Someone will correct me, I'm certain....but from what I remember there was something about the larger cam that severely decreases performance at lower RPMs.

 

The cams aren't really "larger", they are "longer" (which I guess could be equated to as larger *shrug*).

But for the most part, there is no reason that VVT can't kick in any sooner. It's based off oil pressure (estimated through rpm's) and a cpu re-program can change the switch point. The 2nd cam profiles are just more "aggressive," offering longer lift. Gas consumption obviously goes up and you therefore lose gas mileage, and that is the only reason Toyota held off the more gas-hungry cam profile for 6500. What Brennon said is true only when you are trying to run such aggressive cam profiles at an obscenely low rpm like 3500. The 2nd cam profiles offer a high-flow, high-horsepower offering and that obviously is a counter for torque--which we WANT at low rpm's. I'm relatively certain the 2nd cam profile kick-in point could be dropped to 5500 or 5000 without much negative repercussions to the low-end torque that the 1st cam profiles are shaped for.

Ideally, if you're using a car for daily driving, you'd want to see what your average maximum highway speed is (lets say 80mph) and see where your rpm's are at in 5th gear (if you have a 6-speed; 4th gear if you have a 5-speed) then set the VVT kick-in point like 250 above that. That is how we tuned our SAE racecar (sort of; we were dealing with a top speed of 105, so it was a little different, but the same principle).

 

Taken off wikipedia FWIF



Due to the behavior of the gases (air and fuel mixture) before and after combustion, which have physical limitations on their flow, as well as their interaction with the ignition spark, the optimal valve timing, lift and duration settings under low RPM engine operations are very different from those under high RPM. Optimal low RPM valve timing, lift and duration settings would result in insufficient fuel and air at high RPM, thus greatly limiting engine power output. Conversely, optimal high RPM valve timing, lift and duration settings would result in very rough low RPM operation and difficult idling. The ideal engine would have fully variable valve timing, lift and duration, in which the valves would always open at exactly the right point, lift high enough and stay open just the right amount of time for the engine speed in use.

 

i have a lexus is300, and it is reprogrammed to allow the VVT system to engage at 4500 instead of 6200 or so.

 

Brennon, that is absolutely true. But that "low point" is where I was saying down around 3000. Running cam profiles that are trying to allow more gas/air than the engine can suck in (yes, there is a vacuum effect going on in the engine when the intake valves are open). That's why VVT waits until later in the rev cycle to kick in (and for gas mileage benefits on mass production cars). But the capability for an engine to suck in more gas/air increases exponentially with rpm's. If an engine can suck in a measure of 1 at 2500 rpms, it can suck in a measure of 4 at 5000 rpms. Its hot like that

 

VVT, or vtech is simply a tool manufacturers use to get better emissions and gas mileage.

Yes, lowering the cam switchover point will improve performance. It will also have a negative effect on your mpg.

 

For '06 Elise/Exiges this cam crossover point was made lower right? I feel mine comes on way before 6,500 rpms.

 

Can someone explain why Lotus programmed the ecu to require 6500 rpm for the VVT "kick"?

Isn't there potential for similar performance gains at lower rpm's also ?

Yes there is. Lotus is offering a reflash in Europe. 2nd cam comes in at 5.750 RPM.

 

Below is an image of Honda's triple VTEC system.

A specific cam profile provides a specific power curve. By making the lobes more agressive by means of higher lift and/or changing durations, you move that curve to another sweet spot.

Lowering the switch-over point too low will actually hurt your cars performance.

In the Elise's, the crossover point is probably where it is because it provides the smooth transition between the two cam profiles.

Lowering it to 5750 allows the advantage of keeping the car in 2nd cam when shifting.

Sometimes, you'll see tuners (on Honda motors) who move the VTEC transition higher. That way as the 1st cam profile is leveling out, when the second cam profile kick in, it's more of a jump for certain customers who sometimes like to feel their "power" kick in.

 

I guess the answer is to get someone to make a VVT controller like you can get for the Honda engines... My son runs a Civic with a blue printed custom set up B18C JDM Integra Type R motor with a programmer... tune to what you like. It works... between the engine/tranny/suspension set up and his driving skills he can hand a lot of 'high performance' car their lunch. So if any of you Northern Virginia Elise or Exige S drivers see a black with black Kosie wheels rather plain Jane (as in sleeper) looking 95 Civic that has a somewhat suspicious sound... beware

Michael

 

VVT, or vtech is simply a tool manufacturers use to get better emissions and gas mileage.

That is barely the case, and, if I may be so bold, quite far from the whole truth.

VVT, a generic term used to describe the many different design approaches, and to avoid patents, to VTEC, the original engineering advancement made by Honda in the early-mid 80's and introduced in the late 80's on the Prelude SI, was designed for the sole purpose of increasing performance without resorting to larger engine displacement, loss of drive-ability for a mass-produced vehicle, or tossing gas mileage out the door. How's that for a well-constructed sentence!!

