i modifed a set of je pistons for my b18a....i removed all the sharp edges and gave the whole top a once over to smooth it out...now the amount of material removed was not a lot but definitely enough to change the compression....what id like to know is if there is a way for me to calculate the volume of the modified dome? all the pistons weigh within 1-2.5 grams of one another and id like to measure each one to make sure that the dome volume is even across the board....thanks in advance

this may seem dumb, but what if one of the pistons was .1-.2 CC less than another one...what would be the outcome? more prone to detonation,harder to tune,erratic operation? thanks

 

If you were just using an abrasive roll or something similar youll probably be fine. 1 or 2 cc's difference wont do much if anything. The way to calculate CC volume is to place the piston a few inches into the bore, and calculate what the volume would be if the dome were completely flat (in cc's). Lets say it should displace 200cc's. (the numbers mean nothing, just to make it easy.) Now take a burrette and fill the cylinder with liquid and note the reading when youre done. If it took 193cc's then you have a 7cc dome.

 

cool thanks bro....

a quick question for you...since there is a gap between the piston and the cylinder wall, will water be able to leak past and give an inaccurate reading?

also the water will get into the ring gaps as well....is this part of the formula or do you need to compensate for it?

 

Whats the difference between compression height and dome?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Suck my DX &raquo;</TD></TR><TR><TD CLASS="quote">cool thanks bro....

a quick question for you...since there is a gap between the piston and the cylinder wall, will water be able to leak past and give an inaccurate reading?

also the water will get into the ring gaps as well....is this part of the formula or do you need to compensate for it?

</TD></TR></TABLE>

Use vaseline on the sides of the pistons to keep the water from leaking out.
Make sure you clean it out of the piston and cylinder wall very well.

 

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

Use vaseline on the sides of the pistons to keep the water from leaking out.
Make sure you clean it out of the piston and cylinder wall very well.</TD></TR></TABLE>
great idea!

now the only part that confuses me is at which point i should start measuring? do i want the water to be filled starting from the first ring, second ring, top of skirt?
where does dome volume start?

sorry if i am incorrect with the terminology...just trying to understand the process

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by spun Vtec &raquo;</TD></TR><TR><TD CLASS="quote">Whats the difference between compression height and dome?</TD></TR></TABLE>

compression height is from the center of the wrist pin hole to the top of the dome

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by doood &raquo;</TD></TR><TR><TD CLASS="quote">compression height is from the center of the wrist pin hole to the top of the dome</TD></TR></TABLE>
I believe you are mistaken. Compression height is measured from the centre of the wrist pin to the deck / top outer edge of the piston.

 

so where is dome volume measured from?

 

Anything from the deck / top outer edge of the piston to the top of the dome.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Oyvind Ryeng &raquo;</TD></TR><TR><TD CLASS="quote">Anything from the deck / top outer edge of the piston to the top of the dome.</TD></TR></TABLE>
cool, thanks bro!

edit* so the valve reliefs are or are not calculated into dome volume?

 

Yes, they are. The dome adds to the compression ratio, valve relief takes it away. If the spec sheet says you have a so-and-so volumious dome, that measurement has already subtracted the volume of the valve reliefs. If it didn't, the compression ratio stated on the sheet would not match reality. The expextion is if you have a piston without valve reliefs, then the spec sheet will tell you the exact volume of the dome.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Oyvind Ryeng &raquo;</TD></TR><TR><TD CLASS="quote">Yes, they are. The dome adds to the compression ratio, valve relief takes it away. If the spec sheet says you have a so-and-so volumious dome, that measurement has already subtracted the volume of the valve reliefs. The expextion is if you have a piston without valve reliefs, then the spec sheet will tell you the exact volume of the dome.</TD></TR></TABLE>
thanks for clarifying that for me

 

Don't thank me yet; I'm not a 100% on this - just another forum-riding internet superhero.

 

well at least it gives me a good idea...im gonna stop by the machine shop tomorrow and see what he has to say...all in all it sounds like a pretty simple process....pretty straighforward plus it gives me info on the pistons that i can always refer back to if they last.

