Variable Valve Lift

[30yr_old_fogey]

What controlls the low-to-high valve lift transition in the 2zz engine?Can it be messed with? Thanks,Bryan

[lordyogi]

maybe this will help you understand what the VVT does and the Lift does and how it all works. http://web.ask.com/redir?bpg=http%3a%2f ... vvtl-i.htm

[30yr_old_fogey]

Ok, so the higher valve lift occurs when the oil pressure reaches a certain point...Why not sooner in the RPM range? (No just saying "because that's the way it is").Why not have it kick in at 4k instead of 6k. There must be a technical reason..Any insight?

[Chris]

I don't have the precise answer but here's the basic reasoning.1) time to fill combustion chamber2) speed of flow1)the higher the rpm the shorter time available for the fuel mix to fill the cylendar. At low rpm (less than 6k as you state) there is ample time for the mix to get sucked in however above 6k the time is much shorter thus a 'bigger hole' is required for the gas to enter.2) the mix (air fuel) has mass thus inertia thus the intake flow does not stop at BDC but continues "coasting" into the chamber even when the piston is coming up. Slow mix speeds stop sooner thus reducing any "ram" or supercharging effects while higher speeds perform a mini supercharge effect. Therefore, to keep flow speed and the associated inertial effect, high, a small opening is needed at 'low rpm' while the speed will still be high with a bigger opening but a faster piston speed... um, clear as mud? Another example is the corvette. under a certain rpm only one of the two intake valves opens providing the high mix speed which translates to flexability which is really a bit extra torque at low rpm... after a certain speed both valves open for the same reason explained above.hope this helps a bit!

[30yr_old_fogey]

Wow! Very informative!Ok, with all that said, when the high-lift kicks in at ~6k, there is quite a large surge of power.Basically, is this large power surge just waiting to kick in, and the engineers decided that 6k was a good place to cut it loose, or is 6k the earliest point at which all that flow speed stuff will work???Thanks,Bryan

[Chris]

Hey 30 yr fogy... this is 35 yr fogy here I think you're getting WAY beyond me. Another GENERAL answer is that if you could have Infinitely adjustable timing and lift... you could probably have a smoother power band... ie power increasing progressively as rpm's increase (remember hp is a function of torque) rather than the 'spike' at 6k. I would ASSUME (remember, I'm no expert here!) that the cost associated with a more adjustable/flexable timing lift potential would be really expensive thus Toyota probably just set the "switch" at an optimum level that the engineers determined to be 6k.That's about the extent of my knowledge.... anyone else know why 6k is the magic number???Slightly off topic... even though smooth power deliver is technically the best... don't you just love the kick at 6k? I don't have the GT but have tested it a couple times... what a hoot!Happy driving and GREAT QUESTIONS! It's great when people want to learn what makes their cars tick!

[MadBill]

I'm with 30 yr on this: There is such a kick in power when lift comes in at 6,000 RPM, you have to wonder if there wouldn't be at least slightly more power with the big cams from say 5,000 RPM on up and if so, why do they delay so long?PS: Minor clarifacation for Chris: It was just the ZR-1 Corvette (~`91 -'95?) that had a 4 valve engine and the valves always opened, but there were extra throttle blades in 8 of the 16 inlet ports that stayed closed at low RPM and in "Valet" mode. (The effect was the same as if a valve stayed closed though.)

[mu_ohio]

I also think that Toyota wanted the wow effect and you can't have that if it's smooth. What Toyota really needs to do is work on the gear ratio. I would be satisfied if the gearing allowed me to stay in lift even if it was above 6k. That way, you would only use lift if you really wanted to hammer it and had more fuel effeciet range.

[HIGHREV]

I think about old school hot rods. If you got a big cam in your small block chevy it would have trouble idling. I have seen street rods that practically shudder at an idle and you can almost hear each piston fire. But when they get up in the rpm's they make big power.I think our cars would run on the higher lift cam at lower rpms. But the fuel efficiency would go out the window. I can't say they would have more horsepower either. My opinion is that it may be too much fuel for the air mix. If there was a turbo or super charger behind it it would probably make good power. The other variable is the timing. Our cars have brains that are programmed with fuel efficiency in mind. The computer may try to adjust the timing to make sense of the fuel rich output or it might fall on its face trying.I also think they picked the 6G number with driveability in mind. If the high lift was activated at 4 or 5 it would be too weird on normal driving. I think it would be similar to an automatic trans hunting for gears. I personally would like to see it come in around 5500 so you could stay in the high cam between shifts without so much effort.