VTEC (not VTECH), standing for Variable valve Timing and lift Electronic Control, is hailed as one of the greatest advancements in ICE (Internal Combustion Engine) engineering. In a hope of increasing performance, Honda wanted to bring more to the table. Racecars have always employed what we know to be true: Let more gas and air in, get more power out. While so simple in theory, the application of just throwing valves wide-open to drink in the gas and breath in air doesn't work so well. As brennon shared above, gas and air can only flow so quickly. If you've been around a racecar (which do not employ VVT), you'll notice that the engine idles at around 2,000. At this very high idle speed, however, the engine sounds very rough, as if were about to stop because it is starving for fuel (that is exactly its problem at the low speed). The cam profiles are staying open for much longer than the flow of gas and air can enter at the low engine speed.

That does two things we don't like: Kill peformance and obliterate gas mileage. So Honda said, "We need to get those longer cam profiles in an engine when its pistons are creating enough of a vacuum suction to get the right amount of fuel and air in." After years of work, VTEC was born and now we all absolutely love it. It provides that kick-in-the-pants surge of power that says, "Holy ****, there is a God!" as it, quite literally, instantaneously increases power output by as much as 20%.

Gas mileage isn't good when in the longer cam lobes are controlling things, that's why VVT has things set to come in at such a high rpm--while it would be incredibly fun and quite thrilling, can you imagine running on the 2nd cam profiles while cruising down the highway at 70mph in 5th gear (gas mileage = ouch).

So really, what VVT does is put power on-tap when you want to drive it fun. VVT isn't a tool used as a gimmick (it's expensive!) and it certainly doesn't give you better gas mileage. What it does is give you the option of getting near-full potential out of an engine when you really want to.

And FYI, there aren't actually a 2nd set of cams. There are only 2 cams in a 4c DOHC engine. There are 2 sets of lobes on each cam, with the lobes right next to each other. The cam shifts sideways to use the 2nd lobes when the engage point hits. Here is a picture of some cams (B-series Honda motors) with 3 lobe profiles drool: :bow

 

Hmm.
Amazingly enough, no V-tec on Honda's F1 or IRL motors.

 

The original cams are designed to kick in at that RPM, adjusting them might not make the best power...
There are other cams with different profiles, but MUST be paired with ECU tuning

 

i have a lexus is300, and it is reprogrammed to allow the VVT system to engage at 4500 instead of 6200 or so.

I am sorry, but this is not a very smart one. The is300 doesn't have the 2ZZ and it doesn't have a 8000-8500 redline!!!!!!!!!!!!!!!!

 

Can someone explain why Lotus programmed the ecu to require 6500 rpm for the VVT "kick"?

Isn't there potential for similar performance gains at lower rpm's also ?

i have a lexus is300, and it is reprogrammed to allow the VVT system to engage at 4500 instead of 6200 or so.

The thing that kicks in at 6200 RPM isn't VVT it's the "L" (Lift) in VVTi-L. The ECU uses oil pressure to operate a mechanism that shifts cam lobes from ones designed for low-RPM operation to ones designed for high-RPM operation. The switch-over point at 6200 was picked for a couple reasons: gas mileage, emissions and cam lobe wear. It could have been done at a lower RPM, but not much lower. Some have hacked the ECU to switch over at 5700 RPM, but that is about the minimum on a N/A engine without losing power.

Probably the biggest benefit of lowering it to 5700 is to be able to stay on the high cam after a shift while on a track. If you're not very close to redline when shifting up, the RPMs will drop to below the point where it will shift back to the low-speed cam.

A forced induction engine could utilize the high-speed cam to benefit at a much lower RPM.

The IS300 doesn't have VVTi-L, so none of this applies to that engine.

 

ELPTEX: a well written expose of the system.
i haven't been into my corolla XRS engine yet but bought the elise because of the XRS.

 

Hmm.
Amazingly enough, no V-tec on Honda's F1 or IRL motors.

No need. F1 employs the longer cam lobes straight up from the get go--hence their high idling speed and rough idle.

 

The engine in the Lotus doesn't 'slide' the rocker for the high speed cam (as in lobe, not shaft). It engages a pin that allows the high speed cam follower to now touch the high speed cam, opening the valve with more lift and duration both.
Much lower and it would be too much- but I've heard the 5700 figure too, that seems to work ok (rather than the stock 6200).

There are other factors, such as the different types of cam follower for high and low speed. A roller on the low, slider on the high.

 

VVT works in the same manner as it does with a 2jz engine as it does with the lotus elise engine. its the same system. different parents, different engine, except 1 of the injectors doesnt "lift" whereas the lotus has both injectors lift? right? i hear the same kick in my Is300 as i do in a lotus elise.

1 example of my car. the video is shaky, but its an example of the VVTI system kicking in..

http://www.youtube.com/watch?v=TNMSJR21pog

and PS the 2jz engine redlines at 7200rpm... not too far off from your "8000-8500"

 

No need. F1 employs the longer cam lobes straight up from the get go--hence their high idling speed and rough idle.

