CC's larger than the piston bore...
10-04-2015, 01:12 AM
#1
CC's larger than the piston bore... How come CC's that become larger than the piston bore don't suffer from flow loss? For example: a 3.898" combustion chamber growing to 4" when the intake valve is un-shrouded? The benefit of un-shrouding the valve is understood. What's not clear is why the step from the outside edge of the CC to the smaller piston bore doesn't cause an airflow interruption large enough to be an added restriction.
Maybe it's that the overlap only takes place over a small area; approx. 25* over the radius?
Maybe it's that the overlap only takes place over a small area; approx. 25* over the radius?
10-04-2015, 10:44 AM
#2
I had the same thought while reading your other thread on head gaskets for after market heads. My opinion is that it will drop flow to lower than it was before it was opened up. In its unopened form it had the smoothest transition possible from combustion chamber to bore. Now with it opened up and having a step in from the combustion chamber to the bore its creating an area of turbulence that is effectively making the bore even smaller in terms of air flow.
10-04-2015, 02:26 PM
#3
My opinion is that it will drop flow to lower than it was before it was opened up. In its unopened form it had the smoothest transition possible from combustion chamber to bore. Now with it opened up and having a step in from the combustion chamber to the bore its creating an area of turbulence that is effectively making the bore even smaller in terms of air flow.
Yah, it's confusing to me when highly respected head porters routinely de-shroud the intake valve; thereby, causing the step to the bore. Because they choose to do it, there MUST be a benefit greater than any loss caused by the step. Otherwise, they wouldn't do it. As noted, above, I have a$$umed that it has something to do w/ the fact that the step is limited to a very short part (along the radius) above the cylinder bore (maybe 1.75"-2" along the radius on the outside of the intake valve).
Have heard of guys porting/sloping the top edge of the cylinder bore (not so far down as to cause a top ring issue) in order to smooth out the effect of the step. That method is a bit extreme for my taste.
10-05-2015, 08:38 AM
#4
Yah, it's confusing to me when highly respected head porters routinely de-shroud the intake valve; thereby, causing the step to the bore. Because they choose to do it, there MUST be a benefit greater than any loss caused by the step. Otherwise, they wouldn't do it. As noted, above, I have a$$umed that it has something to do w/ the fact that the step is limited to a very short part (along the radius) above the cylinder bore (maybe 1.75"-2" along the radius on the outside of the intake valve).
Have heard of guys porting/sloping the top edge of the cylinder bore (not so far down as to cause a top ring issue) in order to smooth out the effect of the step. That method is a bit extreme for my taste.
Have heard of guys porting/sloping the top edge of the cylinder bore (not so far down as to cause a top ring issue) in order to smooth out the effect of the step. That method is a bit extreme for my taste.
Cylinder head guys are emphasizing low and mid lift flow these days. I suppose the deshrouded chamber helps the low and midlift flow numbers, where the cylinder pressure is lowest, and when the valve is fully open to where that step you're talking about may have an affect, it's not really at a time when the affect is detrimental.
10-05-2015, 10:12 AM
#5
Understood. Was referring to a relief/port/slope added only to the area of the edge (previously chamfered or not) directly below the extended CC for the specific purpose of smoothing out the effect of the step. I don't know or understand anything more about it other than what was read while searching. Found guys discussing it on Yellowbullet.
I suppose the deshrouded chamber...to where that step you're talking about may have an affect, it's not really at a time when the affect is detrimental.
10-05-2015, 10:36 AM
#6
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I'm also guessing that the head gasket makes some difference here as well. That step isn't right up against the head due to the head gasket, so there is still some room (.041") for the air to pull through there. That's where I could see a smaller bore head gasket really hurting you, because it would defeat the shrouding and possibly even be counter-productive.