway, although it may be technically correct to call a transmission an "overdrive" if its output shaft turns faster than its input shaft in top gear(s), Light is correct that the term can be misleading or confusing depending on the overall gearing choices made by the designers. Its really a gray area in FWD trannies where the final drive is built within the same unit, so you never actually "see" that point in the gearbox that is "overdriven" like you do at the tailshaft of a RWD gearbox.

SilverMax_04

Good discussion by both "light" and "jc" on manual trannies. The Max 6-speed manual trannie is technically an overdrive trannie because the top (highest) gear has a ratio that is less than 1 to 1 (1:1). But, unlike rear-wheel drive cars, the trannie output does not go to a shaft but to the final drive gears. According to the info in my 2004 Maxima Product Guide, these are the final drive ratios for '04:

6 Manual = 4.133:1
4 Auto _ = 3.789:1
5 Auto _ = 2.440:1

I'm curious what cruising speed a Max with a CVT is going when the engine is turning at 3,000 RPM? The 6-speed '04 is only going 82 MPH at 3 K RPM. I suspect the '07 is probably closer to 90 MPH at that engine speed -- this should help fuel economy somewhat vs the 6-speed.

jcalabria

I don't recall exact numbers off the top of my head, but I seem to remember about 2600 rpm @ 80mph. The 6MT is about 14% shorter in top gear relative to the CVT. Our relative engine speed at ~80mph matches that percentage drop.

Here's a comparison chart I put together for the low/top/final & overall ratios for the Max trannies:


As I noted above, the CVT has the widest ratio spread low to high. I also just learned while looking this up that the 5AT can definitely NOT be considered an overdrive transmission at all! There's not a shaft in that gearbox that turns faster than the crankshaft. The CVT, on the other hand, is turning more than twice the crank speed somewhere in there! The higher internal speeds coupled with the shorter final drive is likely to reduce the torque loading on the pulleys & belt.

The chart does illustrate how the designers can arrive at relatively similar overall gearing by a variety of gearing combination.

RHMax

5AT is not an overdrive trannie, 90MPH at 3200RPM. It certainly could use an overdrive gear.

jcalabria

No, its not... but it's taller overall in top gear than the 6MT, which technically IS an overdrive tranny, in both 5th & 6th gears.

The term "overdrive" can be misleading and practically meaningless if you don't know the whole picture!

SilverMax_04

Thanks jc.

That chart is quite interesting. What it ignores is the addition of a torque converter to the gearing in the CVT and the 5-auto trannies. The additional multiplication from the torque converter actually give both of these auto trannies an even wider spread than your chart shows. But, because the torque converter is variable, it's hard to put a number to it.

jcalabria

Yeah... the TC definitely helps the launch. I've driven a CVT 3.2L Audi - even though its a decent motor it feels like a slug off the line. Its got a computer controlled clutch rather than a TC... you can really feel the difference.

I'm not 100% sure, but I believe that the torque multiplication ratio for most stock torque converters is just under 2:1, but it lasts for a very short period at that ratio and decreases fairly rapidly once the turbine starts to rotate. But it does help that initial punch.

big dave

What are the drawbacks to the CVT trannys besides heat buildup? Why are manufacturers like Mercedes, Lexus and BMW continuing to add more ratios to their transmissions instead of going the CVT route? I wonder just how much power these trannys can handle? Has CVT been used in any high horsepower applications yet?

jcalabria

It has been used in F1 cars (that were banned pretty quickly) but expense was no object there.

I think the issue is torque capacity at reasonable cost and the higher end mfgs are generally higher output motors. Nissan/Jatco has really stretched the envelope for reasonably priced CVTs that can handle fairly powerful motors. So far (3+ years in the Murano) there have been no major issues. The other issue has been, not surprisingly from our conversation here, developing control strategies to make the thing play nice in a variety of situations.

BTW, Mercedes offers a CVT in the A Class and Audi uses them in cars as large/powerful/upscale as the front drive versions of the A6. Other than in the Mini, BMW hasn't shown much interest.

SilverMax_04

Heat buildup is my major issue. Waste heat indicates a loss of energy that should be going to turning the wheels. Because of this waste heat, these trannies require a special high-temperature lubricant and two trannie coolers to dissapate the waste heat. Normal auto trannies need one trannie cooler. Manual trannies do not need any trannie cooler.

I rest my case, only a manual trannie for my vehicles. Besides, I'm a control freak who does not want a computer controlling my trannie.

big dave

The one thing I did notice is that after 2 or 3 passes the converter appeared to tighten up a bit. The car was no longer going straight to redline at full throttle and it showed in the .3 slower 1/4 mile times. I didn't hot lap so I don't think heat buildup in the trans should've been a problem. I wonder if this a way of "learning" my driving style.

MDS

You got that right!

Maxima1964

Jc, what about your TPMC bothers you? I had to take my Maxima back in two days later to get the light to stop coming on.

lightonthehill

Maxima1964 - jc will probably answer this later, but I remember he not only had trouble with the light, but with the brittleness of the valve stem, which he managed to break very quickly using very little pressure.

Also, if I recall correctly, with Nissan's new TPMS, you can't just replace a broken valve stem, but have to replace everything on that wheel associated with the TPMS.

But jc will elaborate on this.

jcalabria

Light hit the highlights... but one of my valve stems snapped off when adding air with same air chuck I've used on my compressor for probably 20 years... its the same type you see at most gas stations that has a self-clamping action built in - well the self-clamping action snapped the stem right off. I later found some off-the-record information from a Ford technician specifically warning about this happening, but no owner's guides I've seen, including Nissan's, warn about this at all. You can read the whole thread at

http://forums.maxima.org/showthread.php?t=487989

Nissan did replace the stem under warranty, but I've had the warning light ever since, with neither the time nor the inclination to deal with the service department again.

I also resent not being able to rotate your own tires any more, and its absolutely absurd that Nissan has a full size spare option, with a matching aluminum wheel, that doesn't have a TPMS sensor in it so you can't do a five tire rotation.

The whole thing is just a perfect example of government do-gooders trying to protect/compensate for the least-common-denominator idiots at the expense and inconvenience of people who can take care of things themselves.

SilverMax_04

Right. Nanny Government is terrible for people who handle things on their own. We don't need it -- but there are idiots out there who will probably ignore the mandated warnings -- even flashing lights.

Nietzsche

iTrader: (7)

You can still rotate your own tires. You just have to reset the TPMS system. It's not hard, if I can find the instruction I'll send them to you.

Nissan 6

do you remember Williams-Renault F1 CVT