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How does variable valve timing work?

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Old 05-06-2009, 04:53 PM
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How does variable valve timing work?

Especially in the rev-up engines? Does the timing advance?

I was told the reason the rev-up engines consume more oil is because of the addition of the variable timing on the exhaust.
I was also told that they consume oil quickly when running over 4k rpm because this is when the timing system kicks in. (or turns off)
Old 05-06-2009, 05:31 PM
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In a nut shell it adjusts the timing to run more fuel efficient at certaing RPM speeds, but some systems are continuous VVT where it actually uses engine speed and throttle position and engine temp to change the timing on the engine. This sytems assist in providing tourque at lower rpms.this is similar to the system that Honda has but on the Vq35de we dont have lift, only on the 3.7l Vqvhr that has the VVEL system.
Old 05-06-2009, 05:32 PM
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check this out!!

"Variable Valve Timing
*There are a couple of novel ways by which carmakers vary the valve timing. One* system used on some Honda engines is called VTEC.

VTEC (Variable Valve Timing and Lift Electronic Control) is an electronic and mechanical system in some Honda engines that allows the engine to have multiple camshafts. VTEC engines have an extra intake cam with its own rocker, which follows this cam. The profile on this cam keeps the intake valve open longer than the other cam profile. At low engine speeds, this rocker is not connected to any valves. At high engine speeds, a piston locks the extra rocker to the two rockers that control the two intake valves.

Some cars use a device that can advance the valve timing. This does not keep the valves open longer; instead, it opens them later and closes them later. This is done by rotating the camshaft ahead a few degrees. If the intake valves normally open at 10 degrees before top dead center (TDC) and close at 190 degrees after TDC, the total duration is 200 degrees. The opening and closing times can be shifted using a mechanism that rotates the cam ahead a little as it spins. So the valve might open at 10 degrees after TDC and close at 210 degrees after TDC. Closing the valve 20 degrees later is good, but it would be better to be able to increase the duration that the intake valve is open.

Ferrari has a really neat way of doing this. The camshafts on some Ferrari engines are cut with a three-dimensional profile that varies along the length of the cam lobe. At one end of the cam lobe is the least aggressive cam profile, and at the other end is the most aggressive. The shape of the cam smoothly blends these two profiles together. A mechanism can slide the whole camshaft laterally so that the valve engages different parts of the cam. The shaft still spins just like a regular camshaft -- but by gradually sliding the camshaft laterally as the engine speed and load increase, the valve timing can be optimized.

The variable cam system used on some Ferraris

Several engine manufacturers are experimenting with systems that would allow infinite variability in valve timing. For example, imagine that each valve had a solenoid on it that could open and close the valve using computer control rather than relying on a camshaft. With this type of system, you would get maximum engine performance at every RPM. Something to look forward to in the future...

For more information on camshafts, valve timing and related topics, check out the links on the next page
Old 05-06-2009, 07:00 PM
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Great post ^

BTW. variable valve timing has no relation to oil consumption. Many cars feature varible valve event timing on the exhaust side and don't suffer from oil consumption
Old 05-06-2009, 08:16 PM
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Originally Posted by Jeff92se View Post
Many cars feature varible valve event timing on the exhaust side and don't suffer from oil consumption
I have a theory:

The 05-06 have the rev-up engine; i.e. higher redline.
They also have the variable exhaust valves that the 05-06 non rev-up don't have.
They have the same short block (pistons, cranck, etc.)

If the OC is caused by rings then I would assume that the non-rev engines would also be defective if they were run the same way as cars with manual transmissions.

However this was not really a problem on 03-04 cars.

As far as I can see the only way oil can make it in the combustion chamber is under vacuum. So maybe the exhaust valve timing is causing 'excess vacuum'?

The person who told me to run above 4k rpm was the service rep at an Infiniti dealership.

He said that doing this will consume the most oil. He said that long drives on the interstate and leaving it in 5th gear is what will do it.

When he mentioned the variable exhaust valves I became a little puzzled. But I do recall something about the valve guides/seals being crap. So that would be another way to pull oil into the combustion chamber.

Anyways, this is just a little brainstorming. I appologize in advance if any of my facts are wrong.

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