In F1 it's also not allowed in regulations. Rule 5.6.1 - Variable valve timing and variable valve lift systems are not permitted.

http://www.formula1.com/insight/rulesandregs/13/995.html

 

There's variable valve TIMING and variable valve LIFT. VVTi is just variable valve TIMING. The intake camshaft assembly rotates on a gear to change valve timing (they either open and close earlier in the the combustion cycle, or later. This is always continuous and depends on the needs of the engine at any given time. It contributes more to economy.

VVTLi adds variable valve LIFT which, as others have mentioned, kicks in at (in the Lotus application) 6200rpm and causes the valves to open higher for a longer duration. This contributes to horsepower (more specifically, top end horsepower), at the expensive of torque... Think of it as having both a racing cam and a street cam in one engine. The lift side of mechanism is an on/off switch. Either the added lift is engaged or it's not.

Neither system has anything to do with injectors.

The IS300 range (including the new IS350) only uses VVTi... so no lift. If you're sensing a boost in power in your powerband, that's because the engine is tuned for it. It's not because the camshafts are acting more aggressively. The Lotus Elise S, only has VVTi.

The really cool stuff will be electromagnetically actuated valves (BMW is one manufacturer working on a system) that allows completely independent and flexible timing and lift that's directly computer controlled.

Oh yeah, the reason why VVTLi systems have such high redlines is because the internals are built to withstand the speeds... and the amount of horsepower increases with RPM. So you need lots of RPM to generate the power you get from that extra lift. The 2ZZ in the Elise is good for quick bursts to 8500rpm... and I'll bet the engine can take 9000rpm in brief bursts if it had to. That's quite a bit higher than 7200 rpm. In comparison, my old 1G AWD Eagle Talon without any valve assistance redlines at 7300rpm.

VVT works in the same manner as it does with a 2jz engine as it does with the lotus elise engine. its the same system. different parents, different engine, except 1 of the injectors doesnt "lift" whereas the lotus has both injectors lift? right? i hear the same kick in my Is300 as i do in a lotus elise.

1 example of my car. the video is shaky, but its an example of the VVTI system kicking in..

http://www.youtube.com/watch?v=TNMSJR21pog

and PS the 2jz engine redlines at 7200rpm... not too far off from your "8000-8500"

 

No need. F1 employs the longer cam lobes straight up from the get go--hence their high idling speed and rough idle.

I think that was Robert's point. You could never get an engine like that to pass emissions requirements and gas mileage would be horrible...both important to a production car. What he didn't say, and I think is as much to the point, is that drivability would also be horrible for a street car without the low-speed cam profiles.

Bottom-line: You're both right. The VVTi-L either adds more power (in the high-RPM range) to the engine, by adding the high lobes, or it allows a very high-performance engine to pass emissions and have acceptable gas mileage by adding the low-RPM lobes. Same result, just coming at it from opposite perspectives.

 

VVT works in the same manner as it does with a 2jz engine as it does with the lotus elise engine. its the same system. different parents, different engine, except 1 of the injectors doesnt "lift" whereas the lotus has both injectors lift? right? i hear the same kick in my Is300 as i do in a lotus elise.

1 example of my car. the video is shaky, but its an example of the VVTI system kicking in..

http://www.youtube.com/watch?v=TNMSJR21pog

and PS the 2jz engine redlines at 7200rpm... not too far off from your "8000-8500"

I had an IS300 for four years. If there was any kick at 6200, or any other RPM, I didn't notice it. In the Elise, you can't miss it!
:shift:

 

Bottom-line: You're both right. The VVTi-L either adds more power (in the high-RPM range) to the engine, by adding the high lobes, or it allows a very high-performance engine to pass emissions and have acceptable gas mileage by adding the low-RPM lobes. Same result, just coming at it from opposite perspectives.

Eh, that wasn't its original design philosophy, though--which is what I am speaking to. I did an [incredibly awesome] internship with Honda and had a 1-week long indoctrination that highlighted much of the company. Their bit on VTEC was nothing but a non-stop, ecstatic expression of adding more power and performance to a mass-produced, daily-driven engine. Of course they discussed gas mileage and how the first cam profiles were for gas mileage and daily-driveability, but did any of that *really* matter? Nah.

I will never forget the first time I took a friend out in an S2000. He had never been in a car that had VVT and he was utterly blown away with how much of a surge of power came up when the 2nd cam lobes took over.

Another curious bit. I was talking with my dad today about engines (ah, father-son bonding time in the garage, gotta love it) and we made it a point to be absolutely impressed that in any given engine, the valves are opening and closing 20 times per second at about 4,000 rpms. Isn't that freakin' sweet?!?

 

Where did you get that math?

In a coventional four-stroke engine, a valve will complete an open-and-close cycle once for every two crankshaft rotations. So at 4000rpm, a valve with complete a cycle 2000 times. 2000 cycles per minute equals 33-1/3 cycles per second.

A valve would complete a cycle 20 times per second at 2400rpm.

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