Piston Dome Volume & C/R Calc Help Needed

[70rcode]

Hey guys, need a little help Verifying the beauty of the classic 1.0" Down piston Dome volume Calculation is Self-correcting with C/R calculators when piston Ch, piston-deck & block deck height figures are unknown & Zero'd out when plugged into engine C/R calculators to get reasonably accurate (+ -- 0.2) C/R measurement results...Any help is greatly appreciated....thanks,....tom.

 

[451Mopar]

PI*R^2*h
https://www.google.com/search?q=vol...69i57j0l7.10903j0j15&sourceid=chrome&ie=UTF-8

Calculate volume of a 1" height cylinder and the bore size radius (squared) times Pi. That would be in cubic inches, so convert to cc with multiplier 16.3871 to get cc's. At 1" (or what ever distance you want to use where the dome is below the engine deck) fill / cc the chamber to get the volume above the piston and then subtract from the calculated cylinder volume.

 

[33 IMP]

What is your situation? Best way is with an assembled short block, and the heads off the engine. That way , everything can actually be measured. Assuming anything in c.r. calculations can result in a wildly inaccurate result. Guessing at deck height and chamber volume, you can miss actual cr by two points.(not two tenths of a point!)
And most people are doing the dome cc check at 1/2" down now, i'm told. Makes sense, wont need as much fluid, and might not need a refill of the measuring device.

 

[70rcode]

Actually,..how to measure a g2 hemi piston-deck clearance ? There's No piston perimeter flat or lip edge (like in wedge type pistons) to measure off to let me calculate the true C/R ..Maybe someone can share a method ?

 

[70rcode]

Also, this piston's > 0.75" honkin dome height forces me to use the 1.0" down fill distance + the deep plunge cut valve reliefs way below deck is kinda throwing me a curve to measure.

 

[66Satellite47]

The 1" down fill gives you the true "net" volume, accounting for both the dome and the big valve reliefs.

 

[BSB67]

Hey guys, need a little help Verifying the beauty of the classic 1.0" Down piston Dome volume Calculation is Self-correcting with C/R calculators when piston Ch, piston-deck & block deck height figures are unknown & Zero'd out when plugged into engine C/R calculators to get reasonably accurate (+ -- 0.2) C/R measurement results...Any help is greatly appreciated....thanks,....tom.

Seems like 451Mopar answered your question, if not, I guess I don't understand the question.

Give us your measured or known volumes

 

[66Satellite47]

I should have said that with a G2 Hemi you can still get the 1" down location, space the piston stop up & measure the 1" drop.

 

[69Bee]

Put a dial indicator w/stand on the block, and with the piston at TDC, rotate the crank until the dial reads 1 inch down. There is no need to measure from the deck to the piston.

 

[70rcode]

Ok guys,.. 4.252" bore x 1.000" tall calculated volume = 232.7 cc -- 145.4 cc burette measured fill = 87.3 cc Calculated Dome volume result combined with 3.75" stroke, 173.0 cc head volume, 0.020" thick head gasket & Zero'd out unmeasured piston deck all plugged into Summit's racing online calculator resulted to 10.6 CR ...done right ?

 

[33 IMP]

Thats what i got with your numbers on wallace, 10.63. It might be slightly below that if you add in a number for the "other" inputs that wallace mentions. Looks to me like you have around 10.5

 

[70rcode]

Agree...10.5 Cr close enough for my needs...Trying to get my ancient (45 yrs ago) bracket days 'cuda running again & making sure major mechanicals are today's wimpy pump gas compatible..Amazing, now that I can actually measure stuff, how much out of spec mechanical issues I'm finding to correct ! Big thx again guys for all the responses & help.....tom.