[30yr_old_fogey]

Ok, now that I've got you all travelling down this road, why don't we figure out how the oil pressure causes the transition from low- to high- lift at a specific RPM (currently ~6,000)...Is it all internal, or does the ECM (correct me if my acronym is wrong) have anything to do with it?If it's all internal, there must be some kind of pressure sensing / steering mechanism the Toyota (or is it Yamaha?) engineers "hard-wired" (pardon my electronics background) into the design. They must have had some way to "tune" this transition point in prototypes, right? The real question is - can we get at it???On the other hand, if it's computer controlled, we should be able to get at it...Let's figure this out. Anyone know any experts with regard to the VVTL-I engine?[End of Brain Dump...]Thanks for listening,Bryan

[MadBill]

There was a previous thread re the programming of lift. I believe it turns out to be pretty complicated. Even if one had the smarts to change the RPM point (I think there must be an ECM-controlled solenoid that directs oil pressure to effect the shift), an earlier shift would change air flow requirements, and thus fuel and spark needs, between the new shift point and the present one, requiring extensive changes in numerous other tables in this range.

[mitro]

Now this is the kind of thinking I like to see! I'm with fogey 100% on this. With the dramatic change in power at lift theres GOT to be a couple extra ponies to be had in the midrange by switching to that cam grind say... 500 rpms sooner. Not only would you get a smoother transition but that may just be enough to stay in lift from first gear @5500 all the way through 4th.

[DABEAR95]

quote:Another example is the corvette. under a certain rpm only one of the two intake valves opens providing the high mix speed which translates to flexability which is really a bit extra torque at low rpm... after a certain speed both valves open for the same reason explained above.hope this helps a bit!The corvette only has one intake valve per cylinder. Except the ZR-1 corvette with the LT-5 engine.

[NovaResource]

quote:The corvette only has one intake valve per cylinder. Except the ZR-1 corvette with the LT-5 engine.Correct, and the LT5 did not shut off valves like Chris claims.

[MadBill]

Hey you two in the back, pay attention! I posted this ZR-1 info on Monday!

[30yr_old_fogey]

One of these days, I'm going to talk to the "2zz trained" mechanic at the dealer (assuming there is one), and find out what this is all about.What I'm really after here is how the lift mechanism is engaged, by what, and if it can be engaged lower, say a bit before 5700 rpm (that's where I seem to land from 1st to 2nd).As we all know, most products in the market for the general public are designed with design margin for longevity (and the occasional misuse) and various regulations. So, Toyota probably could have moved it down, but decided not to for some reason.Bryan

[Chris]

Nice catch, Scott... I was technically incorrect:"A unique feature of the engine was it's intake system. Each cylinder was fed by two runners-one for each intake valve. During normal driving, each cylinder was fed by one runner.Through the use of a dash controlled key switch, whenever additional power was required, the driver could turn the switch allowing the other runner to come into play."The same function as shutting down a valve.In any case, the key is to maintain high intake flow speeds at all rpms.Good question about the 'magic' 6k switchover... I'm interested to know what the deal is.

[QUIKAG]

The high lift crossover point is determined by the ECU. Once parameters are met with oil pressure (if the engine is cold, it will not allow lift to engage) and a few other minor things, lift will engage at the predetermined point.To my knowledge there are no bypasses yet to allow the lift point to be lowered. On Honda's there are a few devices, one popular one is the VAFC, that allows the fuel trim settings to be adjusted as well as lowering the VTEC engagement point. I wish someone would devise a system to allow our engagment points to lower a bit.I normally fall just below the engagement point on the 1-2 shift and sometimes on the 2-3 shift. I'm shifting near the rev limiter too at 8350rpm. One of my problems is that I have the 17 inch wheels which incrases the diameter of the wheel which effectively raises the gearing in the transmission. I've been told that if I go to 16 or 15 inch wheels with low profile rubber, I won't have much of a problem with staying in 'lift' on the upshifts. That said, I like my 17 inch wheels and the Vibe isn't really a drag car anyway, so I'm not too worried about it.

[mu_ohio]

Actually, tire size will not have an impact on where you land after each gear. It will affect what speed you are going at a given rpm. For example, if you just had the engine and tranny not connected to anything, you would still fall out of lift when shifting gears. We actually reach a higher speed in each gear compared to the Celica while it accelerates quicker.

