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Re: Hey Duke
Yes I remember the Varicam from the sixties. I believe they use a flyweight-spring system - similar in concept to the ignition centrifugal advance mechanism - to retard the cam from a relatively "advanced" base condition as revs increase. (All the OE cams from the sixties except the 962 are considered "advanced" in typical hot rod lingo.) This is a good idea, and the same basic principle is used on some modern engines, but the electronic control and mechanical technology is more sophisticated.
This device was ultimately not successful, and I wonder if reliability had something to do with its lack of success as the concept is sound.
Some refer to these devices - both vintage and modern, as "variable valve timing", which is somewhat of a misnomer. Valve timing does vary, but duration is fixed. What they really change is cam indexing relative to the crankshaft so I refer to them as "variable cam phasers". "Variable valve timing" systems change duration such as Honda VTEC system.
Some modern designs use variable cam phasing on the exhaust side (DOHC designs) only, which is done for emission control purposes. At idle the exhaust cam is advanced to keep overlap low. At cruise the exhaust cam is retarded, which increases overlap and exhaust gas residual. It is effectively an EGR system, but since it's all done internally in the engine, it's more reliable than an external EGR valve system. External EGR valves have a tendency to leak and/or seize.
GM Powertrain has tested prototype LS engines with two in-block cams going back over five years. The only reason for two in-block cams is to segregate the exhaust and inlet valves on to different cams to facilitate variable phasing on one or both. Apparently current emission and performance targets are being met without this technology, but I expect it to appear in a production engine at some point in the future.
Duke -
i ran one of those in my Z-28 with a 140 cam
but the pig tail spring kept breaking but when the spring was intact it workedComment
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What I suspected... *NM*
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