As i've been told it is the the volume from the position of the piston at the bottom of the stroke to the position at the top of the stroke? is that correct?

if it is, then the volume between when the piston is at the top of the stroke to the top of the head is not included in the displacement figure?

i got some feed back here
https://honda-tech.com/zerothread?id=734986

i was asking why larger/smaller pistons supposedly only change the compression and not displacement...if anyone with some clear definitions could pop in on the post...plz plz...thank you, thank you.

I'm sure i'll have some picky questions, so put your patience hats no

 

The volume of the combustion chamber (above the piston at TDC) is NOT included.

Displacement is equal to: (pi/4) * (bore)^2 * (stroke) * (# of cylinders)

 

Alrighty, thanks.

So if we could go a little further...

If you got a low compression piston, there would actually be more room in the cylinder, but by definition the "displacement" would not actually change.

Would the "actual space" in the cylinder increase by much, with lower compression pistons? or is that why it'n not considered...because the space between the top of the stroke and the head is so minimum its just not included?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by issues4 &raquo;</TD></TR><TR><TD CLASS="quote">If you got a low compression piston, there would actually be more room in the cylinder, but by definition the "displacement" would not actually change.

Would the "actual space" in the cylinder increase by much, with lower compression pistons? or is that why it'n not considered...because the space between the top of the stroke and the head is so minimum its just not included?</TD></TR></TABLE>

If you get a low compression piston you are NOT changing the displacement in any way, you are just changing the compression.

The space in the cylinder would increase with lower compression pistons so that when the piston moves the same distance it did with the higher compression pistons, it no longer compresses it to the point that the higher compression pistons did, thus resulting in lower compression pistons. If you didn't understand what I just said, you can calculate the "combustion chamber" volume using the displacement of one cylinder and the compression of that cylinder. Divide the displacement by the compression to get a variable we will call "A". Multiply "A" times the compression plus one, or "A x (compression + 1)", to get a variable we will call "B". Subtract the displacement from B and you will get the combustion chamber volume. You will find that when using the same displacement, the lower the compression, the higher the combustion chamber volume will be.

 

Thank you so much, math and all makes very good sense.

 

I wonder why combustion chamber size is never a concern for people around here...after all, the more air you an get in, the bigger the boom....strange....

anyone know why?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by issues4 &raquo;</TD></TR><TR><TD CLASS="quote">

i was asking why larger/smaller pistons supposedly only change the compression and not displacement... </TD></TR></TABLE>

Be careful with your wording. Larger pistons would mean overbored pistons which would change displacement.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by issues4 &raquo;</TD></TR><TR><TD CLASS="quote">I wonder why combustion chamber size is never a concern for people around here...after all, the more air you an get in, the bigger the boom....strange....

anyone know why? </TD></TR></TABLE>

Most people do with what they got. It's hard to determine the ideal combustion chamber size for any one application (except for those guys with years of experience.)

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by issues4 &raquo;</TD></TR><TR><TD CLASS="quote">I wonder why combustion chamber size is never a concern for people around here...after all, the more air you an get in, the bigger the boom....strange....

anyone know why? </TD></TR></TABLE>Just for now, pretend we can ignore everything about non-steady flow, reflection of pressure waves, & all that junk. So this is a pretty crude explanation...

When the piston comes up during the exhaust stroke, it can't push ALL the exhaust out. The combustion chamber volume is still there, filled with exhaust.

Now the exhaust valve closes, the intake valve opens, & the piston goes down. Sucking in a volume of air/fuel equal to the DISPLACEMENT.

 

That is a very good point (ponders for a moment...)

 

In a performance engine you always want the smallest chamber possible for several reasons.1st-smaller chambers=faster burn rate.2nd smaller chambers make higher compression ratios easier to obtain.