Variable valve timing

So how does variable valve timing actually work? Like V-TEC Honda or VVT-i Toyota? Is it working the camshaft or something actually with each valve? Just wondering. Rocketman


Depends on the year, make, and model.

Some cars use variable timing on just intake valves, some on both intake and exhaust.

Some use oil pressure operated variable sprockets to change when the valves open–actually changing camshaft timing to suit the needs.

Hondas used oil pressure to engage/disengage a different set of rockers to different lobes on the camshaft to change the valve lift and timing.

Yes, this means that an engine running low on oil may and/or will have incorrect timing.

Most variable valve timing adjusts the intake cam timing so the overlap varies. I don’t remember which is which but more or less overlap affects the torque curve. The variable valve timing adjusts the overlap so you get max torque available for the specific RPM that you are at.

Honda’s V-TEC is a far more sophisticated animal. It uses two separate cam lobes, one for low RPM and one for High RPM. At low RPM, only one intake valve actually opens completely and at a profile that favors low end torque. The other intake valve only opens a crack to improve swirl in the combustion chamber.

At high RPM, a rod connects both rockers for the intakes together and they ride on the bigger lobe.

Chrysler/Fiat have a new system that used a master/slave cylinder system between the cam lobe and the valve. The cam is always in the High mode, but a solenoid valve is placed in the line to relive hydraulic pressure between them to trim off some timing at both the front and trailing edge of the cam profile, making it a bit milder for low RPM. This system has more potential than either of the other systems, but right now it is kept pretty simple. It only has two steps programmed in it.

Nissan has a system where the cam actually lifts off the followers. At idle, only the very top of the lobe actuates the valves. As throttle is added and RPM increases, the cam is lowered onto the tappets (buckets as they call them) and more of the cam profile is used. This eliminates the need for a throttle body. It seems like a good idea but is only used on the 3,7 engine. They were going to go across board with this on all engines but haven’t for some reason.

I agree with your sarge, Wizards. Rocketman

domo arigato @knfenimore san.

rocketman Even though “Wizards” is an attempt at humor it does describe my lack of knowledge concerning modern vehicles and how they work. My formal training and dealership experience ended in 1976. I worked on cars for about one more year then got a real job driving vehicles where I actually made money.

BMW has a variable lift on the intake and variable camshaft angle system on each cam on some engines. The variable lift system doesn’t require a throttle plate, idle is a very low lift and wide open, high rpm is the highest lift. Clever and complicated.

A lot of car makers have jumped onto this technology because it has remained reliable and trouble free in most cars. It’s amazing how many “moving parts” we have added to the gas engine and it still performs reliably.

Thanks for posting, that is an interesting animation @knfenimore .

sgtrock21 - I found your post interesting, I started out working in gas stations doing repairs long before there was anything such as Certifications for mechanics. I then went into truck driving, mostly for union companies, our mechanics usually made a few cents an hour more than the city drivers but it didn’t come close to paying for the tools they had to have. When I became a road driver we were paid by the mile and made a lot more money. I always contemplated while I was watching a mechanic lying on his back under my broke down truck with all the slop dripping in his face how much easier it was to break’em than to fix’em.

ASE, with great respect, I believe that only low pressure will affect valve timing, not low level… unless, of course, that level drops too low for the pump’s pickup to maintain a reliable supply.

If I’m incorrect in my understanding, I’d truly appreciate technical guidance.

To add my own thoughts, we often hear from people who try to second guess their manufacturer’s oil recommendations, and even those who never check and even rarely change their oil. Wouldn’t it be great of they understood that oil is not longer used just a lubricant, but a hydraulic fluid as well. Perhaps they’d better realize the importance of monitoring the level and of using the correct viscosity.

As long as the oil pump pickup tube is immersed completely in oil I also don’t see the level as having any effect on the valve timing mechanism. Of course if the level is borderline and sloshing occurs due to movement of the vehicle I can see cavitating happening and problems developing.

It would be similar to a poor fitting filter or warped valve body in an automatic transmission which then causes bubbles to occur and which would then affect fluid pressures and so on.

Of course if the engine is chronically low on oil that brings the possibility of oil sludging or coking into the situation due to overheated oil and can possibly screw up the works with the VVT.

The Honda VTEC systems that use two sets of lobes on the camshafts only require 7psi of oil pressure to operate. I remember sitting in class learning about that system and the instructors saying that with only 7psi needed low oil pressure would never cause a malfunction, because any running engine has more than 7psi.

Now 20 years later we regularly see cars 2 or 3 qts low and VTEC faults that are fixed by adding oil. On the Hondas it is my opinion that when the engine is 2 or more qts low there just isn’t enough oil to get way up to the top of the engine and push the rocker pistons back and forth to actuate properly. But the pressure never drops low enough to turn on the oil light.

Nissan V6 engines also seem to be sensitive to oil level/volume. I’ve seen them come in with valve timing codes and noisy chain/valvetrain, adding oil to the proper level fixes the issue, also with no oil light ever coming on.

Because the oil light is an accurate indicator of oil pressure, right?

Interesting information, and I thank you. Also an excellent reason to monitor one’s oil level.