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I think that this phenomenon is a consequence of engine structure. I have noticed the same thing on many cars and trucks. It is the same kind of sensation as when you try to go uphilll slowly in 4th for instance. Essentially there is a low load on the engine when you have engaged the clutch or driving uphill in a high gear, but it is greater than the load at idle (you dont have to move the drivetrain and wheels in neutral) and at low RPM in this kind of situation a relative vaccuum or vapor lock is created in the compression chamber (the pressure is low because the amount of fuel put in is low while the fuel air mix is the same). The engine must work extra then to move the piston down from the chamber and this can cause a vibration in these low load low rpm situations. This happens in all engines at some point in its profile (maybe saabs are designed with lower idle so that fuel consumption is lower at idle, so it seeems like we have to throttle a great deal to engage the drivetrain without this vibration). The solution is to lower piston or chamber size (ie change the engine design) or increase pressure in the chamber (ie add more pedal or downshift to increase RPM and thus fuel delivery and pressure). The new SVC prototype engine elegantly gets around this undesirable effect (changes the size of the compression chamber hydraulically) and the press release nicely describes the phenomenon as 'pumping losses' in the desription (under "1. Reducing the engine displacement - size does matter") of engine design principles. This press release can be viewed here:
http://www.saabnet.com/tsn/press/000318.html
Its taken me a long time to understand this, and I hope my description here helps. Thanks
posted by 138.26.47...
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