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Vvt-i Kickin


theWizard
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Correct VVT-i is an intelligent system that constantly adjusts the Valve timing based upon load and engine speed to give a combination of good economy, idling characteristics, low end torque and high end power.

Highest level of advance occurs around mid revs at high load if I remember correctly

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You will feel it more when you get to aroung 3500rpm.. car wwill pull a little harder.

may pull harder at optimum rev range....nowt to do with VVTI...as said...it deosn't kick in...it's always active.

no matter that the car is..what turbo's etc it has..there will be an optimum power band where she performs better...giving the impression something has kicked in.

you just feel something "kick in" because the engine is producing the most power and torque at a particular rev range. Like the T-sport maximum power output is at 6200 in third if memory serves me correct....maybe wrong gear..but revs is about right.

Vipes

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You will feel it more when you get to aroung 3500rpm.. car wwill pull a little harder.

may pull harder at optimum rev range....nowt to do with VVTI...as said...it deosn't kick in...it's always active.

no matter that the car is..what turbo's etc it has..there will be an optimum power band where she performs better...giving the impression something has kicked in.

you just feel something "kick in" because the engine is producing the most power and torque at a particular rev range. Like the T-sport maximum power output is at 6200 in third if memory serves me correct....maybe wrong gear..but revs is about right.

Vipes

Yes, totally agree although I know what cfc1 means. When the 1 litre gets to around 3500rpm it really starts to happen. On the 1.3 there doesn't appear to be that 'kick in' feeling, it just performs better through each gear at the higher revs (naturally!).

I must have a shot at a T Sport one day. I know there's a big difference but I would like to appreciate it for myself.

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Yes, totally agree although I know what cfc1 means. When the 1 litre gets to around 3500rpm it really starts to happen. On the 1.3 there doesn't appear to be that 'kick in' feeling, it just performs better through each gear at the higher revs (naturally!).

I must have a shot at a T Sport one day. I know there's a big difference but I would like to appreciate it for myself.

Yeah I find that too - took her up to 5500 rpm (the highest ever revs for me) and the power was great, altho she was in 2nd gear at the time....

PS: I was doing it to try and get rid of the idle problem that I posted a thread about yesterday before you say anything ;)

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Cam-Changing + Cam-Phasing VVT

Combining cam-changing VVT and cam-phasing VVT could satisfy the requirement of both top-end power and flexibility throughout the whole rev range, but it is inevitably more complex. At the time of writing, only Toyota and Porsche have such designs. However, I believe in the future more and more sports cars will adopt this kind of VVT.

Example: Toyota VVTL-i

Toyota’s VVTL-i is the most sophisticated VVT design yet. Its powerful functions include:

VVTLi_3.jpg

Continuous cam-phasing variable valve timing

2-stage variable valve lift plus valve-opening duration

Applied to both intake and exhaust valves

The system could be seen as a combination of the existing VVT-i and Honda’s VTEC, although the mechanism for the variable lift is different from Honda.

Like VVT-i, the variable valve timing is implemented by shifting the phase angle of the whole camshaft forward or reverse by means of a hydraulic actuator attached to the end of the camshaft. The timing is calculated by the engine management system with engine speed, acceleration, going up hill or down hill etc. taking into consideration. Moreover, the variation is continuous across a wide range of up to 60°, therefore the variable timing alone is perhaps the most perfect design up to now.

What makes the VVTL-i superior to the ordinary VVT-i is the "L", which stands for Lift (valve lift) as everybody knows. Let’s see the following illustration :

VVTLi2.jpg

Like VTEC, Toyota’s system uses a single rocker arm follower to actuate both intake valves (or exhaust valves). It also has 2 cam lobes acting on that rocker arm follower, the lobes have different profile - one with longer valve-opening duration profile (for high speed), another with shorter valve-opening duration profile (for low speed). At low speed, the slow cam actuates the rocker arm follower via a roller bearing (to reduce friction). The high speed cam does not have any effect to the rocker follower because there is sufficient spacing underneath its hydraulic tappet.

VVTLi.jpg< A flat torque output (blue curve)

When speed has increased to the threshold point, the sliding pin is pushed by hydraulic pressure to fill the spacing. The high speed cam becomes effective. Note that the fast cam provides a longer valve-opening duration while the sliding pin adds valve lift. (for Honda VTEC, both the duration and lift are implemented by the cam lobes)

Obviously, the variable valve-opening duration is a 2-stage design, unlike Rover VVC’s continuous design. However, VVTL-i offers variable lift, which lifts its high speed power output a lot. Compare with Honda VTEC and similar designs for Mitsubishi and Nissan, Toyota’s system has continuously variable valve timing which helps it to achieve far better low to medium speed flexibility. Therefore it is undoubtedly the best VVT today. However, it is also more complex and probably more expensive to build.

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