How many degrees of overlap can you have before you have to start worrying about bleeding off compression?

 

static compression ratio won't change. at low rpm, anything lower than about 112 degrees LSA will cause a noticable difference

 

Basicaly you want no more than +/- ~8.50 DCR

 

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.

 

Thank you for emancipating

 

Your welcome ... I think (now where is that dictionary)

 

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.

 

Damn i think you just rephrased my question with the correct terminology, lol

 

You are correct! I used the wrong term. Thanks for the correction.

 

(blush) You noticed.

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.

 

Why would you care about loosing compression due to overlap?
Put in a big stick and when the tach is at 6000 and climbing fast, you will never miss it.