Calculating compression ratio - a common figure overlooked
03-13-2011, 09:14 PM
#1
Calculating compression ratio - a common figure overlooked When reading another thread where a member was calculating his compression, I noticed that a lot of people were not including the piston volume that exists above the rings.
Most people said:"I have flat tops, my piston volume is 0". This is not correct. If you want to be truly accurate, you have to include that volume above the rings where the piston head is smaller than the bore. This article calls it "piston crevice volume":
http://www.chevyhiperformance.com/te...ide/index.html
Here is the relevant quote from the article:
While the article says:"Typically this will not increase total volume by more than 1 cc", I think an LS1 is probably around 1.2. An LS2 or LQ will likely be slightly more.
Here is an example of a compression calculator's results for an LS6 engine using a piston volume of 1.2cc. This is the calculator used in the example: http://www.kb-silvolite.com/calc.php?action=comp
Most people said:"I have flat tops, my piston volume is 0". This is not correct. If you want to be truly accurate, you have to include that volume above the rings where the piston head is smaller than the bore. This article calls it "piston crevice volume":
http://www.chevyhiperformance.com/te...ide/index.html
Here is the relevant quote from the article:
Speaking of accuracy, there is another small volume that is usually ignored but also contributes to compression. This is called the crevice volume and is the tiny volume between the compression ring and the top of the piston. Typically this will not increase total volume by more than 1 cc, but if you’re looking for complete accuracy, it should be included. The best way to account for crevice volume is to measure the entire piston/cylinder assembly with the engine assembled. Using a flat plastic plate with a hole drilled in the top, you can place the piston ½ inch down the cylinder and then fill that volume from a measured burette. Then compare the volume of a perfect cylinder to the amount you measured. The difference will be the combination of any piston top valve reliefs and the crevice volume.
Here is an example of a compression calculator's results for an LS6 engine using a piston volume of 1.2cc. This is the calculator used in the example: http://www.kb-silvolite.com/calc.php?action=comp
03-13-2011, 09:45 PM
#2
I always include +1cc for this reason but may not be accurate for every single engine. But I include it. The way I was taught. But usually keep to the 5v
I order to use .010-.015 out the bore and thin as possible gaskets. The newer gm are usually .051
I generally shoot for .035-.040 quench on just bout ls engine I assemble for any application
I order to use .010-.015 out the bore and thin as possible gaskets. The newer gm are usually .051
I generally shoot for .035-.040 quench on just bout ls engine I assemble for any application
03-13-2011, 10:43 PM
#3
The compression calculator that I use by Pat Kelly does take that into account. I don't know if he just uses a 1cc fudge factor or takes the bore diameter into account and then works something up. I saw a post by Pat where he incorporated that feature into his calculator.
03-14-2011, 01:09 AM
#4
I've never included it , on purpose, for 2 excellent reasons:
1) When the piston gets up to normal operating temperature, this volume goes away to virtually nothing.
(2) It's better to have the compression slightly lower than you figured, that's safer than having it be slightly higher, like you seem to want.
If you insist on using it, it's not difficult to calculate the exact cold volume, IF you know the exact cold diameter of the pistons at the very top of the sides, the exact bore diameter, and precisely how far down the piston the top ring is.
1) When the piston gets up to normal operating temperature, this volume goes away to virtually nothing.
(2) It's better to have the compression slightly lower than you figured, that's safer than having it be slightly higher, like you seem to want.
If you insist on using it, it's not difficult to calculate the exact cold volume, IF you know the exact cold diameter of the pistons at the very top of the sides, the exact bore diameter, and precisely how far down the piston the top ring is.
03-14-2011, 02:03 AM
#5
I will say after thinking a little more on the subject that this is most likely accurate information. However, if I understand this correctly, that the 1.2cc is a positive volume such as a domed piston would be rated as compared to a negative number for a dished piston, then when calculating it on the Keith Black/Silvolite DCR calculator or any for that matter that the 1.2cc should be entered as negative number just as you enter how far the pistons stick out of the hole for the deck clearance.
Using this method, which I assure you is correct if the 1.2 is implied as a flat dome, then that would put the factory LS6 engine specs used at 10.821 compression ratio.
Using this method, which I assure you is correct if the 1.2 is implied as a flat dome, then that would put the factory LS6 engine specs used at 10.821 compression ratio.
03-14-2011, 08:11 AM
#6
I will say after thinking a little more on the subject that this is most likely accurate information. However, if I understand this correctly, that the 1.2cc is a positive volume such as a domed piston would be rated as compared to a negative number for a dished piston, then when calculating it on the Keith Black/Silvolite DCR calculator or any for that matter that the 1.2cc should be entered as negative number just as you enter how far the pistons stick out of the hole for the deck clearance.
Using this method, which I assure you is correct if the 1.2 is implied as a flat dome, then that would put the factory LS6 engine specs used at 10.821 compression ratio.
Using this method, which I assure you is correct if the 1.2 is implied as a flat dome, then that would put the factory LS6 engine specs used at 10.821 compression ratio.
This is an industry standard being done by every knowledgeable engineer, engine builder and machine shop in the world. No need to re-think it.
03-14-2011, 09:35 AM
#7
Maybe I just misunderstood it. Alright I'm now assuming you're talking about the area inbetween the piston and the cylinder sleeves all the way around the bore from the top ring to the deck is the area you are referring to. Is this correct?
