Cam overlap causes loss of compression?
The amount of compression lost due to overlap is dependent upon when the overlap occurs relative to where the piston is during the compression stroke. In other words, late overlap during the compression upstroke will cause more bleed-off. So valve timing events are critical to determine what the dynamic compression will be.
TECH Fanatic
Joined: Jun 2002
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From: Upstate NY
Originally Posted by nuzee
The amount of compression lost due to overlap is dependent upon when the overlap occurs relative to where the piston is during the compression stroke. In other words, late overlap during the compression upstroke will cause more bleed-off. So valve timing events are critical to determine what the dynamic compression will be.
Overlap occurs when the exhaust is still closing and the intake STARTS to open, right? By the time the piston reaches BDC at the end of the intake stroke, and starts up on the compression stroke, the exhaust valve has been closed a long time and overlap isn't a factor.
The later the intake closes during the COMPRESSION stroke, the more compression is bled off, at least at cranking speed. The intake valve closing (IVC) point is what is used to determing Dynamic Compression Ratio (DCR).
I think this late IVC is what everyone here is referring to, not "overlap" when both valves are open. Right?
Overlap of exhaust/intake does not have a direct bearing on IVC which controls DCR. Of course, if you changed the overlap by altering the LSA and kept the same lobe configuration, you could change IVC and DCR, but that would be an indirect result of varying the overlap. Time to stop.
Not trying to be the grammar police, but specific words mean specific things in engine-speak. You could get pretty confused if you mixed them up. At least you'd get me confused...which is sometimes a short trip.
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Originally Posted by Old SStroker
I'm confused with the terms used in this thread.
Overlap occurs when the exhaust is still closing and the intake STARTS to open, right? By the time the piston reaches BDC at the end of the intake stroke, and starts up on the compression stroke, the exhaust valve has been closed a long time and overlap isn't a factor.
The later the intake closes during the COMPRESSION stroke, the more compression is bled off, at least at cranking speed. The intake valve closing (IVC) point is what is used to determing Dynamic Compression Ratio (DCR).
I think this late IVC is what everyone here is referring to, not "overlap" when both valves are open. Right?
Overlap of exhaust/intake does not have a direct bearing on IVC which controls DCR. Of course, if you changed the overlap by altering the LSA and kept the same lobe configuration, you could change IVC and DCR, but that would be an indirect result of varying the overlap. Time to stop.
Not trying to be the grammar police, but specific words mean specific things in engine-speak. You could get pretty confused if you mixed them up. At least you'd get me confused...which is sometimes a short trip.
Overlap occurs when the exhaust is still closing and the intake STARTS to open, right? By the time the piston reaches BDC at the end of the intake stroke, and starts up on the compression stroke, the exhaust valve has been closed a long time and overlap isn't a factor.
The later the intake closes during the COMPRESSION stroke, the more compression is bled off, at least at cranking speed. The intake valve closing (IVC) point is what is used to determing Dynamic Compression Ratio (DCR).
I think this late IVC is what everyone here is referring to, not "overlap" when both valves are open. Right?
Overlap of exhaust/intake does not have a direct bearing on IVC which controls DCR. Of course, if you changed the overlap by altering the LSA and kept the same lobe configuration, you could change IVC and DCR, but that would be an indirect result of varying the overlap. Time to stop.
Not trying to be the grammar police, but specific words mean specific things in engine-speak. You could get pretty confused if you mixed them up. At least you'd get me confused...which is sometimes a short trip.
TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
Originally Posted by BBADWS6
Damn i think you just rephrased my question with the correct terminology, lol
Any IVC after BDC, and they all are, theoretically bleeds off compression. at least at some engine speeds. That's why DCR is lower than Static Compression Ratio (SCR), of course.
So you design the SCR and IVC point to get the DCR you want.
CAM? (Clear As Mud)
The pun was intended.
