Anyone know how to find area above top ring on a piston?
08-03-2007, 05:03 PM
#1
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Anyone know how to find area above top ring on a piston? Someone explained it to me this way but I need further clarification and how I should measure piston to wall clearance. The guy who gave me this info was not the originator of this info so I am unsure it is even correct.
"The volume above the top ring is small. For example, given that the area of a circular ring is pi (x2 - y2), if the piston to wall clearance is .005" in a 4.030" bore, the area is (4.030^2-4.025^2)3.14 = 0.129in^3 = 2.1cc. This is a large example, as most top rings are less than 0.25" down."
I have a stock lt1 block with stock 4.00 bore pistons and rings that I am trying to figure this out with. thanks in advance
"The volume above the top ring is small. For example, given that the area of a circular ring is pi (x2 - y2), if the piston to wall clearance is .005" in a 4.030" bore, the area is (4.030^2-4.025^2)3.14 = 0.129in^3 = 2.1cc. This is a large example, as most top rings are less than 0.25" down."
I have a stock lt1 block with stock 4.00 bore pistons and rings that I am trying to figure this out with. thanks in advance
08-03-2007, 05:39 PM
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All he is doing is taking the difference between the area of the two circles created by the piston and the cylinder. The problem I see with his math is that it looks like he did Pi x diameter^2 instead of Pi x radius^2.
You can break it down like this...
Find the area of the cylinder, Pi*r^2 (Radius = 1/2 of bore, or 2.015 in your case)
Find the area of the piston, Pi*r^2 (Radius = 1/2 of piston diameter, or 2.0125)
Subtract piston area from cylinder area. This leaves you the area of the piston-to-wall clearance, in inches. Now take this and multiple it by the "top ring down" distance, and that will give you the volume of the top ring down.
I get .0041 in^3 or .0672cc. Someone check my math?
You can break it down like this...
Find the area of the cylinder, Pi*r^2 (Radius = 1/2 of bore, or 2.015 in your case)
Find the area of the piston, Pi*r^2 (Radius = 1/2 of piston diameter, or 2.0125)
Subtract piston area from cylinder area. This leaves you the area of the piston-to-wall clearance, in inches. Now take this and multiple it by the "top ring down" distance, and that will give you the volume of the top ring down.
I get .0041 in^3 or .0672cc. Someone check my math?
Last edited by thunder550; 08-03-2007 at 05:46 PM.
08-03-2007, 10:56 PM
#3
TECH Fanatic
Originally Posted by thunder550
All he is doing is taking the difference between the area of the two circles created by the piston and the cylinder. The problem I see with his math is that it looks like he did Pi x diameter^2 instead of Pi x radius^2.
You can break it down like this...
Find the area of the cylinder, Pi*r^2 (Radius = 1/2 of bore, or 2.015 in your case)
Find the area of the piston, Pi*r^2 (Radius = 1/2 of piston diameter, or 2.0125)
Subtract piston area from cylinder area. This leaves you the area of the piston-to-wall clearance, in inches. Now take this and multiple it by the "top ring down" distance, and that will give you the volume of the top ring down.
I get .0041 in^3 or .0672cc. Someone check my math?
You can break it down like this...
Find the area of the cylinder, Pi*r^2 (Radius = 1/2 of bore, or 2.015 in your case)
Find the area of the piston, Pi*r^2 (Radius = 1/2 of piston diameter, or 2.0125)
Subtract piston area from cylinder area. This leaves you the area of the piston-to-wall clearance, in inches. Now take this and multiple it by the "top ring down" distance, and that will give you the volume of the top ring down.
I get .0041 in^3 or .0672cc. Someone check my math?
FWIW:
With a 4.030 bore and a 4.025 piston with the top ring .250 down and no chamfer on the piston, I got .1296 cc or about twice what you got. You need to use the .250 height.
Anyway, even at .13 cc does it matter in the whole scheme of things? Even if you are using it to figure SCR, the difference in swept volume (displacement) of a 4.03 (nomional) bore a 3.48 stroke changes about .454 cc for every .001 extra bore, or 3.5 times the volume you are talking about.
Don't get lost in the math.
Jon
08-04-2007, 09:13 AM
#4
Originally Posted by White_LightningZ
Someone explained it to me this way but I need further clarification and how I should measure piston to wall clearance. The guy who gave me this info was not the originator of this info so I am unsure it is even correct.
"The volume above the top ring is small. For example, given that the area of a circular ring is pi (x2 - y2), if the piston to wall clearance is .005" in a 4.030" bore, the area is (4.030^2-4.025^2)3.14 = 0.129in^3 = 2.1cc. This is a large example, as most top rings are less than 0.25" down."
I have a stock lt1 block with stock 4.00 bore pistons and rings that I am trying to figure this out with. thanks in advance
"The volume above the top ring is small. For example, given that the area of a circular ring is pi (x2 - y2), if the piston to wall clearance is .005" in a 4.030" bore, the area is (4.030^2-4.025^2)3.14 = 0.129in^3 = 2.1cc. This is a large example, as most top rings are less than 0.25" down."
I have a stock lt1 block with stock 4.00 bore pistons and rings that I am trying to figure this out with. thanks in advance
The lt1 top land thickness according to my sources is about .230". Let's calculate the volume of a 4.000" bore @ .230" deep, - 4.000" x 4.000 x .230" x .7854 x 16.39 = 47.37 cc's.
Assuming ~1 degree land taper (most aftermarket shelf piston manufacturers will be close to this spec) we will be ~3.960" top land diameter at the bottom and ~3.952" at the top of the top land. Using 3.956" as an average land diameter above the top ring we can find the volume of the top land of the piston. 3.956" x 3.956" x .230" x .7854 x 16.39 = 46.33 cc's
47.37 - 46.33 = 1.04 cc's
Keep in mind that this is a cold calculation, as the piston gets to operating temperature this volume is much less due to expansion.
08-04-2007, 06:31 PM
#5
Top lands will swell to within a few thousandths of the bore under running conditions.. It's not something to look at when calculating a compression ratio because your results will be off if you use the static number.
08-16-2007, 02:43 PM
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I dunno about you guys, but I never put the area above the top ring in the mix, as I don't see it making any more of a difference than it would if an overzealous dude at the SRP factory cut a valve notch just a kiss too deep.
And I'm with briannutter, it's really impossible to tell EXACTLY how much the piston will swell.
Doing the math using LSPerformance's numbers, a flat top piston with no notches, a gasket I made up (but used the same gasket each time), .008 in the hole, a 64cc chamber, and a piston that is made of unobtanium and doesn't swell, we have 11.0593:1 without the top ring land area, and 11.0742:1 if we factor in the top ring land area. That's a difference of .0149:1, even less once we're up to operating temps... So that's the first and last time I'll ever figure the top ring land space.
As for finding piston to bore, OldSStroker has it covered.
And I'm with briannutter, it's really impossible to tell EXACTLY how much the piston will swell.
Doing the math using LSPerformance's numbers, a flat top piston with no notches, a gasket I made up (but used the same gasket each time), .008 in the hole, a 64cc chamber, and a piston that is made of unobtanium and doesn't swell, we have 11.0593:1 without the top ring land area, and 11.0742:1 if we factor in the top ring land area. That's a difference of .0149:1, even less once we're up to operating temps... So that's the first and last time I'll ever figure the top ring land space.
As for finding piston to bore, OldSStroker has it covered.
