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PostPosted: Thu Sep 05, 2019 4:47 pm 
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Joined: Sun Dec 14, 2014 11:06 pm
Posts: 110
Location: Portland, OR
On the SCCA website:

https://dk1xgl0d43mu1.cloudfront.net/us ... 1462546777

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Ian
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PostPosted: Thu Sep 05, 2019 5:59 pm 
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Joined: Fri Dec 23, 2011 6:59 am
Posts: 1215
Location: Spring Hill, FL.
Ian. If you think you can sort out that sheet, Please run the example I used. Show the math and where the dome of the piston is subtracted from the intake BDC. Thanks. It make very little sense to me as far as the equation goes. The rest is the math to measure the bits.
:Like I said by far the best method is simple. Measure the CC at BDC. Measure the CC at TDC. It's not hard and works well .

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PostPosted: Thu Sep 05, 2019 6:34 pm 
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Location: Portland, OR
My engine also has a protruding dome. I used the previous version of that spreadsheet on my previous builds. It’s just what Jesse said here’s the basics as I would explain it:

-lower piston in block until dome is below deck.
-measure height of piston below deck (a main part not a dome)
-using that “h” dimension and bore, calculate the volume of the imaginary cylinder (image cylinder created by the bore, a plane at the deck and a plane at the top of piston. We’re ignoring the dome at this step.
-seal the sides of piston with grease, seal your cc plate to deck with grease, measure cc of enclosed volume.
-subtract measured cc from cc of imaginary cylinder. This is dome cc.
-measure cc of head
-subtract dome cc from head cc. This is your combustion chamber cc.
-proceed with compression ratio calc.

The spreadsheet does the math behind the scenes on some of that but the process of measuring the dome below the deck is the important part.

This is also spelled out on the instructions on the second tab. It may take trying it for it to make sense.

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PostPosted: Thu Sep 05, 2019 7:09 pm 
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Location: Spring Hill, FL.
I can measure the dome .That is not in question at all ..

Again I ask. Does the equation subtract the dome volume from the BDC or cylinder volume at BDC. ? I dont see any where to subtract the dome .

If it not subtracted, the equation is wrong .
I dont know how to ask any other way.

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PostPosted: Thu Sep 05, 2019 7:16 pm 
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Joined: Sun Dec 14, 2014 11:06 pm
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Location: Portland, OR
Subtract the dome from the volume of the head if you’re thinking of it as you calc a flat top piston. Imagine the dome is glued to the head.

Protech Racing wrote:
The equation should be 445- 6 +40=479/40 = 11.9
.


445+(40-6)=479
40-6=34
479/34=14.1

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PostPosted: Fri Sep 06, 2019 3:31 am 
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Protech Racing wrote:
I can measure the dome .That is not in question at all ..

Again I ask. Does the equation subtract the dome volume from the BDC or cylinder volume at BDC. ? I dont see any where to subtract the dome .

If it not subtracted, the equation is wrong .
I dont know how to ask any other way.

The way Jesse (and others described) does subtract the dome.
The dome is subtracted when you drop the piston a know amount of stroke, cc, then subtract the cylinder volume of that known stroke. The result will be a negative number that matches the dome volume. When entered into the sheet it works out fine.

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Chris Schaafsma


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PostPosted: Fri Sep 06, 2019 4:47 am 
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Joined: Sun Aug 18, 2002 6:52 pm
Posts: 2187
Location: Topeka, Kansas
Chris that's the way I understand it as well. I think we're just having a communication issue. I'm not sure I understand the BDC reasoning as long as you know stroke and bore. You do just as Chris described, drop the piston a specified amount from TDC and then measure volume, deducting the amount of volume you dropped the piston in the bore. Maybe I'm confused. Good discussion though!

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