Skatethesea

So I am on deployment and instead of lifting weights or sleeping i am readin as much as possible to better understand how my baby works.

So i am to the point of Static and Dynamic compression. I understand the algorythems to figuring the SCR and am getting into the cam timing portion of it all. Now my question lies in the compression/power stroke cycle. The intake valves do not completely close until a certain part of the compression stroke. Letting what would be compressed air and fuel vent out and leak by. By advancing the cam timing you can prevent this to a certain degree. But obviously our engines and all others are designed to do this for a reason.

I was wondering why you would want to let the air/fuel mixture escape and where does it go? Wouldnt this cause back pressure on the intake side of the head. Or is it going out the PCV, which is the only thing i can think of.

I hope this isnt a noob question. I am just curious as weird times of night.

(that sounded gay)

Skate

RedY2KS2k

The incoming air column has mass and velocity, thus momentum. As the compression stroke begins, there's a short period where the momentum of the incoming air overcomes the push of the compression stroke and air continues to ENTER rather than being pushed out. Eventually, the force of the compression will overcome the momentum of the incoming air. You close the valve at or before that point.

At higher RPM, the column of air has greater velocity, thus you want to leave the valve open a bit longer to take advantage of this greater momentum.

"Miller Cycle" is a whole different discussion.

ahrmike

this is more noticeable @ higher RPMS - thats why mroe agressive cams have lopey idles (non variable timing cars) - because not only does the intake cam stay open longer than "jsut at the beginning" of the stroke, but the exhaust also overlaps with the intake, making it appear to have "bad compression".

SgtB

It also lets velocity pull more exhaust out. The piston doesn't pull or push directly. There is a lag on both intake and exhaust.

Skatethesea

very informative. I appreciate that. i am going to keep this thread open and add to it as i find more puzzling facts. I appreciate your input. I hope that you dont become bothered.

SgtB

Not at all.