: Particulars of VVT?

12-30-04, 05:50 PM
How does the VVT system work in the 3.6L V6? What does the actuator look like? What is the control circuit? What is the feedback loop?

12-31-04, 01:46 AM
You've probably seen this already...

he V-6 VVT uses electro-hydraulic vane-type phasers to rotate the camshafts relative to the cam-drive sprockets. Managed by the engine control module (ECM), these vane phasers maximize control and minimize response time, turning the exhaust and intake cams (and cam lobes) in o*ne direction or the other in infinitely variable combinations over a range of 50 degrees. Moreover, the cam phasing system was developed for maximum durability and outstanding noise, vibration and harshness control. It is virtually impervious to particles or contaminants in the engine oil and minimizes the chance that the phasers can stick, even in the most demanding operating conditions. At idle the V-6 VVTs exhaust cams operate at the full advanced position for minimum valve overlap. At other engine speeds over the full range of operating conditions, the phasers adjust cam timing quickly and seamlessly for optimum performance, driveability, fuel economy and emissions control.

The result is smooth, even torque delivery without sacrificing high-rpm horsepower, and excellent specific fuel consumption. Cam phasing also pays big dividends in reducing exhaust emissions by optimizing exhaust valve overlap and eliminating the need for a separate exhaust gas recirculation (EGR) system. By closing the exhaust valves late at appropriate times, the cam phasers force the desired amount of exhaust gas back into the combustion chamber for more complete burning in the next combustion cycle, greatly reducing oxides of nitrogen (NOX) emissions. As a result, the High Feature V-6 meets all emissions mandates without complex, weight-increasing emissions control systems such as EGR and air injection reaction (AIR).

Phasor pics: http://60degreev6.com/albums/36VVT/cutaway_inlet_and_exhaust_cam_phasers.sized.jpg

12-31-04, 05:02 PM
What?? Now we have vtec engines??, cool!! Im guessing these V6 are in the cts? or what, guess i havent read up on the newer cars.

01-01-05, 09:08 PM
What?? Now we have vtec engines??, cool!! Im guessing these V6 are in the cts? or what, guess i havent read up on the newer cars.

The new V6 in the CTS has variable valve timing on the intake and exhaust cams.

The rear wheel drive Northstar in the SRX/XLR/STS also has variable valve timing on the intake and exhaust cams.

This system is not to be confused with the VTEC system on the Honda engines as they are not the same at all.

The variable valve timing system on the Cadillac engines adjusts the cam timing as engine RPM and load change to optimize torque/HP/emissions/idle quality. This sytems only adjusts cam timing not valve lift or the valve event....i.e..the lift and duration of the cam lobe stays the same...just the timing of the cam lobe is adjusted. The VVT system allows nearly instantaneous adjustment of the cam timing over a wide range and in nearly infinite increments. Since the intake is controlled separately from the exhaust this system also allows the valve overlap to be adjusted as well as simply the intake and exhaust cam timings. The advantage to this system is that individual cam timing and valve overlap can be adjusted to a nearly infinite combination allowing idle quality, emmissions, torque and power to be optimized depending on a variety of speed/load combinations. The disadvantage is that the system is "stuck" with the lift and duration of the cam lobe on the cam shaft as it can only change the cam timing not the lobe.

The VTEC system on the Honda engines uses two separate cam lobes for each valve lineup. The cam followers for the two different lobes are connected/disconnected via a mechanism operated by engine oil pressure under control of the PCM to use one of the cam profiles or the other. One cam profile is a "low speed" profile for good torque and idle quality. The other cam profile is a "racing cam" profile that is good for high engine speed operation. As the engine RPM increases the PCM switches from one profile to the other. The advantage of this system is that is allows the actual cam profile and valve event to change...i.e..more duration and more lift for the hot rod "power" profile. The disadvantage is that cam timing changes are limited to the different locations of the two cam lobes and that the system can only select between one of the lobes or the other.

Both systems fall under the guise of "variable valve timing" but are quite different both in their operating mechanism and what they do.

Honda and Porsche are just now offering systems that have variable lift as well as variable timing as they are putting BOTH mechanisms in the valve train to allow selecting cam lobe profile AND adjusting intake and exhaust cam timing simultaneously.

Understand that none of the variable valve timing mechanisms "create" any power. If the engine system is optimized for peak power the variable sytems will not make the lift or timing any "better" so it will not change the peak power point. BUT, by changing the timing or lobe profile, the systems can broaden the power band and allow the engine to make more low end power (with the VVT) than it would have normally if restricted to the valve arrangement selected for peak power.

In the case of the Northstar RWD engines the VVT allows the engines to make the peak power of an L37 engine while maintaining the low end torque of the LD8 engine. Putting more area under the torque curve will always accelerate the vehicle faster so it is a net performance improvement but the peak power numbers do not necessarily reflect the changes from the VVT.

The other real advantage of the Cadillac VVT system is that by adjusting the exhaust cam timing judiciously it allows emissions control (NOx specifically) without an EGR valve or external EGR system. During the range of operation where NOx generation is an issue the exhaust cam timing is severely retarded so as to allow exhaust gas reversion from the exhaust port back into the cylinder thus supplying "EGR" for the next cylinder event. Much more controlable, more precise, better quality of EGR gases and no EGR system to plug over time. Plus a simpler emission control system with no external EGR system to plumb or control.

01-02-05, 11:24 AM
Wow thats awesome info.

Never knew honda had 2 different cams running, or posiitons i guess.

Well the engines are going continuously variable, when do the trannys start??

I remember reading an artilce a few years ago with a cvt tranny in some foriegn car i believe, that would rule!!

01-02-05, 08:00 PM
Thanks Bbob, I knew that Mercedes had a centrifugal cam timing adjuster back in the '30s an some racing straight eights. Also, I experimented with a kit for advancing and retarding cam timing on small block Chevies back in the 70's that operated on engine oil pressure. It came in two versions. One responded to a manual control and the other was automatic on-the-fly.