Thread: CR question?

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How do I do the math for Compression?

I would like to check it for a 64cc head and a 72cc head and 76cc head.

Piston TRW H617P .275 dome.

Deck is +.0045(piston below deck. .0045)

Head gasket thickness .051(MLS Cometic)

What else do I need?

Thanks

Jimmy

Originally Posted by JBoss

What else do I need?

Bore and stroke is about all else you need... here's a calculator (there are several on the 'net):
http://www.kb-silvolite.com/calc.php?action=comp

-Chris

Originally Posted by skids72

Bore and stroke is about all else you need... here's a calculator (there are several on the 'net):
http://www.kb-silvolite.com/calc.php?action=comp

-Chris

Thank you.

I hope I got this right.

something is wrong with your piston head volume.....need to enter volume instead of height of pop up..... your entry of 0.275 is not in cubic centimenter units

TRW catalog should give piston dome (or recess) volume.

Thanks Robot, you were posting while I was still typing.....

Not even close Jimmy.
To find static compression ratio, you need 5 values.
1. cylinder volume in cc's
2. chamber volume in cc's
3. piston dish or dome volume in cc's
4. compressed gasket volume in cc's
5. piston deck height in cc's

The main mistake you made was in not finding the dome volume. It is 11.8 cc's and took me nearly a half hour to find, searching through the net. With a dome piston, you deduct the dome volume from the chamber volume, then treat the piston as a flat top with no value.

Here's how the math would look for the 64cc chambers.....

.7854 times 4.03 times 4.03 times 3.48 times 16.387 = 727.4 cc's in the cylinder.

64cc's less 11.8 cc's would equal 52.2 cc's in the chamber.

.7854 times 4.03 times 4.03 times .0045 times 16.387 = .94cc's in the piston deck height.

.7854 times 4.06 times 4.06 times .051 times 16.387 = 10.82cc's in the gasket

0cc's in the piston crown, because we have deducted the dome volume from the chamber volume, so the piston is zero.

We will add the 5 values together. 727.4 + 52.2 + .94 + 10.82 + 0 = 791.36cc's displacement drawn in with the intake valve open.

Now, we will drop out the cylinder volume and add the remaining 4 values together. 52.2 + .94 + 10.82 + 0 = 63.96cc's, the space into which the mixture is squeezed with the piston at TDC and the intake valve closed.

Now, we will divide the larger number by the smaller number and find static compression ratio.....791.36 / 63.96 = 12.37:1 SCR.

Now, following my example, your homework is to figure the SCR with 72cc chambers and with 76cc chambers. Using an online calculator is fine, but if you plan to be a motorhead, you need to know how to do it yourself with pencil and paper.

.7854 is a constant that we use. It is 1/4 of pi.
16.387 is another constant. It is the number of cc's in a cubic inch. When you start off measuring cylinder volume, you find cubic inches. This must be converted to cubic centimeters (cc's) in order to match up with the other values.

With a true flat-top piston (with no valve relief), you do not deduct any value from the chamber. You simply assign a value of zero to the piston and use the chamber volume as it is. In the case of this motor, you would add 727.4 + 64 + 0 + 10.82 + .94 and would find a total of 803.16 cc's drawn in with the intake valve open. Dropping out the cylinder volume and adding the remaining 4 values, you would add 64 + 0 + 10.82 + .94 and find 75.76cc's.
Dividing the larger by the smaller (803.16 / 75.76) would yield 10.60:1 SCR.

With a flat-top piston with eyebrows or a valve relief trough or with a piston using a dish, use the published volume for the piston value. For instance, using a piston with a 12cc dish in this motor we're discussing.....
727.4 + 64 + 12 + 10.82 + .94 = 815.16cc's. Dropping out the cylinder and adding the remaining 4 values....64 + 12 + 10.82 + .94 = 87.76cc's. Now, 815.16 divided by 87.76 = 9.28:1 SCR.

Some of the online calculators can be very confusing because you do not know whether the dome is figured as a negative value or a positive value. This even varies among piston manufacturers, so unless you are prepared to compute the SCR yourself, it's pretty much a guess with an online calc.

Oh chit! I need to find more paper.

Thanks Tech.

Alright. After much hair pulling and thowing pens across the room a few times.

I have came up with a workable CR of 10.58 with a 76cc head useing the info tech gave me.

Thank You!

Originally Posted by JBoss

Alright. After much hair pulling and thowing pens across the room a few times.

I have came up with a workable CR of 10.58 with a 76cc head useing the info tech gave me.

Thank You!

Excellent!! I came up with the same answer.

Now, you are in a small, elite group of enthusiasts who have the knowledge to figure static compression ratio on any motor without using an online calculator. Congratulations!!!

You have the same static compression ratio I do in my 383. Now you need to learn about Dynamic Compression Ratio (assuming you are not knowledgeable already) so you can choose the proper camshaft so you can run on pump gas.

Originally Posted by rumrumm

You have the same static compression ratio I do in my 383. Now you need to learn about Dynamic Compression Ratio (assuming you are not knowledgeable already) so you can choose the proper camshaft so you can run on pump gas.

OK. I need more enlightenment. Please explain "Dynamic CR"

I will be using a comp cam. 270H

Isnt Google wonderful?

http://www.empirenet.com/pkelley2/DynamicCR.html

Originally Posted by robot

Isnt Google wonderful?

http://www.empirenet.com/pkelley2/DynamicCR.html

Thats the same one I was reading. And YES Google is wonderful.

Originally Posted by JBoss

OK. I need more enlightenment. Please explain "Dynamic CR"

I will be using a comp cam. 270H

Go back to the calculator:
http://www.kb-silvolite.com/calc.php?action=comp
Enter all of your information and then plug in your rod length (5.7 inches) and the intake closing angle for the cam (61 degrees) and this will give you your dynamic compression ratio. It is much more accurate than static compression ratio in determining whether your engine will run on pump gas. As a general rule of thumb, iron headed engines should be 8.0 or less while aluminum headed engines should be 8.5 or less. If you want to read up no DCR, check out this thread:
http://www.chevytalk.org/fusionbb/sh...php?tid/92966/