compression ratio again

[badsix]

i'm still having trouble getting this down. I've used some online calculators but come up with different answerers. i'm having trouble getting a grip on the deck height. my pistons and cylinders are NPR's and i had the domes cut down to fit my 1600n A heads. now I know what the deck height is and my pistons are down in the cyl. a little. but they are machined at a slope almost right out to the cyl. there's like a small triangle shaped area around the piston that's below the top of the cyl.
here are some numbers that I have com up with.
cyl. head vol.= 56.5 cc
cyl. bore = 86.5 mm
stroke = 74 mm
piston dome = 7.7
I got the dome volume by machining a 1" piece of Lexan with a chamber. cc that then put it on the cyl. and with the piston at TDC cc again. I came up with a difference of 7.7 cc for the dome. this also includes the little triangle area below the top of the cyl. so total compressed volume would be 56.5 cyl. head volume - the 7.7 for the piston dome total compressed volume is 48.8. I want to get this right the first time and would like to be around 9.0 to 9.5 I think I'm close but not quite sure.
thanks Jay D. I think this is going to kill me! and its suppose to be fun.

[Don C]

Jay,
You have to figure in the volume for the "deck height" between the top of the piston and top of cylinder. Here's some good reading to confuse you some more:
http://www.precisionmatters.biz/pdf/compression-ratio.pdf

[jacques]

Hi
the methodology used should work since he eliminates the deck height contribution. i question the 86.5 using 86 i calculate 9.8 CR
j

[badsix]

Jacques, i'm not correct with the 86.5 IT SHOULD BE 85.5
thanks Jay D.

[badsix]

DON, thanks good information, I've been reading it over and over.
Jay D.

[badsix]

I think I have a problem figuring the dome area, my tool is reading with the piston about 2mm below the deck. i'm going to put the piston up flush with the top of the cyl. then use my tool to figure the area of the dome. so yes my deck height is 2mm or really close.
Jay D.

[jacques]

if the piston is at TDC and you install the head then measure the volume you can calculate the CR. if you use a dummy head and make the appropriate corrections you have the actual combustion volume you can calculate the CR the deck height is irrelevant.
j

[badsix]

i'm going to try it with the head and see what happens.
here are some figures I did today
bore = 85.5 mm
stroke = 74 mm
swept v = 424.87
deck height = 2.25 mm
deck height v = 28.26 cm
head v = 56.5 cm
piston dome = 15.5 cm
so as per Ron LaDows how to find your compression, this is what I did.
I added the deck volume 28.26 to the head chamber volume 56.5 this = 84.76 net volume
then I subtracted the piston dome v of 15.5, 84.76 - 15.5 = 69.71 new net volume.
so 69.71 + 424.87 = 494.58 dived by 69.71 = CR of 7.094 is this correct?
Jay D.

[jacques]

hi i get 12.92 for the deck height contribution.
j

[badsix]

AH YES YOU ARE CORRECT HERE IS WHAT IT SHOULD BE.
BORE = 85.5
STROKE 74
SWEPT VOLUME = 424.87
DECK HIGHT = 2.25 MM
DECK V = 12.92
PISTON DOME V. = 15.5
CYL HEAD V. = 56.5
so if I
add 56.5 head v. to 12.918 deck v. = 69.418 - 15.5 dome v. = net v. 53.918

so 53.918 + 424.87 = 478.788 divide by 53.918 = CR-8.8799
could someone please check if my figures are correct
thanks Jay D.

[jacques]

I GET 8.88

[badsix]

GREAT, thanks Jacques. now things are starting to mean something. today i'm going to put the head on and check the volume just for kicks. i'm also going to recheck all my measurement.

now that I have a figure to work on I would like to up the compression to around 9-1
the 8.88 was with NO base gasket so add 1 base gasket apr.1.45cc or a net v. of 55.368 lowers it down to 8.67 then cut .50mm from the head gets me 2.90 cc net v. of 52.468

53.918+1.45 base gasket= 55.368 net v. now -.50mm= 2.90cc from the 55.368 = 52.468 new net v.
so 52.468 + 424.87 = 477.338 divide by 52.468 = new CR-9.097
just right I believe, any thoughts
thanks Jay D.

[bbspdstr]

Jumping in...and then quickly out: Don't forget that it all means something if all 4 are the same, starting with CC-ing each combustion chamber and assuming the stamped number is meaningless unless proven correct. Also factor real fuel octane, etc, etc. Anything over 9 to 9.5 CR is fine even with today's 'unpredictable' fuels. But then, I don't build engines anymore...so I'll butt out. Good luck,

Bruce

[badsix]

i'm buggered up with a back injury, hope to have surgery in November early December. in the meantime I should be able to get going on the engine again. I have some minimal body work and a lot of misl. small stuff. but that will have to wait until I recover. I had to put the Porsche on hold. as we bought a foreclosed house about a year ago to flip or rent and that's used up all my time. yep 9.0 to 9.5 is what i'm shooting for, yes i'm going to recheck all my measurements I want to put this together one time only.
Jay D.

[jacques]

Hi
i would use the copper base shim and be happy with the compression. if you measure the volume assembled to the of the spark plug deduct 2 ml for the plug. cutting down cylinders is tricky it should be done with two cutting tools like turning a rotor. if your heads are still marked say 58 you can add an additional base gasket. i am not a fan of getting a certain CR at the expense of the geometry.
cheers
j