waht excactly is VTC i see it in alot of the odl posts but cant figure out much info can someone fill me in a bit?

 

Man I've been wondering myself. I wish someone would faq this kind of stuff! https://maxima.org/forums/showthread...threadid=14814

 

Someone is bound to have explained this before, but without having to search, here is my summary explanation

The 4 cam VE engines have a mechanism that gives the intake cams variable timing. It functions under
low to mid range rpm (less than 4600 and coolant temps are less than 230F or 110C -non overheated-)
while the engine is under moderate to heavy load. The advancing of the intake cam timing "moves"
torque curve "to he left" meaning power comes on early and peaks at lower rpm. This marvel of
engineering is achieved by a mechanism that allows the camshaft to rotate about 20 degrees relative to
the cam gear. This gives it 28 degrees of valve overlap in stead of 8. If you can imagine how a gear
wheel can slide on spline axle, back and forth without turning; Now picture the ridges on the spline
not going straight, but gently turning around the axle. Now when you slide the gear back and forth, it
turns slightly as it is following the curved splines.
This is in principle how the camshaft is rotated. A strong spring holds it in the advanced position, and
oil pressure pushes it back to the neutral position. The valve to admit oil pressure is a solonoid
driven by the ECCM which determines the proper advance conditions. The VTC problems commonly
written about is a weakening of the return spring that is then no longer holding the gear firm to the one
end with enough force, allowing it to make the rattling VTC failure sounds. A "rig" fix sometimes
aplied is to rewire the VTC solenoid forcing them to engage oil pressure all the time, thus forcing the
cams to the neutral position. This disables the advance mechanism, and often eliminates the noise.

Hope this helps
John

 

so it basically works like the VVT-i ( varible valve timing with intellegence) found on other cars such as lexus.

ahhhh i see said the blind man, i thought thats what it was but kept seeing how people were having problems and it ocnfused me.
VG all the way baby

 

I've read the Nissan FSM also. It's weird that when the pressure is in mode II, it increases valve overlap. That usually decreases the dynamic compression ratio and promotes better breathing(intake out exhaust valve schavanging effect) on the upper rpms. That somehow doesn't jive w/ better low end power(less overlap would promote this) But the fsm does seem to show the torque curve moving to the left for mode II though. It seems odd that Nissan would have to develope such a complicated system just to aid in low end power when they are working w/ a 3.0 V6. Logic would seem to develope a system that would promote good low end(default) and better high end when that system is "activated" ie.. vtec variable cam lift tech. If what you say is actually the way it is, Nissan developed a vtc for low end then turned around and developed the power valve in the intake for low and high end. Very odd indeed.

In short, the Nissan fsm sucks when it trys to describe how this system works.

 

hey guys thanx alot

one mroe thing, waht exactly is your sig pic of jeff92se? ive asked once be4 but you must have not seen my post.

 

Not being an automotive engineer, but with a decent understanding of the mechanical intricacies of engines and drivetrains, I usually extrapolate intentions by observing. My take is as follows.

Four valve per cylinder engines with a valve train tuned for speed and power have a notoriously lousy bottom end. In the VE we have variable intake runners (if you have the 5-speed) to optimize mid range and to boost high rpm ability.

The VTC scenatio is an engineering solution to the bottom end problem.

In the Honda VTEC approach, engineering is applied to optimize the super high end, and as a result, the engine sings in rpm ranges where few others can exist for more than mere seconds. It is also a race bread design

I'd venture the guess that most Maximas, other than the few in the hands of us nut cases, rarely see the better end of 4000 rpm, so the design is probably optimized to the expected most common conditions. The VTEC is a showcase application installed in a small percentage of the fleet. Different design philosophy, different end result.

I'd be curious to read an engineer's comment on the different design option.

Note to Jeff: My take on reading the FSM is when the system goes to mode II is that the VTEC valves bleed the oil pressure and the spring advances the camshaft. Applied oil pressure restores it back to base timing