[Stang2Vibe]

When winding it out for max acceleration, I am able to shift at about the point where the rev limiter kicks in and fall back at about 6,000 rpm when the shift is completed. Just outside the point where the lift kicks in. It would be nice to be able to shift and remain in the lift stage. Maybe there is some reason that the engine has to go back into regular cam lift before the lift can be reengaged? Or maybe lift should be disengaged for some reason when rpms fall when you shift? I'm sure if it was meant to be done, the engineers would have designed it to stay in lift after the shift either by programming it electronically that way or by designing the gearing for it. There is some reason why lift is disengaged in beween the shifts.Another question. Since the Vibe GT/Matrix XRS has the same drivetrain as the Celica GTS, how close are acceleration times between our GT's and Celica GTS's. And would a Vibe GT be faster than a Celica GT with the base 1zz engine in it?

[DABEAR95]

If I powershift, I can keep it in lift. I don't plan on doing it again though. It's not a race car and it didn't feel too good! Felt much better in my Eclipse.

[QUIKAG]

quote:When winding it out for max acceleration, I am able to shift at about the point where the rev limiter kicks in and fall back at about 6,000 rpm when the shift is completed. Just outside the point where the lift kicks in. It would be nice to be able to shift and remain in the lift stage. Maybe there is some reason that the engine has to go back into regular cam lift before the lift can be reengaged? Or maybe lift should be disengaged for some reason when rpms fall when you shift? I'm sure if it was meant to be done, the engineers would have designed it to stay in lift after the shift either by programming it electronically that way or by designing the gearing for it. There is some reason why lift is disengaged in beween the shifts.Another question. Since the Vibe GT/Matrix XRS has the same drivetrain as the Celica GTS, how close are acceleration times between our GT's and Celica GTS's. And would a Vibe GT be faster than a Celica GT with the base 1zz engine in it?A Vibe GT is faster than a Celica GT. A Celica GT-S is faster than a Vibe GT due to the lower weight. Quite a few Celica GT-S with a few breathing mods are solidly in the 14's in the 1/4 mile.I need to think about how all the systems work, but I know that wheel/tire size plays a role in effective gearing of the motor. Lowering the diameter of your wheel/tires will virtually gear the tranny lower. From my roadracing and autocrossing experience, it's a fact. You will be getting less speed in each gear because of the smaller diameter of your wheel/tire setup which is the same thing that happens when you put a higher gear ratio. Your driveshaft/transaxle thing spins only so fast, so when your wheel/tires are smaller, you won't be generating as much rolling speed in each gear. Finally, I don't think the engineers set the transmission up to fall out of the 'lift' point on upshifts in lower gears, it's just the way it works out on the Vibe GT especially with the 17 inch wheel/tire package. It's the same transmission and motor as the Celica GT-S and the bigggest wheels you could get with the GT-S are 16 inchers.I, too, have been able to stay in lift on the 1-2 and 2-3 upshift, but I had to powershift which entails leaving my foot fully on the gas, jamming the clutch, very quickly grabbing another gear, and popping the clutch. Sometimes the tires grab (damn sticky Dunlop 9000 tires) and the car wheelhops (does not sound good at all) and other times the tires spin a bit which works nicely.To summarize, if you have a Vibe GT and want to stay in lift, get some 15 inch tires, put some slippery tires on them and shift hard near the rev limiter at 8350 rpm.

[Kissfan79]

I was just looking at some stats from Car & Driver and they have the Vibe Gt and Celica GT-S basically dead even in 0-60 and 1/4 mile times. Another site Iwas looking at had the Vibe GT a HAIR quicker than the Celica GT-S. Jim

[QUIKAG]

The Celica is lighter with the same drivetrain and motor, so the Celica is going to be faster given equal conditions and drivers. Also, the Celica should handle slightly better too. I wouldn't take a Celica GT-S to a roadrace course and expect to win.

[MadBill]

Quote> To summarize, if you have a Vibe GT and want to stay in lift, get some 15 inch tires, put some slippery tires on them and shift hard near the rev limiter at 8350 rpm.Most of what you say is correct, QUIKAG, but although the true road speed at a given RPM is affected by the tire size, the RPM drop between shifts is determined only by the transmission gear ratios. Mental experiment:If you put the car up on stands, took off the wheels and ran it up to say 6,000 RPM in 3rd gear, checked the speedo reading, then shifted to 4th and maintained the same MPH reading, the revs would be 'X' % lower, even though there were no wheels at all.(On second thought, maybe you just mean't the "lower gearing effect" of smaller tires would give enough wheel spin after a rev-limiter 1-2 shift to keep the engine on lift?)

