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deck height - cc' vs calculation
https://slantsix.org/forum/viewtopic.php?t=43886		 Page 1 of 1
Author:  gtdart [ Sat Mar 05, 2011 8:35 am ]
Post subject:  deck height - cc' vs calculation
Wanting to raise my compression ratio from 7.8:1 - 9:1, I had may head shaved. After re-measuring the head, I got 9.36:1, a little too much. Seeing how far I was off, I decided to repeat all of my compression measurements to insure accuracy. I have flat top pistons.

When measuring the deck height and doing the cylinder volume math, I get 28.5cc. When measuring with a graduated syringe (same method for cc'ing head volume), I repeatedly get 31cc. This discrepancy yields a difference of 2.5cc which put me at 9.1:1 compression.

I suspect I'm finding the additional volume because the syringe method includes the chamfers of the piston and bore as well as the area between the top of the piston and the compression ring.

Based on my cam choice (RV-10) 9.3:1 CR is pushing my DCR into 8.5:1 territory where 9.1:1 is much more comfortable.

Is the discrepancy I'm seeing between the syringe method vs cylinder volume calc valid?

thanks -shane
Author:  sandy in BC [ Sat Mar 05, 2011 4:17 pm ]
Post subject: 
Id listen to the syringe.....math is always subject to variables ....where volume of a liquid is pretty constant.

Good on ya for checking your math and finding a closer measure.
Author:  gtdart [ Sat Mar 05, 2011 6:34 pm ]
Post subject: 
That was the opinion I was hoping for... although it's easy to believe in something I WANT to be true! :wink:

With the "cylinder volume calc" being the most widely used method of determining deck height volume, does anyone really know what their true compression ratio is? I recognize that it's not that big of an issue unless you've gone too far and that last 2.5cc suddenly becomes so precious.

Thanks Sandy. I'm gonna stop fussing with this issue and close this motor up tonight!

-shane
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