rcldesign

Ok, I have a pretty straight forward question on my mind... how variocam is controled. Does it simply get an RPM based switching signal from the DME (i.e. if RPM > x, change cam profile), or does the cam change all the time based on rpm?

yoyoguy2

i was under the impression that it changes all the time (hence continuously variable) but i would also like to know where the signal comes from and how the changing is engaged....

Travis - sflraver

An in depth answer to this question would be awesome. This is something I have kicked around in the back of my mind for some time now but have never delved to deeply into researching.

led

<a href="http://leo.worldonline.es/jaumepor/angles/tecnica/varicamen.htm" target="_blank">Both of the individual systems making up VarioCam Plus ( camshaft adjustment and valve lift control ) are controlled by the ME7.8 Motronic unit designed especially for these requirements and offering superior computation performance. Particularly engine speed, gas pedal position, engine oil and coolant temperature as well as gear recognition are essential for VarioCam Plus engine management.</a>

rcldesign

My interpritation of that article is that there are essentially 2 cam profiles and the Motronic determines when to use each of them... and from the article I also gathered that the Vario cam only changes lift and overlap (no duration controls?). Someone with a bit more technical know-how please let me know if I'm right or wrong here...

Riff

This is the later Variocam as used in the 993 and later cars. The version used in the 968 functioned differently. For an expaination of the earlier version check out this site:

<a href="http://www.928sg.com/heads.htm" target="_blank">928 Engine mods</a>

Basically the earlier Variocam only changed the advance of the intake cam. He also has some interesting other changes to his 928 motor.

Lift and duration are functions of the cam's profile. Overlap is a function of the intake and exhaust profiles relation to one another. For a nice explaination of this check out: Nice little animation

<a href="http://www.howstuffworks.com/camshaft1.htm" target="_blank">How Stuff Works: Cams</a>

From the later version looks like the Motronic can choose between 2 different profiles, one is low lift/short overlap to improve efficency and the other is a higher lift/longer overlap to improve performance.

HTH,

PorscheG96

Wrong.

The primary physical component of 968 Variocam is a hydraulic tensioner with upper and lower 'ramps' just like the 944 S/S2. At 2500 RPM the Variocam unit receives a 12 volt signal from the DME and is fed oil to pressurize a piston. The lower ramp lifts and geometry of the chain is altered to retard timing of the intake camshaft by 15 degrees. What this does is impart valve overlap to reburn exhaust gases and meet new emissions standards by the DOT. The Variocam tensioner remains 'activated' until 5500 RPM, when the DME cuts the signal and standard timing is resumed [just like on a 944 S2]. Nothing very complicated like the modern variable timing in Variocam Plus, VANOS, VTEC, and the trick stuff used by Ferrari.

Danno

"The lower ramp lifts and geometry of the chain is altered to retard timing of the intake camshaft by 15 degrees."

Yup right on, except that it's advanced in the mid-range for more torque, not retarded. There's only one cam lobe and profile. The Variocam just changes the timing of the intake cam. The timing at idle and high-RPM is retarded compared to the mid-range timing.

PorscheG96

Crap, you caught me Danno!

I messed up...the Variocam is DEactivated between 2500 and 5500 RPM, meaning that the intake cam is in its advanced orientation and there is minimal valve overlap. The Variocam is active UNTIL 2500 RPM [for emissions purposes] and then AFTER 5500 RPM to help push the exhaust gases out for power.

Riff

What part did I get wrong??? I pointed out the 968 variocam only changed the advance of the intake cam ie changed the overlap, not that is could choose between 2 cam profiles....

Phooz Busta

Wasn't replying to you.

jim968

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by Danno:
<strong>"The lower ramp lifts and geometry of the chain is altered to retard timing of the intake camshaft by 15 degrees."

Yup right on, except that it's advanced in the mid-range for more torque, not retarded. There's only one cam lobe and profile. The Variocam just changes the timing of the intake cam. The timing at idle and high-RPM is retarded compared to the mid-range timing. </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Correctomundo as always, Danno. And trust me, you _can_ feel it happen. Not a big kick like coming on boost, but it's there. It's definitely a power-enhancing event.

And for all, yes, it just varies timing between the cams (changes intake, specifically). Totally a binary on-off device. AFAIK, just a solenoid valve allowing oil pressure into the "hydraulic" cylinder in the chain tensioner. Some of the confusion is because Porsche has chosen to keep using the basic name for more complex systems.

Jim, already confused...

Danno

Yah, the later VariocamPlus is more like VTEC with two distinct cam-lobe profiles. A locking mechanism in the lifter then determines which cam-lobe actually drives the valves. But it's still only a binary ON/OFF system like VTEC.

My personal favorite is the 3D cam lobe used in the Ferrari F355 5-valve heads. It provides for continously variable valve-lift, timing, duration (and therefore overlap)! The torque curve on this thing is perfectly FLAT! Fully optimized flow at all RPMs... wow....

roadrunner

Ferrari also uses variable back-pressure too, but I don't remember if it was only on the F360...