Comp Ratio question
12-12-2009, 12:08 AM
#1
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Comp Ratio question Need a little help
I have a LQ4 engine with the stock heads I believe there 74cc I have just purchased a set of dart heads that are 62cc.
The comp ratio was 9.5 to 1 Will the new ratio be 11 to 1 .
Thanks for any help
I have a LQ4 engine with the stock heads I believe there 74cc I have just purchased a set of dart heads that are 62cc.
The comp ratio was 9.5 to 1 Will the new ratio be 11 to 1 .
Thanks for any help
12-12-2009, 04:29 AM
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Formulas for compression ratio and combustion chamber size Handy formulas:
cylinder volume = bore * bore / 4 * stroke * pi.
engine displacement = cylinder volume * number of cylinders.
compression ratio = (cylinder volume + combustion chamber volume) / combustion chamber volume.
combustion chamber volume = cylinder volume / (compression ratio - 1).
---
Example usage, given your figures:
Bore x stroke: 101.6 mm x 92.0 mm.
Cylinder volume: 101.6 * 101.6 / 4 * 92.0 * 3.14159 = 745873 cubic millimeters = 746 cc.
Stock compression ratio: (746 + 74) / 74 = 11.081.
Compression ratio with 62 cc heads: (746 + 62) / 62 = 13.032.
I don't think this is accurate. The stock compression ratio on an LQ4 (Vortec 6000) should be about 9.4:1, and CR on the LQ9 (VortecMAX) should be about 10.0:1. You can use the last formula to calculate the stock combustion chamber volume:
combustion chamber volume = cylinder volume / (compression ratio - 1).
combustion chamber volume = 746 / (9.4 - 1) = 88.810 cc.
Keep in mind that "combustion chamber" refers to ALL of the space between the top of the piston and the head; this includes the thickness of the head gasket and deck clearance. After accounting for the gasket, and possibly a dished piston, and possibly some deck clearance, the combustion chamber may be a lot bigger than 62 cc. If the piston has negative deck clearance and/or has a dome, the combustion chamber may be smaller than 62 cc.
I'm not sure what the formula for combustion chamber volume would be. Something like:
head volume + deck clearance + gasket + piston crown.
The combustion chambers in the head, and fancy piston crowns, are irregular shapes and have to be measured by filling them with liquid and then measuring the amount of liquid. The space for the gasket is just a short tube: gasket bore * gasket bore / 4 * gasket thickness * pi. Ditto for deck clearance.
If 74 cc is an accurate number, that means there are approximately 14 cc of volume between the gasket, deck clearance and piston crown (if the pistons are dished or reverse dome). (88 - 74 = 14.) So the "62 cc" heads actually give you a 76 cc combustion chamber (62 + 14). That would give you a compression ratio of:
compression ratio = (746 + 76) / 76 = 10.8.
Take all these numbers (especially the decimals) with a grain of salt. I just threw this up for you and I was pretty sloppy with rounding and significant figures. I just did this to show how the formulas can be used.
cylinder volume = bore * bore / 4 * stroke * pi.
engine displacement = cylinder volume * number of cylinders.
compression ratio = (cylinder volume + combustion chamber volume) / combustion chamber volume.
combustion chamber volume = cylinder volume / (compression ratio - 1).
---
Example usage, given your figures:
Bore x stroke: 101.6 mm x 92.0 mm.
Cylinder volume: 101.6 * 101.6 / 4 * 92.0 * 3.14159 = 745873 cubic millimeters = 746 cc.
Stock compression ratio: (746 + 74) / 74 = 11.081.
Compression ratio with 62 cc heads: (746 + 62) / 62 = 13.032.
I don't think this is accurate. The stock compression ratio on an LQ4 (Vortec 6000) should be about 9.4:1, and CR on the LQ9 (VortecMAX) should be about 10.0:1. You can use the last formula to calculate the stock combustion chamber volume:
combustion chamber volume = cylinder volume / (compression ratio - 1).
combustion chamber volume = 746 / (9.4 - 1) = 88.810 cc.
Keep in mind that "combustion chamber" refers to ALL of the space between the top of the piston and the head; this includes the thickness of the head gasket and deck clearance. After accounting for the gasket, and possibly a dished piston, and possibly some deck clearance, the combustion chamber may be a lot bigger than 62 cc. If the piston has negative deck clearance and/or has a dome, the combustion chamber may be smaller than 62 cc.
I'm not sure what the formula for combustion chamber volume would be. Something like:
head volume + deck clearance + gasket + piston crown.
The combustion chambers in the head, and fancy piston crowns, are irregular shapes and have to be measured by filling them with liquid and then measuring the amount of liquid. The space for the gasket is just a short tube: gasket bore * gasket bore / 4 * gasket thickness * pi. Ditto for deck clearance.
If 74 cc is an accurate number, that means there are approximately 14 cc of volume between the gasket, deck clearance and piston crown (if the pistons are dished or reverse dome). (88 - 74 = 14.) So the "62 cc" heads actually give you a 76 cc combustion chamber (62 + 14). That would give you a compression ratio of:
compression ratio = (746 + 76) / 76 = 10.8.
Take all these numbers (especially the decimals) with a grain of salt. I just threw this up for you and I was pretty sloppy with rounding and significant figures. I just did this to show how the formulas can be used.
Last edited by m477; 12-12-2009 at 05:00 AM.
12-12-2009, 01:30 PM
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While we're on the topic of compression calculations, does anyone know the dish volume of 5.3L pistons (on the LM7) compared to the flat-top pistons (on the 4.8L and also the L33)?
I've searched around and couldn't seem to find a number.
I've searched around and couldn't seem to find a number.
