VVT What is it and when did it Vanish?

[Ex-CA Driver]

I have been trying to figure out what Variable valve timing is and if I have it. 91 q45 made in Feb. If I do what are the benefits and when did it go away? My father has a 1997 Q45 but he says he does not have it nor does he want it. Umm ok. That's alright, Cuz secretley I know he really drives a Q41. Ahahahhahah

[911/Q45]

All Qs have VVT. The cam changes timing with rpm to optimize low and high rpm power and emmissions.

[Q45tech]

96 Q does not have VVT and the Q41 VVT action is different as is the reduced lift and duration of the cams [96Q also has low performance cams].

The VVT optimizes the idle for smoothness and the torque in the 2500-4600 rpm range [creates 10% more than you would get without VVT. Very reliable design on all years never creates a problem like the system on the J30 [at high mileage].

[aaacomp]

The VVT is what the old time drag racers wanted. Something to change the timing curve, that's one reason the Q has such a strong top end. We used to put a button in the cam sprocket to change our need for more top end etc.............:thumbup

[VimyJ]

The VVT optimizes the idle for smoothness and the torque in the 2500-4600 rpm range [creates 10% more than you would get without VVT. Very reliable design on all years never creates a problem like the system on the J30 [at high mileage].

This is interesting. I haven't heard of this VVT timing problem on high mileage VG30DEs. Q45tech, in your experience, when do these problems manifest themselves (what is your definition of high mileage?) and what are the symtoms? The valves are hydrolicly controlled so I would imagine good oil maintenance would have an effect on the the "problem's" onset. Yes?

[VimyJ]

After studying the service manual, I notice there are valve timing control solenoid valves. Are these the problem areas on the the high mileage VG30DE's or the contact and sliding surfaces of the lifters?

[Chally]

Ex-Ca Driver

In laymens terms, the Inlet camshaft is advanced forward so that it gets opened earlier. This is done at 4000 rpm on the Q45. Your 91 model has it.

It basically changes the overall Cam profile to what we call a Lumpy Cam. These cams make the engine work very well at high speeds but give it a poor idle. Just look at a drag car at idle, the car rocks badly & not the result you want in a Luxury Vehicle.

Most run-of-the-mill cars are made to idle nicely & these really don't rev very well, & if you do try to rev them, they don't have any power at revs.

So Nissan, & other manufacturers, designed a system to give us the best of both worlds, Smooth idle & great power at revs.

If you look at the front of the engine at the front of the Inlet cams, you will see a little solenoid poking out of the plastic covers, this is what activates the cam change. It just lets Oil Pressure in which forces the cam around a few degrees, & when the switch is off, the cams return to their nornal position.

Hope this has helped.

[Q45tech]

The 248 degree intake cam's normal position is such that at idle and above 4600 rpm the overlap in relation to the exhaust cam closing [8 deg ATC] is TDC or 8 degrees of overlap.When oil pressure builds above 14 psi and the ecu open a valve the intake cam is forced around to a position where the intakes open at 20 degrees before TDC increasing the overlap to 28 degrees.When 4600 rpm is reached the air velocity in the runners is so high that an early opening is not required so the ecu shuts the valve and the lack of oil pressure lets the intake cam return to the normal position.

Unlike old single intake valve v8 the 2 valves provide around 30% more air flow potential so cam duration is much shorter as is valve lift [function of valve diameter].32 valve engines have so much [too much] air flow at low rpms that they appear sluggish till 3500 rpms. The cam opening point change helps maximize the low rpm velocity so that 10% more air follows in the 2500-4600 range than normally would.

More modern engines with additional exhaust VVT are able to optimize the timing events so that the torque curve is flat from 1500 to whatever rpm the torque peak occurs [4000?]

Acceleration is all about creating the maximum torque at any rpm old performance engines tended to be peaky whereas modern ones tend to have broad useful ranges.

The low over lap position [4600-7300] is not optimized for maximum power beyond 6,000 [where the torque is down to 243 lb/ft or 278 HP but it is useful out to redline [6900] where it is still above 170 lb/ft [roughly 233 HP].

Much of early Q VVT was optimized for a very smoth idle.

Generally 4 valve per cylinder engines can be built to have a reliable maximum torque of 75-80 lb/ft per liter......3.5=262; 4.0 liter =300; 4.5 liters =337.