[mu_ohio]

MadBill, you are correct and backup my earlier post. The tires affect the final gear ratio which is the ratio of your transmission to tire speed. The entire gearing needs to be correct with maybe the exceptioni of 6th. The taller tires to make a difference between our car and the Celica GT-S. hopefully someone will have an ECU programmer that will allow us to adjust the speedo for different tire sizes. If so, I would go with a lower tire height.

[QUIKAG]

quote:Quote> To summarize, if you have a Vibe GT and want to stay in lift, get some 15 inch tires, put some slippery tires on them and shift hard near the rev limiter at 8350 rpm.Most of what you say is correct, QUIKAG, but although the true road speed at a given RPM is affected by the tire size, the RPM drop between shifts is determined only by the transmission gear ratios. Mental experiment:If you put the car up on stands, took off the wheels and ran it up to say 6,000 RPM in 3rd gear, checked the speedo reading, then shifted to 4th and maintained the same MPH reading, the revs would be 'X' % lower, even though there were no wheels at all.(On second thought, maybe you just mean't the "lower gearing effect" of smaller tires would give enough wheel spin after a rev-limiter 1-2 shift to keep the engine on lift?)MadBill, I understand what you're saying and it has made me confused. I know for a fact that putting smaller diameter tires/wheels on your car will allow your car to accelerate faster at the expense of absolute velocity in each gear. How that helps keep the car in lift is a question I'm now not sure about...I know that wheelspin would be easier, so staying in 'lift' would be much easier, but I don't know if the smaller wheel size in and of itself would do it...Okay, let's try this. I agree that upshifting will drop your car x% rpm. But since you have smaller wheels/tires the difference in speed drop will be narrower giving you a much better chance of staying lift.Does anyone have some 15 inch snow wheels/tires and 17 inch factory wheels/tires to compare? When I went from the stock 3.42 gear ratio in my Vette to 4.10 gears, on upshifts my rpm DEFINITELY stayed up higher than it did with the 3.42 gears. Going to smaller wheels/tires has a similar effect, you will fall into a higher rpm on upshifts.What is our stock final drive gear ratio in the GT? Isn't it like a 4.30 or something? I know it's higher numerically than the base Vibe which is another thing that helps our speed. Instead of messing with the computer, it would be cool if we could get an aftermarket 4.80 gearset or something like that. Increased acceleration in all gears without touching the motor.

[DABEAR95]

I have 16 inch snow tires. It was a bit easier to keep it in lift (if I got traction). I think it was because I was accelerating a bit faster and when I let off, there was a bit of a kick left. I can't really explain it, as it didn't happen often. Most of the time once I hit lift the tires would break loose, unless I was on flat dry ground going straight.The tires were 1/2 inch shorter.Jason

[MadBill]

Quote> Okay, let's try this. I agree that upshifting will drop your car x% rpm. But since you have smaller wheels/tires the difference in speed drop will be narrower giving you a much better chance of staying lift. Err... No.Let's try an example where wheelspin can't confuse the issue: We'll assume good pavement and decent 25" diameter tires. If you run up to the rev limiter in your GT in second gear, ~ 8,350 RPM and 60 MPH (I'm guessing re the speed, since I've got a base 5 speed), the gear ratios alone will determine what RPM you will see after a fast shift to 3rd. If the overall second gear ratio is for example 10.0:1 and third gear is 7.2:1, then you'll still be doing 60 MPH, but at 7.2/10.0 x 8,350 RPM =6012 RPM. If you now fit a set of ultra-short 23" tires and repeat the exercise, 8,350 RPM will be an actual 55 MPH (but your speedo will still read 60, because it is calibrated for the stock tires) and after the shift you will still see 6,012 RPM.The only way short, slippery tires might help is on a 1-2 shift, where wheel spin in second gear might keep the RPM a little higher and thus the engine would stay in lift.Hope this helps!

[QUIKAG]

Okay, I understand. Nonetheless, smaller diameter wheels/tires will help acceleration, but may not keep the motor in 'lift.' Interesting description, thanks for taking the type to explain the err of my ways!!

[MadBill]

That's it!(Always glad for an opportunity to rattle on re technicalities!)

[Chris]

Just for the techies around...I was hanging out at canadiandriver.com (great place) and someone posted a link to this site where these guys built a continually variable valve timing unit (www.pattakon.com ).Now, I'm not endorsing this thing nor do I even know if it works but, it is an interesting place to browse and sheds some light on how vvt works and stuff.Check it out if you're so inclined.Have a great weekend all!