hoodeng wrote: ↑Fri Aug 21, 2020 11:05 pm
What was the manufacturers claimed volume for the crown? Can you wet out a cylinder? Did they have a reason for a two step squish shelf? Who made the pistons? Can you post a photo of the combustion chamber including the head surface?
Your red marked band i would expect be the datum for piston deck height, your blue marked shelf in pistons of this type would be to accommodate a recessed combustion squish shelf.
Cheers.
Hi,
Well, unfortunately none of this information is available from the manufacturer. I wrote to Asso Werke but no reply yet. It seems that these pistons somehow modified by a previous builder and that red area machined afterwards so it is not the original design. Why two step squish I couldn’t figure it out but I believe the blue area is the real squish area by looking at the chamber. I also believe that previous builder adapted this piston to this engine and meanwhile targeted a CR which was not possible unless he took of some volume by machining down that red area. I am having a difficulty sometimes posting photos but I will try now.
Cheers
Do your test with kerosene to the bottom of the plug hole, trans fluid hangs up in the burette, move your assembly to purge any air,. Stick with method 'B' all variables are discounted.
Ahh the old previous owner mods with no apparent reason,, it'll get you every time! we have all been there at some time.
Cheers.
Last edited by hoodeng on Sun Aug 23, 2020 4:18 am, edited 1 time in total.
hoodeng wrote: ↑Sun Aug 23, 2020 4:12 am
Do your test with kerosene to the bottom of the plug hole, trans fluid hangs up in the burette, move your assembly to purge any air,. Stick with method 'B' all variables are discounted.
Cheers.
Yes, I thought ATF was not the correct stuff and I will do it by kerosene again on Monday.
You would have thought he would adjust the comp cutting the crown and retaining the squish shelf ?, unless the under crown thickness is under .200" or so.
hoodeng wrote: ↑Sun Aug 23, 2020 4:21 am
You would have thought he would adjust the comp cutting the crown and retaining the squish shelf ?, unless the under crown thickness is under .200" or so.
I really don’t know but certainly the squish band was neglected in such a machining. I haven’t seen 16v pistons like this before.
I was asked to report the engine’s CR and I am not probably going to use these pistons during the build since one of them has a damaged pin bore and secondly I will have to build the engine with 11:1 max 11,25:1 CR.
The only mistake that I can think of in the method A could be; although I am calculating/measuring the dome volume as described it is the dome volume itself. But due to the fact that the piston’s deck height ( if we accept the red area ) is not actually in the equation since at TDC the deck height is 2.20 mm below the deck.
So this -2.20 mm equates to an additional 12,37 cc of volume. In my opinion to be added to chamber volume and gasket volume then the calculated dome volume will be subtracted from that total.
12,37 + 41,30 + 6,85 = 60,52 cc - 14,57 cc ( dome volume as calculated) = 45,95 cc ( total compressed volume )
This 45,95 cc figure is much closer ( 0,95 cc mistake probably because of the ATF ) to the volume that I found with Method B.
Correct. You don't need the deck height to measure CR by this method. Just start at TDC and move the piston down by a carefully measured distance 'A' and compare the measured resulting volume to that of the calculated value for a pure cylinder of height 'A'. The resulting value combined with the gasket and chamber volumes is all you need for computing CR.
MadBill wrote: ↑Sun Aug 23, 2020 9:58 am
Correct. You don't need the deck height to measure CR by this method. Just start at TDC and move the piston down by a carefully measured distance 'A' and compare the measured resulting volume to that of the calculated value for a pure cylinder of height 'A'. The resulting value combined with the gasket and chamber volumes is all you need for computing CR.
Hi MadBill,
In my 11:00 o’clock post above, I actually did that to find out the absolute dome volume. I found it as 14,57 cc. Since it is a dome it should be subtracted from the chamber vol + gasket vol to find the compressed volume.
But when I did that I have ended up extremely high CR figure as I described in my method A. That CR figure was hugely different than the method B in my post.
But then in my 3:35 pm post above, I took the piston’s deck height into calculation and that produced a CR figure which is much closer to my simple method B again as described in my 11:00 o’clock post.
Agreed, you do not need the deck height when doing the dropped piston in bore method, but you will need an absolute zero deck on assembly for the numbers to be relevant.
Like Hoodeng said, you do need to account for the deck height at TDC. The OP is on the right track there when he add the 2.2mm below deck bore volume to the compressed vol.
Nut124 wrote: ↑Sun Aug 23, 2020 8:41 pm
Like Hoodeng said, you do need to account for the deck height at TDC. The OP is on the right track there when he add the 2.2mm below deck bore volume to the compressed vol.
The deck height volume is included in the figure you get with the piston drop* method and the piston never encroaches on it, so the deck height measurement is irrelevant for the CR calculation. *Recall that the drop in the described method is measured from TDC, not the top of the block. (Of course, the desired deck height must be established before doing any CR calculation.)
Nut124 wrote: ↑Sun Aug 23, 2020 8:41 pm
Like Hoodeng said, you do need to account for the deck height at TDC. The OP is on the right track there when he add the 2.2mm below deck bore volume to the compressed vol.
Huh? He didn’t say that.. he said the opposite
He said if deck ht is zero it does not matter, meaning that if not zero it must be included in the calc. Same thing.
Nut124 wrote: ↑Sun Aug 23, 2020 8:41 pm
Like Hoodeng said, you do need to account for the deck height at TDC. The OP is on the right track there when he add the 2.2mm below deck bore volume to the compressed vol.
Huh? He didn’t say that.. he said the opposite
He said if deck ht is zero it does not matter, meaning that if not zero it must be included in the calc. Same thing.
It doesn’t need a deck height doing the down bore thing
Read how madbill explains it....zero has nothing to do with it
