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Car Forum / Porsche / Porsche Cars / January 2004Vario Cam
What exactly is this, and how does it work?? > What exactly is this, and how does it work?? http://www.google.com/search?hl=en&ie=ISO-8859-1&q=variocam DS Dan, The genius of Porsche come through again... Basically the variocam (introduced on the 968 in 1992) is a system in which the Motronic computer adjusts the valve timing depending on situation. It would retard on start up or in the cold and advance during hard driving. It accomplished many things. Fuel economy, motor efficiency, reduction of motor noise and the ability to use lower grades of gasoline without detonation or pinging. Simply put, it varied the valve timing. Good enough ? A 968 owner would probably give better insight than I could. I want to try out a 968 and check the difference with my S2 which basically ran the same motor minus Variocam. > What exactly is this, and how does it work?? >Basically the variocam (introduced on the 968 in 1992) is a system in which >the Motronic computer adjusts the valve timing depending on situation. It [quoted text clipped - 3 lines] >pinging. >Simply put, it varied the valve timing. The main point is that it changes the relative timing between the inlet and exhaust valves, rather than the usual advance/retard, which has existed since about the 1930s and just changes the timing of the spark relative to the piston position.  SignatureP >>Basically the variocam (introduced on the 968 in 1992) is a system in which >>the Motronic computer adjusts the valve timing depending on situation. It [quoted text clipped - 8 lines] > has existed since about the 1930s and just changes the timing of the > spark relative to the piston position. And in 2002 the normally-aspirated Carreras got "VarioCam Plus" which also modifies valve lift in addition to timing....... > Dan, > The genius of Porsche come through again... Wonder how long before Chevy puts this on the Corvette and calls it "all new" ? > Basically the variocam (introduced on the 968 in 1992) is a system in which > the Motronic computer adjusts the valve timing depending on situation. It [quoted text clipped - 5 lines] > > Good enough ? Yup, just the explanation I wanted to hear....basically like the Honda VTEC, except it works at all engine speeds rather than an on/off switch. > A 968 owner would probably give better insight than I could. I > want to try out a 968 and check the difference with my S2 which basically > ran the same motor minus Variocam. That S2 is a sweet ride. I checked one out prior to buying my 86 944, wish I would have spent a few extra bucks for it. Amazing what they can do with a four cylinder! > > What exactly is this, and how does it work?? > Yup, just the explanation I wanted to hear....basically like the Honda VTEC, > except it works at all engine speeds rather than an on/off switch. I belive that there is a difference - Variocam changes the timing but VTEC actually changes the cam profile. The reasons to do it are the same - but there are advantages and disadvantages to both. When you change the profile you can have more efficient breathing because of longer opening "period" at the different profiles - where with Variocam the opening "period" is the same - what changes is just the relation between the times when the intake and exhaust valves are open. When the engine is turning very fast you actually need to "pre-open" the intake so that breathing will start while exhaust is still happening - but the time it is open is still the same with Variocam where with a different lobe profile - you can control both the timing and the duration of the open period. On the other hand - Variocam is a simpler system and thus easier to manufacture and probably more reliable (not to mention that it is more linear). I belive that in Honda's newer i-VTEC they also change the cam profile on the exhaust valves. To the best of my knowledge - Ferrari actually goes the "VTEC" route only with very expensive cam lobes that have very gradual changes and a system that positions them based on RPM - so they enjoy the benefit of gradual changes and the more optimized profiles at different RPM ranges. I might be wrong however - can't remember where I read it. Ron. >>Yup, just the explanation I wanted to hear....basically like the Honda > [quoted text clipped - 27 lines] > > Ron. Good info on the Variocam, Ron. We've got a 2003 Civic Si and I have no idea how the i-VTEC works, but...... The 2002 and later normally aspirated Carreras now sport "Variocam Plus", which not only varies opening duration but also valve lift. The low valve lift is 3.6MM and the high is 11MM. Timing variations can run up to 40 degrees. > Good info on the Variocam, Ron. We've got a 2003 Civic Si and I have no > idea how [quoted text clipped - 3 lines] > opening duration but also valve lift. The low valve lift is 3.6MM and > the high is 11MM. Timing variations can run up to 40 degrees. Jim, Thanks for the info. I looked it up and it seems that Variocam Plus is basically the same as the lastest VTEC offerings from Honda - they use 2 lobe profiles too and that's how they change the valve lift and duration. They still use timing retardation as well as the original Variocam did - but at some point switch to the different profile. As for i-VTEC - it is hard to get through the marketing speak - it seems that i-VTEC is different on different Honda models. For example - on the RSX-S they do vary the profile on the exhaust valves but on some other models they do not. They also have a system that basically does not use one of the intake valves at low speeds and only opens it at higher speeds - this system is implemented on the latest VFR motorcycle (and maybe elsewhere too). It sounds as if most manufacturers are slowly converging on the way that Honda did it in the original VTEC and adding some smarts. Ferrari's system still strikes me as the most elegant. http://auto.howstuffworks.com/camshaft3.htm Ron. |
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