 

Well, best of luck to you on your build all the same. But if the difference between the original and the modified piston is in the area of just ~0.2 cm³, that'll just have a marginal effect on the compression ratio. A stock PR3 combustion chamber has how big of a volume? Over 40 cm³, isn't it so? Add the volume of the headgasket, and we're talking only a very minor change to the compression ratio.

 

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

compression height is from the center of the wrist pin hole to the top of the dome</TD></TR></TABLE>
After Looking further I can confirm that doood's comment was incorrect. <TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Oyvind Ryeng &raquo;</TD></TR><TR><TD CLASS="quote">
I believe you are mistaken. Compression height is measured from the centre of the wrist pin to the deck / top outer edge of the piston.</TD></TR></TABLE>

Thats right. The piston deck. Not to be confused with the deck of the block.

Im still a little unclear If this is true:

The dome Is the total height of the dome of the piston with account for the valve reliefs subtracted out.?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Oyvind Ryeng &raquo;</TD></TR><TR><TD CLASS="quote">
I believe you are mistaken. Compression height is measured from the centre of the wrist pin to the deck / top outer edge of the piston.</TD></TR></TABLE>

thanks for the correction

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by spun Vtec &raquo;</TD></TR><TR><TD CLASS="quote">
Im still a little unclear If this is true:

The dome Is the total height of the dome of the piston with account for the valve reliefs subtracted out.?

</TD></TR></TABLE>

no it has nothing to do with height .... from what i understand its the total of volume displaced by the dome and the recession of the valve reliefs(which will be negative)...of course the dome of a high compression piston will have more volume than the total of the recessed area so the positive remainder will be the dome volume...

combustion contraption explained the process to measure dome volume in the second post...

the only thing im kinda confused about though is that people in this thread say you measure dome volume from the edge of the piston deck up....now doesnt compressed air also go between the sides of the piston and cylinder wall till the rings? or does that volume not equate into dome volume measurments?

 

If you want to get that nitty-gritty you might as well try to measure dynamic compression.
The space between the piston and the cylinder is negligible at best don't think so hard.

Just CC your chambers and measure your dome volume and do the math.

 

compression height and dome volume are in no way related.

like you said before, compression height is measured from the center of the wrist pin to the deck of the piston. if you can imagine your piston flat at the deck of it, then your dome volume would be zero. adding dome will increase the volume, and valve reliefs take away from it. i dont know if that will help you understand or not

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Phil M &raquo;</TD></TR><TR><TD CLASS="quote">If you want to get that nitty-gritty you might as well try to measure dynamic compression.
The space between the piston and the cylinder is negligible at best don't think so hard.

Just CC your chambers and measure your dome volume and do the math. </TD></TR></TABLE>

cool thanks....lol id rather over think it than underthink it and lead to problems later cuz my measurments were inaccurate ya know. but thatnks for reassuring me the space is negligible...


yeah i definitely do need to cc the chambers, since ive had a valve job done in the past and dont know how deep they sit in the seat...also i believe the head has been resurfaced at some point, so id like to get an accurate measurment of where it stands now...

im still a bit unsure how to measure dynamic compression, just trying to gather data now and calculate it when i find out....
doesnt dynamic compression have to do with valve timing events and RPM?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Suck my DX &raquo;</TD></TR><TR><TD CLASS="quote">now doesnt compressed air also go between the sides of the piston and cylinder wall till the rings? or does that volume not equate into dome volume measurments?</TD></TR></TABLE>

depending how you measure the volume, that difference should be added to both volumes and therefore cancelled out anyway.

 

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

depending how you measure the volume, that difference should be added to both volumes and therefore cancelled out anyway.</TD></TR></TABLE>
that makes sense... thanks bro!

can you recommend any good books for performance oriented engine rebuilds?

 

does anyone know the variables in calculating dynamic compression ratio? i would think its alot more complex that the static calculations. ive got that down and even made my own calculator, so its time to move on to better things. can someone lead me in the right direction?