If I'm cruising at, say, 120kph on the highway (manualn trans), and abruptly take my foot off the gas pedal, you can feel a bit of a 'bump'. Seems like it's coming from the engine/transmission/clutch. Same if you abruptly put your foot back on the gas. What exactly is that? Something to do with the flywheel or something?

Not worried about it .. I've felt it on every car I've driven. I was driving a Saturn Ion for a couple days (it's crap, BTW), and I noticed the same thing, only it was about 5x more pronounced in the Ion. Just wondering what that was...

 

If by bump you mean you physically feel a bump then it's merely force either being applied or un-applied. Its normal. If its a sound like a bump then I have no clue, heh.

 

It's just a sort of jerk you can feel. Can't hear anything. But it's more than just the force you feel going from a constant velocity to a slight deceleration when you take your foot off the gas. It's almost like it's something to do with the engine stopping producing power, but then being forced to turned anyway by the rest of the car .. something like that.

I know it's normal .. I was just curious as to what exactly caused it, because of the fact that it was SO much more apparent on that Saturn.

 

Yup, what you said is exactly what it is and also what causes it. But if it feels like it loses power thats very odd. However as to why its more pronounced in the Saturn I have no clue. One possibility could be the fact that the saturn has a bit more torque so when force is reapplied its simply jus applying more force thus you feel it more? You would think the Saturn would be less pronounced being that it weighs a bit more so any change in momentum would be reduced. lol, who knows man.

 

So it's sorta like you stop fueling the engine, which in turn stops applying force to the drivetrain, and then the drivetrain 'catches up' or whatever and starts driving the engine?

More torque makes sense .. the Ion has the 2.2L Ecotec, which is actually a pretty torquey engine (only part of the car I liked). The car is less than 100kg heavier, though. It felt much more clunky on the Ion. I'm not sure how to describe it, other than it jerked more and felt a heck of a lot cheaper. I wondered if it had something to do with the transmission or flywheel design or something..?

I suppose it could just be that Honda uses better mounts and does a better job of preventing those kinds of vibrations from reaching the cabin.

 

yeah, a bump ( a slight one) is normal, but you might wanna check your engine/tranny mounts just to be on the safe side.

 

Yea part of that bump is just the motor being jerked back and forth in the squishy rubber motor mounts.